Neurophysiology PDF

NEUROPHYSIOLOGY UNIT 1 → week 1 Neurons are the functional anatomical units of the nervous system. They consist of a cell body, soma, and specialised processes, dendrites and one axon; they also have axon terminals ( synaptic terminals) and a synapse. Axon can extend far away from the soma. For ins...

NEUROPHYSIOLOGY UNIT 1 → week 1 Neurons are the functional anatomical units of the nervous system. They consist of a cell body, soma, and specialised processes, dendrites and one axon; they also have axon terminals ( synaptic terminals) and a synapse. Axon can extend far away from the soma. For instance, axons from the motor neurons innervating right or left foot originate in the spinal cord at the level of the lumbar vertebral column bones where motor neurons bodies are located in the spinal cord. Thus the length of these axons is around 1 metre! Dendrites (more) and axons are ramified close to their end. In the soma of the neuron takes place the majority of all metabolic activities leading to the build of neuronal molecular structures and functional proteins and molecules. In the soma is also located the nucleus, where there are present genes which are expressed according to the functional demands of the neuron. Proteins and small molecules are transported to the terminal ending of neuronals processes using specific molecular structures like microtubules or microfilaments which act as tracks for the anterograde or retrograde transports. In Endoplasmic Reticulum (ER) proteins are synthesised (1); then they are assembled in Golgi apparatus (2) and transported along axon to its endings using microtubules (3) or microfilaments (4) (anterograde transport). Retrograde transport of molecules from axonal endings to the neuronal body utilizes the same mechanisms. Other important functional structures of the neuron are: the axon hillock where the electrical response of the neuron (the action potential) originates; the myelin sheath which is fundamental in regulating how fast action potentials move towards axonal endings. TYPES AND QUANTITY OF NEURONS According to a study, a normal 1.5 kg brain should contain about 86-100 billion neurons and an equal number of non neuronal cells (mostly glial cells). Neurons are not homogeneous. There are around 40.000 different types of neurons. These neurons differ for 3D shapes and biophysical properties. These differences are responsible for their different functional responses once they are stimulated. 1. Sensory neurons ( afferent n ) → they convert external stimuli (light,sound,touch) into electrical impulses, they send info from sensory receptors to the central nervous system. 2. Motor neurons ( efferent n ) → from cns to muscles and glands to trigger movement or secretion. 3. Interneurons → they act as connectors between afferent and efferent n, they are only found in the cns, they process and interpret sensory info and decide the body's response. FUNCTIONAL REGIONS OF THE NEURON Despite differences in shape, in each neuron we can identify 4 functional regions: 1. input region, where contacts (or synapses) with axons from other neurons are located; 2. integrative region, where signals from other neurons are integrated generating neuron own response (action potentials); 3. conductive region, where neuron responses (action potentials) are travelling towards axon endings; 4. output region, where axon terminals (synapses) contacting other neurons are located. SYNAPSES Axon endings terminate in contact with the input region (dendrites and or soma) of other neurons.The number of synaptic contacts on a single neuron can be enormous. It is supposed that there are about 1015 synapses in our brain. Considering the number of neurons (see previously), it could be calculated an average number of 1000 synapses for each neuron. Substantially each synapse represents the input of a single specific neuron. As a consequence, each neuron must integrate the input arriving from about 1000 different neurons. There is obviously an extreme variability in these numbers. Some neurons receive one or very few synapses, while others receive more than 1 million synapses. Through synaptic connections, neurons are anatomically organised in circuits. These anatomical circuits are also functional circuits since signals (action potentials) generated by one neuron are transmitted to another one at the level of their synaptic connection. UNIT 2 → week 1 GLIAL CELLS They modulate neuronal function and signalling and they provide support to the nervous system. They help nerves to be stuck in one place and they make them work the way they should. Glial cells are the most numerous cells in the brain. They are also considered as the “sleeping giants” of neuroscience, because their functional role, even if progressively revealed in the last years, remains still unknown. In any case, it seems clear that glial cells contribute to brain function supporting neuronal function. Nevertheless it is clear that without glia, the brain cannot function properly: an example is multiple sclerosis, a disease that specifically leads to the destruction of glial cells (oligodendrocytes) associated with the development of signs of mild or significant nervous system dysfunction. GLIAL CELLS CLASSIFICATION (4 types) 1. Astrocytes ○ They form the Blood Brain Barrier 2. Myelinating glia ○ Oligodendrocytes ○ Schwann cells ○ They regulate Action Potential conduction along axons 3. Microglia ○ They are involved in brain inflammatory processes 4. Ependymal cells ○ They form the wall of ventricular spaces, i.e. the internal empty spaces of the brain filled by extracellular fluid or liquor ASTROCYTES The most numerous glia in the brain are astrocytes. These cells fill most of the spaces between neurons. The space remaining between neurons and astrocytes in the brain is calculated to be 20 nm on average. They play a fundamental role in regulating the extracellular space milieu. They have a stellate shape with processes spreading from the cell body to the periphery (A). At synapse level, astrocyte processes regulate extracellular space milieu, an important function which stabilises synaptic function by maintaining a constant ion concentration, scavenging and eliminating neurotransmitters or other molecules which could interfere with synaptic function (B). ASTROCYTES AND BLOOD BRAIN BARRIER (BBB) Astrocyte processes endings or end feet are part of the so-called Blood Brain Barrier (BBB), an anatomical and functional barrier between blood and brain extracellular space. BBB is made by the capillary (peripheral blood vessel) wall (endothelium) and astrocyte end foot. Gas (O2 and CO2, H+ and other ions, molecules, chemicals (and drugs) must physically cross this barrier to access neuronal extracellular space. BBB is particularly important because it maintains brain extracellular space with respect to blood and body extracellular fluids and because it is a key point for possible useful drugs for brain dysfunctions. In fact, chemical compounds showing promising results in vitro which are not able to cross BBB, are of no therapeutic utility unless directly injected (intrathecal injection) in the brain. MYELINATING GLIA 1. Both central and peripheral axons are wrapped by oligodendrocytes (central axons) and Schwann cells (peripheral axons), which form myelin sheaths around them; 2. Myelin sheath are periodically (every 1 mm) interrupted at the so called Ranvier nodes; 3. Myelin function is to create an insulator layer around the conductive membrane, which localises neuronal membrane electrical phenomena to Ranvier nodes. ○ In this way, the conduction of Action Potential from axon hillock to its endings, is “jumping” from one Ranvier node to the closest one. Action potential conduction is therefore “saltatory” and faster than normal action potential conduction in unmyelinated axons. Lesson 2 GROSS ANATOMY OF CNS → central nervous system Main divisions of the Central Nervous System: Encephalon with Cerebral hemispheres (Cerebral Cortex) and deep structures (Thalamus Hypothalamus Basal Ganglia + other internal structures), Cerebellum, Brainstem (Midbrain+Pons+Medulla), Spinal cord. FUNCTIONAL ORGANISATION OF CNS The nervous system is organized in a peripheral nervous system and a central nervous system (see figure below). PERIPHERAL NERVOUS SYSTEM: ○ Afferent division: it collects sensory informations from the close external environment (somatic inputs from cutaneous and skeletal-muscle receptors) or far external environment (special sensory inputs from visual, acoustic, olfactory, vestibular and gustatory receptors), and sensory informations from the internal body environment (visceral sensory inputs from internal viscera). ○ Efferent division: it sends motor commands to body muscles (somatic motor efferents to skeletal muscles and visceral motor efferents to visceral smooth muscle). Visceral smooth muscle is the muscle of the gastrointestinal wall, urinary bladder, vessels of the cardiovascular system, exocrine glands and smooth muscle. CENTRAL NERVOUS SYSTEM: ○ Processing of sensory inputs to generate motor outputs. BRAINSTEM Brainstem is a region that relays information from and to the cortex integrates sensory and motor information from and to the head. Brainstem is subdivided in: 1. Midbrain ○ sound (inferior colliculi) and gaze (superior colliculi) orientation. ○ locomotor center. 2. Pons ○ connective bridge to the cerebellum. ○ respiratory control centers. 3. Medulla ○ control of basic functions (respiratory and cardiovascular control, sleep-wake cycle, attention and arousal). ○ posture control. SPINAL CORD Spinal cord is the region where the first sensory neurons relay sensory information to the brain, and where are located motoneurons which send to the muscles the motor commands originated from the brain. Both sensory and motor information are collected or sent to the rest of the body through spinal nerves (peripheral nerves). CEREBELLUM Cerebellum is connected both with the spinal cord and encephalic structures. It is organized in a peripheral 3-layers cortex, site of cerebellar input processing, and deep nuclei which receive cerebellar cortex output. Cerebellar cortex output is always inhibitory. Cerebellum plays a role in: balance and motor control; fluidity and precision of mental processes. CEREBRAL HEMISPHERES → DIENCEPHALON Thalamus is a large relay center for almost all sensory information directed to the cortex and of much motor information originating from the motor cortex. It is also a center for the control of the wake/sleep state of the cerebral cortex. Hypothalamus controls behaviors that help the body satisfy its needs to maintain the internal equilibrium (homeostasis). When organisms have a particular need, they generally generate a behavior designed to maintain the body back to this stable state, known as homeostasis. For example, when hungry or thirsty, a person will engage in behaviors that lead to ingesting food or drink; if cold, the person will search for a blanket or a warmer location. The hypothalamus provides the signals telling the brain that these sorts of behaviors are needed. CEREBRAL HEMISPHERES → SUBCORTICAL SYSTEMS Limbic system → Scientists thought that these structures functioned mainly to process emotional information, by linking information from the sensory world and from an individual’s internal state with information from the cortex. Even if these concepts are retained, new functions are now attributed to the limbic system, such as quick response to salient information (amygdala), selection of action or motivation (cingulate cortex), memory formation and place learning (hippocampus). CEREBRAL HEMISPHERES → SUBCORTICAL SYSTEMS Basal Ganglia involved in motor control. involved in cognitive functions. CEREBRAL HEMISPHERES → CEREBRAL CORTEX Cerebral cortex is made by 6 layers of neurons (4 in the hippocampus). Cerebral cortex is the higher and more complex sensorimotor information processing and other cognitive functions site in our brain. At cortical level cognitive functions are processed, for instance object recognition, spatial processing, and attention. PERIPHERAL NERVOUS SYSTEM -→ SOMATIC DIVISION All nervous information is collected from the rest of the body and sent to the rest of the body through peripheral nerves which are mixed (they contain sensory and motor axons), pure sensory (only sensory axons) or motor (only motor axons). MOTOR INFORMATIONS Motor neurons, whose body is located in the Central Nervous System (CNS), innervate peripheral skeletal muscles (voluntary muscles) through their axons. At neuromuscular synapses, the neurotransmitter Acetylcholine is released. The venoms curare or bungarotoxin selectively block neuromuscular synapses causing a paralysis leading to death as a consequence of the block (paralysis) of respiratory muscles. PNS → SOMATIC DIVISION Therefore the somatic division of the peripheral nervous system consists in the motor subdivision and in the sensory subdivision. - The motor subdivision consists of spinal motor neurons (arrow in Figure A) and their axons which innervate their target skeletal muscles. - The sensory subdivision consists of a first sensory neuron (1 in the figure B), whose body is peripherally localized in a spinal sensory ganglion. This neuron is connected to a second sensory neuron (2 in Figure B) whose body is in the spinal cord. The different sensory informations (modalities) carried by sensory fibers are: a. pain; b. temperature; c. touch; d. proprioception (muscle and joint receptors). PNS (peripheral) → AUTONOMIC DIVISION The autonomic division of the peripheral nervous system consists in a motor subdivision and in a sensory subdivision. Both innervate smooth and cardiac muscles, glands and the complex neuronal network of the gastrointestinal wall. Autonomic motor innervation (see figure and table) is a two neuronal chain in which the body of the first motor neuron, or preganglionic neuron is located in the spinal cord, while the body of the second motor neuron, or postganglionic neuron, is located in a peripheral autonomic ganglion. AUTONOMIC NERVOUS SYSTEM An overview of the innervation of our viscera by sympathetic and parasympathetic divisions of the autonomic nervous system. It innervates and regulates the functions of: 1. internal viscera. 2. internal organs. 3. glands. Lesson 3 BASIC PRINCIPLES OF ELECTRICITY I= ∆V/R Where I is current or movement of an electrical charge; ∆V is voltage or potential difference across two points; R is the resistance encountered by current to move from one point to the other in the electrical circuit. In biology, an “electrical charge” is an ion: anions (negatively charged ions) or cations (positively charged ions). Thus, current, in biology, is the movement of an ion across two points with different voltages (for instance between the inner and the external surface of cellular membranes). CELL POTENTIAL ACROSS MEMBRANE By inserting a microelectrode in the cell through the plasma membrane and placing another electrode in the extracellular fluid bathing the cell (a), a potential difference or membrane potential across the cell plasma membrane is measured (b). By convention, the extracellular fluid is designated as the voltage reference point, and the inside voltage of the cell is designated as the excess of charge with respect to the external reference voltage (which is by convention equal to 0). Thus membrane potential is the excess of charge (negative or positive) of the inside of the cell with respect to the outside. In neurons membrane potential is -70 mV; in cardiac muscle cells, for instance is -90 mV like in skeletal muscle fibres. The sign of inside voltage is designated as the sign of the excess of charge. If negative charge prevails, membrane potential is negative (b); if positive charge prevails, membrane potential is positive. ORIGIN OF MEMBRANE POTENTIAL Ions can cross the membrane only through specific channels (ion channels) where they encounter a resistance to cross from one side to the other of the membrane. Thus they move across the membrane encountering a "resistance" (R in the equation; see previous slides); more channels equals lower R; less channels equals higher R. K+ is more concentrated inside the cell than outside the cell; Na+ is more concentrated outside the cell than inside the cell. Ions diffuse through the membrane towards the zone where they are less concentrated, but when they move across the membrane also an electrical charge (positive or negative) is moved from one side to the other side of the membrane. Thus an excess of charges is accumulated on the side of the cell where ion moves. Ions cross the membrane through selective specific channels. There are K+ selective channels and Na+ selective channels, for example. Selective channels densities are enormously different. K+ channels are extremely dense with respect to Na+ channels. Thus, in resting conditions, R to K+ movement is low; R to Na+ is very high. ION CHANNELS Ions move from a compartment where they are more concentrated to a compartment where they are less concentrated. This is a diffusion process. Ion channels are specific for each ion. This signifies that K+ crosses the membrane through potassium channels and Na+ crosses the membrane through sodium channels. The specificity of an ion channel is determined by ion dimension and biophysical properties allowing only the specific ion to get through the channel itself. EQUILIBRIUM POTENTIAL When one ion diffuses toward a compartment where it is less concentrated, an electrical charge is lost in one compartment and acquired in the other. A net charge increase is observed in the compartment where ion is moved. As charges accumulate in the compartment where ions are moving, an electrical attraction induced by the change of charge in the compartment abandoned by ions is gradually generated, which is evoking an opposite flux of ions back towards the original compartment. Eventually an equilibrium between the diffusion flux and the opposite electrically induced flux is reached. Membrane potential at the equilibrium is called Nernst’s equilibrium potential. It is particular for each ion, depending on ion concentrations on one and other compartments. In the neuron it depends on ion concentrations inside and outside the cell. Resting membrane potential is the weighted (depending on the relative capacity to cross the membrane of each ion) summation of K+ and Na+ Nernst’s equilibrium potentials. Because in resting conditions, K+ is far more “permeable” than Na+ it derives that membrane potential is close to Nernst’s K+ equilibrium potential, which is approximately -90 in physiologic conditions. MAINTAINING ION CONCENTRATION OUTSIDE AND INSIDE A membrane molecular mechanism (Na+/K+ pump) actively pumps 3 Na+ outside the cell and 2 K+ inside the cell, consuming energy or ATP.ATP is obtained by O2 and Glucose consumption. Then K+ back diffuses outside across the membrane through selective channels densely present (low resistance or high permeability). Then Na+ back diffuses inside across the membrane through selective Na+ channels scarcely present. There’s a net movement of positive charges outside the cell, which establishes a potential gradient across the plasma membrane. Note: an infinitesimal fraction of the total number of charges number of charges in the two compartments movie. Thus the total number of positive and negative charges in each of the two compartments is in equilibrium. LOCAL POTENTIALS Electrical stimulation of a cell membrane region results in an increase (hyperpolarization) or reduction (depolarization) of its negative potential. Electrical stimulation induces a flow of charges (ions) through the passive ion channels of the stimulated membrane region which is responsible for its hyperpolarization or depolarization. These local and transient changes in membrane potential (local potentials) propagate to adjacent membrane regions and gradually dissipate away from the point where they were originating due to the biophysical properties of cell membrane. Part of the current is dissipated in the extracellular space and part is internally dissipated in the neuron. ACTION POTENTIAL When membrane depolarization reaches a particular value (threshold value; -55 mV) a stereotyped response of the excitable cell membrane, called action potential, is triggered. Once triggered, action potential proceeds without stopping. Cell membrane continues to depolarize (depolarization phase), becoming positive inside than outside (inversion of membrane polarity) until it reaches a maximum peak (peak of the action potential) in which the membrane potential is positive about 20-30 mV inside than outside (see figure in the next slide). Once action potential has peaked, the membrane actively repolarizes (repolarization phase) by rapidly restoring the previous resting membrane potential value (-70 mV). Action potential is very fast in neurons (about 1 ms of duration), slightly longer in skeletal muscle fibers (about 2 ms) and much longer in cardiac myocytes (about 250 ms). An example of action potential → After the threshold is reached, it is triggered by an all or nothing response with a progressive depolarization of the membrane which reaches a peak. At the peak membrane potential is reversed (positive inside with respect to outside). Then a repolarization phase is observed which restores the original membrane potential (negative inside at -70 mv). Action potential, differently from local potential, is all or nothing. Once the threshold is reached it develops completely and fully. It does not matter the intensity of the stimulation of the membrane, as long as it is sufficient to depolarize it at the level of the threshold value. We identify three main characteristics of the Action Potential. 1. the presence of a membrane potential threshold value for its trigger; 2. once triggered, it fully develops; 3. It is characterized by a strong and rapid depolarization of the membrane with reversal of its polarity at the peak of the action potential. Repolarization is very fast and strong, too. The characteristics of the action potential are determined by the rapid modification of membrane permeability for the two main ions: sodium and potassium. Under resting conditions, membrane potential is basically determined by the membrane potassium permeability, far higher than that for sodium. At the threshold, membrane channels open, which were previously closed. They are the voltage-sensitive channels for sodium. The opening of these channels dramatically changes the membrane permeability for sodium which becomes much higher than that for potassium. This induces the membrane potential to assume values similar to the Nernst equilibrium potential for Sodium (+30 mV inside than outside), and therefore the membrane depolarizes rapidly approaching, at the peak of the action potential, the value of the Nernst equilibrium potential for Sodium. Near the peak of the Action Potential, the sodium channels close inactivating, and sensitive voltage channels open up for potassium. The membrane permeability for this ion again becomes much higher than for sodium, and the membrane potential rapidly tends to summarize values similar to Nernst equilibrium potential for potassium (-90 mV indoors than outside). When the resting membrane potential is restored, the sodium-dependent voltage channels return ready to reopen again as soon as the membrane depolarizes at the threshold. ACTION POTENTIAL CONDUCTION ALONG CELL MEMBRANE The all or nothing properties of the Action Potential make it possible to propagate to the entire membrane of the neuron without any energy dissipation as it is observed for local potentials. Local potentials, i.e. those characterized by a transient hyperpolarization of the membrane or its depolarization below the threshold, propagate with decrement to the adjacent regions of the cell membrane dissipating at a certain distance from the point of origin depending on the biophysical characteristics of the membrane itself. AP ORIGIN In the neuron, action potential usually originates at the level of the point of origin of the axon: the axon hillock. From the axon hillock, along the entire axon are present the voltage-sensitive channels specific for sodium and potassium (i.e. the channels responsible for the development of action potentials). In the soma of the neuron and its dendrites these active channels are not present. Therefore, if the membrane potential of the axon hillock reaches the threshold, an action potential is evoked that then propagates along the entire axon to its end. AP PROPAGATION The propagation (or conduction) of the action potential along the axon occurs for continuity or in a “saltatory” manner. - Continuous conduction (between the region where the action potential is triggered and the adjacent regions) takes place in the non-myelinated axons. - Saltatory conduction is observed in myelinated axons, i.e. wrapped in the membrane of myelin cells (oligodendrocytes or Schwann cells). In these axons, the insulating sheath formed by the membrane of the myelinic cells stops about every mm (Ranvier nodes) leaving not wrapped the axon membrane, which at that point has dependent voltage channels. Action Potential therefore "jumps" from one Ranvier node to the next. The rate of propagation of the Action Potential is therefore higher in myelinated axons than in those not myelinated by its ”saltatory" nature. With the same myelination (or non-myelination) the rate of propagation of the action potential is higher in axons with a larger diameter. SALTATORY CONDUCTION At the Ranvier node, the axon is not myelinated and its membrane is directly in contact with extracellular space. Here voltage-sensitive channels are present, making possible ion fluxes responsible for action potential generation. Action potential generated in one Ranvier node, then depolarizes the next close Ranvier node, till threshold evoking in this last an action potential. Therefore action potential propagates along the axon in a “saltatory” manner. Lesson 4 SYNAPSE Neurons connect to each other at the synapse level. Synapses are formed by the axonal bouton endings of a presynaptic neuron and the membrane of dendrites, soma or, sometimes even axons, of the postsynaptic neuron. In the human nervous system there are an estimated 1012 neurons and about 1015 synapses, which gives an incredible number of about 103 synapses on average for each neuron. Considering that the diameter of a neuron is on average about 10 micrometers, one can imagine the extreme density of these synapses on the soma and dendrites of the neuron. STRUCTURE OF SYNAPSE At synapse, the membranes of the presynaptic neuron (presynaptic membrane) and postsynaptic neuron (postsynaptic membrane) are very close, but separated by a narrow space: synaptic cleft. Therefore, the electrical signal (action potential) of the presynaptic neuron is not directly transmitted to the postsynaptic neuron, but is transformed in the presynaptic terminal into a chemical signal that is then translated again into an electrical signal at the postsynaptic membrane level. These physiological processes occur because of the different characteristics of pre- and postsynaptic structures. PRESYNAPTIC EVENTS: neurotransmitter release Inside the presynaptic bouton are vesicles that contain the molecules of a substance: the neurotransmitter. These vesicles are free to move inside the bouton. Some of them are attached at particular points in the presynaptic membrane (active zones) with a multiprotein complex, the SNARE complex, formed by the vesicle protein (Synaptobrevin) and two presynaptic membrane proteins (SNAP25 and Synapsin). When action potential arrives at the end of the axon, voltage-dependent calcium channels which are present only at this level, open. Calcium goes into synaptic bouton, activating a protein (synaptotagmin) that, once bound to the SNARE complex, further brings the vesicle membrane and the presynaptic membrane closer together. The fusion of the vesicle with the presynaptic membrane activates a process of neuron exocytosis that in about 0.3 ms releases into the synaptic space the neurotransmitter molecules contained in the vesicle. The electrical signal (action potential) of the presynaptic neuron is then transformed into a chemical signal (release of the neurotransmitter) for the post-synaptic neuron. A single neuron generally releases only one type of neurotransmitter molecule. This allows to classify neurons on the basis of the released neurotransmitter molecule. POST-SYNAPTIC EVENTS: post synaptic potentials Neurotransmitter molecules, once released into the synaptic cleft, bind to specific receptors located in the postsynaptic membrane. These receptors are: 1. Active ion channels that are opened by the bound neurotransmitter; 2. Protein complexes that, once bound the neurotransmitter, activate the formation of second intracellular messengers in the postsynaptic neuron. These can in turn open specific active ion channels of the membrane of postsynaptic neurons. In both cases there is a flux of specific ions through the membrane of the postsynaptic neuron that induces the appearance of a transient depolarization or hyperpolarization. These are local potentials and postsynaptic potentials. Then neurotransmitter molecules are degraded by specific enzymes localized in the pre- or postsynaptic membrane or they are reuptake by specific transporters back again in the synaptic bouton and utilized to refill new vesicles. EXCITATORY POSTSYNAPTIC POTENTIALS (EPSPs) Post-synaptic potentials will be excitatory if they depolarize the membrane of the postsynaptic neuron approaching the potential threshold for triggering the action potential. INHIBITORY POSTSYNAPTIC POTENTIAL (IPSP) Post-synaptic potentials will be inhibitory if they hyperpolarize neuronal membrane potential away from the threshold of action potential. DIFFERENT TYPES OF NEUROTRANSMITTERS There are different types of neurotransmitters. Glutamate and Acetylcholine are always excitatory neurotransmitters. ○ Acetylcholine is the excitatory neurotransmitter of the neuromuscular synapse, that is, the synapse through which the spinal motor neuron activates muscle contraction. ○ Glutamate is the main excitatory neurotransmitter of the CNS. The release of Glutamate from dead neurons after an ischemic stroke could be the cause of an increased neuronal damage due to the increased neuronal activity requiring more oxygen supply. Norepinephrine, Serotonin and Dopamine may be excitatory or inhibitory neurotransmitters, depending on the type of receptor present in the membrane of the postsynaptic neuron. Glycine and GABA (gamma-amino-butyrric acid) are neurotransmitters that are always inhibitory. Benzodiazepines are drugs binding GABA postsynaptic receptors, activating them as done by GABA. Glycine release is blocked by strychnine and tetanus toxin. This induces the simultaneous increase of flexor and extensor joint muscles contraction with consequent “spastic” paralysis. NEUROTRANSMITTERS BIND POST-SYNAPTIC RECEPTORS Released neurotransmitter molecules diffuse in the synaptic cleft and bind to the postsynaptic receptors. These can be ligand-gated ion channels (i.e. neurotransmitter molecules bind to the receptor-channel and open it). These are the so-called ionotropic receptors. Or they can be receptors activating intracellular enzymes. In this case, a second cytoplasmic messenger is synthesised. This last, in turn, opens other ion channels generating the postsynaptic potential. These receptors are called metabotropic receptors. TYPES OF NEURONS It is possible to classify different types of neurons in accordance with the released neurotransmitter. Different types of neurons can be distributed widely in the central nervous system, or be located in specific and limited regions.In the figure: dopaminergic neuronal bodies site and their projections to the subcortical and cortical structures are shown. NOREPINEPHRINE SYSTEM The following image shows the norepinephrine system, with the locus coeruleus sending projections to key brain areas like the neocortex, thalamus, and hypothalamus, influencing arousal and attention. SEROTONIN SYSTEM SYNAPTIC INTEGRATION On average about 1000 synapses are present on each neuron. Each of these synapses will be active in relation to the activity of the presynaptic neuron. If at each action potential arriving at the synapse, a depolarizing or hyperpolarizing post-synaptic potential is generated on the membrane of the postsynaptic neuron, these synaptic potentials will add together in time and space like the waves generated on the surface of a pond by a handful of small stones thrown into the pond itself. Spatial summation is the sum of postsynaptic potentials generated by different synapses and temporal summation is the sum of postsynaptic potentials generated by sequences (bursts) of action potentials arriving at a single synapse. If, as a result, the potential of the postsynaptic neuron axon hillock membrane is sufficiently depolarized to reach the threshold, an action potential will be evoked. In this case the neuron is excited by synaptic activity. If, on the other hand, the membrane potential of the axon hillock hyperpolarizes moving away from the action potential threshold, a higher depolarization will be necessary to evoke an action potential. In this case the neuron is inhibited by synaptic activity. SPATIAL SUMMATION At the axon hillock, excitatory and inhibitory postsynaptic potentials, originate from different synapses, algebraically summate. TEMPORAL SUMMATION At the hillock excitatory postsynaptic potentials summate, depolarizing the hillock membrane to the Action potential threshold and evoking an action potential. If there is a temporal summation of inhibitory postsynaptic potentials, hillock membrane potential is hyperpolarized. NEURONS HUB Therefore, neurons that are connected to each other to form networks and circuits, are hubs which receive multiple information (in the form of sequences of action potentials) from other neurons, integrate them through synaptic integration, and generate in response sequences of action potentials which they send to other hub neurons. DIGITAL CODING Therefore all information (sensory, motor, higher and complex neuronal activities) are coded as sequences (bursts) of action potentials. It is a sort of digital processing of all information, which is integrated and processed in each neuron of neuronal networks. Each vertical bar represents an extracellularly recorded action potential from a neuron. It is clear the change of burst frequency, the presence of silent periods, or the presence of single action potentials. This is the coded information. Lesson 5 → Functional organization of spinal cord segments: input/output SPINAL CORD GROSS ANATOMY The spinal cord is the part of the central nervous system that is located within the vertebral canal and extends from the foramen magnum of the skull to the third lumbar vertebra. Initially, during development, the spinal cord occupies the entire length of the vertebral canal. Subsequently it grows slower than the vertebral canal, and in the adult it occupies only two-thirds of the upper spine. As a result, the lumbar and sacral nerve roots descend for a while inside the vertebral canal before exiting their respective intervertebral foramina. The spinal cord is divided into segments (myomers) according to the points of origin of the spinal nerves. Each spinal nerve consists of an anterior (motor) root and a posterior (psychic) root that merge together forming the spinal nerve exiting the respective intervertebral foramen. There are 8 cervical myomers, 12 thoracic, 5 lumbar, 5 sacral and 1 coccygeal that give rise to 31 pairs of spinal nerves. SPINAL CORD MICROSCOPIC ANATOMY The transverse section of the spinal cord shows a white peripheral region and an H-shaped gray central matter. 1. The peripheral region is formed by bundles of nerve fibers projecting towards upper structures (spinal segments, brainstem, encephalon) or projecting from these last to the spinal cord segments. 2. The central region can be divided into two specular regions (left and right) each of which has a posterior (dorsal horn) and an anterior extension (anterior horn). 3. In the dorsal horn are located the sensory neurons that receive, through the sensitive nerve fibers of the spinal nerves, the sensory information collected by the peripheral cutaneous or internal receptors. 4. In the anterior horn, motor neurons innervate skeletal muscles, that is, voluntary muscles. Together with sensory and motor neurons there are numerous interneurons both excitatory and inhibitory, which form simple and complex circuits generating local motor responses to sensory inputs arriving to the same spinal segment/s. SECTIONAL ORGANISATION OF THE SPINAL CORD Peripheral white matter is the spinal cord region where are located the bundles of nervous fibres which relay sensory information to brain higher centers (brainstem or encephalon) and relay higher centers motor and control information directed to the spinal cord segments and neurons. Sensory signals ascending to higher levels travel along posterior columns and lateral columns. Motor signals arriving from higher centers travel along anterior columns. In the gray matter somatic neurons are separated from visceral neurons. SPINAL CORD: INPUT/OUTPUT Action potentials originating from peripheral sensory receptors are directed towards the spinal cord along the centrifugal branch of the first sensory neuron, whose cell body is located in the sensory ganglia close to the spinal cord but outside the central nervous system. From the cell body of the first sensory neuron, action potentials continue towards the spinal cord through the centripetal branch of the sensory neuron and enter the spinal cord through the posterior roots of the spinal nerve. In the posterior horn of the spinal cord the centripetal branch of the first sensory neuron makes synaptic contact with the second sensory neuron. Second sensory neuron, directly (see figure) or indirectly, by way of interneurons, excites motor neurons. Action potentials generated by motor neurons (located in the anterior horn) exit the spinal cord through the anterior roots of the spinal nerve which innervates skeletal muscles. Other motor neurons innervate the involuntary smooth muscle of : 1. internal organs: e.g. gastrointestinal tracts or bladder; 2. cardiovascular system: e.g. heart and parietal blood vessels smooth muscle; 3. exocrine glands. SEGMENTAL CIRCUITS IN THE SPINAL CORD At the level of a single or more spinal segments, there are local circuits which generate automatic direct motor responses or reflexes, to sensory stimuli that reach the same spinal cord segments. These automatic motor responses are the elementary motor responses of our nervous system. The coordinate organization of these elementary (simple and complex) motor responses by the supraspinal structures (brain and brainstem) allows our nervous system to generate more complex motor responses, such as automatic postural adjustments in static and dynamic conditions that allow us to maintain posture, locomotion, or voluntary movements directed towards a specific goal constituting behavior. SPINAL CORD: SENSORY ASCENDING PATHWAYS The main ascending pathways relay sensory information coming from cutaneous and deep (visceral and muscle-skeletal) receptors. This information reaches the thalamus where there is the third neuron of the sensory pathways. Thalamic sensory neurons relay sensory information to the primary somatosensory cortex. SPINAL CORD: DESCENDING PATHWAYS The main descending pathways relay motor signals originating from the upper motor neurons of the primary motor cortex and stimulate the response of spinal cord motor neurons (lower motor neurons). Usually spinal cord motor neurons are not directly stimulated. They are indirectly stimulated through interneurons. This can be viewed as an upper “control” of spinal cord motor circuits, rather than a pure direct stimulation of a specific motor neuron. BASIC REFLEXES Reflexes are a way to organize fast motor responses to sensory external or internal stimuli. They are automatic responses which can be very simple (monosynaptic reflexes) or more (polysynaptic reflexes) or extremely complex (central pattern generators, which generate the limb extensor/flexor muscle alternating contraction typical of locomotion). In reflexes, the fine goal directed feature of voluntary movements is lacking in favour of a fast motor response whose intensity is proportional to sensory stimulus intensity. According to the famous neurophysiologist Charles Scott Sherrington, reflexes are the basis of voluntary movements. We can imagine that, in voluntary movements, the cerebral cortex plays a role in “organizing” these elementary motor circuits by synchronizing them and directing them towards a goal. A similar role is played by the brainstem in organizing spinal cord segmental sensory-motor circuits for brainstem multisegmental reflexes regulating static and dynamic posture of our body, or in generating locomotion. AUTONOMIC REFLEXES Similarly to the above described somatic motor reflexes evoked by sensory inputs, it must be considered that visceral sensory inputs evoke autonomic or vegetative motor signals regulating the activity of cardiovascular system or gastrointestinal system, as well as regulating glands secretion. These segmental responses can be also organized by brainstem inputs, in turn evoked by visceral sensory signals (an example is the baroreceptors reflex). REFLEX ARC A reflex arc is the neural pathway that controls a reflex. A reflex arc is made by: an afferent sensory fiber which carries sensory inputs from specific receptors (in this case cutaneous pain receptors stimulated by a heat source; an efferent motor fiber which carries the automatic (reflex) motor response to the sensory stimulus (in this case pain); a central (spinal cord gray matter) integrating center which processes sensory stimulus and organises motor response. FLEXOR ( WITHDRAWAL ) REFLEX The flexor (withdrawal) reflex is evoked by the stimulation of the cutaneous pain receptors of a limb. 1. Nociceptive signals reach the spinal cord where through interneurons they activate the motor neurons for the flexor muscles of the same limb.  2. At the same time, inhibitory interneurons activated by the same nociceptive signals inhibit motor neurons for the extensor muscles of the same limb, which can thus flex and move away from what caused the pain. 3. Through other interneurons activated by nociceptive signals, motor neurons are stimulated for the extensor muscles of the contralateral limb and inhibited those for their flexor muscles. In this way there will be an extension of the contralateral limb that aims to maintain the postural stability of the body preventing it from falling to the ground when the injured limb has flexed. STRETCH REFLEX The stretch reflex is evoked by the stimulation of stretch receptors arranged between muscle fibers of skeletal muscles. These receptors inform the central nervous system of excessive passive elongation of the muscle fiber themselves, evoking an automatic (reflex) contraction of the same muscle fibers that opposes their elongation. This is achieved through the direct synaptic activation of motor neurons by sensitive fibers from neurons of the corresponding sensory ganglion. At the same time, collaterals of the same sensitive fibers synaptically activate the inhibitory interneurons which block motor neurons for muscles that antagonize the movement induced by the contraction of the passively elongated muscle. Blocking the release of the neurotransmitter glycin at the level of these inhibitory synapses results in spastic paralysis of movement. Movement is blocked (paralysis) due to the simultaneous contraction of the agonist and antagonist muscles. A similar condition is observed as a result of tetanic toxin envenomation which, in fact, blocks the release of the neurotransmitter glycine. Stretch reflex of Quadriceps femori induced by patellar hammer percussion. Inhibitory block of the motoneurons of antagonist muscle (Hamstrings) is also shown. LESSON 6 Functional view - brainstem BRAINSTEM: FUNCTIONAL OVERVIEW The brainstem is the part of the central nervous system that joins the spinal cord with the encephalon. It is divided into three regions, in rostro-caudal direction: midbrain, pons and medulla. In the brain stem are located neurons that collect the sensory information of the head and that send motor signals to the voluntary muscles of the same region. From this point of view we therefore find a functional organization of sensory inputs and motor outputs similar to that observed in the spinal cord. Sensory signals travel along the peripheral sensory fibers of the primary sensory neurons located in the peripheral sensory ganglia and, along the centripetal projection of the same neurons, they reach the brain stem where they make synaptic contact with the sensory neurons of the sensory nuclei. Motor signals travel along axons of neurons of the motor nuclei of cranial nerves. Sensory and motor fibers can be together (mixed sensory and motor cranial nerves), or be the only component (motor or sensory) of the cranial nerves. In the brain stem, moreover, we can find sensory neurons that process special senses (acoustic; vestibular; gustatory) or particular forms of visceral (internal) senses such as sensory signals about blood pressure (baroreceptors of the aorta and carotid) or about the oxygenation of the arterial blood (chemoreceptors of the aorta and carotid). BRAINSTEM MAIN NEURONAL CENTERS In the brain stem are also localized: neurons and circuits that control, through direct pathways to the spinal cord, the functional activity of the spinal cord segments; neurons and circuits that receive sensory informations from spinal cord segments through direct or collateral pathways of the ascending projections to the brain; neurons and circuits that control the activity of high brain centers or networks; neurons and circuits that control the state of contraction of visceral smooth muscles (gastrointestinal motility and exocrine glandular secretion), cardiovascular and respiratory responses in different states of activity of our body (control of cardiac output and blood pressure; control of ventilatory activity). MOTOR DESCENDING PATHWAYS Major descending pathways originating from brainstem. These pathways control static and dynamic posture as well as elementary distal movements of the limbs. The reticulospinal motor projections are responsible for static postural reflexes maintaining posture in orthostatic conditions (upright posture). A reduction of brainstem blood supply could cause a sudden loss of reticulospinal signals leading to a sudden loss of upright posture associated or not with loss of consciousness (drop attack). The vestibulospinal projections regulate dynamic postural reflexes during body/head movements. The rubrospinal projections function is less known. There are no descriptions of a pure lesion of red nucleus in humans. In primates, its stimulation generate gross movements in the limbs distal parts BRAINSTEM: CONTROL OF DYNAMIC AND STATIC POSTURE Important are the neurons that control the dynamic and static posture of our body (vestibulo-spinal pathways; reticulo-spinal pathways) and of our gaze (vestibulo-oculomotor pathways; reticular oculomotor centers). In static or dynamic conditions (when we are moving or we are moving segments of our body) we must always maintain our center of gravity within the body support area on the ground. This is achieved through postural adjustments involving all body skeletal parts and the muscles that move them. These postural adjustments are controlled by brainstem neurons and their projections to the spinal cord segments. Static posture is mainly controlled by reticulo-spinal pathways and dynamic posture by vestibulo-spinal pathways. At the same time, a focussed vision must be maintained by preventing the retinal slip of the visual field through gaze adjustments induced by vestibulo-oculomotor signals. Brainstem gaze-related neurons and circuits are also involved in helping to focus salient static or moving visual objects on the fovea, the retinal region with the maximum visual acuity. VESTIBULO-OCULAR REFLEX Head angular and linear accelerations are sensed by Vestibular receptors of the semicircular canals and maculae (utriculus and sacculus). These signals are transmitted to the vestibular nuclei where they are processed to generate vestibulo-ocular movements (vestibulo-ocular reflex) with the same, but opposite, velocity of the head movements. In this way the visual field is stabilized on the retina, despite head movement. The circuit of horizontal vestibulo-ocular reflex is described in the figure. Sensory vestibular afferents stimulate sensory vestibular nuclei neurons, which, in turn, activate ocular muscle motor neurons localized in abducens and oculomotor nuclei of the brainstem. VESTIBULO-SPINAL REFLEX The same vestibular inputs to vestibular nuclei evoke vestibulo-spinal signals (vestibulo-spinal reflex) exciting antigravity muscles (extensor muscles) motoneurons, preventing body falling. For instance, if the body is falling on the right side, homolateral hindlimb extensor muscles are quickly excited by vestibulo-spinal excitatory signals and equilibrium is restored. CARDIOVASCULAR AND OTHER CONTROLS 2 BRAINSTEM: CAC (cardiovascular activity control) In the medulla of the brainstem are localized neurons that control the activity of the autonomic nervous system. The neurons of the autonomic system regulate the frequency of the heartbeat and the diameter of the small arterial vessels, in order to maintain a pressure gradient between arteries and veins allowing a constant and adequate flow of blood from the heart to the peripheral tissue and from this back to the heart. BARORECEPTOR REFLEX Flowchart showing brainstem cardiovascular responses generated by baroreceptors stimulation. Baroreceptors are arterial pressure sensors localized in the aorta and carotid. An increase of arterial blood pressure, increases their outputs to the nervous system, while a decrease in blood pressure, decreases their signals to the nervous system BRAINSTEM: RESPIRATORY ACTIVITY CONTROL In the brain stem there are also neurons that control the respiratory rhythm, the depth of breathing and the alternation between inspiration and expiration through the sequential activation of the inspiratory and expiratory muscles. A pneumotaxic center located in the pons regulates the respiratory rhythm, increasing it when the need for oxygen supply is rised. An apneustic center also located in the pons, controls the switching between inspiration and expiration. Two columns of medullary neurons control the activity of segmental spinal cord inspiratory and expiratory motor neurons, respectively. These motor neurons innervate respiratory skeletal muscles, among which is the diaphragm, the most important inspiratory muscle. BRAINSTEM: CONTROL OF LOCOMOTION In the more rostral region of the brainstem are localised neurons that control locomotion (mesencephalic locomotor centre). These neurons, through their projections to the spinal cord, control the function of the spinal cord segmental circuits (Central Pattern Generators) that regulate the alternating contraction of flexor and extensor muscles of the limbs necessary for locomotion. Stimulation of these mesencephalic neurons in decerebrate animals (in which all connections between brain stem and encephalon were dissected) initiates locomotion and controls its speed. In quadrupeds the frequency of stimulation of the locomotor centre, in addItion to regulating the speed of locomotion, controls the transition from trot to gallop. BRAINSTEM: ASCENDING OUTPUTS An example of brainstem nervous information ascending to the encephalon (nonspecific thalamus and cortex) is given by the neurons of the so-called ascending activating reticular system. These neurons are responsible for the waking state and the electroencephalographic desynchronized activity of the cerebral cortex during wakefulness. Their lesion induces the appearance of coma. Neuronal activity, largely excitatory, originating from the activating reticular substance, reaches the reticular nuclei of the thalamus stimulating the thalamic-cortical circuits responsible for the de-synchronized activity of the cortex.The absence of these inputs from the activating reticular substance, releases the thalamic-cortico-thalamic oscillatory networks, responsible, on the contrary, for the cortical synchronized activity characterising Non-REM sleep. LESSON 7 ENCEPHALON: GROSS ANATOMY In the encephalon we can observe two fundamental regions. A central region that contains structures which control the activity of the cortex and the rest of the nervous system. An external region composed of the two cerebral hemispheres, connected through the corpus callosum, which includes the cerebral cortex where the most complex neural processing activities of nervous signals take place. ENCEPHALON: CENTRAL REGION In the central region of the encephalon, three basic structures are evident: 1. the thalamus, centrally located, 2. the hypothalamus, formed by numerous nuclei located under the thalamus, 3. the basal ganglia, a set of nuclei located above, to the side and below the thalamus. THE THALAMUS The thalamus plays an important role in the function of the cerebral cortex. It is in fact a site for the processing of sensory information which is then related to specific regions of the cerebral cortex. It is a site for processing information that comes from the cortex or from the basal ganglia or from the cerebellum which is then related to specific regions of the cerebral cortex. It is a site for the processing of information which is relayed in a non-specific way to the cerebral cortex, such as the thalamic-cortico-thalamic connections described above for the sleep / wakefulness mechanisms. HYPOTHALAMUS The hypothalamus plays an important role in autonomic functions and in the expression of visceral behavioral processes.The hypothalamus, in fact, controls: 1. the secretion of hormones, another way in which the nervous system (through hypothalamic neurons) controls the activity of specific tissues / organs of our body; 2. the thermoregulation of our body; 3. the water balance of our body by regulating fluid intake (sense of thirst) and fluid loss by the kidneys (ADH hormone); 4. the introduction of food (sense of hunger and sense of satiety); 5. the sexual behavior; 6. the behaviors of anger, aggression and escape; 7. the cardiovascular and respiratory responses in these situations. BASAL GANGLIA Basal ganglia are formed by:  neostriatum (nucleus caudatus and putamen), paleostriatum (globus pallidus), subthalamic nucleus and substantia nigra. BASAL GANGLIA: OVERVIEW The basal ganglia are formed by different structures: neostriatum (caudate nucleus and putamen), the paleo striate (globus pallidus) , the subthalamic nucleus and the substantia nigra. Considering the motor deficits associated with basal ganglia lesions, it appears clear that these structures play a critical role in gating movement. However, clinical data derived by in deep analyses of patients, neurophysiological data, neuroanatomical and neuroimaging studies, have also demonstrated the existence of several non motor pathways through the basal ganglia, including an emotional (limbic) channel and an associative or cognitive channel. These pathways appear to serve distinct but related functions in emotion and cognition, which appear particularly prominent in humans and primates. According to this view, the same principles governing movement disinhibition in the motor pathway apply to emotional or cognitive processing in the limbic and associative channels. FUNCTIONAL ASPECTS Each of the channels described in the previous slide comprises a feedback loop beginning in the cortex, projecting through the basal ganglia, and ultimately providing excitatory feedback to the cortex. In this model, cortical inputs to the basal ganglia serve as a source of potential variability in behavior, and the basal ganglia themselves contribute to the selection of behavior on the basis of prior outcomes. Therefore, in patients with basal ganglia lesions associated with motor gating deficits, there are observed deficits in performing or learning new actions to acquire rewards or avoid punishments. Impairments in probabilistic classification tasks that require subjects to make predictions based on a series of cues are also observed. Finally the ability to produce learned sequences of movements is also disrupted. DIRECT + INDIRECT PATHWAYS The afferents from the cortex to the basal ganglia are excitatory and end in the neostriatum. Neostriatal neurons send inhibitory signals to neurons located in both the medial and lateral regions of the globus pallidus. Neurons in the medial region of the globus pallidus send inhibitory signals out to the neurons of the thalamus. The latter sends excitatory signals to the cortex closing the feedback loop. The neurons of the lateral region of the globus pallidus, send inhibitory signals to the subthalamic nucleus. These neurons are excitatory neurons that stimulate inhibitory neurons of the medial region of the globus pallidus, which, in turn, inhibit thalamus-cortical excitatory neurons. In conclusion, observing the functional motor effects and considering that inhibition of an inhibitory neuron is equivalent to increasing the excitability of its target neuron, stimulation of the direct pathway is equivalent to a facilitation of movements, while stimulation of the indirect pathway is equivalent to an inhibition of movements. Under physiological conditions the two pathways are in continuous dynamic equilibrium. BG. AND DOPAMINE The dopaminergic pathway from the substantia nigra to the striatum plays a prominent regulatory role on the function of the basal ganglia. It is thought that this pathway has opposite effects on the direct and indirect pathways, activating the former and inhibiting the latter. It follows that dopamine has a role in facilitating movements. Thus, it can be well understood how in Parkinson's disease, where dopamine is missing, motor disorders are characterized by a difficulty in movements. CEREBRAL CORTEX OVERVIEW The cerebral cortex represents the maximum evolution of the nervous system in terms of processing complexity. It is estimated that there are about 159 neurons covering an area of 220 cm2. The cortex has numerous introflexions and convexities (gyri) related to its folding, so in large part it is not visible. The cortex is connected to the deep encephalic structures (thalamus, hypothalamic region), the brain stem, cerebellum and spinal cord, but is also richly interconnected at the level of the single hemisphere (intracortical association fibers) and between the two hemispheres (fibers of the corpus callosum). CORTEX FUNCTIONAL UNITS: CORTICAL COLUMNS The complexity of cortical functions can be traced back to its histological structure and the presence of functional (non-anatomical) units named cortical columns. The cortex is generally divided into 6 layers, identified (in Roman numerals) starting from the surface of the cortex itself. Layers II and IV receive input signals to the cortex itself and consist of small granular-shaped neurons (granular neurons) tightly packed together. In layers III and V are instead contained neurons of large size and pyramidal shape (pyramidal neurons) that give rise to the output signals of the cortex. Each cortical column is formed by about 10000 neurons and is characterized by the ability to process a single input (for example, the visual signal collected from a single visual sensory fiber of the retina or the tactile signal of a certain skin region collected by a single tactile sensory fiber. CORTICAL COLUMNS: VISUAL CORTEX The orientation of the bar focused on the retina is represented in the cortical columns of the visual cortex. Each column represents a specific orientation of the bar. Columns activated by visual stimuli of one eye are close to columns activated by visual stimuli of the other eye. They are called ocular dominance columns. EXAMPLE: MOTOR CORTEX In the motor cortex, cortical columns represent the vector of the movement generated by their stimulation. They are not represented by single muscles or single joint flexo/extensions, but rather the entire movement which will be generated through its vector. Lesson 8 Memory → overview MEMORY Memory is the series of processes whereby the nervous system acquires information from new experiences, retains this information over time, and eventually uses it to guide behavior and plan future action. This definition points to the 3 basic memory phases shared by all forms of memory encoding, storage, and retrieval. 1. Encoding consists of the processes whereby experiences can alter the nervous system. These alterations, known as memory traces, are believed to involve changes in the strength and number of synaptic connections between neurons. 2. Storage is the retention of memory traces over time; this retention requires stabilization or consolidation processes. 3. Retrieval is the processing of stored memory traces. Learning is used as a synonym of encoding and can also describe gradual changes in behavior as a function of training. In this second meaning, the term learning refers to the combined effect of all encoding storage and retrieval in gradually enhancing the performance of the task. For this reason, this second use of learning is popular in contexts with multiple learning trials, such as school education and for instance most of the animal memory paradigms. GENERAL TAXONOMY Scientists and clinicians investigating memory in normal human participants found evidence that retaining information across delays of seconds or minutes involve mechanisms that are fundamentally different from those utilized for retaining information across delays of hours, days, or weeks. In the first case we talk about short term memory; In the latter case we talk about long term memory. The concept of a short term memory, which originally was about the simple maintenance of information, was later expanded into the notion of working memory, which includes not only simple maintenance but also operations performed on the information being maintained and mechanisms of attention allocation. Eventually, we arrived at the classification illustrated in the figure. Working memory mediates the maintenance and manipulation of information online for a few seconds or minutes. Long term memory mediates the retention of information for longer periods (days, months, decades). MEMORY AND SYNAPTIC PLASTICITY At the cellular and molecular levels, however, all the forms of memory seem to depend on changes in neural connectivity and the relative strength of the synaptic transmission. The Canadian psychologist Donald Hebb suggested that memories are stored in the brain in the form of networks of neurons called cell assemblies. To explain how relevant neurons come to be linked, he proposed that when presynaptic and postsynaptic neurons are simultaneously active, the synaptic connections between them are strengthened. This is called hebbian learning. More recently, the scientist Eric Kandel who won for this the Nobel Prize, found the molecular basis of the changes in synaptic strength hypothesized by Hebb. By studying a simple reflex in the sea slug Aplysia Californica, he found two forms of learning that occur in many animals, including humans: habituation and sensitization. APLYSIA GILL WITHDRAWAL REFLEX Touching the siphon skin evokes gill withdrawal. The response is generated by a nervous circuit made by a siphon skin sensory neuron which excites the gill retracting muscle motor neuron. The importance of this animal model is due to the nervous system simplicity (40.000 neurons) and an easy access to intracellular recording of neuronal activity. Molecular mechanisms of synaptic plasticity SHORT TERM MEMORY MECHANISMS Habituation is a reduced response when the same stimulus is repeated over time; Sensitisation is an increased response to the habituated stimulus when it is paired with an aversive stimulus. In Aplysia Californica, if gills are touched, they immediately retract. This is the gill withdrawal automatic response or gill withdrawal reflex. If this tactile stimulus is repeated over time, gill retraction is more and more less evident, until there is no more retraction. We say that there is habituation of this reflex. If, now, we pair the tactile stimulus of the gills with a tactile stimulus of the tail, the gills withdrawal reflex is immediately rescued. The neuronal circuit responsible for this reflex is made by a sensory neuron which is connected with a tactile receptor of the siphon mantle region. This excitatory sensory neuron is synaptically connected to the motoneurons which activate gill withdrawal muscles. A modulatory neuron excited by tail skin sensory neurons, activates siphon skin sensory neuron/motor neuron synapse. SENSORY NEURON-MOTOR NEURONAL SYNAPSE Normal neurotransmitter release in physiological conditions. Upon arrival of sensory neuron action potentials, Glutamate is released by the presynaptic sensory axon terminals. The released glutamate binds to ionotropic postsynaptic receptors inducing a postsynaptic excitatory potential in the postsynaptic neuron (motor neuron). Part 2 Reduction of neurotransmitter release: Habituation → The continuous stimulation of sensory-motor synapse leads to a decrease in neurotransmitter release. The reduced released neurotransmitter induces smaller Postsynaptic excitatory potentials in the motor neuron leading to a reduced number of motor action potentials stimulating gills retractile muscles. Part 3 Return to normal neurotransmitter release: Sensitization → The skin tail sensory neuron stimulates an excitatory modulatory neuron which, in turn, activates the skin siphon sensory neuron axon terminal to restore a normal neurotransmitter release. SHORT TERM AND LONG TERM PLASTICITY MECHANISMS Short-term memory mechanisms → By making intracellular electrophysiological recordings Kandel found that habituation involves a decrease in neurotransmitter release at these synapses between sensory neurons and motor neurons, whereas sensitization involves an increase in neurotransmitter release at the same synapses. Both are forms of synaptic plasticity and they can be regarded as a model for short term memory because these changes last for minutes. LONG TERM PLASTICITY MECHANISMS Long-term memory mechanisms → A second major advance in understanding the cellular mechanisms of memory was obtained by Llomo and Blyss in their studies of hippocampal synaptic transmission in rabbits. These scientists stimulated different afferent pathways and the recorded responses in a postsynaptic neuron. They found that when one pathway was stimulated with a high frequency electrical train, after this high frequency stimulation, the postsynaptic neuron showed stronger responses when the same afferents were again stimulated with single stimuli. Because this increased synaptic response lasted for a long time, they called the phenomenon long term potentiation (LTP). Subsequently other scientists found this phenomenon in other regions of the brain of other animals and according to Llomo and Blyss they found that LTP can last tens of minutes, hours, or longer. The molecular basis of LTP is an increased synthesis of an intracellular second messenger due to the increased synaptic activity. The increased intracellular second messenger activates the phosphorylation of cytoplasmic proteins which then migrate to the nucleus and induce the expression of genes codifying proteins involved in synaptic functions and structures. As a consequence of these morphological changes, synaptic strength is increased and these changes last for long periods (long-term plasticity). More recently a similar phenomenon of plasticity was also discovered for inhibitory synapses from which it's term Long Term Depression. THE CASE OF PATIENT HM AND TYPE OF MEMORIES HM CASE One of the most important cases for the discovery of memory mechanisms in humans is the case of the patient H.M. At 27 years old he underwent neuro surgery for the treatment of intractable epilepsy. He also suffered 10 epileptic seizures a day, a condition not compatible with life. His surgeon removed much of both temporal lobes including the amygdala and entorhinal cortex and 2/3 of hippocampus. At that time this was the only way to treat these forms of intractable epilepsy. Surgery was successful in relieving seizures. However, the bilateral removal of the above mentioned structures induced in the patient the appearance of a devastating memory deficit (amnesia). He was studied until his death in 2008, because his memory deficits were extremely pure and not contaminated by other neurological deficits like in other cases.First of all, HM was severely impaired in memory but he didn't suffer any other cognitive deficit. Sensory and perceptual functions were normal as well as his IQ score. This last was also increased due to the relief from seizures. He had no deficits in executive tasks including those which measure frontal lobe function. HM memory deficits included all kinds of information and sensory modalities. He remembered neither verbal stimuli, such as new names and words, nor non verbal stimuli, such as faces and figures. He did not remember any event after surgery. Although HM was unable to remember new events and facts (declarative memory), he was absolutely normal in retaining information for brief periods of time (working memory). He performed well in the digit span task where he was able to repeat as a normal person about 7 to 9 digits. HM memory deficits did not affect nondeclarative memories. For example he was absolutely normal in performing the mirror drawing task where he showed daily improvements and these last persisted overtime. CONCLUSIONS In conclusion, H.M. suffered from a deep anterograde declarative amnesia, which lasted for his entire life until death. Particularly interesting was his retrograde declarative amnesia. He also was unable to remember facts that happened in the months preceding surgery, even if he was able to remember events of his youth. This retrograde amnesia shows that memory needs to be consolidated before being an information which can be reutilized by the nervous system. The dissected structures (medial temporal lobe) are therefore important for the consolidation process. Another important finding is that consolidated memories are clearly not stored in the medial temporal lobe. In fact, if this was the case, H.M. could not remember facts and events that happened in his early life. GROSS ANATOMICAL LESION IN HM Neurosurgeon dissected out the medial temporal lobe and its internal structures. This lesion caused: 1. a severe anterograde amnesia; 2. a retrograde amnesia for the months/years preceding the surgery (failure in consolidating recent memories before surgery); 3. amnesia was a declarative type amnesia; 4. no impairment in nondeclarative memory. DECLARATIVE MEMORY Declarative memory also known as explicit memory refers to conscious memory for events and facts. Declarative memory was originally distinguished by having a component of consciousness. Researchers developed tasks that depend on brain regions in rodents and monkeys similar to those studied in humans, and that can be used to study declarative memory in non-human animals. H.M. case shows that medial temporal lobe is involved in declarative memory. NON DECLARATIVE MEMORY Nondeclarative memory (implicit memory) in all its forms is characterized by the fact that it is expressed through performance and it is independent of conscious awareness. Nondeclarative memory is evidenced by a change in the behavior, even if the person is unaware that memories from specific past experience are being accessed. The major forms of non declarative memory are very different from each other. Priming is a change in the processing of stimulus due to a previous encounter with the same or a related stimulus, such as completing an award fragment with the previously read word. Skill learning is a gradual improvement in performance due to repeated practice, such as mirror drawing or riding a bicycle. Conditioning is formed by simple responses to associations between stimuli, as Pavlov's dog salivating at the sound of a can opener associated with the food.