Organization of
Somatic Nervous system
Spinal nerve and Reflex arc
Dr. Shaheen Haroon Rashid
OBJECTIVES
1. Recall various components of somatic nervous system.
2.
Explain structure of typical spinal nerve.
3. Describe reflex arc.
4. Identify clinical application.
Nervous System
1.CNS
2.PNS
1. SOMATIC
1. Brain
2. Spinal Cord
2. AUTONOMIC
Somatic nervous system (SNS)
All parts of the nervous system outside of the brain and
spinal cord
1. Somatic System: Links spinal cord with body and
sense organs; controls voluntary behavior
2. Autonomic System: Serves internal organs and
glands; controls automatic functions such as heart
rate and blood pressure
3. Enteric System
Functional Classification
BRAIN
SPINAL CORD
AFFERENT
EFFERENT
NERVES
NERVES
EXTERORECEPTORS
EFFECTOR
ORGANS
INTERORECEPTORS
SKELETAL
MUSCLES
(CNS)
PNS
SOMATIC
AUTONOMIC
SMOOTH AND
CARDIAC MUSCLES
AND GLANDS
Peripheral Distribution of Spinal nerve
Nerves Spinal nerves
1.Form lateral to intervertebral foramen
A.Where dorsal and ventral roots unite
B.Then branch and form pathways to destination
Motor nerves---
first branch
1. White ramus Carries visceral motor fibers to sympathetic ganglion
of autonomic nervous system
2. Gray ramus
Unmyelinated nerves , Return from sympathetic
ganglion to rejoin spinal nerve
Spinal Nerves
1. Based on vertebrae where
spinal nerves originate
1. Positions of spinal segment and
vertebrae change with age
1. Cervical nerves Are named
for inferior vertebra
• All other nerves Are named
for superior vertebra
Spinal nerves continued
1. Note: cervical spinal nerves exit from
above the respective vertebra
1. Spinal nerve root 1 from above C1
2. Spinal nerve root 2 from between C1
and C2, etc.
2. Clinically, for example when referring
to disc impingement, both levels of
vertebra mentioned, e.g. C6-7 disc
impinging on root 7
3. Symptoms usually indicate which
level
Peripheral Nerves
1. Epineurium
wraps entire nerve
2. Perineurium
wraps fascicles of tracts
3. Endoneurium
wraps individual axons
Nerve structure
1. Nerves are only in the periphery
2. Cable-like organs in PNS = cranial and spinal nerves
3. Consists of 100-100,000 of myelinated + unmyelinated axons
(nerve fibers)+ connective tissue + blood vessels
4. Support Cells of the PNS
Satellite cells ---Protect neuron cell bodies
Schwann cells---Form myelin sheath
Morphology of neuron
1.membrane
Cell body (soma)
2.nucleus
Two parts
Dendrites
Processes
Axon
Presynaptic terminals.
terminal (bouton / button)
AXON
1.Plasmalemma--axolemma
2.Cytoplasm--axoplasm
(mitochondria,microtubues,
Neurofilaments,)
3. Axon hillock;Origin
4. No rough ER--No protein synthesis
5. Axon terminal
6. Chromatophilic----- no Nissl body
FUNCTIONAL PARTS OF AXON
1. Processes
Integration zone
2.Axon hillock
1ST portion of the axon plus the region of
the cell body fro m which the axon leaves
Neuron’s trigger zone
3.Nerve fiber
Single, elongated tubular extensio
that conducts AP away from the ce
Conducting zone of the neuron
4..Collaterals
Side branches of axon
5.Axon terminals
Release chemical messengers other cells
with which they come into close
Output zone of the neuron
REFLEX = reflection
is an involuntary, immediate, automatic and
stereotyped response to a specific sensory stimulation.
Classification
1. Clinical
2. Physiological
3. Number of synapses
4. Site
5. Anatomical
6. Development
7. Functional
8. On purposes
9. Response is confined
10. Depending on the part involved
11. Character of the response
12. Other reflexes
SIGNIFICANCE
HOMEOSTASIS (autonomic reflexes)
1. TONE DURING RESTING STATE
2. TONE DURING TENSE MOTOR ACTIVITY
3.
POSTURE
4.
EQUILIBRIM
5.
EXECUTION OF MOVEMENTS
6.
SMOOTHNESS
7.
DAMPNESS during resting , walking, running, states
8.
ROLE AS PROPRIOCEPTOR( unconcouscious+ concious
kinaesthetic sensations)
R-SIM Reflex arc pathway
1. R
receptor neuron receives the stimuli
2. S
sensory neuron passes the impulse on
3.
I
interneuron at the spinal cord processes
4. M
motor neuron acts
Simplified reflex arc
stimulus
Simplified reflex arc
stimulus
receptor
Simplified reflex arc
sensory neurone
stimulus
receptor
Simplified reflex arc
stimulus
sensory neurone
receptor
spinal
cord of
central
nervous
system
Simplified reflex arc
stimulus
sensory neurone
receptor
spinal
cord of
relay
central
neurone
nervous
system
Simplified reflex arc
stimulus
sensory neurone
receptor
spinal
relay
cord of
neurone central
nervous
system
motor neurone
Simplified reflex arc
stimulus
sensory neurone
receptor
relay
neurone
effector
motor neurone
spinal
cord of
central
nervous
system
Simplified reflex arc
stimulus
sensory neurone
receptor
relay
neurone
response
effector
motor neurone
spinal
cord of
central
nervous
system
Spinal Reflexes
1. Somatic reflexes mediated by the spinal cord
are called spinal reflexes
2. These reflexes may occur without the
involvement of higher brain centers
3. Additionally, the brain can facilitate or inhibit
them
R 3 Inputs to
Alpha
Motor Neurons
DRG
(1) Afferent
(sensory) neuron
(2) Upper
motor
(3) Spinal interneuron
neurons
31
Monosynaptic Reflexes
= Excitatoryinterneuron
=Inhibitory interneuron
= Synapse
= Stimulates
=Inhibits
Thermal
pain receptor
in finger
Afferent
Pathway
Ascending pathway
to brain
Stimulus
Biceps
(flexor)
contracts
Hand
withdrawn
Efferent pathway
Triceps
(extensor)
relaxes
Effector
organs
Response
Integrating center
(spinal cord)
UMN lesions
LMN lesions
1. Weakness, paralysis
1. weakness, paralysis
2. Spasticity
2. flaccidity, hypotonia
3. tendon reflexes
3. tendon reflexes
4. +ve Babinski sign
4. -ve Babinski sign
5. Little,if muscle atrophy
5. Muscle atrophy
6. No fasiculation
6. Fasiculation of muscle
UMN v LMN
Cortex
UMN
LMN
Muscle
Flaccidity
Spasticity
Reflex testing
0 = ABSENT
1+ = HYPOREFLEXIA
2+ = NORMAL
3+ = HYPERREFLEXIA
4+ = HYPERREFLEXIA & CLONUS
SPINAL SHOCK
Spinal shock is a state of transient physiological (rather
than anatomical) reflex depression of cord function
below the level of injury with associated loss of all
sensori-motor functions.
An initial increase in blood pressure is noted due to the
release of catecholamines, followed by hypotension.
Shingles ( of the herpes family)
In dorsal root ganglia and cranial nerves
Initial infection:
chicken pox virus
Peripheral Neuropathy
Regional loss of sensory or motor function
Due to trauma or compression R
metabolic causes