Download Spinal Nerves and Reflexes

Central Nervous
System
- Spinal Nerves and
Reflexes
Chapter 13B
Spinal Nerves - Number
There are 31 pairs of spinal nerves…a total of
62 nerves.
Spinal cord is located in the vertebral canal.
Spinal nerves exit vertebral column through
intervertebral foramina.
Intervertebral
foramen
Vertebral
canal
Spinal Nerves
Interneuron
Sensory neuron
Sensory fiber
Motor neuron
Spinal nerve
Motor fiber
All spinal nerves are mixed nerves….contain sensory and motor fibers.
Spinal Nerves - Supply
NV
C2–C3
C2
C3
C3
C4
T2
C6
L1
L2
C8
T1
C7
L3
L4
KEY
L5
Spinal cord regions
C5
T1
T2
T3
T4
T5
T6
T7
T8
T9
T10
T11
T12
S2
T2
T3
T4
T5
T6
T7
T8
T9
T10
T11
T12
L1
L2
L4 L3
L5
Spinal nerves go to skin, muscles and some
of the internal organs.
C4
C5
Dermatomes: areas of the skin that is
connected to a specific spinal nerve.
T2
C6
T1
C7
S4S3
L1
S5
C8
S1 L5
L2 S2
L3
= Cervical
= Thoracic
= Lumbar
S1
= Sacral
L4
ANTERIOR
POSTERIOR
Myotomes: specific muscles that are
supplied by a specific spinal nerve.
Spinal Nerves - Branches
Spinal nerve
Dorsal
ramus
Dorsal root Dorsal root
ganglion
Spinal
nerve
Ventral
ramus
Dorsal horn
Ventral
horn
Ventral root
Rami
communicantes
After exiting vertebral column, EACH spinal nerve splits into branches, called rami:
1. Dorsal ramus: contains nerves that serve the dorsal portions of the trunk- carry visceral motor, somatic
motor, and sensory information to and from the skin and muscles of the back.
2. Ventral ramus: contains nerves that serve the remaining ventral parts of the trunk and the upper and
lower limbs- carry sensory information from the body wall and the limbs and motor information to
smooth muscle, skeletal muscle and glands.
3. Rami communicates: Contain autonomic nerves that carry visceral motor and sensory information
to and from the visceral organs.
Roots: each root is either sensory or motor; Rami: each rami is sensory and motor.
Spinal Nerves - Plexus
Reminder:
Each spinal nerve gives off a branch called
ventral/anterior ramus  goes to the muscles and skin
on the anterior surface of the trunk, arms, legs, head
and shoulders.
Ventral/anterior rami branches of thoracic nerves T2-T12
(Do not form plexus)  give rise to intercostal nervesinnervates intercostal muscles, muscles and skin of
anterolateral thorax and most of the abdominal wall.
Ventral/anterior rami branches of other spinal nervesC1-C8, L1-L5, S1-S5 reorganize and form network on
both sides of the spinal cord before going to the muscles
and skin of their respective areas.
This network of intersecting nerves is called plexus.
Nerve plexus are found only on --------? Ventral rami
Spinal Nerves - Plexus
Plexus:
Refers to a network formed by the ventral
rami branches of the spinal nerves (exception
of T1-T12).
There are four major plexi formed next to the
spinal cord:
Cervical plexus: in the neck region.
Brachial plexus: in the shoulder region.
Lumbar plexus: in the lower back region.
Sacral plexus: in the buttock region.
Spinal Nerves – Cervical Plexus
Cervical Plexus:
Formed by the ventral rami branches of
the spinal nerves C1 – C5.
C1
C2
C3
C4
C5
Cervical
plexus
Supplies to the skin and muscles of the
head, neck and upper part of the
shoulders.
Phrenic nerve- A major nerve of cervical
plexus-originates from C3, C4 and C5 spinal
nerves and supplies diaphragm, a key
respiratory muscle.
“C3, C4, C5 keeps the diaphragm alive”
Spinal Nerves – Brachial Plexus
Brachial Plexus:
Formed by the ventral rami branches of
the spinal nerves C5 – C8 and part of T1.
Supplies to the skin and muscles of the
shoulders and upper extremities (arms).
C5
C6
C7
C8
T1
Brachial
plexus
1) Axillary nerve- innervates deltoid muscle.
2) Musculocutaneous nerve- innervates flexor muscles on arm
(biceps brachii, brachialis and coracobrachilais).
3) Radial nerve-Innervates the extensor muscle of the arm
(triceps brachii) and forearm.
4) Ulnar nerve- innervates flexor muscles of wrist (flexor carpi
ulnaris).
5) Median nerve- innervates flexor muscles of the forearm.
Spinal Nerves – Lumbar Plexus
T12
L1
L2
L3
L4
Lumbar
plexus
Lumbar Plexus:
Formed by the ventral rami branches of
the spinal nerves part of T12, L1 – L4.
Supplies to the skin and muscles of the
lateral and anterior abdominal wall,
external genitals and part the thighs.
Major nerves include:1) Genitofemoral nerve
2) Lateral femoral cutaneous nerve
3) Femoral nerve
Spinal Nerves – Sacral Plexus
Sacral Plexus:
Formed by the ventral rami branches of
the spinal nerves L4 – L5 and S1 – S4.
Supplies to the skin and muscles of the
buttocks and lower extremities (legs).
L4
L5
S1
S2
S3
S4
Sacral
plexus
Major nerve:Sciatic nerve (originates from L4-S3)-longest
and thickest nerve in the body-innervates
gluteal muscle, hamstrings and calf muscle.
Sciatic nerve
Reflex
Sensory fiber
Cell body of
Sensory neuron
Dorsal/posterior
root ganglion
Denticulate
ligament
Spinal nerve
Motor fiber
Reflex:
Refers to a fast response to an external or internal stimulus  to maintain homeostasis.
Example:
Prick your finger  muscles of the arm contract  you pull your arm to prevent further
injury and fluid loss.
Brain is not usually involved in reflex action.
With age, conduction rate decreases  Does that increase or decrease reflex time??
Reflex Arc
1
2
Arrival of
stimulus and
activation of
receptor
Dorsal
root
Activation of a
sensory
neuron
Sensation
relayed to the
brain by axon
collaterals
Spinal cord
3
Information
processing
in the CNS
REFLEX
ARC
Receptor
Stimulus
Response by a
peripheral effector
Effector
4
Ventral root
Activation of a
motor neuron
KEY
Sensory neuron
(stimulated)
Interneuron
Motor neuron
(stimulated)
Reflex arc: refers to the pathway followed in order to cause a reflex. It involves:
1. Receptor- receives the stimulus  causes generation of an impulse.
Could be dendrite of the sensory neuron or a specialized cell.
2. Sensory neuron- carries impulse to CNS (spinal cord/brain).
3. Integration center- CNS where information is analyzed.
Association neuron/interneurons are involved.
4. Motor neuron- takes impulses from CNS to where the response occurs.
5. Effector- the structure that responds….muscle or a gland.
Neuronal Circuits
Human body has about:
10 million sensory neurons  that bring impulses to CNS.
Half million motor neurons  that take impulses from CNS to the effectors 
to get a response.
20 billion interneurons  that connect sensory neurons to correct motor
neurons  to get correct response!
How interneurons coordinate all body functions….billions of interneurons are organized in much
smaller number of units called neuronal pools- a group of neurons dedicated to a particular
function/functional group of neurons that process & integrate information.
Neuronal circuit-structural orientation of neurons in the neuronal pool.
Neuronal Circuits
Divergence
Convergence
Serial processing Parallel processing
Reverberation
Neuronal circuits can be of different types:
1. Divergence: one neuron triggers many other neurons-broad distribution of a specific input- causes amplification of signal,
e.g.- a motor neuron innervating 1000’s of muscle fibers.
2. Convergence- many neurons converge to one neuron-causes concentration of signal, e.g.-different types of sensory
stimuli can have same ultimate effect-linking different senses like sight, sound and smell to associate it with a memory.
3) Serial processing- a type of neuronal processing where one neuron stimulates the next, eventually causing a specific
anticipated response.
4) Parallel processing-several neurons/neuronal pools process information simultaneously- many responses can occur at
the same time-e.g. step on a nail  pain, “ouch”, lift foot, balance your body…all at the same time.
5) Reverberation- a positive feedback mechanism-collateral branches of axon extends back toward the source of an impulse
and further stimulates the presynaptic neuron to produce prolonged and repetitive effect, e.g. complex circuits that control
consciousness, muscle coordination, breathing.
Reflex Arc - Types
Patellar reflex
A. Innate reflexes: reflexes you are born with…blinking, suckling, chewing.
Acquired reflexes: reflexes you learn…driving, jumping ropes.
Withdrawal reflex
B. Somatic reflex arc: results in contraction of skeletal muscle…voluntary effector.
Autonomic/Visceral reflex arc: results in contraction of smooth/cardiac muscle or secretion of a
gland…involuntary effectors.
C. Monosynaptic reflex arc: does not involve interneuron…one synapse…sensory  motor, e.g. patellar
reflex.
Polysynaptic reflex arc: has one or more interneurons…2 or more synapses,
e.g. withdrawal reflex
Intersegmental reflex arc: sensory neurons enter at certain level/segment of the spinal cord 
interneurons take to another level/segment  motor neurons exit at different level.
D. Spinal reflex arc: involves spinal cord as the integration center…simpler.
Cranial reflex arc: involves brain as the integration center…more complex.
Reflexes & Neurological Impairment
Somatic reflexes are often used to diagnose nervous system injuries.
Lack of reflex indicates pathway injuries.
Patellar reflex: patellar ligament is tapped  sensory impulses are sent to the spinal cord
 motor impulses exit on the same side  thigh muscle- quadriceps femoris contracts
 extension of the leg at the knee.
Achilles reflex: achilles tendon is tapped  contraction of gastrocnemium muscle  foot
extends at the ankle.
Reflex Arc - Types
Spinal Cord Disorders
Spinal cord trauma: damage to the spinal cord  loss of muscle control  paralysis.
Paraplegia: where both the lower extremities are paralyzed.
Quadriplegia: where all four limbs are paralyzed.
Neuritis: inflammation of the nerves.
Due to injuries, bone fracture, drugs, etc.
Painful and affects muscle contraction.
Sciatica: a type of neuritis.
Compression of sciatic nerve against coxal bone or hip injury  injury to sciatic nerve 
numbness of the lower extremity or severe pain radiating from the lower back to the
leg.
Shingles: where chicken pox virus, hiding in the peripheral nerves  becomes re-activated
when immune system in compromised  starts multiplying  spreads along the
peripheral nerve  form painful blisters on the area of the skin served by that nerve
(dermatome).
Poliomyelitis: caused by poliovirus  attacks motor neurons  paralysis of a specific
muscle  affecting movement, respiratory/heart failure.
Rabies: virus infection transmitted by animal bites (dogs, bats)  infection spreads
through the nerves  to CNS  seizures, coma, death.