The Nervous System PDF

The Nervous System Associate Prof. Ts. Dr. Richard Muhammad Johari James Integrative Pharmacogenomics Institute (iPROMISE) Learning Outcome At the end of the lecture, students will be able to: Describe the fundamentals of the nervous system and nervous tissue, the central nervous system, the p...

The Nervous System Associate Prof. Ts. Dr. Richard Muhammad Johari James Integrative Pharmacogenomics Institute (iPROMISE) Learning Outcome At the end of the lecture, students will be able to: Describe the fundamentals of the nervous system and nervous tissue, the central nervous system, the peripheral nervous system and reflex activity and the autonomic nervous system Lecture outline 1. Fundamentals of the nervous system and nervous tissue 2. The central nervous system 3. The peripheral nervous system and reflex activity 4. The autonomic nervous system 1. Fundamentals of the nervous system and nervous tissue 1.1 Functions and divisions of the nervous system 1.2 The nervous tissue 1.3 Membrane potentials 1.4 Synapse 1.5 Neurotransmitters and receptors 1.1 Functions and divisions of the nervous system Sensory Motor Integration Input Output  Peripheral Central Nervous Nervous System System  The peripheral nervous system has two functional subdivisions The sensory, or afferent, division consists of nerve fibers that convey impulses to the central nervous system from sensory receptors located throughout the body Sensory fibers conveying impulses from the skin, skeletal muscles, and joints are called somatic afferent and those transmitting impulses from the visceral organs are called visceral afferent fibers  The motor, or efferent, division of the peripheral nervous system transmits impulses from the central nervous system to effector organs, which are the muscles and glands These impulses activate muscles to contract and glands to secrete The motor division also has two main parts  The somatic nervous system is composed of somatic motor nerve fibers that conduct impulses from the central nervous system to skeletal muscles The autonomic nervous system consists of visceral motor nerve fibers that regulate the activity of smooth muscles, cardiac muscles, and glands 1.2 The nervous tissue The nervous system consists mostly of nervous tissue, which is highly cellular Less than 20% of the central nervous system is extracellular space, which means that the cells are densely packed and tightly intertwined Nervous tissue is made up of just two principal types of cells: neuroglia and neurons Neuroglia are supporting cells They are smaller cells that surround and wrap the neurons Neurons are the excitable nerve cells that transmit electrical signals  There are six types of neuroglia—four in the central nervous system and two in the peripheral nervous system. PERIPHERAL NERVOUS CENTRAL NERVOUS SYSTEM SYSTEM Astrocytes Oligodendrocytes Schwann cells Microglia Satellite cells Ependymal cells  provide a supportive scaffolding for neurons segregate and insulate neurons guide young neurons to the proper connections promote health and growth Astrocytes Astrocytes are the most abundant, versatile, and highly branched glial cells They cling to neurons and their synaptic endings, and cover capillaries  Support and brace neurons Anchor neurons to their nutrient supplies Guide migration of young neurons Maintain the appropriate chemical environment for generation of nerve impulses Take up excess neurotransmitters Help to form the blood-brain barrier Help the formation of synapses in learning and memory Oligodendrocytes Oligodendrocytes are myelinating glia in the in central nervous system while Schwann cells in the peripheral nervous system They provide layers of membrane that insulate the axon which are called the myelin sheath The sheath is periodically interrupted by nodes of Ranvier Myelin sheath speeds the propagation of nerve impulses down the axon  Neurilemma encloses outer nucleated cytoplasmic layer of the Schwann cell and aids regeneration in axon injury The white matter in the regions of the brain and spinal cord is a dense collections of myelinated fibers While the gray matter is mostly soma and unmyelinated fibers Ependymal cells Ependymal cells provide lining of fluid- filled ventricles within the brain It directs cell migration during brain development Microglia Microglia phagocytes to remove debris left by dead or degenerating neurons and glia Satellite cells Satellite cells are a type of glial cell that line the exterior surface of neurons in the peripheral nervous system They also surround neuron cell bodies with ganglia They provide structural support and regulate the exchanges of materials between neuronal cell bodies and interstitial fluid Neuron A prototypical neuron consists of 3 main parts: the soma, axons and dendrites The soma is the spherical central part of neuron The cytosol is enclosed by the neuronal membrane The soma contains the nucleus and organelles It has well-developed endoplasmic reticulum, Golgi apparatus, mitochondria and ribosomes However, it has no centrioles, hence its amitotic nature  There are three types of cytoskeleton that holds the neuronal membrane: The microtubules are nucleated at the centrosome, then released and delivered to either the dendrites or the axon Neurofilaments are abundant in axons and the spacing of neurofilaments is sensitive to the level of phosphorylation The microfilaments are dispersed within the cell and they are most abundant near the plasma membrane  The axon starts from the axon hillock No rough endoplasmic reticulum extend into the axon and there are very few free ribosomes The protein composition of the axon membrane is fundamentally different from that of the soma membrane Axons may extend from less than a millimeter to a meter long The branches of axons called axon collaterals The end of axon is called axon terminal The site where axon comes into contact with other neurons or cells and passes the information to them is called a synapse Branches at the end of axon that forms a synapse on dendrites and cell bodies in the same region are called terminal arbor  The cytoplasm content is different between axon and axon terminal: microtubules do not extend into the terminal terminal contains numerous small bubbles of membrane called synaptic vesicles inside surface of the membrane that faces the synapse has a particularly dense covering of proteins numerous mitochondria  Dendrites of a single neuron is called a dendritic tree. The wide variety of shapes and sizes of dendrites is used to classify groups of neurons. Dendritic membrane under the synapse has many specialized protein molecules called receptors that detect the neurotransmitters in the synaptic cleft. Electrical signals in dendrites are conveyed as graded potentials and not action potentials.  Neurons can be classified according to: the number of neurites such as unipolar, bipolar or multipolar the dendrites such as stellate cells, pyramidal cells the connections such as primary sensory neurons, motor neurons, interneurons the axon length such as projection neurons or circuit neurons and neurotransmitter, such as cholinergic, adrenergic or dopaminergic neurons 1.3 Membrane potentials Membrane potential changes are produced by changes in membrane permeability to ions and alterations of ion concentrations across the membrane Two types of signals could be produced when the membrane potential changes : graded potentials and action potentials Graded potential Graded potentials are small deviations from the membrane potential They decrease in intensity with distance Current generated is quickly dissipated due to the leaky plasma membrane and can only travel over short distances Their magnitude varies directly with the strength of the stimulus Graded potentials can summate or even cancels each other out Sufficiently strong graded potentials can initiate action potentials  Changes in the membrane potential could result in three events: depolarization, repolarization and hyperpolarization A depolarization occurs when the inside of the membrane becomes less negative A repolarization occurs when the membrane returns to its resting membrane potential And a hyperpolarization occurs when the inside of the membrane becomes more negative than the resting potential Resting potential The potential difference across the membrane of a resting neuron is about –70 mV It is generated by different concentrations of sodium, potassium, chloride and proteins anions Ionic differences are the consequence of differential permeability of the membrane to sodium and potassium and, also the operation of the sodium-potassium pump  The membrane is more permeable at rest to potassium, so more potassium diffuses out of the cell than sodium diffuses in  Sodium potassium pump moves three sodium ions out of the cell and two potassium ions into the cell, creating concentration gradients Thus, more positive charges leave the cell than enter, leaving behind many negatively charged ions and proteins and making the inside of the cell more negative than the outside Action potential An action potential is a brief reversal of membrane potential with a total amplitude of about 100 mV Action potentials are only generated by muscle cells and neurons They do not decrease in strength over distance An action potential in the axon of a neuron is called a nerve impulse  At the resting stage, the sodium and potassium voltage-gated channels are closed When a stimulus is received by the dendrites of a nerve cell, it causes the sodium channels to open If the opening is sufficient to drive the interior potential from -70 mV up to -55 mV, the process continues Having reached the action threshold, more sodium voltage-gated channels open The sodium influx drives the interior of the cell membrane up to about +30 mV The process to this point is called depolarization  During repolarization, the sodium channels close and the potassium channels open Since the potassium channels are much slower to open, the depolarization has time to be completed Having both sodium and potassium channels open at the same time would drive the system toward neutrality and prevent the creation of the action potential With the potassium channels open, potassium exits the cell and internal negativity of the resting neuron is restored  The repolarization typically overshoots the resting potential to about - 90 mV This is called hyperpolarization Hyperpolarization prevents the neuron from receiving another stimulus during this time, or at least raises the threshold for any new stimulus Hyperpolarization assures that the signal is proceeding in one direction Repolarization restores the resting electrical conditions of the neuron, but it does not restore the resting ionic conditions After hyperpolarization, the sodium-potassium pump eventually brings the membrane back to its resting state of -70 mV and restores the resting ionic conditions All-or-none law The law of all-or-none law is the principle that the strength by which a nerve or muscle fiber responds to a stimulus is not dependent on the strength of the stimulus If the stimulus is any strength above threshold, the nerve or muscle fiber will give a complete response or otherwise no response at all Note that the amplitude of an action potential is always the same The intensity of a stimulus is not reflected by the amplitude of the action potential, but by the frequency of the action potential Refractory period Each sodium channel has two voltage-regulated gates the activation gate and the inactivation gate The time from the opening of the sodium activation gates until the closing of inactivation gates is the absolute refractory period  The absolute refractory period prevents the neuron from generating an action potential, ensures that each action potential is separated and enforces a one-way transmission of nerve impulses The interval following the absolute refractory period is the relative refractory period  It takes about 3 to 4 ms for all sodium channels to come out of inactivation in order to be ready for activation opening again During this period, the threshold level is elevated, allowing strong stimuli to increase the frequency of action potential events Propagation of action potential In response to a signal, the soma end of the axon becomes depolarized The depolarization spreads down the axon Meanwhile the first part of the membrane repolarizes Because sodium channels are inactivated and additional potassium channels have opened, the membrane cannot depolarize again The action potential continues to travel down the axon  The conduction velocities vary widely among neurons The rate of impulse propagation is determined by the axon diameter and the presence of a myelin sheath The larger the diameter of an axon, the faster the impulse The presence of myelin sheath dramatically increases impulse speed The uninsulated nodes of Ranvier are the only places along the axon where ions are exchanged across the axon membrane, regenerating the action potential between regions of the axon that are insulated by myelin  The propagation of impulse from one node of Ranvier to an adjacent node of Ranvier along the axon is called saltatory conduction The largest and most heavily myelinated fibers conduct quickly, for example to convey motor, touch and proprioceptive impulses The less myelinated and unmyelinated fibers conduct more slowly for example to convey pain, temperature, and autonomic impulses 1.4 Synapse Synapses between the axon endings of one neuron and the dendrites of other neurons are axodendritic synapses Those between axon endings of one neuron and cell bodies of other neurons are axosomatic synapses There two types of synapses, electrical synapses and chemical synapses Electrical synapse Electrical synapses consist of gap junctions that intimately connect the cytoplasm of adjacent neurons and allow ions and small molecules to flow directly from one neuron to the next Neurons joined in this way are said to be electrically coupled, and transmission across these synapses is very rapid Chemical synapses Chemical synapses are specialized for release and reception of chemical neurotransmitters Although close to each other, presynaptic and postsynaptic membranes are always separated by the synaptic cleft, a fluid filled space approximately 30 to 50 nm wide  First a nerve impulse arrives at a synaptic end bulb of presynaptic axon and opens the voltage-gated Ca2+ channels Then, the increase of Ca2+ triggers exocytosis of synaptic vesicles and the release of neurotransmitter to synaptic cleft The neurotransmitter crosses the synaptic cleft and binds to receptors on the postsynaptic neuron The postsynaptic membrane permeability changes, causing an excitatory or inhibitory effect When the potential reaches the threshold it triggers one or more nerve impulses  The neurotransmitter receptors mediate changes in membrane potential according to: the amount of neurotransmitter released the duration the neurotransmitter is bound to the receptor There are the two types of postsynaptic potentials: EPSP – excitatory postsynaptic potentials IPSP – inhibitory postsynaptic potentials EPSP EPSPs are graded potentials that can initiate an action potential in an axon At excitatory synapses, neurotransmitter binding causes depolarization of the postsynaptic membrane Each EPSP lasts for a few milliseconds and then the membrane returns to its resting potential The only function of EPSPs is to help trigger an action potential distally at the axon hillock of the postsynaptic neuron IPSP Neurotransmitter binding to a receptor at inhibitory synapses causes the membrane to become more permeable to potassium and chloride ions This leaves the charge on the inner surface negative and reduces the postsynaptic neuron’s ability to produce an action potential Summation A single EPSP cannot induce an action potential EPSPs must summate temporally or spatially to induce an action potential In temporal summation – presynaptic neurons transmit impulses in rapid-fire order In spatial summation – the postsynaptic neuron is stimulated by a large number of terminals at the same time IPSPs can also summate with EPSPs, canceling each other out Synaptic Delay Synaptic delay is the rate-limiting step of neural transmission The neural transmission across a chemical synapse is comparatively slow compared to the speed of an impulse Neurotransmitters must be released, diffuse across the synapse, and bind to receptors causing the synaptic delay of 0.3 to 5 ms Termination of Neurotransmitter Effects Neurotransmitter bound to a postsynaptic neuron produces a continuous postsynaptic effect It blocks reception of additional “messages” and must be removed from its receptor 1.5 Neurotransmitters and receptors Neurotransmitters can be classified chemically and functionally Classification of neurotransmitters by chemical structure Neurotransmitters fall into several chemical classes based on molecular structure There are conventional and unconventional neurotransmitters Conventional neurotransmitters are stored in synaptic vesicles. They are released when calcium enters the axon terminal in response to an action potential They act by binding to receptors on the membrane of the postsynaptic cell Unconventional neurotransmitters are such as endocannabinoids and gasotransmitters CLASSIFICATION OF NEUROTRANSMITTERS BY CHEMICAL STRUCTURE Conventional neurotransmitters Small molecule Amino acids neurotransmitters - glutamate, GABA (γ- neurotransmitters aminobutyric acid), and glycine. Biogenic amines -dopamine, norepinephrine, epinephrine, serotonin, and histamine. Purinergic neurotransmitters - neurotransmitters ATP and adenosine, Acetylcholine Neuropeptides Endorphins and enkephalins Substance P Neuropeptide Y CLASSIFICATION OF NEUROTRANSMITTERS BY CHEMICAL STRUCTURE Unconventional neurotransmitters Endocannabinoids Tetrahydrocannabinolic Acid (THCA) Tetrahydrocannabinol (THC) Cannabidolic Acid (CBDA) Cannabidiol (CBD) Cannabinol (CBN) Gasotransmitters CO NO Classification of neurotransmitters by function The neurotransmitters can be classified by their effects: inhibitory versus excitatory They could also be classified by their actions: direct versus indirect Receptors There are two types of receptors – ionotropic and metabotropic Ionotropic receptors are membrane-spanning ion channel proteins that open directly in response to ligand binding, while metabotropic receptors trigger a signaling pathway, which may indirectly open or close channel, or have some other effect entirely Ionotropic receptors Large protein complexes with specific binding sites for the neurotransmitters When the neurotransmitter binds to the receptors, the protein complex changes in shape causing the channel to open They may have either an excitatory or an inhibitory effect, depending on the ions that can pass through the channel and their concentrations inside and outside the cell They typically produce very quick physiological responses.  The current starts to flow within tens of microseconds of neurotransmitter binding, and the current stops as soon as the neurotransmitter is no longer bound to its receptors In most cases, the neurotransmitter is removed from the synapse very rapidly An example of ionotropic receptor is nicotinic cholinergic receptor Metabotropic receptors Only affects ion channel opening and closing indirectly The receptor is not an ion channel Signaling through these metabotropic receptors depends on the activation of several molecules inside the cell and often involves a second messenger pathway Because it involves more steps, signaling through metabotropic receptors is much slower than signaling through ligand-activated ion channels  Some metabotropic receptors have excitatory effects when they're activated, while others have inhibitory effects Often, these effects occur because the metabotropic receptor triggers a signaling pathway that opens or closes an ion channel Alternatively, a neurotransmitter that binds to a metabotropic receptor may change how the cell responds to a second neurotransmitter that acts through a ligand-activated channel Signaling through metabotropic receptors can also have effects on the postsynaptic cell that don’t involve ion channels at all An example of metabotropic receptor is muscarinic cholinergic receptor 2. The central nervous system  The central nervous system consists of the brain and the spinal cord The brain can be divided into the cerebrum, cerebellum and brain stem The cerebral hemispheres form the superior part of the brain Together they account for about 83% of total brain mass and are the most conspicuous parts of an intact brain  The median longitudinal fissure separates the cerebral hemispheres Each cerebral hemisphere has three basic regions: a superficial cortex of gray matter, which looks gray in fresh brain tissue; an internal white matter; and the basal nuclei, islands of gray matter situated deep within the white matter  The transverse cerebral fissure, separates the cerebral hemispheres from the cerebellum below  Several sulci divide each hemisphere into five lobes—frontal, parietal, temporal, occipital, and insula The insula, is buried deep within the lateral sulcus and forms part of its floor The insula is covered by portions of the temporal, parietal, and frontal lobes  The cerebral cortex is the “executive suite” of the nervous system, where the conscious mind is found It enables us to be aware of ourselves and our sensations, to communicate, remember, and understand, and to initiate voluntary movements In general, the right hemisphere receives sensations from, and controls movements of the left side of the body and vice versa  The cerebellum contains as many neurons as the cerebrum It is the movement connection center where there is extensive connections with the cerebrum and the spinal cord The left side controls the movements of the left side of the body and vice versa. Brainstem The brainstem forms the stalk from which the cerebral hemispheres and cerebellum sprout It consists of the medulla oblongata, pons and midbrain The brainstem is a complex nexus of fibers and cells that in part serves to relay information from the cerebrum to the spinal cord and cerebellum and vice versa It is also the site where vital functions such as breathing, consciousness and body temperature are regulated Spinal cord The spinal cord is encased in the bony vertebral column and attached to the brain stem It is the major conduit of information from the skin, joints and muscles of the body to the brain and vice versa It communicates with the body via spinal nerves which are part of the peripheral nervous system Meninges The central nervous system is protected by meninges They are three types of membranes : dura mater, arachnoid membrane and pia mater The dura mater is tough, inelastic bag that surrounds the brain and spinal cord The arachnoid membrane appearance and consistency resemble a spider web And the pia mater is a thin membrane that adheres closely to the surface of the brain Ventricular system The ventricular system is the fluid filled caverns and canals inside the brain The cerebrospinal fluid is produced by choroid plexus in the ventricles of the cerebral hemispheres The Ventricular System of the Brain COMPONENT RELATED BRAIN STRUCTURES Lateral ventricles Cerebral cortex Basal telencephalon Third ventricles Thalamus Hypothalamus Cerebral aqueduct Tectum Midbrain tegmentum Fourth ventricle Cerebellum Pons Medulla Blood-brain barrier The blood-brain barrier is the separation of circulating blood from the brain extracellular fluid in the central nervous system It occurs along all capillaries and consists of tight junctions around the capillaries that do not exist in normal circulation Endothelial cells restrict the diffusion of microscopic objects and large or hydrophilic molecules into the cerebrospinal fluid, while allowing the diffusion of small hydrophobic molecules such as oxygen, carbon dioxide and hormones The cells of the barrier actively transport metabolic products such as glucose across the barrier with specific proteins 3. The peripheral nervous system and reflex activity  The PNS includes all neural structures outside the brain and spinal cord, that is, the sensory receptors, peripheral nerves and their associated ganglia, and efferent motor endings Sensory receptors are specialized to respond to changes in their environment, which are called stimuli Typically, activation of a sensory receptor by an adequate stimulus results in graded potentials that in turn trigger nerve impulses along the afferent peripheral nervous system fibers coursing to the central nervous system Classification of sensory receptor by stimulus type The receptor classes are named according to the activating stimulus Mechanoreceptors respond to mechanical force such as touch, pressure, vibration, and stretch Thermoreceptors are sensitive to temperature changes Photoreceptors, such as those of the retina of the eye, respond to light energy Chemoreceptors respond to chemicals in solution Nociceptors respond to potentially damaging stimuli that result in pain Classification of sensory receptor by location The receptors can also be recognized according to either their location or the location of the activating stimulus Exteroceptors are sensitive to stimuli arising outside the body, so most exteroceptors are near or at the body surface They include touch, pressure, pain, and temperature receptors in the skin and most receptors of the special senses  Interoceptors, also called visceroceptors, respond to stimuli within the body, such as from the internal viscera and blood vessels They monitor a variety of stimuli, including chemical changes, tissue stretch, and temperature Proprioceptors, like interoceptors, respond to internal stimuli; however, their location is much more restricted Proprioceptors occur in skeletal muscles, tendons, joints, and ligaments and in connective tissue coverings of bones and muscles Classification of sensory receptor by structural complexity On the basis of overall receptor structure, there are simple and complex receptors, but the overwhelming majority are simple The simple receptors are modified dendritic endings of sensory neurons They could be in the form of encapsulated dendritic endings and free nerve endings They are found throughout the body and monitor most types of general sensory information Complex receptors are localized collections of cells associated with the special senses Nerves A nerve is a cordlike organ that is part of the peripheral nervous system Nerves vary in size, but every nerve consists of parallel bundles of peripheral axons enclosed by successive wrappings of connective tissue  Nerves are classified according to the direction in which they transmit impulses Nerves containing both sensory and motor fibers and transmitting impulses both to and from the central nervous system are called mixed nerves Those that carry impulses only toward the central nervous system are sensory or afferent nerves, and those carrying impulses only away from the CNS are motor or efferent nerves Most nerves are mixed Pure sensory or motor nerves are rare The peripheral nerves are classified as cranial or spinal depending on whether they arise from the brain or the spinal cord. Cranial nerves Twelve pairs of cranial nerves are associated with the brain The first two pairs attach to the forebrain, and the rest are associated with the brain stem Other than the vagus nerves, which extend into the abdomen, cranial nerves serve only head and neck structures Spinal nerves Thirty-one pairs of spinal nerves, each containing thousands of nerve fibers, arise from the spinal cord and supply all parts of the body except the head and some areas of the neck. All are mixed nerves. These nerves are named according to their point of issue from the spinal cord. There are 8 pairs of cervical spinal nerves (C1–C8), 12 pairs of thoracic nerves (T1–T12), 5 pairs of lumbar nerves (L1–L5), 5 pairs of sacral nerves (S1–S5), and 1 pair of tiny coccygeal nerves (Co1).  Notice that there are eight pairs of cervical nerves but only seven cervical vertebrae. The first seven pairs exit the vertebral canal superior to the vertebrae for which they are named, but C8 emerges inferior to the seventh cervical vertebra.  Each spinal nerve connects to the spinal cord by a dorsal root and a ventral root Each root forms from a series of rootlets that attach along the length of the corresponding spinal cord The ventral roots contain motor or efferent fibers that arise from ventral horn motor neurons and extend to and innervate the skeletal muscles Dorsal roots contain sensory (afferent) fibers that arise from sensory neurons in the dorsal root ganglia and conduct impulses from peripheral receptors to the spinal cord  The spinal nerve rami and their main branches supply the entire somatic region of the body from the neck down The dorsal rami supply the posterior body trunk The thicker ventral rami supply the rest of the trunk and the limbs  Except for T2–T12, all ventral rami branch and join one another lateral to the vertebral column, forming complicated interlacing nerve networks called nerve plexuses Nerve plexuses occur in the cervical, brachial, lumbar, and sacral regions and primarily serve the limbs The terminals of somatic motor fibers that innervate voluntary muscles form elaborate neuromuscular junctions with their effector cells  The cerebral cortex is at the highest level of our conscious motor pathways Motor control exerted by lower levels is mediated by reflex arcs Many of the body’s control systems belong to a general category known as reflexes, which can be either inborn or learned Reflexes are classified functionally as somatic reflexes if they activate skeletal muscle, or as autonomic reflexes if they activate visceral effectors  Somatic reflexes mediated by the spinal cord are called spinal reflexes Many spinal reflexes occur without the direct involvement of higher brain centers However, the brain is “advised” of most spinal reflex activity and can facilitate, inhibit, or adapt it, depending on the circumstances 4. The autonomic nervous system  Also called the involuntary nervous system, which reflects its subconscious control, or the general visceral motor system, which indicates the location of most of its effectors The autonomic divided into sympathetic and parasympathetic nervous system The sympathetic and parasympathetic divisions typically function in opposition to each other The parasympathetic division is the brake and involves in actions that do not require immediate reaction. Think of "rest and digest“ The sympathetic division is the accelerator and involves inactions requiring quick responses. Think of "fight or flight“ These two systems permanently modulating vital functions, in usually antagonistic fashion, to achieve homeostasis Sympathetic division Also called the thoracolumbar division There are two kinds of neurons involved in the transmission of any signal through the sympathetic system: pre-ganglionic and post- ganglionic The shorter preganglionic neurons originate from the thoracolumbar region of the spinal cord. They travel to a ganglion, often one of the paravertebral ganglia, where they synapse with a postganglionic neuron. From there, the long postganglionic neurons extend across most of the body.  At the synapses within the ganglia, preganglionic neurons release acetylcholine which activates nicotinic acetylcholine receptors on postganglionic neurons In response to this stimulus postganglionic neurons release norepinephrine, which activates adrenergic receptors on the peripheral target tissues The activation of target tissue receptors causes the effects associated with the sympathetic system Exceptions: The postganglionic neurons of sweat glands release acetylcholine for the activation of muscarinic receptors, except for areas of thick skin, the palms and the plantar surfaces of the feet, where norepinephrine is released and acts on adrenergic receptors Chromaffin cells of the adrenal medulla are analogous to post- ganglionic neurons; the adrenal medulla develops in tandem with the sympathetic nervous system and acts as a modified sympathetic ganglion. Within this endocrine gland, pre-ganglionic neurons synapse with chromaffin cells, stimulating the chromaffin to release norepinephrine and epinephrine directly into the blood Postganglionic neurons for renal vasculature release dopamine at renal vasculature. Sympathetic dopamine activation to the kidneys dilates renal vessels which increases blood flow and in turn increases the glomerular filtration Parasympathetic division Also called the craniosacral division As in the sympathetic nervous system, efferent parasympathetic nerve signals are carried from the central nervous system to their targets by a system of two neurons The first neuron in this pathway is referred to as the preganglionic or presynaptic neuron Its cell body sits in the central nervous system and its axon usually extends to synapse with the dendrites of a postganglionic neuron somewhere else in the body. The axons of presynaptic parasympathetic neurons are usually long, extending from the central nervous system into a ganglion that is either very close to or embedded in their target organ  As a result, the postsynaptic parasympathetic nerve fibers are very short The parasympathetic nervous system uses mainly acetylcholine as its neurotransmitter Most transmissions occur in two stages: When stimulated, the preganglionic neuron releases acetylcholine at the ganglion, which acts on nicotinic receptors of postganglionic neurons The postganglionic neuron then releases acetylcholine to stimulate the muscarinic receptors of the target organ  The neurons of the somatic nervous system innervate directly the effector organs and are highly myelinated compared to the neurons of the autonomic nervous system The preganglionic neurons of both sympathetic and parasympathetic divisions are lightly myelinated and releases acetylcholine The postganglionic neurons of both sympathetic and parasympathetic divisions are unmyelinated The postganglionic neurons of the sympathetic divisions release mainly norepinephrine while the postganglionic neurons of the parasympathetic nervous system release acetylcholine  A yellow background with black text Description automatically generated A logo with a red and black logo Description automatically generated https://www.youtube.com/@Dr.RichardsPhysiologyNotes https://youtube.com/playlist?list=PLhQ PgS4- I3jScbyOK3YW1AITpIW_oam23&si=dKs G7C7XZwn8s6b8 References Marieb E.N., Hoehn K. , Human Anatomy & Physiology. 8th Edition, Pearson International. An online version of this book is also available. The specific chapters are - Chapter 11: Fundamentals of the Nervous System and Nervous Tissue - Chapter 12: The Central Nervous System - Chapter 13: The Peripheral Nervous System and Reflex Activity - Chapter 14: The Autonomic Nervous System