Spinal Cord and Nerves

Functions of the spinal cord

The spinal cord is a vital link between the brain and the rest of the body.

Functions of spinal nerves

Spinal nerves serve two important functions: they are a pathway for sensory and motor impulses and are responsible for reflexes.

Regions of the spinal cord

The five parts of the spinal cord, listed superior to inferior, are: Cervical part, Thoracic part, Lumbar part, Sacral part, and Coccygeal part.

Conus medullaris

The tapering inferior end of the spinal cord.

Spinal cord termination

The spinal cord extends from the foramen magnum to the L1 vertebra.

Cauda equina

Axons that extend inferiorly from the spinal cord's conus medullaris.

Filum terminale

A thin strand of pia mater within the cauda equina that attaches the conus medullaris to the coccyx.

Cervical enlargement

Innervates the upper limbs.

Lumbosacral enlargement

Innervates the lower limbs.

Number of spinal nerves

There are 31 pairs of spinal nerves.

Epidural space

Sits between the dura mater and the periosteum of the vertebra.

Subdural space

A narrow potential space separating the dura mater from the deeper arachnoid mater.

Subarachnoid space

A real space filled with cerebrospinal fluid (CSF).

Denticulate ligaments

Paired, lateral triangular extensions of the pia mater that suspend and anchor the spinal cord laterally to the dura mater.

Gray matter location

Located centrally (inner region) and contains dendrites, cell bodies of neurons, unmyelinated axons, and glial cells.

White matter location

Located externally (outer region) and contains myelinated axons.

Lateral horns

Contains cell bodies of autonomic motor neurons.

Posterior horns

Contains axons of sensory neurons and cell bodies of interneurons.

Anterior horns

Contains cell bodies of somatic motor neurons.

Gray commissure

For communication between right & left side.

Central canal

The central canal is found within the gray commissure.

Posterior funiculus

Contains only sensory (ascending) tracts.

Lateral funiculus

Contains both ascending (sensory) and descending (motor) tracts.

Anterior funiculus

Contains both ascending (sensory) and descending (motor) tracts.

Spinal nerves

There are 31 pairs of spinal nerves.

Anterior roots

Contain motor axons only.

Motor axons origin

Arise from cell bodies in the anterior and lateral horns of the spinal cord.

Posterior roots

Contain sensory axons only.

Sensory axons origin

Arise from cell bodies in the posterior root ganglion, which is attached to the posterior root.

Numbering of spinal nerves

Spinal nerves are numbered according to the locations of the intervertebral foramina they use to exit.

C1-C7 spinal nerves exit

Exit the intervertebral foramen above the vertebra of the same number.

8th pair of spinal nerves exit

The eighth cervical spinal nerve (C8) exits above the first thoracic vertebra.

Remaining pairs of spinal nerves exit

Exit below the vertebra of the same number.

Ramus

A branch that a typical spinal nerve splits into after leaving the intervertebral foramen.

Posterior ramus

Innervates the deep muscles and skin of the back.

Anterior ramus

Innervates the anterior and lateral portions of the trunk and the upper and lower limbs.

Distribution of spinal rami

The spinal nerve divides into posterior rami (innervate deep muscles and skin of the back) and anterior rami (innervate anterior and lateral trunk, upper and lower limbs, forming nerve plexuses and rami communicantes).

Dermatome

A segment of skin supplied by a single spinal nerve; it is a map of sensory regions of the skin.

Importance of dermatome

Important for identifying potential sites of spinal nerve damage, understanding referred visceral pain, and diagnosing a shingles rash.

Nerve plexus

A network of interweaving anterior rami of spinal nerves that split into multiple named nerves that innervate body structures.

Formation of nerve plexus

The anterior rami of spinal nerves form the nerve plexuses.

Major nerve plexuses

Cervical (C1-C4), brachial (C5-T1), lumbar, and sacral.

Cervical plexus

The major plexus formed by spinal nerves C1-C4.

Brachial plexus

The major plexus formed by spinal nerves C5-T1.

Lumbar plexus

One of the major plexuses in the nervous system.

Sacral plexus

One of the major plexuses in the nervous system.

Intercostal nerves

The anterior rami of spinal nerves T1-T11 that travel in the spaces between adjacent ribs.

Subcostal nerve

Spinal nerve T12, called so because it arises below the ribs.

Innervation of intercostal muscles

Innervated by T3-T6.

Innervation of arms

Innervated by the brachial plexus (C5-T1).

Innervation of armpit

Innervated by T2, which is sensory for the axilla.

Innervation of abdominal muscles and skin

Innervated by T7-T12.

Innervation of anterior chest wall

Innervated by T3-T6, which are sensory for the anterior chest wall.

Reflex

A rapid, preprogrammed, involuntary reaction to a stimulus.

Components of a reflex

A stimulus is required, a rapid response, a preprogrammed response, and an involuntary response.

Reflex arc

The neural circuit of a reflex.

Parts of a reflex arc

Stimulus activates sensory receptor, nerve impulse travels through sensory neuron to the CNS, interneurons process information, motor neurons send impulses to effector, effector brings about response.

Ipsilateral reflex arc

A reflex arc where both the receptor and effector organs are on the same side of the body.

Contralateral reflex arc

A reflex arc where impulses from a receptor cross the spinal cord to activate effector organs on the opposite limb.

Monosynaptic reflex

A reflex where sensory axons synapse directly on motor neurons, with no interneurons involved.

Polysynaptic reflex

A reflex with more complex pathways exhibiting a number of synapses involving interneurons.

Stretch reflex

A monosynaptic reflex arc where the sensory axon from the muscle spindle synapses directly on the motor neuron.

Golgi tendon reflex

A polysynaptic reflex that involves the activation of interneurons by Golgi tendon organs to inhibit motor neurons.

Development of the spinal cord

The CNS develops from the neural tube, with the neural canal becoming the central canal of the spinal cord.