Divisions of the nervous system
CNS
PNS
EFFERENT
Somatic
Skeletal
muscle
-voluntary
ANS
Cardiac &
smooth muscles
Glands
-involuntary
AFFERENT
Somatic
Visceral
Cardiac &
Skeletal
muscle, tendons smooth muscles
Glands
joints, skin
PNS Terminology
• Ganglia – neuron
cell bodies
• Peripheral nerves –
neuronal axons
• PNS neuroglia
– Satellite cells
• Enclose neuron cell
bodies in ganglia
– Schwann cells
• Cover peripheral
axons
Efferent Division of the PNS
• the somatic nervous system and part of
the autonomic nervous system
• the somatic – control of skeletal muscle
• the ANS – involuntary control over cardiac
and smooth muscle + gland secretion
I - Olfactory
II - Optic
III - Oculomotor
IV-Trochlear
V - Trigeminal
VI - Abducens
VII - Facial
VIII - Acoustic
IX - Glossopharyngeal
X - Vagus
XI - Accessory
XII - Hypoglossal
-cranial nerves – 12 pairs
-considered part of the peripheral nervous system (PNS)
-olfactory & optic contain only sensory axons = sensory nerves
-remaining are motor or mixed nerves (both motor and sensory axons)
Spinal Nerve
•after passing through
intervertebral foramina
the spinal nerve branches
= ramus/rami
•Dorsal ramus
Sensory/motor
innervation to skin
and muscles of back
•Ventral ramus
-Ventrolateral body
surface, body wall
structures, muscles
of the upper and
lower limbs
•in addition to these rami - the spinal
nerves also give off a meningeal branch reenters the vertebral canal and supplies
the vertebrae, vertebral ligaments and
meninges
• rami communicantes = branches from
the spinal nerve
-defined as a connection between a spinal
nerve and the sympathetic trunk of
the ANS
sympathetic trunk
Nerve Plexuses
• Four major plexuses
– Cervical plexus
– Brachial plexus
– Lumbar plexus
– Sacral plexus
• Joining of ventral rami
of spinal nerves to form
nerve networks or
plexuses
• Found in neck, arm, low
back & sacral regions
• No plexus in thoracic
region
Somatic Nervous System
• considered the voluntary aspect of the PNS
– but the muscles of posture and balance are controlled involuntarily by
the lower brain centers (brain stem, cerebellum)
• cell bodies located in the ventral gray horn of the spinal cord
• the axon of a somatic motor neuron extends from the CNS
continuously to its skeletal muscle target
• terminals release acetylcholine – contraction
• can only stimulate its target
Somatic Nervous System
- somatic/motor axons
emerge from the ventral
gray horn and travel into
the spinal nerve
- they then travel through
either the:
– dorsal ramus to end up at
the muscles of the back
– OR the ventral ramus to
end up at the muscles of
the limbs and body wall
(chest/abs/pelvis)
Somatic Nervous System
• somatic motor neurons that originate in
the ventral gray horn (or the brain
stem) receive incoming information
from many converging presynaptic
neurons
– both excitatory and inhibitory on these
motor neurons
– the neurons that synapse with these motor
neurons are:
– 1. reflex neurons originating in the spinal
cord
– 2. neurons from motor areas of the brain –
form the descending white matter tracts
– these neurons synapse with the somatic
motor neurons and regulate their activity
• activation – impulse sent to muscles
• inhibition – no impulse, no contraction
Somatic Nervous System
• no matter what motor pathway you learn – they eventually
control the somatic motor neuron in the ventral gray horn
or the brain stem
• therefore the somatic motor neuron is considered the
final common pathway
– considered the only way any other part of the nervous system can
influence muscle activity
Somatic Motor pathways
• all excitatory and inhibitory signals that control skeletal
muscle movement converge on the somatic motor neurons
• these somatic motor neurons originate in one of two places
– 1. Brain Stem
– 2. Ventral Gray Horn of Spinal Cord
• these motor neurons extend from the brain stem and SC to
innervate the skeletal muscles
– also called lower motor neurons (LMNs)
– their axons extend through the cranial and spinal nerves to skeletal
muscle
– only LMNs provide output from the CNS to skeletal muscle fibers
Somatic Motor pathways
• neurons in four distinct circuits control
movement by providing input to these LMNs
–
–
–
–
1. local circuit neurons
2. upper motor neurons (UMNs)
3. basal ganglial neurons
4. cerebellar neurons
Somatic Motor pathways
• 1. local circuit
– input arrives at LMNs from nearby interneurons called local circuit
neurons
– receive input from somatic sensory receptors and higher centers of
the brain
– help coordinate rhythmic activities in muscle groups
•
2. UMNs: Upper Motor Neurons
– provide input to the local circuit and LMNs
– essential for planning, initiating and directing sequences of
voluntary movements
– extend from the brain to the LMNs via two types of
somatic motor pathways
• 1. direct motor pathways: nerve impulses for
voluntary movement
– lateral corticospinal, anterior corticospinal and
corticobulbar (brain stem)
– UMNs originate in the motor cortex and travel
down the spinal cord as the corticospinal tracts
to synapse with the LMN
– OR – UMNs exit the brain stem
– the LMN emerges as spinal nerves or through
the brain stem and out as cranial nerves
• 2. indirect motor pathways: or
extrapyramidal pathways
– nerve impulses follow
complicated circuits that involve
the cortex, basal ganglia,
thalamus and brain stem
– descending axons/tracts pass
outside the pyramids of the
medulla
– 1. rubrospinal
– 2. reticulospinal
– 3. vestibulospinal
• 3. Basal ganglia pathways
– assist movement by providing input to
the UMNs
– “okays” the motor pathways that
emerge from the motor cortex
– also suppresses unwanted
movements and initiates and
terminates movement
– the production of dopamine by the
substantia nigra also effects muscle
tone by modifying this path
– caudate nucleus and putamen receive
sensory input from several areas of
the brain – to know what muscles are
doing
Motor Cortex
Basal Ganglia
Thalamus
Somatic Motor pathways
• 4. Cerebellar
– function involves four activities
• 1. monitoring intentions for
movement
• 2. monitoring actual movement
• 3. comparing the command
(intention and movement) with
sensory information
• 4. correction – to UMNs
– travels via the thalamus to the
UMNs in the cerebral cortex
– or can go directly to the UMNs
in the brain stem
Cerebellum
Thalamus
Motor Cortex
Midbrain
The Neuromuscular Junction
• end of the lower motor neuron (synaptic
terminal or axon bulb) in very close association
with a muscle fiber/cell
• distance between the bulb and the folded
sarcolemma = synaptic cleft
• nerve impulse leads to release of a specific
neurotransmitter (acetylcholine)
•this release will result in activation of the muscle
cell and contraction
•therefore the NMJ is ALWAYS excitatory
•the only way inhibition can take place is through
the inhibition of the neuron “connecting” with the
muscle –i.e. upper motor neurons
http://www.blackwellpublishing.com/matthews/neurotrans.html
ANS
• two divisions that innervate the same organs
• efferent branch regulates “visceral” activities
(motor commands, involuntary, organs)
• also has an afferent branch that receives
sensory information from these areas
• generally divided into the:
– A. Parasympathetic
– B. Sympathetic
ANS
• involuntary motor commands and sensory information
• supplies cardiac and smooth muscle, glands (i.e. viscera)
• comprised on two neurons
– preganglionic and postganglionic
– preganglionic synapses with the cell body of the postganglionic within
the ganglion
– the pregang and postgang neurotransmitters can differ
– the postganglionic neuron is unmyelinated
– glands are innervated by the preganglionic neuron – e.g adrenal gland
which then releases epinephrine or norepinephrine in response
Parasympathetic Division
• cell bodies of the preG neurons
are located in the brain stem
– axons form the four cranial nerves III,
VII, IX and X
– known as cranial parasympathetic
outflow
• also found in the lateral gray
horns of the sacral spinal nerves
2 through 4
– emerge as part of the cranial or spinal
nerve
– known as sacral parasympathetic
outflow
Parasympathetic Division
• parasympathetic ganglia are located
near or in the target
• called terminal ganglia
– the preG fibers are very long because
they must extend from the CNS to an
organ
– synapse with postG within the terminal
ganglia
– four major terminal ganglia are located
close to the organ they innervate
– 1. otic (parotid gland)
– 2. submandibular (submandibular and
sublingual glands)
– 3. pterygopalatine (lacrimal gland)
– 4. ciliary (pupils)
Sympathetic Division
•
•
•
for visceral motor commands
cell bodies of the preG neurons are located in
the lateral gray horns of T1 to L2
sympathetic ganglia:
– site of the synapse between the preG and
postG neurons
– short preG lead into these ganglia
– long postG axons lead out to target
– two groups:
1. sympathetic trunk ganglia: or
paravertebral chain ganglia
-vertical row lateral to the vertebral
column
-3 cervical, 11 or 12 thoracic, 4 or 5
lumbar and 4 or 5 sacral
2. prevertebral ganglia: or the collateral
ganglia
-postG neurons innervate the abdominal
organs
-anterior to the vertebral column and are
near the large abdominal arteries
-three major prevertebral ganglia: celiac,
superior mesenteric and inferior
mesenteric
celiac
ganglion
Sympathetic Division
•
•
•
•
axons exit the lateral gray horn through the
ventral root of the spinal cord
axons form part of the spinal nerves T1 to
L2
form part of the spinal nerve along with
somatic motor nerve axons and
parasympathetic preG axons
BUT the axons then enter the rami
communicantes and pass to the nearest
sympathetic trunk ganglion – synapse with
the postG neuron
celiac
ganglion
*** whether it is sympathetic or
parasympathetic – the preG
neurons release AcH
Sympathetic Dominance
• fight or flight
• protective response
• elevated heart rate, blood pressure,
respiration rate
• increase blood flow to skeletal muscles, lungs,
heart, brain
• decrease blood flow to digestive, reproductive
and urinary organs
Parasympathetic Dominance
• “rest and digest” response
• dominates in quiet, stress-free situations
• resets the system after sympathetic
stimulation
– e.g. slow the heart rate and lower blood pressure
ANS Neurotransmitters
• specific neurons release specific NTs – have distinct names
• cholinergic neurons –release of AcH
– all preG neurons from sympathetic and parasympathetic neurons
– all parasympathetic postG neurons
– two types of receptors
• 1. nicotinic
• 2. muscarinic
• adrenergic neurons – release of NE
– most sympathetic postG are adrenergic
– two types of receptors
• 1. alpha – a1 and a2
• 2. beta – b1 and b2 and b3
ANS receptors
• the NTs released by the ANS can either stimulate or inhibit its target –
depends on the receptors located in the target
1. Cholinergic receptors – respond to AcH
• a. nicotinic – named because they are also activated by nicotine
– found on the cell bodies of the postG neurons within the ganglia of the symp. and
parasymp. division
– i.e. respond to AcH release from symp and parasympathetic preG fibers
– binding of AcH opens channels for the movement of multiple ions including Na and K
– if more positive ions (e.g. Na) enter the postG neuron within the ganglion –
depolarization and initiation of an AP by the postG neurons
adrenergic R
nicotinic R
muscarinic R
• b. muscarinic receptors
• can bind either Ach or muscarene (Amanita muscaria mushroom)
• called metabotropic receptors - specific for one kind of ion
– e.g. ligand-gated Na channel
• expressed on tissues “downstream” of post-ganglionic neurons – at the
target tissue
– e.g. neuromuscular junction (ligand-gated sodium channel)
adrenergic R
nicotinic R
muscarinic R
2. Adrenergic receptors – respond to NE/Epi
• alpha and beta classes – a1, a2, b1, b2, b3
• distributed in a specific pattern in target tissues and respond to either NE
or Epi or both
– epinephrine made by the adrenal glands and NOT by neurons
• respond to activation by activating G proteins -> second messengers
(cAMP or Ca)
• therefore they are called G protein coupled receptors
adrenergic R
nicotinic R
muscarinic R
Reflexes
Reflex arc
•Neural “wiring” of reflex
•Requires 5 functional components: 1.
sensory receptor, 2. sensory neuron, 3.
integrating center (SC or BS), 4. motor
neuron & 5. effector
• By development
– Innate, acquired
• Where information is processed
– Spinal, cranial
• Motor response
– Somatic, visceral
• Complexity of neural circuit
– Monosynaptic
Classification of
Reflexes
• Stretch reflex: causes contraction
in response to stretch
• regulates skeletal muscle
length and tone
• monosynaptic - only one
synapse in the CNS - between
and single sensory and motor
neuron
• all monosynaptic reflexes are
ipsilateral reflexes - input and
output on same side
• sensory receptors are found in
muscle spindles – activated
when stretched
– e.g. Patellar reflex – hit
with a mallet stretches
the quadriceps and its
tendon - results in
contraction
– muscle spindles in the
quadriceps muscles are
activated
– reflex results
Spinal
Reflexes
•Tendon reflexes: controls muscle
tension by causing muscle relaxation
before muscle contraction rips tendons
•generally polysynaptic - more
than one CNS synapse involved
between more than two different
neurons
•sensory receptor synapses with 2
interneurons
• 1. an inhibitory IN synapses with motor
neurons and causes
inhibition and relaxation of
one set of muscles
• 2. a stimulatory IN synapses
with motor neurons and
causes contraction of the
antagonistic muscle
Spinal
Reflexes
Postural
Reflex
withdrawl
crossed
extensor