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

Vertebralcanal

Spinal Nerves

All spinal nerves are mixed nerves….contain sensory and motor fibers.

Motor fiber

Spinal nerve

Sensory neuron

Sensory fiber

Motor neuron

Interneuron

Spinal Nerves - SupplyN V

C2–C3

C2C3

C4

C3

C4

C5

C5T1T2

T2

T2 T7

T3T4T5T6

T8T9T10T11T12

T2

T1

T2

T7

T3T4T5T6

T8T9T10T11T12 C6

C7C6

C7

C8

T1C8

L1L2

L3L4L5

L1

L2

L3

L4

L1

S2

S1

S2

S3S4

S5L5

L1

L2

L3L5

L4

S1

ANTERIOR POSTERIOR

KEYSpinal 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 supplied by a specific spinal nerve.

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 informationto 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 ganglion

Dorsal root

Ventral root

Spinalnerve

Spinal nerve

Dorsal horn

Ventralhorn

Spinal Nerves - PlexusReminder: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 nerves-innervates intercostal muscles, muscles and skin of anterolateral thorax and most of the abdominal wall.

Ventral/anterior rami branches of other spinal nerves-C1-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.

Supplies to the skin and muscles of the head, neck and upper part of the shoulders.

Cervical plexus

C2C3

C5

C1

C4

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 PlexusBrachial 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

Brachial plexus

C6

C8T1

C7

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

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 nerve2) Lateral femoral cutaneous nerve3) Femoral nerve

Lumbar plexus

T12

L1

L2L2

L3

L4

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

L4

L5S1

S2S3

S4

Major nerve:-Sciatic nerve (originates from L4-S3)-longestand thickest nerve in the body-innervatesgluteal muscle, hamstrings and calf muscle.

Sciatic nerve

Reflex

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??

Denticulateligament

Motor fiber

Sensory fiber

Spinal nerve

Cell body ofSensory neuron

Dorsal/posteriorroot ganglion

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 Sensation

relayed to the brain by axon collaterals

Spinal cord

ReceptorStimulus

EffectorResponse by a peripheral effector

Activation of a motor neuron

Ventral root

REFLEX ARC

KEYSensory 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

Neuronal Circuits

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.

Divergence Convergence Serial processing Parallel processing Reverberation

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 reflexIntersegmental 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.

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.