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Anatomy and Function of the Spinal Cord and Spinal Nerves, Study notes of Physiology

Clayton State University Physiology

An in-depth exploration of the structure, functions, and clinical correlations of the spinal cord and spinal nerves. Topics include the spinal cord's relationship to the brain, its protective coverings, the inferior end of the spinal cord, the gray and white matter, tracts, reflexes, and clinical considerations. Additionally, the document covers spinal nerves, their branching, and associated nerve plexuses.

Typology: Study notes

Pre 2010

Uploaded on 08/04/2009

koofers-user-3ft 🇺🇸

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Chapter 13 The Spinal Cord & Spinal Nerves

Objectives:

•Spinal Structure

•Spinal Functions

•Reflexes

•Plexus

•Clinical correlations

•Dermatomes & Myotomes

•Disorders

The Spinal Cord & Spinal Nerves

•CNS = brain + spinal cord

•Spinal cord is continuous withy medulla oblongata

•Functions

–spinal cord reflexes

–integration (summation of inhibitory and excitatory) nerve impulses

–“highway” for upward and downward travel of sensory and motor information

Structures Covering the Spinal Cord

•Vertebrae

•Epidural space filled with fat

•Dura mater

–dense irregular CT tube

•Subdural space filled with interstitial fluid

•Arachnoid = spider web of collagen fibers

•Subarachnoid space = CSF

•Pia mater (innermost)

–thin layer covers BV

–denticulate ligs hold in place

External Anatomy of Spinal Cord

•Flattened cylinder

•16-18 inches long &

3/4 inch diameter

•In adult ends at L2

•In newborn ends at L4

•Growth of cord stops at age 5

•Cervical enlargement

–upper limbs

• Lumbar enlargement

–lower limbs

1

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Chapter 13 The Spinal Cord & Spinal Nerves Objectives:

Spinal Structure
Spinal Functions
Reflexes
Plexus
Clinical correlations
Dermatomes & Myotomes
Disorders The Spinal Cord & Spinal Nerves
CNS = brain + spinal cord
Spinal cord is continuous withy medulla oblongata
Functions
spinal cord reflexes
integration (summation of inhibitory and excitatory) nerve impulses
“highway” for upward and downward travel of sensory and motor information Structures Covering the Spinal Cord
Vertebrae
Epidural space filled with fat
Dura mater
dense irregular CT tube
Subdural space filled with interstitial fluid
Arachnoid = spider web of collagen fibers
Subarachnoid space = CSF
Pia mater (innermost)
thin layer covers BV
denticulate ligs hold in place External Anatomy of Spinal Cord
Flattened cylinder
16-18 inches long & 3/4 inch diameter
In adult ends at L
In newborn ends at L
Growth of cord stops at age 5
Cervical enlargement
upper limbs
Lumbar enlargement
lower limbs

Inferior End of Spinal Cord

• Conus medullaris

cone-shaped end of spinal cord

• Filum terminale

thread-like extension of pia mater
stabilizes spinal cord in canal

• Caudae equinae (horse’s tail)

dorsal & ventral roots of lowest spinal nerves

• Spinal segment

area of cord from which each pair of spinal nerves arises Spinal Cord & Spinal Nerves
Spinal nerves begin as roots (exiting through intravertebral foramina)
Dorsal or posterior root is incoming sensory fibers
dorsal root ganglion (swelling) = cell bodies of sensory nerves
Ventral or anterior root is outgoing motor fibers Spinal tap or Lumbar Puncture

• Technique

– long needle into subarachnoid space

– safe from L4 to L

• Purpose

– sampling CSF for diagnosis

– injection of antibiotics, anesthetics or chemotherapy

– measurement of CSF pressure

Gray Matter of the Spinal Cord

• Gray matter is shaped like the letter H or a butterfly

contains neuron cell bodies, unmyelinated axons & dendrites
paired dorsal and ventral gray horns
lateral horns only present in thoracic spinal cord
gray commissure crosses the midline

• Central canal continuous with 4th ventricle of brain

White Matter of the Spinal Cord

White matter covers gray matter
Anterior median fissure deeper than Posterior median sulcus
Anterior, Lateral and Posterior White Columns contain axons that form ascending & descending tracts

Reflex Arc

• Specific nerve impulse pathway

• 5 components of reflex arc

– receptor

– sensory neuron

– integrating center

– motor neuron

– effector

• 4 important somatic spinal reflexes

– stretch, tendon, flexor(withdrawal) & crossed extensor reflexes

Stretch Reflex (patellar reflex)

• Monosynaptic, ipsilateral reflex arc

• Prevents injury from over stretching because muscle contracts when it is stretched

• Events of stretch reflex

muscle spindle signals stretch of muscle
motor neuron activated & muscle contracts

• Brain sets muscle spindle sensitivity as it sets muscle tone (degree of muscle

contraction at rest)

• Reciprocal innervation (polysynaptic- interneuron)

antagonistic muscles relax as part of reflex Tendon Reflex

• Controls muscle tension by causing muscle relaxation that prevents tendon

damage

• Golgi tendon organs in tendon

– activated by stretching of tendon

– inhibitory neuron is stimulated (polysynaptic)

– motor neuron is hyperpolarized and muscle relaxes

• Both tendon & muscle are protected

• Reciprocal innervation (polysynaptic)

– causes contraction of ipsilateral muscle group

Flexor (withdrawal) Reflex

• Step on tack (pain fibers send signal to spinal cord)

• Interneurons branch to different spinal cord segments

• Motor fibers in several segments are activated

• More than one muscle group activated to lift foot off of tack

Crossed Extensor Reflex

• Lifting left foot requires extension of right leg to maintain one’s balance

• Pain signals cross to opposite spinal cord

• Contralateral extensor muscles are stimulated by interneurons to hold up the body

weight

• Reciprocal innervation - when extensors contract flexors relax, etc

Clinical Considerations

• Checking a patient’s reflexes may help to detect disorders/injury

• Plantar flexion reflex -- stroke the lateral margin of the sole

– normal response is curling under the toes

– abnormal response or response of children under 18 months is called Babinski

sign (upward fanning of toes due to incomplete myelination in child) Spinal Nerves

31 Pairs of spinal nerves
Named & numbered by the cord level of their origin
8 pairs of cervical nerves (C1 to C8)
12 pairs of thoracic nerves (T1 to T12)
5 pairs of lumbar nerves (L1 to L5)
5 pairs of sacral nerves (S1 to S5)
1 pair of coccygeal nerves
Mixed = sensory & motor nerves Connective Tissue Coverings

• Endoneurium = wrapping of each nerve fibers

• Perineurium = surrounds group of nerve fibers forming a fascicle

• Epineurium = covering of entire nerve

dura mater blends into it at intervertebral foramen Branching of Spinal Nerve

• Spinal nerves formed from dorsal & ventral roots

• Spinal nerves branch into dorsal & ventral rami

– dorsal rami supply skin & muscles of back

– ventral rami form plexus supply anterior trunk & limbs

– meningeal branches supply meninges, vertebrae & BV

A Nerve 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

intercostal nn. innervate intercostal spaces
T7 to T12 supply abdominal wall as well

Sacral Plexus

• Ventral rami of L4-L5 & S1-S

• Anterior to the sacrum

• Supplies buttocks, perineum & part of lower limb

• Sciatic nerve = L4 to S3 supplies post thigh & all below knee

Peroneal nerve injury produces foot drop or numbness
Tibial nerve injury produces calcaneovalgus (loss of function on anterior leg & dorsum of foot) Sciatic Nerve Branches

• Notice: Common Peroneal nerve and Tibial nerve behind the knee

• Notice: Sciatica pain extends from the buttock down the leg to the foot

• May be sign of herniated disc

Dermatomes & Myotomes

• Each spinal nerve contains both sensory & motor nerve fibers

• Dermatome

– area of skin supplied by one spinal nerve

– overlap prevents loss of sensation if one damaged

– sensory anesthesia requires 3 spinal nerves to be blocked

• Skin on face supplied by Cranial Nerve V

Dermatomes

• Damaged regions of the spinal cord can be distinguished by patterns of numbness

over a dermatome region

• Infusing local anesthetics or cutting roots must be done over 3 adjacent spinal

nerves.

• Spinal cord transection

injury that severs the cord loss of sensation& motor control below the injury Disorders

• Neuritis

– inflammation of nerves

– caused by injury, vitamin deficiency or poison

• Shingles

– infection of peripheral nerve by chicken pox virus

– causes pain, skin discoloration, line of skin blisters

• Poliomyelitis

– viral infection causing motor neuron death and possible death from cardiac failure

or respiratory arrest