Anatomical Organization of
the Nervous System
• Central nervous system (CNS)
– consists of the brain located
within the skull and the spinal
cord located within the vertebral
foramen
– integration and command center
of the body
• Peripheral nervous system (PNS)
– consists of nerves (extensions of
the CNS) that connect the CNS to
all other locations in the body
Nervous System
• One of 2 controlling and communicating systems of
the body (other is the endocrine system)
• Transmit sensory information
– send electrical impulses called action potentials
(APs) to the CNS
• from eyes, skin, blood vessels, ears, digestive
tract, joints, muscles, lungs…
• Integration
– interpretation of sensory information by the CNS
• type, location and magnitude of stimulus
• Transmit motor information
– send APs from the CNS to various effector organs
throughout the body
• provides a way to respond to stimuli
Cells of the Nervous System
• The two principal cell types of the nervous system are:
– Neurons
• hundreds of thousands of neurons extend axons
and make synapses all over the body with other
neurons, muscles and glands
• communicate through action potentials
• allows for short response times to changes in
homeostasis
– Neuroglia
• guide developing neurons to make synapses
• provide a supportive scaffolding for developed
neurons
Neuron Types of the Nervous System
• Sensory (afferent)
– associated with sensory receptors
– send APs via the PNS toward the CNS
• Interneurons
– integrate information within the CNS
– receive APs from sensory neurons and initiate APs
in motor neurons
• Motor (efferent)
– send APs via the PNS away from the CNS
• All 3 neuron types are used to respond to stimuli
– reflex
Basic Function of the Nervous System
Membrane Potential
• Although the total solute concentration in the ICF and
ECF are equal, there is an uneven distribution of
charged substances across the cell membrane of
every cell in the body
– creates an electrical potential (energy) between the
ICF and ECF
– measured as a voltage
• in millivolts (mV)
– causes the cell membrane to be polarized
• a measurable charge difference between the ICF
and ECF
• The ICF is negatively charged compared to the ECF
– a typical membrane potential is –70 mV
• In an UNSTIMULATED (resting) cell this potential
remains constant and is referred to as the resting
membrane potential (RMP)
Resting Membrane Potential
Basis of the Resting Membrane Potential
• Due to the permeability characteristics of the plasma
membrane to charged (polar) substances
– permeability is the ease in which one substance
can move through another substance
• Permeable charged substances
– K+
– Na+
Basis of the Resting Membrane Potential
• In a resting cell, Na+ and K+ are constantly pumped
across the cell membrane by the Na+,K+-ATPase
maintaining:
– a high Na+ concentration in the ECF
– a low Na+ concentration in the ICF
– a high K+ concentration in the ICF
– a low K+ concentration in the ECF
Basis of the Resting Membrane Potential
Diffusion of Na+ and K+
• There is a constant diffusion of Na+ into the cell by:
• Na+ channels that are always open (leaky)
• There is a constant diffusion of K+ out of the cell by:
• open K+ channels that are always open (leaky)
• The permeability of the cell membrane in a resting cell
to potassium is approximately 40 times greater than
the permeability to sodium
– due to a much larger number of potassium leak
channels compared to sodium leak channels
• When a cell is at rest, the pumping of the Na+,K+ATPase, exactly equals the diffusion of Na+ and K+
– results in a steady state condition
Contribution of Na+ to the RMP
• If the cell membrane were permeable only to sodium
then sodium would diffuse into the cell
– as sodium diffuses into the cell it causes the inside
of the cell to become positively charged (it only
takes a few ions because each ion has a large
charge) which begins to reduce additional sodium
ion entry (due to repulsion)
• Sodium diffusion stops when the inside of the cell has
58 more mV of charge compared to outside
(membrane potential = +58 mV)
– at this potential, the concentration gradient moving
Na+ into the cell exactly balances the positive
electric charge repelling Na+ out of the cell
– Equilibrium potential for Na+ (ENa)
Contribution of K+ to the RMP
• If the cell membrane were permeable only to
potassium then potassium would diffuse out of the cell
– as potassium diffuses out of the cell it causes the
inside of the cell to become negatively charged (it
only takes a few ions because each ion has a large
charge) which begins to reduce additional
potassium ion exit (due to attraction)
• Potassium diffusion stops when the inside of the cell
has 90 less mV of charge compared to outside
(membrane potential = -90 mV)
– at this potential, the concentration gradient moving
K+ out of the cell exactly balances the negative
electric charge attracting K+ into the cell
– Equilibrium potential for K+ (EK)
RMP
• Note that the RMP is neither equal to ENa or EK, but is
somewhere between these 2 values
• If the permeability of these 2 ions through the cell
membrane were exactly the same, the RMP would be
exactly between the values of ENa and EK, or -16 mV.
• However, the permeability of the cell membrane to
potassium is approximately 40 times greater than that
of sodium due to a much greater number of potassium
leak channels.
• This causes potassium to have a much greater
influence on the RMP compared to sodium, which is
why at -70 mV the RMP is closer to -90 mV than +58
mV.
Changes in the Resting Membrane Potential
• Many cells of the body use the electric potential
across the cell membrane to function
– the membrane potential changes from its resting
value due to a change in the environment of the cell
• the change in the membrane potential causes the
cell to “respond” to the change in its environment
• Changes in the membrane potential from resting
values are due to the function of gated ion channels
– these channels remain closed (while a cell is at
rest) until a change in the environment of the cell
(STIMULUS) causes them to open
Types of Gated Ion Channels
• Gated ion channels only allow the diffusion of 1
(sometimes 2) type of ion across the cell membrane
– Ligand-gated channels
• open when a specific chemical binds to the
extracellular portion of the channel
– Stretch-gated channels
• open when the plasma membrane is stretched
– Voltage-gated channels
• open when the membrane potential deviates
from resting and reaches a specific voltage
Gated Channels
• Channel types include some of the following examples
– Voltage-gated Ca2+ channels
– Stretch-gated Cl- channels
– Voltage-gated K+ channels
– Ligand-gated Na+ channels
• The diffusion of any additional ions across the plasma
membrane occurs at a much faster rate than the rate
of pumping of the Na+,K+-ATPase
– this causes the cell membrane potential to deviate
from the resting value
Operation of a Ligand-Gated Channel
Example: ligand-gated Na+ channel
• Closed when a chemical is NOT bound to the
extracellular portion of the channel
– Na+ cannot enter the cell
• Opens when a specific chemical attaches to the
extracellular portion of the channel
– Na+ diffuses into the cell
Operation of a Ligand-Gated Na+ channel
Deviations in the Resting Membrane Potential
• The opening of a gated ion channel will allow a
specific ion to diffuse down its respective gradient
across the cell membrane
• The membrane potential will deviate from the resting
value (-70mV) based on 2 criteria:
– the charge of the diffusing ion
• either positive (cation) or negative (anion)
– the direction of the diffusion
• either into or out of the cell
Deviations in the Resting Membrane Potential
• The ICF becomes less negative when:
– a cation diffuses into the cell
– an anion diffuses out of the cell
• depolarization
–reduces the polarity of the membrane as the
membrane potential moves toward 0mV
• The ICF becomes more negative when:
– a cation diffuses out of the cell
– an anion diffuses into the cell
• hyperpolarization
–increases the polarity of the membrane as the
membrane potential moves further away from
0mV
Deviations in the Resting Membrane Potential
• When the gated ion channels close, the cell
membrane potential returns to its resting value
Gated Channels and the Membrane Potential
• When gated channels open:
– ions move across the cell membrane down its
concentration gradient (HIGH → low)
– the number of ions that move across the membrane
is relatively small and thus DOES NOT CHANGE
the concentration gradient of the ion
• The membrane potential deviates because each ion
has a large charge associated with it
– the movement of only a “few” ions creates a large
change in the distribution of electric charge across
the cell membrane
• After the gated channels have closed, the “few” ions
that diffused are quickly moved up the gradient to
return the membrane potential to resting
Responses to Stimuli
• Stimulation of various cells (receptors/sensors) in the
body causes the opening of gated channels which
changes in the resting membrane potential initiating
an electrical impulse
– ligand-gated channels are opened in taste buds by
the food that is ingested
– stretch-gated channels are opened in free nerve
endings in the dermis of the skin when bitten by a
mosquito
– voltage-gated receptors are opened when your lab
partner uses an electrical stimulating electrode on
your arm
Responses to Stimuli
• The electrical impulse travels from the stimulated
receptor cell to an effector cell (muscle and/or gland)
• A change in the membrane potential of the effector
cell causes a functional change in the cell allowing for
an appropriate response
– the salivary glands will secrete saliva into the mouth
while the tongue and muscles controlling the jaw
will contract, allowing you to chew and swallow or
spit out the ingested food
– the muscles controlling the arm and hand will
contract, allow you swat the mosquito
– the muscles of the hand will contract, causing the
fingers and wrist to flex
Neurons (Nerve Cells)
• The transfer of these electrical impulses over large
distances is accomplished by the cells of the nervous
system called neurons
– capable of:
• generating/initiating an electrical impulse
• sending electrical impulses very rapidly from one
location in the body to another
• changing the resting membrane potential of other
cells within the body including:
–other neurons
–effector cells of the body
• The nervous system is made up of millions of neurons
that connect all parts of the body to one another
Neuron Anatomy
• Dendrites
– branched appendages that receive stimuli
– respond to a stimuli by opening gated channels
• location of stretch or ligand-gated channels
• change in the membrane potential of the neuron
at the precise location of the stimulus on the cell
• Body (soma)
– location of organelles, but can also receive stimuli
– respond to a stimuli by opening gated channels
• location of stretch or ligand-gated channels
• change in the membrane potential of the neuron
at the precise location of the stimulus on the cell
• Axon
– long extension of the cell body, that can branch
many times which sends the electrical impulse to
other cells in the body
• location of voltage-gated channels
Neuron
Initiation of an electrical impulse
• The initiation of an electrical impulse occurs at either
the dendrites or the body of a neuron
– the opening of stretch or ligand-gated channels
causes EITHER a depolarization or a
hyperpolarization, depending on the charge and the
direction of movement of the ion at the location of
the opened gated channels
• this type of membrane potential change is called
a graded (local) potential
–a brief, localized change in the membrane
potential
Graded Potentials
• The grade or magnitude of depolarization or
hyperpolarization is directly related to the size of the
stimulus
– determines the number of gated channels that is
opened
• determines the number of ions that cross the
plasma membrane
Graded Potentials of Stretch-gated Channels
• A small pressure applied to the skin:
– causes a small amount of stretch of the cell
membrane of the pressure sensing cells of the skin
• causes few stretch-gated channels to open
–allows few ions to cross the cell membrane
• causes a small change of the membrane
potential from the resting value
• A large pressure applied to the skin:
– causes a large amount of stretch of the cell
membrane of the pressure sensing cells of the skin
– causes more stretch-gated channels to open
–allows more ions to cross the cell membrane
• causes a larger change of the membrane
potential from the resting value
Graded Potentials
• Decrease in magnitude with distance from the site of
stimulation
– as ions move into/out of the cell through opened
gated channels, they diffuse away from the opened
gated channel
• as the ions diffuse away from the opened gated
channel the concentration of the ion decreases
–as the ion concentration decreases, so does
it’s influence on the membrane potential
• the further away from the stimulus, the
closer the membrane potential is to the
resting value
Function of Graded Potentials
• The purpose of graded potentials in the dendrites or
soma is to cause (or prevent) the opening of voltagegated ion channels in the axon of the neuron
– open when the membrane potential in the axon has
been depolarized to a minimum value
– the opening of voltage-gated channels in the axon
will create a membrane potential change in the
axon called an action potential
• the action potential will “travel” down the length of
the axon and all of its branches to the axon
terminus
Action Potentials (APs)
• A very rapid sequence of membrane potential
changes due to the opening and closing of voltagegated Na+ and voltage-gated K+ channels
• There are 3 sequential phases to an AP in a neuron:
– Depolarization
• a reduction in the polarity of the membrane
potential
– Repolarization
• a return of the membrane potential towards the
resting value
– Hyperpolarization
• the membrane potential reaches values more
negative than the resting value
• All APs in a neuron have the same magnitude
regardless of the size of the stimulus (not graded)
Action
Potential
Threshold and Action Potentials
• The initiation of an AP occurs at the beginning of the
axon called the initial segment and requires that the
membrane potential at the axon be depolarized to
threshold
– the minimum amount of depolarization required to
initiate an action potential
• typically -55mV
• causes the opening of voltage-gated Na+
channels
Threshold and Action Potentials
• Threshold can be reached by a depolarizing graded
potential in the dendrites or soma of a neuron
– small (weak) stimuli DO NOT initiate an AP
because the magnitude of the graded potential at
the axon is TOO SMALL to depolarize the
membrane at the axon to threshold
• subthreshold stimuli
– large (strong) stimuli DO initiate an AP
• threshold stimuli
• All-or-none phenomenon
– action potentials either completely, or not at all
Ionic Basis of Action Potential (Resting State)
• Na+ and K+ channels are closed
Ionic Basis of Action Potential (Depolarization)
If a strong enough stimulus is presented to the cell, the
membrane potential depolarizes to threshold
(-55mV) causing:
– Na+ channels to open
• Na+ enters the cell (diffusion)
–membrane potential continues to depolarize to
+30mV
• K+ channels slowly
begin to open
Ionic Basis of Action Potential (Repolarization)
Membrane potential reaches peak depolarization of
+30mV causing:
• Na+ channels to close
• K+ channels to open
– K+ exits the cell (diffusion)
• the membrane potential
returns toward resting
values (repolarization)
Ionic Basis of Action Potential
(Hyperpolarization and Return to Resting)
• K+ channels remain open
• This causes more than enough K+ to leave the cell
resulting in hyperpolarization of the membrane
potential
• Eventually, the K+
channels
close, allowing the
membrane potential to
return to resting
Refractory Periods
Absolute Refractory Period
• The absolute refractory period:
– is the time during an action potential that another
action potential CANNOT be initiated
• no matter how strongly the dendrites/soma are
stimulated
– ensures that each action potential created is
separated from one another so that the body can
interpret stimuli accurately
– is time required for the voltage-gated Na+ channels
to be “reset”
• required for the channels to open again
Relative Refractory Period
• The relative refractory period:
– is the time after the absolute refractory period until
the membrane potential returns to the resting value
• During this time another action potential CAN be
initiated
– requires a stronger than normal stimulus at the
dendrites
• during this time some of the voltage-gated Na+
channels have been “reset” while others have not
Propagation of an Action Potential
• Once an action potential has been initiated at the
beginning of the axon, it must “travel” (propagate)
along the length of the axon to the axon terminus
• The influx of Na+ into the cell during depolarization
causes the membrane potential in “front” of the
opened Na+ channels to depolarize to threshold
• Reaching threshold opens up the Na+ channels in
“front” of the site of the action potential causing an
action potential to be created in this new location
• As the next group of Na+ channels begins to open, the
ones “behind” them are closing
• The impulse continues to propagate away from its
point of origin to the axon terminus
• “the domino effect”
Propagation of an
Action Potential
Propagation Velocity of an Action Potential
• The propagation velocity is the speed at which the
action potential propagates along the length of the
axon
• Conduction velocity depends on:
– axon diameter (thickness)
• the larger the diameter, the greater the
conduction velocity
– presence of a myelin sheath
• dramatically increases impulse speed
–to speeds up to 300 mph
• more effective than increasing axon diameter
• The human body uses both methods to maximize
propagation velocity
Myelin Sheath
• White, fatty (lipid), segmented covering around most
long axons
• Increases propagation velocity of APs by electrically
insulating the axon
• Formed by Schwann cells
– wraps around the axon many times with its plasma
membrane
– encloses the axon with many concentric layers of
lipid bilayers
Myelin Sheath
Myelin Sheath Formation
Nodes of Ranvier
• The nodes of Ranvier are:
– gaps between the Schwann cells
• naked axon segments
– the ONLY locations of voltage-gated Na+ and K+
channels
• in large densities
• ONLY locations where an AP can be generated
along the length of the axon
Saltatory Conduction
• Ions pass through a myelinated axon only at the
nodes of Ranvier creating an action potential
– due to the large density of voltage-gated Na+
channels creates a large electrical field surrounding
the node
• causes the cell membrane to reach to threshold
at a large distance away (the next node)
–creates and AP at the next node
• The action potential jumps from node to node
– much faster conduction rate compared to
unmyelinated axons (of the same diameter)
Nodes of Ranvier and Saltatory Conduction
Saltatory Conduction
Axon Termini and Synapses
• When the AP reaches the axon termini the impulse
must be transmitted to the next cell in the path to the
effector
• A synapse is the junction between 2 cells where the
impulse is transmitted from one cell to another :
– Presynaptic cell (before synapse)
– Postsynaptic cell (after synapse)
– found between:
• 2 neurons
• a neuron and an effector cell (muscle or gland)
– 2 general types include:
– chemical
– electrical
Axon Termini and Synapses
Chemical Synapses
• Composed of 3 parts:
– axonal terminal of the presynaptic neuron
• contains synaptic vesicles
–filled with a neurotransmitter (chemical/ligand)
– receptor region on the postsynaptic cell which
contains ligand-gated channels
– fluid-filled space between the cells (synaptic cleft)
• separates the presynaptic and postsynaptic cells
Chemical Synapse
Synaptic Cleft: Information Transfer
• An action potential that arrives at the axon terminus of
the presynaptic cell causes the opening of voltagegated Ca2+ channels
– causes Ca2+ to diffuse into the cytoplasm of the
presynaptic cell
• triggers the exocytosis of neurotransmitters into
the synaptic cleft
• The neurotransmitters diffuse across the cleft and
open the ligand-gated channels on the postsynaptic
cell
– causes ions to cross the cell membrane and result
in a graded potential
• postsynaptic potential
–depolarization or hyperpolarization
Synaptic Cleft: Information Transfer
Postsynaptic Potentials
The 2 types of postsynaptic potentials are:
• EPSP (excitatory postsynaptic potentials)
– depolarizing graded potentials
– causes the membrane potential move towards
threshold which increases the chances that an AP
will be initiated in an axon
• IPSP (inhibitory postsynaptic potentials)
– hyperpolarizing graded potentials
• causes the membrane potential move away from
threshold which reduces the chances that an AP
will be initiated in an axon
Excitatory and Inhibitory Postsynaptic Potentials
EPSPs and IPSPs Summate
• A single EPSP CANNOT initiate an action potential
– EPSPs must summate (add) to bring the membrane
potential to threshold at the axon
• Temporal summation
– postsynaptic potentials are generated at a single
location at a high frequency
• Spatial summation
– postsynaptic potentials are generated at different
locations at the same time
• IPSPs can also summate with EPSPs
– cancel each other out
Temporal Summation
Temporal
Summation
Spatial Summation
Myelination of Neurons of the Nervous System
• Some neurons in the CNS are myelinated, while most
are unmyelinated
• All of the neurons in the PNS are myelinated
• Areas of the CNS that are made of myelinated
neurons are called white matter
– represent the locations of long sensory and motor
neurons
• Areas of the CNS that are made of unmyelinated
neurons are called gray matter
– represent the locations of short interneurons which
make many synapses for integration to process
sensory information and initiate motor information
Spinal Cord
• The spinal cord is attached to the brain and extends to
the lumbar region of the vertebral column
• Functions include:
– integration of basic stimuli presented to the body
below the neck through simple reflexes
• withdrawal reflex in response to pain
– sending sensory and motor information to and from
the brain
Spinal Cord Anatomy
• Dorsal (posterior) horns (left and right)
– sensory information enter the cord on the dorsal
aspect where they synapse with interneurons or
motor neurons
– extend into dorsal roots and ganglia (group of cell
bodies outside the CNS)
• Ventral (anterior) horns (left and right)
– motor information exits the cord on the ventral
aspect where they control effectors (muscle or
glands)
– extend into motor roots
• Dorsal and ventral roots merge together to form spinal
nerves
Spinal Cord Anatomy
Brain
Cerebral Cortex
• 4 lobes
– frontal, parietal, temporal and occipital
– location of interneurons for perception of all senses
– site of memory, emotion, learning
– site of initiation of voluntary skeletal muscle
contraction
The Cerebellum
• Protrudes under the occipital lobes of the cerebrum
• Makes up 11% of the brain’s mass
• Modifies the motor information leaving the motor
cortex
– provides precise timing and appropriate patterns of
skeletal muscle contraction to maintain balance and
coordination
• Cerebellar activity occurs subconsciously
Brain Stem
• Comprised of the pons and the medulla oblongata
• Clusters of neurons (brain centers) in regions of the
pons and medulla control the basic life functions:
– heart rate
• controlled by the cardioacceleratory and
cardioinhibitory centers in the medulla
– blood pressure
• controlled by the cardioacceleratory,
cardioinhibitory, and vasomotor centers in the
medulla
– breathing rate
• controlled by the inspiratory and expiratory
centers in the medulla and pons, respectively
• Control of effectors occurs through the Autonomic
Nervous System
Peripheral Nervous System
• The PNS consists of nerves (bundles of axons)
– send APs to and away from the CNS
– 12 pairs (left and right) of cranial are connected to
the brain and 31 pairs (left and right) of nerves are
connected to the spinal cord
• Sensory (afferent)
– all axons carry impulses from sensory receptors via
the PNS to the CNS
• Motor (efferent)
– all axons carry impulses via the PNS from CNS
• Mixed
– a mixture of sensory and motor neurons that carry
impulses via the PNS to and from CNS
– most common type of nerve in the body
Nerves
• Nerve
– cordlike organ of the PNS consisting of axons
enclosed by connective tissue
• Connective tissue coverings include:
– Endoneurium
• loose connective tissue that surrounds each
individual axon
– Perineurium
• coarse connective tissue that bundles axons into
fascicles
– Epineurium
• tough fibrous connective tissue around a nerve
Structure of a Nerve
Reflexes
• A rapid, predictable motor response to a stimulus
• Reflexes can be:
– simple
• involve peripheral nerves and the spinal cord
–rapid
– learned (acquired)
• involve peripheral nerves and require thought
–slower
• Following a stimulus, the sensory and motor
information of a reflex follows a pathway called a
reflex arc
– in many spinal reflexes, the effector is nearby the
location of the stimulus
Reflex Arc
• There are five components of a reflex arc
– Receptor
• detect stimulus
– Sensory neuron
• transmits the afferent impulse to the CNS
– Integration (control) center
• region within the CNS where synapses
(processing of sensory info) occur
– Motor neuron
• sends efferent information to an effector
– Effector
• muscle fiber or gland that responds to the
efferent impulse
• the activity of the effector depends upon the
magnitude of the stimulus
Sensory Receptors
• Structures specialized to respond to stimuli:
– nerve endings (dendrites of neurons)
– sense organs
• nerve endings combined with other tissue types
to enhance detection of a stimuli
–example: taste buds
• Mechanoreceptors
– respond to touch, pressure, stretch and itch
• Thermoreceptors
– respond to changes in temperature
• Photoreceptors
– respond to light
• Chemoreceptors
– respond to chemicals
• Nociceptors
– respond to pain
Neural Integration of the CNS
• Qualitative information (salty, pain or temperature)
depends upon which neurons are propagating APs
• Quantitative (strength) information depend on:
– the number of neurons that are firing APs
– the frequency of APs fired per neuron
Functional Organization of the Nervous System
Sensory Division of the Peripheral NS
Sensory division
• made of afferent neurons
– somatic
• sensory neurons send APs from skin, skeletal
muscles, and joints
– visceral
• sensory neurons send APs from organs within
the abdominal and thoracic cavaties
Motor Division of the Peripheral NS
Motor division
• made of efferent neurons control the action of
effectors
– somatic
• motor neurons send APs to voluntary skeletal
muscle
– visceral
• motor neurons send APs to involuntary cardiac
muscle, smooth muscle and glands
–a.k.a. the Autonomic Nervous System (ANS)
–2 antagonistic (opposing) divisions
• Sympathetic
• Parasympathetic
–the two divisions control the same effectors
(with few exceptions) but create opposite
responses in the effectors
Motor Pathways of the Somatic Nervous Division
vs. Autonomic Nervous Division
Autonomic Nervous System
• Visceral motor neurons of the Peripheral NS control
the activity of involuntary effectors such as cardiac
muscle, smooth muscle and glandular secretion
affecting:
– heart rate
– breathing rate
– sweating
– digestion
– blood pressure
• Action potentials in these motor neurons are initiated
in the medulla oblongata and the pons
– these motor neurons exit the brain by:
• descending tracts of the spinal cord
–exit spinal cord via spinal nerves
• cranial nerves
Function of the Sympathetic Division
• The sympathetic division is called the “fight or flight”
system
– activated when the body needs to expend energy
• Involves E activities
• exercise, excitement, emergency, and embarrassment
• Promotes necessary changes during these activities
– increases heart rate, blood pressure, respiration
rate, blood flow to skeletal muscles, glucose
metabolism
– decreases the activity of and blood flow to the
digestive system organs
• Its activity is illustrated by a person who is threatened
Function of the Parasympathetic Division
• The parasympathetic nervous system is called the
“rest and digest” system
– activated when the body needs to conserve energy
• Involves the D activities
– digestion, defecation, and diuresis (urination)
• Promotes necessary changes during these activities
– decreases heart rate, blood pressure, respiration
rate, blood flow to skeletal muscles, glucose
metabolism
– increases the activity of and blood flow to the
digestive system organs
• Its activity is illustrated in a person who relaxes after
eating a meal
Efferent Pathways of the ANS
• Efferent pathways of the ANS consist of a two-neuron
chain between the brain or spinal cord and the effector
– synapses between the neurons occur at ganglions
– The cell body and dendrites of the preganglionic
neuron is located in the CNS and the axon extends
along a nerve to the ganglion
– The cell body and dendrites of the postganglionic
neuron is located in the ganglion and the axon
extends to an effector organ
Organization of
the Sympathetic
Division
Organization of the
Parasympathetic
Division
Motor Pathways of the Somatic Nervous Division
vs. Autonomic Nervous Division
• All somatic motor neurons exocytose ACh
– ACh binds to nicotinic acetylcholine receptors on
the skeletal muscle fiber leading to its contraction
• All preganglionic motor neurons exocytose ACh
– ACh binds to nicotinic acetylcholine receptors on
the postganglionic neuron creating an AP
• All parasympathetic postganglionic motor neurons
exocytose ACh
– ACh binds to muscarinic acetylcholine receptors on
the effector tissue/organ causing a response
• All sympathetic postganglionic motor neurons
exocytose norepinephrine NE
– NE binds to adrenergic receptors on the effector
tissue/organ causing a response
Efferent Sympathetic vs. Parasympathetic
Effects of Neurotransmitters of the ANS
• The way the 2 divisions of the ANS can create
opposite responses in the effectors that they control is
by the release of different neurotransmitters onto the
cells of the effectors
• The cells of each organ controlled by the ANS have
membrane receptors to BOTH ACh and NE
– organs are dually controlled
• The response of the organ is determined by the
identity of the neurotransmitter released
– the binding of ACh to its receptor will cause the
effector to respond in one way
– the binding of NE to its receptor will cause the
effector to respond in the opposite way
• The effect of ACh and NE is effector specific
– NE increases heart rate, ACh decreases heart rate
– NE decreases the secretion of saliva, ACh
increases the secretion of saliva
Dual Control by the
Sympathetic and
Parasympathetic
Systems