Spinal Nerves and Reflexes: A Comprehensive Guide to the Central Nervous System, Lecture notes of Neurobiology

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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 - Supply

ANTERIOR POSTERIOR KEY Spinal cord regions = Cervical = Thoracic = Lumbar = Sacral

Spinal nerves go to skin, muscles and some

of the internal organs. Dermatomes : areas of the skin that is connected to a specific spinal nerve. Myotomes : specific muscles that are

 - C 2 –C N V - C - C - C - C - C - C - C - T - T - T - T 2 T - T - T - T - T - T - T - T - T - T - T - T - T - T - T - T - T - T - T - T - T - T - T 12 C - C 6 C 

• C
  • C - T 1 C - L - L - L 4 L - L - L - L - L - L - L - S - S - S - S 4 S - S - L - L - L - L 5 L - L - S

Spinal Nerves - Branches

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.

Rami communicantes

Ventral ramus

Dorsal ramus

Dorsal root Dorsal root ganglion

Ventral root

Spinal nerve

Spinal nerve

Dorsal horn

Ventral horn

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.

Supplies to the skin and muscles of the

head, neck and upper part of the

shoulders.

Cervical plexus

C 2 C 3

C 5

C 1

C 4

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 – 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

Lumbar plexus

T (^12)

L 1

LL 22

L 3 L 4

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).

Sacral plexus

L 4 L 5 S 1

S 2 S 3

S 4

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 Arc

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.

Arrival of stimulus and activation of receptor

Activation of a sensory neuron

Dorsal root Sensationrelayed to the brain by axon collaterals Spinal cord

Receptor Stimulus

Effector

Response by a peripheral effector

Activation of a motor neuron

Ventral root

REFLEX ARC

KEY Sensory neuron (stimulated ) Interneuron

Motor neuron (stimulated)

Information processing in the CNS

3

2

4

1

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

Reflex Arc - Types

D. Spinal reflex arc : involves spinal cord as the integration center…simpler. Cranial reflex arc : involves brain as the integration center…more complex.

A. Innate reflexes : reflexes you are born with…blinking, suckling, chewing. Acquired reflexes : reflexes you learn…driving, jumping ropes.

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.

Patellar reflex

Withdrawal reflex

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.

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.