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VT 105
Comparative Anatomy and Physiology
Nervous System
Functions of the Nervous System
sensory function – senses stimuli (changes in internal or external environment)
integrative function – processes sensory inputs and decides on appropriate
responses
motor function – sends signals to effector cells, which respond to the stimuli
DIVISIONS OF THE NERVOUS SYSTEM
Central Nervous System (CNS) – brain and spinal cord
main integrative center
contains the cell bodies of most neurons
Peripheral Nervous System (PNS) – all nervous tissue outside the CNS
nerves – bundles of axons following specific paths outside the CNS
cranial nerves (12 pairs) – arise from the brain
spinal nerves (many pairs) – arise from the spinal cord
ganglia – small clusters of neuron cell bodies outside the CNS
sensory receptors – dendrites of neurons or specialized cells
Functional Divisions of the PNS
somatic nervous system (SNS) – voluntary
sends sensory information about the external environment or body
position to the upper brain, where the inputs are consciously
perceived
sends motor impulses to skeletal muscles to cause body movements
autonomic nervous system (ANS) – involuntary (self-regulated)
sends sensory information about the internal environment to the
lower brain (not consciously perceived)
sends motor impulses to effectors such as smooth muscle, glands,
and cardiac muscle
HISTOLOGY OF NERVOUS TISSUE
Neurons – 3 basic parts
1) cell body – contains the nucleus and cellular organelles
carries out vital functions of the cell
2) dendrites – branched receiving portion of neuron
receive stimuli from the environment or other neurons
vary in number (more dendrites = more stimuli can be received)
3) axon – single, long sending portion of neuron
synaptic end bulb – bulb at end of axon which synapses with effector cell
synaptic vesicles – store neurotransmitters
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sensory (afferent) neuron – axon sends impulses to the CNS
motor (efferent) neuron – axon sends impulses away from the CNS
synapse – site of communication between a neuron and an effector cell
(neuron, muscle fiber, gland)
Neuroglia
Types in CNS:
astrocytes – surround and support neurons, structurally and functionally
help form the blood-brain barrier
oligodendrocytes – produce myelin sheath in the CNS
Types in PNS:
Schwann cells – produce myelin sheath in PNS
satellite cells – surround and protect cell bodies in ganglia
Myelination
myelin sheath – layers of cell membrane(lipid) wrapped around the axon
electrically insulates the axon
increases rate of impulse conduction
PNS – Schwann cells wrap segments of axons
cytoplasm and nucleus of Schwann cell form the outermost layer
nodes of Ranvier – gaps in myelin sheath between Schwann cells
CNS – oligodendrocytes have multiple, flat processes which wrap several
adjacent axon segments
White and Grey Matter
white matter – areas of CNS that appear white and shiny
contains many myelinated axons – myelin is white (lipid)
grey matter – grey areas of CNS
composed of neuron cell bodies, neuroglia, unmyelinated axons
NEURON PHYSIOLOGY – Production of Electrical Impulses
electrical current – flow of charged particles (ions in cells)
Ion Channels in the Cell Membrane
chemically-gated channels – open or close when a particular molecule binds
(eg. taste & smell molecules, neurotransmitters)
mechanically-gated channels – open or close in response to mechanical forces
(eg. touch & pressure, sound waves)
voltage-gated channels – open or close in response to change in membrane
potential (charge inside cell)
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Resting Membrane Potential – at rest, neuron cell membrane is polarized
(different charges on inside and outside of membrane)
Na+/K+ pumps
pump Na+ out of neuron (high Na+ concentration outside neuron) K+
positive Na+ balanced by negative Clpump K+ into neuron (high K+ concentration inside neuron)
positive K+ balanced by negative protein molecules
Neuron has different permeability to ions
K+ permeability is 50-100 times greater than Na+
(many K+ leakage channels, almost no Na+ leakage channels)
K+ leaks out of neuron, down its concentration gradient
Interior of neuron becomes increasingly negative
(negative proteins in neuron too large to diffuse out – impermeable)
Negative charge in neuron draws some K+ back into cell
At equilibrium, resting membrane potential is about -70mV
(70mV more negative inside cell than outside cell)
Stimulation of Neuron – small changes in resting membrane potential caused by
opening chemically- or mechanically- gated channels on dendrites
depolarization – membrane becomes less polarized (less negative inside)
Action Potentials (nerve impulses) – large change in resting membrane potential
caused by opening voltage-gated channels on axons
begins near the cell body and travels down axon to synaptic end bulbs
1) Neuron Stimulated to Threshold – specific level of depolarization that triggers
opening of voltage-gated Na+ channels (about -55mV)
all-or-none response – if neuron reaches threshold, an action potential
occurs (a signal is sent)
if threshold isn't reached, no action potential
2) Depolarization Phase – neuron becomes less polarized (less negative inside)
voltage-gated Na+ channels open – Na+ rushes into neuron
(charge inside neuron becomes positive)
3) Repolarization Phase – neuron becomes polarized again (negative inside)
voltage-gated K+ channels open – K+ rushes out of cell
(charge inside neuron becomes negative again)
(voltage-gated Na+ and K+ channels close again)
4) Na+/K+ pumps restore resting membrane potential
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Refractory Period – time after action potential begins when cell can’t generate another
action potential because voltage-gated channels are not reset
Conduction of Action Potentials – traveling of nerve impulse down the axon
refractory period results in one-way conduction
continuous conduction – step-by-step depolarization of the entire length of an
unmyelinated axon
relatively slow
saltatory conduction – occurs along myelinated axons
depolarization leaps from one node of Ranvier to the next
the entire axon does not completely depolarize
impulse is conducted very rapidly
opening ion channels only at nodes means less Na+ and K+ pass through
membrane and less ATP (energy) is used to pump them back
Synapses Between Neurons
presynaptic neuron – sending neuron (axon synaptic end bulb)
postsynaptic neuron – receiving neuron (dendrite)
synaptic cleft – small space between 2 communicating neurons
an action potential in the presynaptic neuron triggers release of neurotransmitter
from synaptic vesicles
neurotransmitter diffuses across synaptic cleft and binds to receptors
(membrane proteins on the postsynaptic neuron that cause change in charge)
excitatory neurotransmitter – depolarizes the postsynaptic neuron
brings it closer to threshold (may cause an action potential)
inhibitory neurotransmitter – hyperpolarizes the postsynaptic neuron
postsynaptic neuron becomes more negative (farther from threshold)
the postsynaptic neuron can have many synapses
summation of all of the excitatory and inhibitory synapses determines whether the
postsynaptic neuron reaches threshold and produces an action potential
Neurotransmitters – there are many different kinds of neurotransmitters
Acetylcholine (ACh) – acts in PNS and CNS
excitatory at skeletal muscles – causes contraction
inhibitory in the heart – decreases heart rate
acetylcholinesterase – enzyme inactivates acetylcholine in synaptic cleft
gamma aminobutyric acid (GABA) – common inhibitor in CNS
some tranquilizers (valium) enhance action of GABA
Catecholamines – excitatory or inhibitory depending on the receptors
norepinephrine (NE) – “fight-or-flight” responses
epinephrine (E)– hormone from adrenal gland (similar to NE)
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Removal of Neurotransmitter – effect of neurotransmitter continues until it is
removed from the synaptic cleft
3 mechanisms of removal:
enzymatic degradation (eg. acetylcholinesterase)
uptake by cells – neuron that released it or neuroglial cells
diffusion away from synaptic cleft, degraded by other cells
How Drugs and Toxins Modify Nervous System Function
stimulate or inhibit neurotransmitter synthesis
stimulate or inhibit neurotransmitter release
block or activate neurotransmitter receptors
agonists activate receptors (mimic neurotransmitter)
antagonists block receptors (prevent neurotransmitter function)
stimulate or inhibit neurotransmitter removal
THE BRAIN
PROTECTION AND NOURISHMENT OF THE BRAIN
Cranium – bones surrounding and protecting the brain
Cranial Meninges – 3 connective tissue membranes around brain
pia mater – inner membrane which adheres to surface of brain
contains blood vessels which supply the brain
arachnoid mater – delicate middle membrane
has web-like collagen and elastic fibers that extend to the pia mater
subarachnoid space – space between arachnoid and pia that
contains cerebrospinal fluid (CSF)
dura mater – tough, protective outer membrane
fuses with periosteum of cranium
folds between cerebral hemispheres and between cerebrum and
cerebellum help secure brain's position
contains large, open veins that collect excess CSF
Blood-Brain Barrier – protects brain by preventing passage of many substances
from the blood to brain tissue
brain capillaries have tight junctions between cells
astrocyte processes surround capillaries – selectively pass some substances
to neurons but block others
glucose crosses by active transport – main energy source for neurons
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Cerebrospinal Fluid (CSF) – clear fluid which circulates through cavities in
brain, spinal cord, and in subarachnoid space
Functions of CSF:
chemical content helps regulate autonomic functions
cushions delicate neurons of brain and spinal cord
Formation and Circulation of CSF
ventricles – 4 cavities in brain filled with CSF
capillary networks in each ventricle filter blood to form CSF
neuroglial cells lining ventricles regulate content of CSF
CSF circulates from ventricles to central canal of spinal cord and the
subarachnoid space
CSF returns to blood in veins within dura mater
hydrocephalus – excess accumulation of CSF resulting in increased
pressure on the brain
4 DIVISIONS OF THE BRAIN - brainstem, diencephalon, cerebellum, cerebrum
1) Brainstem – connects to the spinal cord
controls vital autonomic functions and autonomic reflexes such as; swallowing,
coughing, sneezing, vomiting
Medulla oblongata – caudal brainstem
vital centers – control vital autonomic functions
cardiovascular center – regulates heart and blood vessels
respiratory center – controls respiratory muscles
Pons – middle region of brainstem
regulates respiratory rhythm
Midbrain (Mesencephalon) – cranial brainstem
contains reflex centers for vision and hearing
2) Diencephalon – (between brain) between brainstem and cerebrum
thalamus – 80% of diencephalon
relay station for sensory impulses traveling to the cerebrum
hypothalamus – ventral to (below) thalamus
has no blood-brain barrier – senses changes in blood and CSF
regulates the ANS – involuntary organ functions
regulates eating and drinking – thirst center, feeding center
regulates body temperature via ANS
link between the nervous and endocrine systems
produces hormones that regulate anterior pituitary gland
produces oxytocin and antidiuretic hormone
participates in emotional behavior (eg. fight-or-flight responses)
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3) Cerebellum – attached to dorsal brainstem
coordinates skeletal muscle movements
receives voluntary motor impulses from cerebrum
receives sensory impulses related to body position and balance
the cerebellum compares intended movements with actual movements
sends feedback to cerebrum for corrections
disorders result in hypermetria – voluntary movements are jerky and exaggerated
4) Cerebrum – largest, most dorsal portion of brain
origin of voluntary actions, site of conscious perceptions, center of intellect
longitudinal fissure – deep groove that divides cerebrum into 2 hemispheres
sulci – shallower grooves that divide hemispheres into lobes
Cerebral Cortex – outer gray matter
contains neuron cell bodies controlling conscious functions
Functional Areas of the Cerebral Cortex
Sensory areas – caudal cerebrum
primary sensory cortex – receives sensations of pain, touch,
temperature from the opposite side of the body
parietal lobe
visual cortex – receives visual sensations
occipital lobe
auditory cortex – receives sensations of hearing
temporal lobe
Motor areas – cranial cerebrum
primary motor cortex – controls voluntary contractions of
skeletal muscles
frontal lobe
the cortex sends motor impulses to the opposite side of the body
Association areas – located within or near motor and sensory areas
allow recognition of sensations
control complex, learned motor skills
performs abstract functions – prediction, reasoning
Cerebral White Matter – deep to cortex
contains axon tracts running to and from spinal cord, between the cerebral
hemispheres, and within the same hemispheres
corpus callosum – main tracts connecting the 2 cerebral hemispheres
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CRANIAL NERVES – 12 pairs arising mainly from brainstem
sensory nerves – only sensory axons
motor nerves – only motor axons
mixed nerves – sensory and motor axons
Cranial nerve I – olfactory nerve
sensory – olfaction (smell)
Cranial nerve II – optic nerve
sensory – vision
Cranial nerve III – oculomotor nerve
motor – somatic – most eyeball movements
autonomic – inner eye movements (pupil size, focusing lens)
Cranial nerve IV – trochlear nerve
motor – eyeball movements
Cranial nerve V – trigeminal nerve
mixed – sensory from face, jaw, and teeth
motor to muscles of mastication (chewing)
has 3 branches;
ophthalmic nerve – sensory
maxillary nerve – sensory
mandibular nerve – mixed
Cranial nerve VI – abducens nerve
motor – eyeball movements
Cranial nerve VII – facial nerve
mixed – sensory from taste buds
motor somatic – facial expressions, lip movement
autonomic – secretion of tears, saliva, nasal secretions
Cranial nerve VIII – vestibulocochlear nerve (auditory, acoustic)
sensory – 2 branches
vestibular nerve – balance
cochlear nerve – hearing
Cranial nerve IX – glossopharyngeal nerve
mixed – sensory from taste buds, throat
motor somatic – swallowing and tongue movement
autonomic – secretion of saliva
Cranial nerve X – vagus nerve
mixed – sensory from larynx, visceral organs, carotid artery
motor somatic – swallowing
autonomic parasympathetic to most viscera
Cranial nerve XI – accessory nerve
motor – head and shoulder movements
Cranial nerve XII – hypoglossal nerve
motor – tongue movements
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SPINAL CORD AND SPINAL NERVES
EXTERNAL ANATOMY OF SPINAL CORD
extends from brainstem to lumbar vertebrae in adult
cauda equina – bundle of nerve roots in caudal vertebral canal after spinal
cord ends
spinal nerves – emerge in pairs through the intervertebral foramina
most emerge caudal to corresponding vertebra (except cervical nerves)
cervical nerves – 1 pair/cervical vertebra
+ 1 pair between skull and atlas vertebra
thoracic nerves – 1 pair/ thoracic vertebra
lumbar nerves – 1 pair/lumbar vertebra
sacral nerves – 1 pair/sacral vertebra
coccygeal nerves – variable numbers
SPINAL MENINGES – 3 connective tissue membranes surrounding the spinal cord
similar to, and continuous with cranial meminges
pia mater – thin, inner membrane on surface of spinal cord with many blood
vessels supplying the spinal cord
arachnoid mater – thin, middle membrane with a spider’s web of collagen and
elastic fibers extending to the pia mater
subarachnoid space – space beneath the arachnoid mater containing
cerebrospinal fluid (CSF)
site for spinal tap to collect CSF
dura mater – outer, dense connective tissue sheath surrounding spinal cord and
cauda equina
suspends spinal cord within vertebral canal
epidural space – space above dura mater
contains adipose tissue and blood vessels
cushions spinal cord
site for epidural anesthetic injections
SPINAL NERVES – mixed nerves (contain sensory and motor axons)
spinal nerves arise at specific spinal cord segments
dorsal root – contains sensory axons
dorsal root ganglion – swelling on dorsal root containing cell
bodies of sensory neurons
ventral root – contains motor axons
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Distribution of Spinal Nerves – spinal nerves branch immediately after passing through
intervertebral foramina (gaps between vertebrae)
phrenic nerve (C5-C6) – innervates diaphragm
thoracic nerves (intercostal nerves)
innervate muscles and skin of thorax and abdominal skin
nerve plexuses – complex networks of nerves on either side of body
brachial plexus (C6-C8 and T1 in cat)
innervates shoulder and forelimb
musculocutaneous, radial, ulnar, median nerves
lumbosacral plexus (L4-L7 and S1-S3 in cat)
innervates abdominal muscles, perineum (genitals, anal and
urethral sphincters) and lower limb
sciatic, femoral, and obturator nerves
spinal cord damage causes loss of sensation and voluntary muscle control
caudal to site of injury
INTERNAL ANATOMY OF SPINAL CORD
Gray Matter – butterfly or H shape located centrally
contains nuclei – clusters of cell bodies in CNS
ventral gray horns – somatic motor nuclei
dorsal gray horns – sensory nuclei
central canal – contains CSF, lined by ependymal cells
White Matter – mainly myelinated axons located peripherally
sensory (afferent) tracts – axons carrying sensory impulses to the brain
motor (efferent) tracts – axons carrying impulses from the brain to
skeletal muscles or autonomic effectors
SENSORY AND MOTOR PATHWAYS
sensations – nerve impulses stimulated by internal or external stimuli
perception – conscious awareness and interpretation of sensations (occurs in
cerebral cortex)
SOMATIC SENSORY PATHWAY
Sensory Receptors – specialized cell or dendrites that detect stimuli in the internal or
external environment
touch receptors – have mechanically-gated channels stimulated by touch
pain receptors – have chemically-gated channels stimulated by chemicals
released by tissue damage or inflammation
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First-order neurons – carry nerve impulses from receptors to CNS
cranial nerves – from face and mouth to brainstem
spinal nerves – from head, neck, thorax, abdomen, and limbs to spinal cord
Second-order neurons – carry nerve impulse from brain stem or spinal cord to thalamus
cross over in medulla or spinal cord before going to thalamus
almost all sensory information from one side of the body goes to the opposite
cerebral cortex
Third-order neurons – carry nerve impulses from thalamus to cerebral cortex
impulses transmitted to the appropriate sensory area of the cortex
SOMATIC MOTOR PATHWAYS – from cerebral cortex to skeletal muscles
Upper motor neurons (UMNs) – from motor area of cerebral cortex to brainstem
or spinal cord
initiate voluntary movements
Lower motor neurons (LMNs) – from CNS to skeletal muscles
cranial nerves – brainstem to face, mouth, and neck
spinal nerves – spinal cord to thorax and limbs
direct motor pathways – UMNs from cortex synapse directly with LMNs
cross over in medulla or spinal cord
(motor impulses from cortex control skeletal muscles on opposite side of
body)
indirect motor pathways – UMNs from cortex form complex pathways that
regulate muscle tone, posture and balance, coordinate movements
cerebellum – compares motor commands from UMNs with actual
movements and provides feedback to UMNs to correct
REFLEXES – fast, automatic responses to specific stimuli
somatic reflexes – involve skeletal muscle
protective reflexes
autonomic reflexes – involve smooth muscle, cardiac muscle or glands
maintain homeostasis in the body
Reflex Arc – pathway for nerve impulses of a reflex
1) sensory receptor – detects a stimulus
2) sensory neuron – generates a sensory impulse and carries it to the CNS
3) integrating center in CNS – interneurons in gray matter of spinal cord or
brainstem synapse with sensory neuron and other neurons to determine
a response
4) motor neuron – carries impulse from CNS to an effector
5) effector – part of body that responds to the motor impulse (muscle, gland)
the effector’s automatic response to the stimulus is called a reflex
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STRETCH REFLEX – suddenly stretching skeletal muscle causes it to contract
protects the muscle from overstretching; helps set muscle tone
1) muscle spindles – sensory receptors in muscle; stimulated by stretching
2) sensory neuron excited – sends an impulse through dorsal root of spinal nerve
into spinal cord
3) spinal cord gray matter – sensory neuron makes an excitatory synapse with a
motor neuron
4) motor neuron excited – sends an impulse through ventral root of spinal
nerve to neuromuscular junctions of the muscle that was stretched
5) muscle excited to contract by release of acetylcholine at NMJ
ipsilateral reflex – sensory impulse triggers a motor reflex on the same side of
spinal cord
reciprocal innervation – an axon collateral of the sensory neuron synapses with
inhibitory interneurons that cause simultaneous relaxation of antagonistic
muscles
WITHDRAWAL REFLEX – a painful stimulus causes withdrawal of a body part
1) pain receptors stimulated (eg. stepping on thorn)
2) sensory neuron excited and sends an impulse to spinal cord
3) spinal gray matter – sensory neuron synapses with multiple excitatory
interneurons, which then synapse with multiple motor neurons
4) motor neurons send impulses to NMJs of flexor muscles of limb
5) flexor muscles contract, pulling limb away from stimulus
reciprocal innervation also occurs; this reflex is also ipsilateral
CROSSED EXTENSOR REFLEX – initiated by same painful stimulus that causes the
withdrawal reflex
causes extension of opposite limb to support body weight and maintain balance
1) pain receptor stimulated
2) sensory neuron sends nerve impulse to spinal cord
3) spinal gray matter - sensory neuron synapses with multiple excitatory
interneurons, which cross to the other side of the spinal cord to synapse
with multiple motor neurons
4) motor neurons conduct impulses to NMJs of extensor muscles of limb
opposite to the one that stepped on the thorn
5) extensors of opposite limb contract to support extra weight as stimulated limb
is lifted
contralateral reflex arc – motor reflex on side opposite to sensory stimulus
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Animals with brain or spinal cord injuries can still have reflexes if the reflex arc is intact.
hyperreflexive – exaggerated reflexes due to loss of UMN regulation of muscle
tone
hyporeflexive – decreased reflexes due to damage to any part of the reflex arc
NEURONAL CIRCUITS – pathways for impulses
diverging circuits – spread of an impulse from 1 neuron to multiple neurons
one stimulus produces multiple responses
eg. pain causes the withdrawal reflex, cross extensor reflex, and reciprocal
innervation of many muscles
converging circuits – multiple neurons synapse with 1 postsynaptic neuron
multiple stimuli can produce the same response
eg. moving the limbs can be due to voluntary impulses from UMNs or
reflex responses to external stimuli
AUTONOMIC NERVOUS SYSTEM – regulates activities of smooth
muscle, cardiac muscle, and glands
operates mainly via reflex arcs
Comparison of Somatic and Autonomic Nervous Systems
SOMATIC
stimuli – conscious sensations
higher brain functions
integration – cerebral cortex,
lower brain & spinal cord
voluntary control
motor – somatic motor neurons
1-neuron pathways from CNS
effectors – skeletal muscles
AUTONOMIC
stimuli – mainly subconscious sensations
integration – hypothalamus
brain stem, spinal cord
little voluntary control
motor – autonomic motor neurons
2-neuron pathways from CNS
effectors – smooth muscles, cardiac
muscle, glands
neurotransmitters – ACh & NE
response – excitation or inhibition
neurotransmitter – ACh
response – excitation
AUTONOMIC REFLEX PATHWAYS
sensory receptors – mainly receptors in organs, blood vessels, or hypothalamus
sensory neurons – carry impulses to spinal cord, brainstem, or hypothalamus
integrating centers – highest level of integration is in the hypothalamus
little conscious perception or voluntary control
autonomic motor neurons – 2-neuron pathways from CNS
preganglionic neurons – from brainstem or spinal cord to autonomic ganglia
autonomic ganglion – site of synapse between 2 autonomic motor neurons
postganglionic neurons – run from autonomic ganglia to effectors
autonomic effectors – smooth muscle, cardiac muscle, glands
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DUAL INNERVATION – many organs receive autonomic motor innervation from
2 divisions separate divisions of the ANS
one division is usually excitatory, the other inhibitory
Sympathetic division – motor neurons that trigger fight-or-flight
responses due to physical or emotional stress
pupils dilate
increased heart rate and force
increased blood flow to skeletal muscles, lungs
airways dilate, respiratory rate increased
energy released from storage in liver and adipocytes
increased metabolism and sweating
inhibition of non-essential activities – digestive & urinary function
Parasympathetic division – motor neurons that trigger rest-and-digest
activities when no stress is occurring
function in production and storage of energy
pupils constrict
decreased heart rate and force
airway constriction
stimulation of digestive and urinary functions
increased nutrient absorption in cells
increased energy storage
Sympathetic Division (Thoracolumbar Division)
preganglionic neurons – arise at thoracic and lumbar spinal cord segments
T1-L2
axon releases ACh at synapse in the autonomic ganglion
always excitatory to postganglionic neuron
autonomic ganglia – near spinal cord
sympathetic chains – rows on either side of spine
prevertebral ganglia – within abdominal cavity ventral to spine
celiac, cranial and caudal mesenteric ganglia
postganglionic neuron – long; from ganglia to effectors all over body
most release NE, which may be excitatory or inhibitory
adrenal medulla – modified sympathetic ganglion in center of adrenal gland
preganglionic neurons trigger release of NE (noradrenalin) and
epinephrine (adrenalin) as hormones
released into interstitial space and diffuse into bloodstream
circulate throughout body and affect any tissue with receptors
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Summary of Sympathetic Division:
thoracolumbar division
preganglionic neuron – short axon; releases ACh
arise at spinal cord segments T1-T12 and L1-L2
autonomic ganglia
sympathetic chain ganglia – lateral to spinal column
prevertebral ganglia – ventral to spinal column
postganglionic neuron – long axon; most release NE
has rapid, widespread affect
innervates most tissues and organs of the body
circulation of adrenal hormones through the blood
Parasympathetic Division (Craniosacral Division)
preganglionic neurons – arise as cranial nerves or sacral nerves
cranial nerves III, VII, IX, X
spinal cord segments S2-S4
axons release ACh at synapse in autonomic ganglia
always excitatory to postsynaptic neuron
autonomic ganglia – near or within effector organs
terminal ganglia
postganglionic neurons – short
release ACh, which may be excitatory or inhibitory
Summary of Parasympathetic Division
craniosacral division
preganglionic neuron – long axon; releases ACh
arise at brainstem nuclei of c.n.III, VII, IX, X
and spinal cord segments S2-S4
autonomic ganglia
terminal ganglia – near or within effectors
postganglionic neuron – short axon; releases ACh
affect is less widespread
innervates only specific glands and organs
no hormonal component as in sympathetic division
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NEUROTRANSMITTERS OF THE ANS
CHOLINERGIC NEURONS – release acetylcholine (ACh)
all preganglionic neurons
all parasympathetic postganglionic neurons
(also somatic neurons at NMJ)
cholinergic receptors – 2 types
nicotinic receptors – found on all postganglionic neurons and
skeletal muscles
always cause excitation (open Na+ channels)
muscarinic receptors – on all parasympathetic effectors
may cause excitation or inhibition
acetylcholinesterase – enzymatically inactivates ACh rapidly
ACh has a short duration of effect at cholinergic receptors
agonist – mimics neurotransmitter and activates receptor
(eg. nicotine, muscarine)
antagonist – binds or blocks receptor and prevents activation
(eg. atropine – blocks muscarinic receptors = parasympatholytic)
ADRENERGIC NEURONS – release norepinephrine (NE)
most sympathetic postganglionic neurons
adrenergic receptors – most sympathetic effectors
activated by NE or epinephrine (from adrenal medulla)
may cause excitation or inhibition due to different receptors
alpha receptors
beta receptors
NE and E are inactivated by;
reuptake into synaptic end bulbs where it is broken down by
monoamine oxidase (MAO)
diffusing away to be removed by other cells which break it down
using catechol-O-metyl transferase (COMT)
effect lasts longer than ACh
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EFFECTS OF SYMPATHETIC AND PARASYMPATHETIC DIVISION
EFFECTOR
adrenal medullae
lacrimal glands
pancreas
salivary glands
posterior pituitary
sweat glands
digestive glands
liver
iris of eye
ciliary muscles of eye
lungs – bronchiole muscles
gall bladder and ducts
stomach and intestines
spleen
urinary bladder
arrector pili muscles
blood vessels
CARDIAC MUSCLE
SYMPATHETIC
GLANDS
secretion of epinephrine
and norepinephrine
no known effect
inhibits secretion of
digestive enzymes
inhibits secretion
secretion of ADH
increases secretion
decreases secretion
release of glucose (various
mechanisms)
decreases bile secretion
SMOOTH MUSCLE
dilation of pupil
relaxation – distant vision
dilates airways
relaxation – no bile release
decreased motility and tone;
sphincter contraction
contraction – releases stored
blood into circulation
relaxation of muscle wall;
sphincter contraction
erection of hairs
alters diameter
increased heart rate and
force of contraction
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PARASYMPATHETIC
no known effect
secretion of tears
increases secretion of
digestive enzymes
increases secretion
no known effect
no known effect
increases secretion
glucose storage
(as glycogen)
increases bile secretion
contraction of pupil
contraction – close vision
constricts airways
contraction – releases bile
increased motility and tone;
sphincter relaxation
no known effect
contraction of muscle wall;
sphincter relaxation
no known effect
no known effect
decreased heart rate and
force of contraction