Histology Ch 12 352-381
Overview of the Nervous System
CNS is brain and spinal cord, PNS is cranial, spinal and peripheral nerves that conduct impulses
from efferent fibers to afferent fibers of CNS
-PNS has collections of nerve cell bodies called ganglia and specialized nerve endings
-interactions between sensory nerves and CNS, and motor responses is called neural pathway
-these pathways mediate reflex actions called reflex arcs
Somatic Nervous System contains functions under voluntary conscious control (except reflex); it
provides sensory and motor innervatino to all parts of body except viscera, smooth and cardiac
muscle, and glands
Autonomic Nervous System – involuntary motor innervation to smooth muscle, heart, glands,
and afferent sensory innervation from viscera; subdivided into
Sympathetic Division, Parasympathetic Division and Enteric Division
Composition of Nerve Tissue – consists of neurons and glia
-neurons are function units of nervous system contacted by synapses
-supporting cells are nonconducting cells located near neurons called glia
-CNS has 4 types: oligodendrocytes, astrocytes, microglia, and ependymal cells (central)
-PNS is called peripheral neuroglia: Schwann cells, satellite cells; satellite cells surround
neuron bodies and are analogous to Schwann cells in ganglia
-ganglia physically support neurons, insulate bodies, speed transmission, repair injury, regulate
internal fluid movement of CNS, clear neurotransmitters, metabolic exchange
-in humans, somatic nervous system has ability to respond to stimuli from environment through
action of effector cells
Autonomic nervous system regulates function of internal organs – helps contract smooth
muscle in blood vessels, gut, gallbladder, and urinary bladder
-innervates cardiac conducting cells in heart to regulate heart contraction
-innervates gland epithelium to regulate synthesis, composition and release of secretions
The Neuron – structural and functional unit of nervous system; 10 billion in the human
Sensory Neuron – convey impulses from receptors to CNS, included in somatic and visceral
afferent nerve fibers
-somatic afferent fibers convey pain, temperature, touch, and pressure; proprioception
-visceral afferent fibers transmit pain from internal organs, membranes, glands, vessels
Motor Neuron – somatic efferent and visceral efferent nerve fibers
-somatic efferent fibers send voluntary impulses to skeletal muscles
-visceral efferent fibers transmit involuntary impulses to smooth muscle, cardiac
conducting cells, and glands
Interneurons (intercalated neurons) - form a communicating network between sensory and
motor neurons.
Functional Components of Neuron – cell body of neuron has nucleus + organelles, most
neurons have only one axon that transmits impulses away from the cell to a synapse, which
makes contact on an effector cell
-dendrites are shorter processes that transmit impulses from periphery toward cell body
-neurons are classified on basis of number of processes extending from cell body:
1. Multipolar Neurons – neurons have 1 axon and 2+ dendrites; direction of impulse is
dendrite  cell body  axon
a. Motor Neurons and Interneurons are multipolar neurons
2. Bipolar Neurons – 1 axon and 1 dendrite, most often in special senses (taste, smell, etc…)
and found in retina of eye and ganglia of vestibulocochlear nerve of ear
3. Pseudounipolar – 1 process, the axon, that divides close to the cell body into 2 long axonal
branches; one extends to periphery and the other extends to CNS
a. Both axons are conducting units and impulses travel from peripheral arborization of
neuron and are the receptor of cell
b. Majority are sensory neurons close to the CNS
c. Cell bodies of sensory neurons are situated in dorsal root ganglia and cranial nerve
ganglia
Cell Body – has characteristics of protein producing cell and a large, euchromatic nucleus +
prominent nucleolus and surrounding perinuclear cytoplasm with large rER and free ribosomes
-ribosomes appear as small bodies called NIssl bodies that stain basic and corresponds
to 1 stack of rER
-also have mitochondria, golgi, lysosomes, microtubules, and neurofilaments
-axon hillock end of cell body into axon, LACKS large cytoplasmic organelles and can distinguish
axon from dendrites
-neurons do not divide; neural stem cells exist and can differentiate to replace damaged cells
-Neural Stem Cells – able to divide and characterized by intermediate filament protein nestin
Dendrites and Axons – main function of dendrites = receive information and carry it to cell body
-dendrites have > diameter than axon and are unmyelinated, tapered, and form dendritic trees
-axons convey information away from cell body to another neuron or muscle; each neuron has
only one axon, and it may be extremely long
-Golgi type I neurons – axons originating from neurons in motor nuclei in CNS (travel far)
-Golgi type II neurons – very short axons of interneurons in CNS
-axon originates from axon hillock, which lacks large cytoplasmic organelles such as Nissl bodies
and golgi cisternae
-region of axon between axon hillock and beginning of myelin sheath = initial segment
-this is the site at which action potential is generated in the axon
-some large axon terminals can undergo local protein synthesis, but most protein synthesis
occurs at cell body and is transported down axon via axonal transport systems
-areas that can synthesize protein in axons = periaxoplasmic plaques
Clinical Correlation: Parkinson’s Disease – slow neurologic disorder caused by loss of dopamine
secreting cells in substantia nigra and basal ganglia of brain. Losing dopamine causes:
1. resting tremor in limb, especially hand in resting position
2. rigidity or increased tone in all muscles
3. slowness of movement and inability to initiate movement
4. lack of spontaneous movements
5. loss of postural reflexes (poor balance and abnormal walking)
6. slurred speech, slowness of thought, and cramped handwriting
-idiopathic parkinson’s disease has unknown etiology, and secondary parkinsonism due to drug
treatment of neurologic disorders
-microscopically, degeneration of neurons is evident and increase in glial cells (gliosis) present
-nerve cells in region display intracellular inclusions called Lewy Bodies having proteins like
alpha-synuclein and ubiquitin
-treatment is with L-Dopa, a dopamine precursor that can cross blood-brain barrier
Synapses – the way neurons communicate with other neurons/effector cells; specialized
junctions to facilitate impulses from one cell to another; 3 different kinds:
1. Axodendritic – between axons and dendrites
2. Axosomatic – between axons and cell body
3. Axoaxonic – between axons and axons
a. Can see synapses with SILVER STAIN
-often, neuron makes several synaptic connections called boutons en passant; axon then
continues and ends as a terminal branch with enlarged tip, the bouton terminal
-synapses are chemical or electrical, depending on mechanism of conduction and the way action
potential is generated
Chemical Synapses – achieved by release of neurotransmitters from presynaptic neuron
Electrical Synapses – synapses contain gap junctions to permit movement of ions between cells
and direct spread of electrical current; in mammals, exist in smooth and cardiac muscle
Components of Chemical Synapse – a presynaptic element is the end of a neuron process from
which neurotransmitters are released, characterized by presence of synaptic vesicles
-binding of vesicles to presynaptic membrane is mediated by transmembrane proteins called
SNAREs; specifically v-SNARE (vesicle bound) and t-SNARE (target-membrane bound)
-another vesicle-bound protein called synaptotagmin 1 replaces SNARE complex which is
dismantled and recycled by NSF/SNAP5 complexes
-presynaptic dense protein accumulation are called active zones where synaptic vesicles are
docked and neurotransmitters are released
-active zones are rich in Rab-GTPase docking complex, t-SNAREs, and synaptotagmin
binding proteins
-synaptic cleft – 20 to 30nm space separating presynaptic from postsynaptic neuron
-postsynaptic membrane – contains receptors with which neurotransmitter interacts
-postsynaptic density – proteins which mediate how neurotransmitter acts on postsynaptic cell
Synaptic Transmission – impulse reaching synaptic button causes voltage gated Ca channels to
open in plasma membrane of bouton; influx of Ca causes synaptic vesicles to migrate, anchor,
and fuse with presynaptic membrane to release neurotransmitter into synaptic cleft
-vesicle docking and fusion is driven by SNARE and synaptotagmin
-porocytosis – alternative to massive neurotransmitter release following vesicle fusion; here
vesicles anchored to active zones release neurotransmitters through pore connecting lumen of
vesicle with synaptic cleft
-the neurotranmistter binds extracellular part of postsynaptic membrane receptor called
transmitter-gated channels; binding causes conformational change in these channels to cause
them to open
-influx of Na causes local depolarization, prompting the opening of voltage gated Na
channels
-some amino acid neurotransmitters can bind G-protein-coupled receptors to produce longerlasting and more diverse postsynaptic responses through effector proteins, which may be G
protein gated ion channels or enzymes that synthesize second messengers
Porocytosis – Ca influx causes vesicle and presynaptic membrane to reorganize and create a
1nm transient pore so neurotransmitters can be released into synapse
Excitatory Synapses – release of acetylcholine, glutamine, or serotonin opens transmitter-gated
Na channels to depolarize membrane and initiate action potential
Inhibitory Synapses – release of GABA or glycine opens transmitter gated Cl channels causing Cl
to enter and hyperpolarize the membrane
-ultimate generation of nerve impulses depends on summation of excitatory and inhibitory
impulses
Acetylcholine – common transmitter between axons and striated muscle at neuromuscular
junction and serves as a neurotransmitter in the autonomic nervous system
-ACh is released by presynaptic sympathetic + parasympathetic fibers and postsynaptic
sympathetic neurons for sweat glands
-neurons using ACh as neurotransmitter are called cholinergic, and receptors for ACh are called
cholinergic receptors; subdivided into 2 classes:
1. Muscarinic ACh Receptor – in the heart is a G-protein receptor linked to K channels;
parasympathetic stimulation causes opening of K channels for hyperpolarization of
heart muscle to slow rhythmic contraction
2. Nicotinic ACh Receptor – in skeletal muscles is a transmitter gated Na channel, which
will cause depolarization of skeletal muscle fibers and contraction
Curare – binds Na channels and blocks action of nicotinic ACh receptors causing paralysis
Atropin – blocks action of muscarinic ACh receptors
Botulinum Toxin – inhibits ACh release to decrease receptor stimulation  paralysis including
respiratory paralysis
Catecholamines – such as norepinephrine (NE), epinephrine (EPI), and dopamine (DA) are all
synthesized from TYROSINE; neurons using these are called catecholaminergic neurons
-secreted by ells in CNS that are involved in regulation of movement, food, and attention
-neurons using epinephrine are called adrenergic neurons, and contain enzyme to convert NE 
EPI to serve as transmitter in ANS and can be released into blood for fight/flight response
Serotonin (5-HT) – formed by hydroxylation and decarboxylation of TRYPTOPHAN and functions
as CNS and enteric nervous system transmitter; neurons called serotonergic
-serotonin is recycled by reuptake into presynaptic serotoneric neurons
Amino Acids such as GABA, Glutamate, Aspartate, and Glycine can act as CNS transmitters
Nitric Oxide (NO) – gas with free radical properties can be neurotransmitter, and can carry
signals from one neuron to another
-NO is synthesized WITHIN THE SYNAPSE and used immediately
-excitatory neurotransmitter glutamate induces NO synthase to produce NO, which diffuses into
postsynaptic membrane and generates action potential
Small peptides – such as substance P, hypothalamic releasing hormones, enkephalins,
vasoactive intestinal peptide, cholecystokinin (CCK), and neurotensin
-most are released by neuroendocrine cells of intestine and act on neighboring cells or be
carried in blood as hormones act on distant target cells
-also synthesized by hypothalamus
Neurotransmitter Degradation/Reuptake – most common process is high-affinity reuptake,
where they are bound to specific neurotransmitter transport proteins on presynaptic membrane
-transmitters recycled back to presynaptic cytoplasm are either degraded or reused in vesicles
-action of catecholamines is terminated by reuptake of them into presynaptic cell using Nadependent transporters (can block using amphetamine and cocaine)
-once inside presynaptic button, they are reloaded into vesicles for future use, but
excess catecholamines are inactivated by catechol O-methyltransferase (COMT) or
destroyed by monoamine oxidase
-Acetylcholinesterase (AChE) – secreted by muscle into synapse and degrades ACh into acetic
acid and choline; choline is taken up by cholinergic presynaptic button and remade into ACh
-AChE inhibitors have been used to treat myasthenia gravis
Axonal Transport Systems – axonal transport is required to convey new material to axon
processes; it is a bidirectional mechanism and serves as mode of communication by carrying
molecules to and from axon terminal on microtubules and intermediate filaments
Anterograde Transport – carries material from body to periphery on kinesin
Retrograde Transport – carries material from axon to body on dynein
Slow Transport System – substances travel from cell body to button at 0.2-4mm/day and is
ONLY an anterograde system; tubulin, actin, and neurofilament proteins are carried this way
Fast Transport System – bidirectional at 20-400mm/day
-Fast Anterograde – carries different organelles such as sER, vesicles, mitochondria, and
sugars, AA, nucleotides, neurotransmitters to axon terminal
-Fast Retrograde – carries same materials as well as endocytosed materials from axon
to cell body
-FAST TRANSPORT REQUIRES ATP for migration of microtubule associated proteins
-toxins and viruses enter CNS through retrograde transport
Peripheral Neuroglia- Schwann cells, satellite cells and other cells associated with specific
organs such as terminal neuroglia (associated with motor end plate), enteric neuroglia
associated with ganglia in GI tract, and Müller’s cells in the retina
Schwann Cells and Myelin Sheath – schwann cells support myelinated and unmyelinated fibers;
develop from neural crest and differentiate by expression of SOX-10
-in PNS, schwann cells produce lipid rich myelin sheath that surrounds axons for fast conduction
-axon hillock and terminal arborizations of synapses are NOT covered by myelin
-unmyelinated fibers are still enveloped by Schwann cell cytoplasm
-Schwann cells also clean up PNS debris and guide regrowth of PNS axons
Formation of Myelin Sheath in PNS – axon lies in a groove on surface of Schwann cell; a 0.1mm
segment of axon becomes enclosed within Schwann cell, which then becomes polarized to 2
membrane domains:
-Abaxonal Plasma Membrane – part of Schwann cell surface exposed to external environment
-Adaxonal or periaxonal plasma membrane – Schwann cell membrane directly in contact with
axon
-Mesaxon – occurs when Schwann cell membrane completely encloses axon; this domain is a
double membrane that connects abaxonal and adaxonal membranes and encloses narrow
extacellular space
-myelin sheath develops when Schwann cell mesaxon surrounds axon and wraps around it ina
spiraling motion to form multiple layers; first few layers not completely arranged
-external to developing myelin sheath is a thin outer collar of perinuclear cytoplasm called the
sheath of Schwann; enclosed by abaxonal plasma membrane and contains nucleus and
organelles of schwann cell
-the apposition of mesaxon of last layer to itself as it closes the ring of the spiral
produces the outer mesaxon, the narrow intercellular space adjacent to external lamina
-internal to concentric layers of myelin sheath is a narrow inner collar of Schwann cell
cytosplasm surrounded by adaxonal plasma membrane
-narrow intercellular space between mesaxon membranes communicates with
adaxonal membraneto produce the inner mesaxon
-compaction of sheath corresponds with expression of transmembrane, myelin specific proteins
such as protein 0 (P0), PMP22, and myelin basic protein (MBP)B
-inner (cytoplasmic) leaflets of plasma membrane come together as a result of (+) charged
domains of P0 and MBP
-aligned leaflets appear as major dense lines and concentric dense lamellae alternate with
slightly less dense intraperiod lines
-mutations in human P0 can cause unstable myelin and may contribute to demyelinating disease
Gullain-Barre Syndrome – called acute inflammatory demyelinating polyradiculoneuropathy,
most common life-threatening disease of PNS; examination shows lymphocyte, macrophage,
and plasma cells around nerve fibers within nerve fascicles causing damage to myelin sheath
-T cell-mediated response against myelin to slow nerve conduction, causing muscle paralysis,
loss of coordination, and loss of cutaneous sensation
Multiple Sclerosis – disease that attacks myelin in the CNS, characterized by damage to myelin,
which becomes detached from axon and is destroyed; destruction of oligodendroglia also occurs
-myelin basic protein is main target for autoimmune reaction
-plaques throughout white matter of CNS is seen
-symptoms are unilateral vision impairment, loss of cutaneous sensation, lack of muscle
coordination, and loss of bladder and bowel control
Thickness of myelin sheath is determined by axon diameter and not by Schwann cell –
regulated by a growth factor called neuregulin (Ngr1) that acts on Schwann cells and is
expressed on axon membrane
Node of Ranvier is junction between two adjacent schwann cells – node of ranvier is where
two schwann cells meet and is devoid of myelin; myelin between two sequential nodes is called
intermodal segment
-myelin is mostly lipids because Schwann cell membrane winds around axon and the cytoplasm
of Schwann cell is extruded
-the inner collar of Schwann cell cytoplasm, between axon and myelin is called SchmidtLanterman Clefts (small islands within successive lamellae of myelin)
-Perinodal cytoplasm is at the node of Ranvier and out collar of perinuclear cytoplasm around
myelin
-unmyelinated axons in PNS are wrapped by Schwann cell cytoplasm
Satellite Cells – neuron bodies of ganglia are surrounded by cuboidal satellite cells; in
paravertebral and peripheral ganglia, neural cell processes must penetrate satellite cells to
establish a synapse
-help to establish and control microenvironment around neuron body to provide electrical
insulation as well as pathway for metabolic exchange
-Enteric neuroglial cells are similar to astrocytes (structural, functional, metabolic support)
Central Neuroglia: 4 types of cells:
Astrocytes – provide physical and metabolic support for neurons of CNS
Oligodendrocytes – maintain myelin in CNS
Microglia – possess phagocytic properties
Ependymal Cells – columnar cells lining ventricles of brain and central canal of spinal cord
-brain and spinal cord develop from embryonic neural tube; in head region, the tube undergoes
thickening and folding to create the brain
-in early stages, glial cells extend through entire thickness of neural tube in radial
manner (radial glial cells) to act as physical scaffold to direct migration of neurons to
appropriate positions in brain
Astrocytes – largest of neuroglia, form a network of cells in CNS and communicate with neurons
1. Protoplasmic Astrocytes – more prevalent in outermost covering of brain gray matter
and have many short processes
2. Fibrous Astrocytes – common in inner core of brain’s white matter, and have fewer
processes and are relatively straight
-both astrocytes contain bundles of intermediate filament fibers called glial fibrillary acidic
protein GFAP
-tumors arising from astrocytes account for 80% of adult primary tumors; express GFAP
-astrocytes help regulate metabolites and wastes from neurons and they maintain the bloodbrain barrier
-astrocytes cover “bare areas” on axon (nodes of Ranvier and synapses)
-protoplasmic astrocytes extend their processes to basal lamina of pia mater to form glia
limitans, an impermeable barrier surrounding CNS
Astrocytes modulate neuronal activities by buffering the K concentration in extracellular space
to maintain microenvironment and activities of neuron
-astrocyte membrane has abundance of K pumps/channels to mediate transfer from high to low
concentration
-influx of K into astrocyte depolarizes membrane, and charge is dissipated by extensive network
of astrocyte processes (potassium spatial buffering)
Oligodendrocytes produce/maintain myelin sheath in CNS – formed by concentric layers of
oligodendrocyte plasma membrane
-each process wraps itself around portion of axon to form intermodal segment of myelin;
multiple processes of a single oligodendrocyte may myelinate one axon or several axons
-Myelin in CNS differs from PNS – instead of P0 and PMP-22 in PNS, other proteins called
proteolipid protein (PLP), myelin oligodendrocyte glycoprotein (MOG), and oligodendrocyte
myelin glycoprotein (OMgp) are found in CNS myelin
-CNS myelin has few Schmidt-Lanterman clefts because astrocytes provide metabolic support
for CNS neurons
-nodes of Ranvier in CNS are larger than those in PNS; saltatory conduction more efficient
-unmyelinated axons in CNS are found bare and not embedded in glial cell processes
Microglia Possess Phagocytic Properties – in regions of injury and disease, microglia proliferate
and become actively phagocytic (reactive microglial cells); considered part of mononuclear
phagocytic system and originate from granulocyte/monocyte progenitors
-microglia enter CNS parenchyma from vascular system
Ependymal Cells form Lining of Ventricles of Brain and Spinal Canal – form a single layer of
cuboidal-to-columnar epithelium that have characteristics of fluid-transporting cells; tightly
bound by junctional complexes at apical surfaces
-within the brain ventricles, ependymal cells are modified to produce CSF by transport ans
secretion of materials from capillary loops
-modified ependymal cells + capillaries are called the choroid plexus
Impulse Conduction – electrochemical process involves generation of action potential, a wave
of membrane depolarization that is initiated at the initial segment of axon hillock
-membrane contains large numbers of voltage-gated Na and K channels
-in response to stimulus, the vNa channels in initial segment open and Na rushes in to depolarize
membrane from -70mV to +30mV
-after depolarization, voltage gated Na channels close and voltage gated K channels open
-K rapidly exits axon to return membrane to its resting potential
-depolarization of one part of membrane sends current to adjacent portions of axon to
depolarize it as well
-Myelinated axons – conduct impulses more rapidly than unmyelinated axons due to jumping of
impulse from node to node, called saltatory or discontinuous conduction
-myelin sheath around nerve does not conduct current, and forms insulating layer around axon
-voltage reversal can ONLY occur at nodes of Ranvier, where axolemma lacks myelin sheath and
is exposed to extracellular fluid; axolemma contains Na and K voltage-gated channels
-current jumps from one node to the next
-in unmyelinated axons, Na and K are distributed uniformly along the length of fiber; nerve is
conducted more slowly and moves as a continuous wave of voltages
Origin of Nerve Tissue Cells – Neurons, oligodendrocytes, astrocytes, and ependymal cells are
derived from neural tube; after neurons migrate to predestined locations, they no longer divide
-neural stem cells are a small number of cells that can still divide
-oligodendrocyte precursors are highly migratory cells and share lineage developmentally with
motor neurons migrating from site of origin to developing axon projections in white matter
-astrocytes also derived from neural tube and migrate to cortex where they differentiate into
mature astrocytes
-Ependymal cells are derived from proliferation of neuroepithlial cells that immediately
surround canal of developing neural tube
-Microglia cells are derived from mesodermal macrophage precursors, specifically
granulocyte/monocyte progenitor (GMP) cells in bone marrow
-they are stimulated by growth factors like colony stimulating factor 1 (CSF-1) secreted
by developing neural cells, and undergo differentiation into ameboid cells, motile cells
-microglia posses vimentin class of intermediate filaments, useful for identification
-PNS ganglion cells and peripheral glia are derived from neural crest
-Schwann cells arise from migrating neural crest cells that become associated with axons of
early embryonic nerves
-sex-determining region Y (SRY) box 10 (Sox10) is required for generation of all
peripheral glia from neural crest
-axon-derived neuregulin 1 (Nrg-1) sustains Schwann cell precursors that undergo
differentiation and divide along growing nerve processes
-Schwann cells in contact with large-diameter axons mature into myelinating schwann cells
-Schwann cells associating with small-diameter axons mature into nonmyelinating cells
Peripheral Nerves – bundle of nerve fibers held together by connective tissue
-cell bodies of peripheral nerves are located with in CNS or in peripheral ganglia, neuronal cell
bodies and fibers leading to and from them
-dorsal root ganglia and cranial nerve ganglia belong to sensory neurons (somatic afferents and
visceral afferents that belong to autonomic nervous system)
-cell bodies in paravertebral, terminal ganglia belong to postsynaptic motor neurons (visceral
efferents) of autonomic nervous system
Motor Neuron cell bodies of PNS lie in CNS – in cortex, brain stem, and spinal cord
Sensory neuron cell bodies are in ganglia outside CNS – both somatic and visceral afferents
send single axons to sensory ganglia in spinal cord or brain stem
-sensory ganglia are in dorsal roots of spinal nerves and CN V, VII, VIII, IX, and X
Connective Tissue Components of Peripheral Nerve – bulk consists of nerve fibers and
supporting Schwann cells
-nerve fibers are held together by connective tissue with 3 components
1. endoneurium – loose connective tissue surrounding each individual nerve fiber
2. perineurium – specialized connective tissue surrounding each nerve fascicle
3. epineurium – dense, irregular connective tissue surrounds peripheral nerve and fills spaces
between the nerve fascicles
Endoneurium – collagen fibrils readily apparent running parallel and around nerve fibers and
binding them together into a fascicle (bundle); endoneurium likely to be secreted by Schwann
cells
-only other connective tissue cells in endoneurium are mast cells and macrophages
Perineurium – special connective tissue layer that serves as metabolically active diffusion
barrier contributing to the blood-nerve barrier, which maintains ionic milieu of ensheathed
fibers
-Perineural cells have receptors, transporters, and enzymes for active transport of substances
across the cells; may be 1 or more layers thick and squamous
-the cells are contractile and contain number of actin filaments (like smooth muscle)
-tight junctions provide basis for blood-nerve barrier and are present between cells located
within the same layer of perineurium
Epineurium – dense, irregular connective tissue surrounding nerve fascicles into a common
bundle; blood vessels supplying the nerves travel in epineurium and branches penetrate nerve
Afferent (Sensory) Receptors – at distal tips of peripheral processes of sensory neurons
1. Exterceptors – react to stimuli from external environment: temp, touch, smell, sound, vision
2. Enteroreceptors – react to stimuli within body; degree of filling or stretch of vessels/canals
3. Proprioceptors – react to stimuli within body to provide sensation of body position and tone
-simplest receptor is the free nerve ending
-most sensory nerve endings with connective tissue sheaths are called encapsulated endings
and are mechanoreceptors in skin and joint capsules
-muscle spindles are sensory nerve endings in skeletal muscle
Organization of Autonomic Nervous System – sympathetic, parasympathetic, and enteric
-ANS controls internal environment, impulses to smooth muscle, cardiac muscle, and glandular
epithelium
-Visceral motor (efferent) and sensory (afferent) neurons are the fibers of the ANS; these are
pseudounipolar neurons with cell bodies in sensory ganglia
-one neuron conveys impulses from CNS to somatic effector, whereas a chain of 2 neurons
conveys impulses from CNS to visceral effectors
Sympathetic and Parasympathetic Divisions of ANS – presynaptic neurons send axons from
thoracic and upper lumbar cord to vertebral (sympathetic trunk) and paravertebral ganglia
which contain cell bodies of sympathetic division
-presynaptic parasympathetic neurons send axons from brain stem (midbrain, pons, medulla)
and sacral segments of cord (S2-4) to visceral ganglia; ganglia in wall of abdominal and pelvic
organs and visceral motor ganglia of CN III, VI, IX, X contain cell bodies of postsynaptic effector
neurons of parasympathetic division
-many functions of SNS similar to adrenal medulla, but sympathetic neurons deliver agent
directly to effector, whereas medulla hormones secrete agent into bloodstream
Enteric Division of Autonomic Nervous System – collection of neurons and ganglia that
innervate the GI tract to control motility, exocrine/endocrine functions, and blood flow
-can function independently from CNS, however digestion requires communication between
enteric neurons and CNS provided by parasympathetic and sympathetic fibers
-enteroreceptors in GI tract provide sensory info to CNS regarding digestive functions,
and CNS then coordinates sympathetic stimulation that inhibits GI secretion, motor
activity, and sphincter control; parasympathetic stimuli produce opposite actions
-ganglia/postsynaptic neurons of enteric division are in lamina propria, muscularis mucosa,
submucosa, muscularis externa, and subserosa of GI from esophagus through anus
-peristalsis will continue even if vagus nerve or pelvic splanchnics are cut
-neurons of enteric division are not supported by Schwann or satellite cells, but instead by
enteric neuroglial cells resembling astrocytes
Summarized View of ANS Distribution –
1. Head
a. Parasympathetic Presynaptic Outflow – to the head leaves brain with cranial neves
b. Sympathetic presynaptic outflow – to the head comes from thoracic region of
spinal cord; postsynaptic neurons have cell bodies in superior cervical ganglion and
axons leave ganglion and hug internal/external carotid arteries to form periarterial
plexus
2. Thorax
a. Parasympathetic Presynaptic Outflow – via vagus nerve; postsynaptic neurons have
cell bodies in walls or parenchyma of organs of thorax
b. Sympathetic Presynaptic Outlow – is from upper thoracic segments of spinal cord;
heart are in cervical ganglia and axons make up cardiac nerves; sympathetic trunk
innervates viscera and axons travel through splanchnic nerves from sympathetic
trunk to organs within thorax and from pulmonary and esophageal plexuses
3. Abdomen/Pelvis
a. Parasympathetic Presynaptic Outflow – through vagus nerve and pelvic splanchnic
nerves; postsynaptic neurons terminate on organ such as submucosal (meissner’s
plexus) and myenteric plexus in alimentary canal
b. Sympathetic Presynaptic Outflow – is from lower thoracic and upper lumbar
segments of cord traveling to prevertebral ganglia through abdominopelvic
splanchnic nerves consisting of greater, lesser, and least thoracic splanchnic nerves
and lumbar splanchnic nerves; postsynaptic neurons have cell bodies in prevertebral
ganglia
i. Only presynaptic fibers terminating on cells in medulla of adrenal gland
originate from paravertebral ganglia of sympathetic trunk
4. Extremities and Body Wall – there is no parasympathetic outflow to body wall/extremities,
and there is ONLY sympathetic innervation of body wall
a. Ach spinal nerve contains postsynaptic sympathetic fibers; for sweat glands,
neurotransmitter released by sympathetic neurons is ACh instead of usual NE