Human Anatomy & Physiology I - Nervous System (Chapter 10) PDF

Human Anatomy & Physiology I Overview of the Nervous System Chapter 10 Organization of the Nervous System Central nervous system - CNS Brain and Spinal Cord (in dorsal body cavity) Integration and command center – interprets sensory input and responds to input Peripheral nervous system...

Human Anatomy & Physiology I Overview of the Nervous System Chapter 10 Organization of the Nervous System Central nervous system - CNS Brain and Spinal Cord (in dorsal body cavity) Integration and command center – interprets sensory input and responds to input Peripheral nervous system - PNS Paired Spinal and Cranial nerves Carries messages to and from the spinal cord and brain – links parts of the body to the CNS  Divisions of the Nervous System Central Nervous System brain spinal cord Peripheral Nervous System peripheral nerves cranial nerves spinal nerves  Nervous System Functions: Sensory Input – monitoring stimuli occurring inside and outside the body Integration – interpretation of sensory input Motor Output – response to stimuli by activating effector organs Divisions Nervous System Levels of Organization in the Nervous System Divisions of Peripheral Nervous System Sensory Division picks up sensory information and delivers it to the CNS Motor Division carries information to muscles and glands Divisions of the Motor Division Somatic – carries information to skeletal muscle Autonomic – carries information to smooth muscle, cardiac muscle, and glands  Functions of Nervous System Sensory Function sensory receptors gather information information is carried to the Motor Function CNS decisions are acted upon Integrative Function impulses are sensory information used to carried to effectors create sensations memory thoughts decisions PNS - Two Functional Divisions Sensory (afferent) Division Somatic afferent nerves – carry impulses from skin, skeletal muscles, and joints to the CNS Visceral afferent nerves – transmit impulses from visceral organs to the CNS Motor (efferent) Division Transmits impulses from the CNS to effector organs, muscles and glands, to effect (bring about) a motor response  Classification of Neurons Sensory Neurons afferent carry impulse to CNS most are unipolar some are bipolar Interneurons link neurons multipolar in CNS Motor Neurons multipolar carry impulses away from CNS carry impulses to effectors Motor Division: two subdivisions Somatic Nervous System (voluntary) Somatic motor nerve fibers (axons) that conduct impulses from CNS to Skeletal muscles – allows conscious control of skeletal muscles Autonomic Nervous System (ANS) (involuntary) Visceral motor nerve fibers that regulate smooth muscle, cardiac muscle, and glands Two functional divisions – sympathetic and parasympathetic Levels of Organization in the Nervous System  Histology of Nerve Tissue Two principal cell types in the nervous system: Neurons – excitable nerve cells that transmit electrical signals Supporting cells – cells adjacent to neurons or cells that surround and wrap around neurons Cell Types of Neural Tissue neurons neuroglial cells  Neurons (Nerve Cells) Highly specialized, structural units of the nervous system – conduct messages (nerve impulses) from one part of the body to another Long life, mostly amitotic, with a high metabolic rate (cannot survive more than a few minutes without O2) Structure is variable, but all have a neuron cell body and one or more cell projections called processes. Generalized Neuron Neuron Structure Nerve Cell Body (Perikaryon or Soma) Contains the nucleus and a nucleolus The major biosynthetic center Has no centrioles Has well-developed Nissl bodies (rough ER) Axon hillock – cone-shaped area where axons arise Clusters of cell bodies are called Nuclei in the CNS and Ganglia in the PNS  Processes Extensions from the nerve cell body. The CNS contains both neuron cell bodies and their processes. The PNS consists mainly of neuron processes. Two types: Axons and Dendrites Bundles of neuron processes are called Tracts in the CNS and Nerves in the PNS  Dendrites Short, tapering, diffusely branched processes The main receptive, or input regions of the neuron (provide a large surface area for receiving signals from other neurons) Dendrites convey incoming messages toward the cell body These electrical signals are not nerve impulses (not action potentials), but are short distance signals called graded potentials  Axons Slender processes with a uniform diameter arising from the axon hillock, only one axon per neuron A long axon is called a nerve fiber, any branches are called axon collaterals Terminal branches – distal ends are called the axon terminus (also synaptic knob or bouton)  Axons: Function Generate and transmit action potentials (nerve impulses), typically away from the cell body As impulse reaches the axon terminals, it causes neurotransmitters to be released from the axon terminals Movement of substances along axons: Anterograde - toward axonal terminal (mitochondria, cytoskeletal, or membrane components) Retrograde - away from axonal terminal (organelles for recycling) Anterograde → ←Retrograde  Myelin Sheath Whitish, fatty (protein-lipoid), segmented sheath around most long axons – dendrites are unmyelinated Protects the axon Electrically insulates fibers from one another Increases the speed of nerve impulse transmission  Myelin Sheath Formed by Schwann cells in the PNS A Schwann cell envelopes and encloses the axon with its plasma membrane. The concentric layers of membrane wrapped around the axon are the myelin sheath Neurilemma – cytoplasm and exposed membrane of a Schwann cell Nodes of Ranvier (Neurofibral Nodes) Gaps in the myelin sheath between adjacent Schwann cells They are the sites where axon collaterals can emerge  Myelination of Axons White Matter contains myelinated axons Gray Matter contains unmyelinated structures cell bodies, dendrites  Axons of the CNS Both myelinated and unmyelinated fibers are present Myelin sheaths are formed by oligodendrocytes Nodes of Ranvier are more widely spaced There is no neurilemma (cell extensions are coiled around axons) White matter – dense collections of myelinated fibers Gray matter – mostly soma and unmyelinated fibers  Classification of Neurons Bipolar two processes eyes, ears, nose Unipolar one process ganglia Multipolar many processes most neurons of CNS  Classification of Neurons Structural Multipolar — three or more processes Bipolar — two processes (axon and dendrite) Unipolar — single, short process  Neuron Classification Functional Sensory (afferent) – transmit impulses toward the CNS Motor (efferent) – carry impulses away from the CNS Interneurons (association neurons) – lie between sensory and motor pathways and shuttle signals through CNS pathways  Supporting Cells: Neuroglia Six types of Supporting Cells - neuroglia or glial cells – 4 in CNS and 2 in the PNS Each has a specific function, but generally they: Provide a supportive scaffold for neurons Segregate and insulate neurons Produce chemicals that guide young neurons to the proper connections Promote health and growth  Types of Neuroglial Cells Schwann Cells Astrocytes peripheral nervous CNS system scar tissue myelinating cell mop up excess ions, etc induce synapse formation Oligodendrocytes connect neurons to blood CNS vessels myelinating cell Ependyma CNS Microglia ciliated CNS line central canal of spinal cord phagocytic cell line ventricles of brain  Supporting Cells: Neuroglia Neuroglia in the CNS Neuroglia in the PNS Astrocytes Satellite Cells Microglia Schwann Cells Ependymal Cells Oligodendrocytes Outnumber neurons in the CNS by 10 to 1, about ½ the brain’s mass. Types of Neuroglial Cells  Astrocytes Most abundant, versatile, highly branched glial cells Cling to neurons, synaptic endings, and cover nearby capillaries Support and brace neurons Anchor neurons to nutrient supplies Guide migration of young neurons Aid in synapse formation Control the chemical environment (recapture K+ ions and neurotransmitters)  Microglia Microglia – small, ovoid cells with long spiny processes that contact nearby neurons When microorganisms or dead neurons are present, they can transform into phagocytic cells  Ependymal Cells Ependymal cells – range in shape from squamous to columnar, many are ciliated Line the central cavities of the brain and spinal column  Oligodendrocytes Oligodendrocytes – branched cells that line the thicker CNS nerve fibers and wrap around them, producing an insulating covering – the Myelin sheath  Schwann Cells and Satellite Cells Schwann cells - surround fibers of the PNS and form insulating myelin sheaths Satellite cells - surround neuron cell bodies within ganglia Regeneration of A Nerve Axon  Neurophysiology Neurons are highly irritable (responsive to stimuli) Action potentials, or nerve impulses, are: Electrical impulses conducted along the length of axons Always the same regardless of stimulus The underlying functional feature of the nervous system  Definitions Voltage (V) – measure of potential energy between two points generated by a charge separation (Voltage = Potential Difference = Potential) Current (I) – the flow of electrical charge Resistance (R) – tendency to oppose the current Units: V (volt), I (ampere), R (ohm) Insulator – substance with high electrical resistance Conductor – substance with low electrical resistance  Ohm’s Law The relationship between voltage, current, and resistance is defined by Ohm’s Law Voltage (V) Current (I) = Resistance (R) In the body, electrical current is the flow of ions (rather than free electrons) across membranes A Potential Difference exists when there is a difference in the numbers of + and – ions on either side of the membrane  Membrane Ion Channels Types of plasma membrane ion channels ❖Passive, or leakage, channels – always open ❖Chemically (or ligand)-gated channels – open with binding of a specific neurotransmitter (the ligand) ❖Voltage-gated channels – open and close in response to changes in the membrane potential ❖Mechanically-gated channels – open and close in response to physical deformation of receptors  Ligand-Gated Channel Example: Na+-K+ gated channel Closed when a neurotransmitter is not bound to the extracellular receptor Open when a neurotransmitter is attached to the receptor - Na+ enters the cell and K+ exits the cell  Voltage-Gated Channel Example: Na+ channel Closed when the intracellular environment is negative Open when the intracellular environment is positive - Na+ can enter the cell  Electrochemical Gradient Ions flow along their chemical gradient when they move from an area of high concentration to an area of low concentration Ions flow along their electrical gradient when they move toward an area of opposite charge Together, the electrical and chemical gradients constitute the ELECTROCHEMICAL GRADIENT  Ion Channels When gated ion channels open, ions diffuse across the membrane following their electrochemical gradients. This movement of charge is an electrical current and can create voltage change across the membrane. Voltage (V) = Current (I) x Resistance (R) Ion movement (flow) along electrochemical gradients underlies all the electrical phenomena in neurons.  Resting Membrane Potential A potential (-70mV) exists across the membrane of a resting neuron – the membrane is polarized Resting Membrane Potential inside is negative relative to the outside polarized membrane due to distribution of ions Na+/K+ pump  Resting Membrane Potential Ionic differences are the consequence of: Different membrane permeabilities due to passive ion channels for Na+, K+, and Cl- Operation of the sodium-potassium pump  Membrane Potentials: Signals Neurons use changes in membrane potential to receive, integrate, and send information Membrane potential changes are produced by: Changes in membrane permeability to ions Alterations of ion concentrations across the membrane Two types of signals are produced by a change in membrane potential: graded potentials (short-distance) action potentials (long-distance)  Levels of Polarization Depolarization – inside of the membrane becomes less negative (or even reverses) – a reduction in potential Repolarization – the membrane returns to its resting membrane potential Hyperpolarization – inside of the membrane becomes more negative than the resting potential – an increase in potential Depolarization increases the probability of producing nerve impulses. Hyperpolarization reduces the probability of producing nerve impulses. Changes in Membrane Potential  Graded Potentials Short-lived, local changes in membrane potential (either depolarizations or hyperpolarizations) Cause currents that decreases in magnitude with distance Their magnitude varies directly with the strength of the stimulus – the stronger the stimulus the more the voltage changes and the farther the current goes Sufficiently strong graded potentials can initiate action potentials  Graded Potentials Voltage changes in graded short- distance signal potentials are decremental, the charge is quickly lost through the permeable plasma membrane  Action Potentials (APs) An action potential in the axon of a neuron is called a nerve impulse and is the way neurons communicate. The AP is a brief reversal of membrane potential with a total amplitude of 100 mV (from -70mV to +30mV) APs do not decrease in strength with distance The depolarization phase is followed by a repolarization phase and often a short period of hyperpolarization Events of AP generation and transmission are the same for skeletal muscle cells and neurons  Action Potential: Resting State Na+ and K+ channels are closed Each Na+ channel has two voltage-regulated gates Activation gates – closed in the resting state Inactivation gates – open in the resting state Depolarization opens the activation gate (rapid) and closes the inactivation gate (slower) The gate for the K+ is slowly opened with depolarization.  Depolarization Phase Na+ activation gates open quickly and Na+ enters causing local depolarization which opens more activation gates and cell interior becomes progressively less negative. Rapid depolarization and polarity reversal. Threshold – a critical level of depolarization (-55 to -50 mV) where depolarization becomes self-generating Positive Feedback?  Repolarization Phase Positive intracellular charge opposes further Na+ entry. Sodium inactivation gates of Na+ channels close. As sodium gates close, the slow voltage-sensitive K+ gates open and K+ leaves the cell following its electrochemical gradient and the internal negativity of the neuron is restored  Hyperpolarization The slow K+ gates remain open longer than is needed to restore the resting state. This excessive efflux causes hyperpolarization of the membrane The neuron is insensitive to stimulus and depolarization during this time  Role of the Sodium-Potassium Pump Repolarization restores the resting electrical conditions of the neuron, but does not restore the resting ionic conditions Ionic redistribution is accomplished by the sodium- potassium pump following repolarization  Potential Changes at rest membrane is polarized threshold stimulus reached sodium channels open and membrane depolarizes potassium leaves cytoplasm and membrane repolarizes Phases of the Action Potential Impulse Conduction Action Potentials Propagation of an Action Potential The action potential is self-propagating and moves away from the stimulus (point of origin)  Stimulus Intensity All action potentials are alike and are independent of stimulus intensity How can CNS determine if a stimulus intense or weak? Strong stimuli can generate an action potential more often than weaker stimuli and the CNS determines stimulus intensity by the frequency of impulse transmission Threshold and Action Potentials Threshold Voltage– membrane is depolarized by 15 to 20 mV Subthreshold stimuli produce subthreshold depolarizations and are not translated into APs Stronger threshold stimuli produce depolarizing currents that are translated into action potentials All-or-None phenomenon – action potentials either happen completely, or not at all Stimulus Strength and AP Frequency  Absolute Refractory Period When a section of membrane is generating an AP and Na+ channels are open, the neuron cannot respond to another stimulus The absolute refractory period is the time from the opening of the Na+ activation gates until the closing of inactivation gates  Relative Refractory Period The relative refractory period is the interval following the absolute refractory period when: Na+ gates are closed K+ gates are open Repolarization is occurring During this period, the threshold level is elevated, allowing only strong stimuli to generate an AP (a strong stimulus can cause more frequent AP generation) Refractory Periods  Axon Conduction Velocities Conduction velocities vary widely among neurons Determined mainly by: Axon Diameter – the larger the diameter, the faster the impulse (less resistance) Presence of a Myelin Sheath – myelination increases impulse speed (Continuous vs. Saltatory Conduction)  Saltatory Conduction Current passes through a myelinated axon only at the nodes of Ranvier Voltage-gated Na+ channels are concentrated at these nodes Action potentials are triggered only at the nodes and jump from one node to the next Much faster than conduction along unmyelinated axons Saltatory Conduction  Saltatory Conduction Current passes through a myelinated axon only at the nodes of Ranvier (Na+ channels concentrated at nodes) Action potentials occur only at the nodes and jump from node to node  Synapse A junction that mediates information transfer from one neuron to another neuron or to an effector cell Presynaptic neuron – conducts impulses toward the synapse (sender) Postsynaptic neuron – transmits impulses away from the synapse (receiver)  Types of Synapses Axodendritic – synapse between the axon of one neuron and the dendrite of another Axosomatic – synapse between the axon of one neuron and the soma of another Other types: Axoaxonic (axon to axon) Dendrodendritic (dendrite to dendrite) Dendrosomatic (dendrites to soma) Synapses  Electrical Synapses Less common than chemical synapses Gap junctions allow neurons to be electrically coupled as ions can flow directly from neuron to neuron - provide a means to synchronize activity of neurons Are important in the CNS in: Arousal from sleep Mental attention and conscious perception Emotions and memory Ion and water homeostasis Abundant in embryonic nervous tissue  Chemical Synapses Specialized for the release and reception of chemical neurotransmitters Typically composed of two parts: Axon terminal of the presynaptic neuron containing membrane-bound synaptic vesicles Receptor region on the dendrite(s) or soma of the postsynaptic neuron  Synaptic Cleft Fluid-filled space separating the presynaptic and postsynaptic neurons, prevents nerve impulses from directly passing from one neuron to the next Transmission across the synaptic cleft: Is a chemical event (as opposed to an electrical one) Ensures unidirectional communication between neurons  Synaptic Cleft: Information Transfer Nerve impulses reach the axon terminal of the presynaptic neuron and open Ca2+ channels Neurotransmitter is released into the synaptic cleft via exocytosis Neurotransmitter crosses the synaptic cleft and binds to receptors on the postsynaptic neuron Postsynaptic membrane permeability changes due to opening of ion channels, causing an excitatory or inhibitory effect Synaptic Cleft: Information Transfer Termination of Neurotransmitter Effects Neurotransmitter bound to a postsynaptic neuron produces a continuous postsynaptic effect and also blocks reception of additional “messages” Terminating Mechanisms: Degradation by enzymes Uptake by astrocytes or the presynaptic terminals Diffusion away from the synaptic cleft  Synaptic Delay Neurotransmitter must be released, diffuse across the synapse, and bind to receptors (0.3-5.0 ms) Synaptic delay is the rate-limiting step of neural transmission  Postsynaptic Potentials Neurotransmitter receptors mediate graded changes in membrane potential according to: The amount of neurotransmitter released The amount of time the neurotransmitter is bound to receptors The two types of postsynaptic potentials are: EPSP – excitatory postsynaptic potentials IPSP – inhibitory postsynaptic potentials Excitatory Postsynaptic Potentials EPSPs are local graded depolarization events that can initiate an action potential in an axon Na+ and K+ flow in opposite directions at the same time Postsynaptic membranes do not generate action potentials. The currents created by EPSPs decline with distance, but can spread to the axon hillock and depolarize the axon to threshold leading to an action potential Inhibitory Postsynaptic Potentials Neurotransmitter binding to a receptor at inhibitory synapses reduces a postsynaptic neuron’s ability to generate an action potential Postsynaptic membrane is hyperpolarized due to increased permeability to K+ and/or Cl- ions. Na+ permeability is not affected. Leaves the charge on the inner membrane face more negative and the neuron becomes less likely to “fire”. EPSPs and IPSPs  Summation A single EPSP cannot induce an action potential EPSPs must summate (add together) to induce an AP Temporal Summation – presynaptic neurons transmit impulses in quick succession Spatial Summation – postsynaptic neuron is stimulated by a large number of terminals at the same time IPSPs also summate and can summate with EPSPs. Summation  Neurotransmitters Chemicals used for neuron communication with the body and the brain More than 50 different neurotransmitters have been identified Classified chemically and functionally Neurotransmitters  Neurotransmitters – Chemical classification Acetylcholine (ACh) Biogenic amines Amino acids Peptides Novel messengers: ATP and dissolved gases NO and CO  Neurotransmitters: Acetylcholine Released at the neuromuscular junction Enclosed in synaptic vesicles Degraded by the acetylcholinesterase (AChE) Released by: – All neurons that stimulate skeletal muscle – Some neurons in the autonomic nervous system  Neurotransmitters: Biogenic Amines Include: – Catecholamines – dopamine, norepinephrine, and epinephrine – Indolamines – serotonin and histamine Broadly distributed in the brain Play roles in emotional behaviors and our biological clock  Synthesis of Catecholamines Enzymes present in the cell determine length of biosynthetic pathway Norepinephrine and dopamine are synthesized in axon terminals Epinephrine is released by the adrenal medulla Neurotransmitters: Amino Acids Include: – GABA – Gamma (γ)-aminobutyric acid – Glycine – Aspartate – Glutamate Found only in the CNS  Neurotransmitters: Peptides Include: – Substance P – mediator of pain signals – Beta endorphin, dynorphin, and enkephalins Act as natural opiates, reducing our perception of pain Bind to the same receptors as opiates and morphine Gut-brain peptides – somatostatin and cholecystokinin (produced by non-neural tissue and widespread in GI tract)  Neurotransmitters: Novel Messengers ATP – Is found in both the CNS and PNS – Produces excitatory or inhibitory responses depending on receptor type – Induces Ca2+ wave propagation in astrocytes – Provokes pain sensation Nitric oxide (NO) – Activates the intracellular receptor guanylyl cyclase – Is involved in learning and memory Carbon monoxide (CO) is a main regulator of cGMP in the brain  Functional Classification of Neurotransmitters Two classifications: excitatory and inhibitory – Excitatory neurotransmitters cause depolarizations (e.g., glutamate) – Inhibitory neurotransmitters cause hyperpolarizations (e.g., GABA and glycine) Some neurotransmitters have both excitatory and inhibitory effects (determined by the receptor type of the postsynaptic neuron). ACh is excitatory at neuromuscular junctions with skeletal muscle and Inhibitory in cardiac muscle.  Neurotransmitter Receptor Mechanisms Direct: neurotransmitters that open ion channels – Promote rapid responses – Examples: ACh and amino acids Indirect: neurotransmitters that act through second messengers – Promote long-lasting effects – Examples: biogenic amines, peptides, and dissolved gases  Channel-Linked Receptors (ligand- gated ion channel) Mediate direct neurotransmitter action, action is immediate, brief, and highly localized Ligand binds to the receptor and ions enter the cells Excitatory receptors depolarize membranes Inhibitory receptors hyperpolarize membranes  G Protein-Linked Receptors Responses are indirect, slow, complex, prolonged, and often diffuse These receptors are transmembrane protein complexes Examples: muscarinic ACh receptors, neuropeptides, and those that bind biogenic amines  G Protein-Linked Receptors: Mechanism Neurotransmitter binds to G protein-linked receptor G protein is activated and GTP is hydrolyzed to GDP The activated G protein complex activates adenylate cyclase Adenylate cyclase catalyzes the formation of cAMP from ATP cAMP, a second messenger, brings about various cellular responses G Protein-Linked Receptors: Mechanism  G Protein-Linked Receptors: Effects G protein-linked receptors activate intracellular second messengers including Ca2+, cGMP, diacylglycerol, as well as cAMP Second messengers: – Open or close ion channels – Activate kinase enzymes – Phosphorylate channel proteins – Activate genes and induce protein synthesis Neural Integration: Neuronal Pools Functional groups of neurons that: Integrate incoming information received from receptors or other neuronal pools Forward the processed information to its appropriate destination Simple neuronal pool Input fiber – presynaptic fiber Discharge zone – neurons most closely associated with the incoming fiber Facilitated zone – neurons farther away from incoming fiber Simple Neuronal Pool Types of Circuits in Neuronal Pools Divergent – one incoming fiber stimulates ever increasing number of fibers. These circuits are often amplifying circuits. (an impulse from a single brain neuron can activate 100 or more motor neurons in the spinal cord and → 1000s of skeletal muscle fibers)  Divergence one neuron sends impulses to several neurons can amplify an impulse impulse from a single neuron in CNS may be amplified to activate enough motor units needed for muscle contraction Types of Circuits in Neuronal Pools Convergent – opposite of divergent circuits, resulting in either strong stimulation or inhibition  Convergence neuron receives input from several neurons incoming impulses represent information from different types of sensory receptors allows nervous system to collect, process, and respond to information makes it possible for a neuron to sum impulses from different sources Types of Circuits in Neuronal Pools Reverberating or oscillating– chain of neurons containing collateral synapses with previous neurons in the chain. Involved in the control of rhythmic activities (sleep-wake cycle, breathing)  Types of Circuits in Neuronal Pools Parallel after-Discharge – incoming neurons stimulate several neurons in parallel arrays  Clinical Application Multiple Sclerosis Symptoms Causes blurred vision myelin destroyed in numb legs or arms various parts of CNS can lead to paralysis hard scars (scleroses) form nerve impulses Treatments blocked no cure muscles do not bone marrow transplant receive innervation interferon (anti-viral drug) may be related to a hormones virus