Spinal Cord and Spinal Nerves

The spinal cord may be subdivided into the following parts:

• The Cervical Part

  • It is the superiormost region of the spinal cord. 

  • It is continuous with the medulla oblongata.

  • The cervical part contains motor neurons whose axons contribute to the cervical spinal nerves and receives input from sensory neurons through these spinal nerves.

• The Thoracic Part

  • It lies inferior to the cervical part.

  • It contains the neurons for the thoracic spinal nerves.

• The Lumbar Part

  • It is a shorter segment of the spinal cord that contains the neurons for the lumbar spinal nerves.

• The Sacral Part

  • It lies inferior to the lumbar part and contains the neurons for the sacral spinal nerves.

• The Coccygeal Part

  • It is the most inferior tip of the spinal cord.

The tapering inferior end of the spinal cord is the conus medullaris.

•  The conus medullaris marks the offical end of the spinal cord proper. 

• Inferior to this point, groups of axons collectively called the cauda equina project inferiorly from the spinal cord. 

• These nerve roots are so named because they resemble a horse's tail. 

• Within the cauda equina is the filum terminale.

• The filum terminale is a thin strand of pia mater that helps anchor the conus medullaris to the coccyx.

The spinal cord is protected and encapsulated by spinal cord meninges, which are continuous with the cranial meninges.

The structures and spaces that encircle the spinal cord, listed from outermost to innermost, are as follows:

• The Epidural Space

  • It lies between the dura mater and periosteum covering the inner walls of the vertebra and houses areolar connective tissue, blood vessels and adipose connective tissue.

• The Dura Mater

  • Deep to the epidural space is the most external of the meninges, the dura mater.

  • Although the cranial dura mater has an outer periosteal layer and an inner meningeal layer, the spinal dura mater consists of just one meningeal laye.

  • The dura mater provides stability to the spinal cord.

  • In additition, at each intervertebral foramen, the dura mater extends between adjacent vertebrae and fuses with the connective tissue layers that surround the spinal nerves.

• The Subdural Space

  • A narrow subdural space separates the dura mater from the arachnoid mater.

• Subarachnoid Space

  • Deep to the arachnoid mater is the subarachnoid space, which is a real space filled with cerebrospinal fluid (CSF).

• The Pia Mater

  • The pia mater, deep to subarachnoid space, is a delicate, innermost meningeal layer composed of elastic and collagen fibers.

  • The meninx directly adheres to the spinal cord and supports some of the blood vessels supplying the spinal cord.

• Denticulate Ligaments

  • They are paired, lateral triangular extensions of the spinal pia mater that attach to the dura mater.

  • These ligaments suspend and anchor the spinal cord laterallty to the dura mater.

The spinal cord is partitioned into an inner gray matter region and an outer white matter region.

The gray matter is dominated by the dendrites and cell bodies of neurons and glial cells and unmyelinated axons, whereas the white matter is composed primarily of myelinated axons.

Location and Distribution of Gray Matter

• The Gray Matter in the spinal cord is centrally located, and its shape resembles a letter H or a butterfly.

• The gray matter may be subdivided into the following components:

  • Anterior Horns 

    • They are the left and right anterior masses of gray matter

    • It primarily house the cell bodies of somatic motor neurons.

  • Lateral Horns

    • They are found in the T1-L2 parts of the spinal cord only.

    • The lateral horns contain the cell bodies of autonomic motor neurons, which innervate cardiac muscle, smooth muscle and glands.

  • Posterior Horns

    • They are the left and tight posterior masses of gray matter.

    • The axons of sensory neurons and the cell bodies of interneuron are located in the posterior horns.

  • The Gray Commissure

    • It is a horizontal bar of gray matter that surrounds a narrow central canal.

    • It primarily contains unmyelinated axons and serves as a communication route between the right and left sides of the gray matter.

  • Within these regions of gray matter are various functional groups of neuron cell bodies called nuclei:

    • Sensory Nuclei in the posterior horns contain interneuron cell bodies.

    • Somatic Sensory Nuclei receive information from sensory receptors, such as pain or pressure receptors in the skin.

    • Visceral Sensory Nuclei receive information from sensory receptors such as stretch receptors in the smooth muscles walls of viscera.

    • Motor Nuclei in the anterior and lateral horns contain motor neuron cell bodies that send nerve impulses to muscle and glands.

    • The Somatic Motor Nuclei in the anterior horns innervate skeletal muscle.

    • The Autonomic Motor Nuclei in the lateral horns innervate smooth muscle ,cardiac muscle and glands.

• Location and Distribution of White Matter

  • The white matter of the spinal cord is external to the gray matter.

  • White Matter on each side of the cord is also partitioned into three regions, each called a funiculus.

    • A Posterior Funiculus lies between the posterior gray horns on the posterior side of the cord and the posterior median sulcus.

    • The white matter region on each lateral side of the spinal cord is the lateral funiculus.

    • The anterior funiculus is composed of tracts of white matter that occupy the space on each anterior side of the cord between the anterior gray horns and the anterior median fissure, the anterior funiculi are interconnected by the White Commissure.

  • The axons within each white matter funiculus are organized into smaller structural units called tracts or fasciculi.

  • Individual tracts conduct either sensory impulses or motor commands only.

  • Each funiculus region contains both ascending and descending tracts.

The 31 paris of spinal nerves connect the central nervous system to muscles, glands and receptors.

Motor axons in a spinal nerve originate from the spinal cord. Anteriorly, multiple anterior rootlets arise from the spinal cord and merge to form a single anterior root (or ventral root), which contains motor axons only. 

These motor axons arise from cell bodies in the anterior and lateral horns of the spinal cord. Likewise, the posterior aspect of the spinal cord contains multiple posterior rootlets that enter the posterior aspect of the spinal cord. 

These rootlets were derived from a single posterior root (or dorsal root), which contains sensory axons only. The cell bodies of these sensory neurons are located in a posterior root ganglion, which is attached to the posterior root.

Each anterior root and its corresponding posterior root unite within the intervertebral foramen to become a spinal nerve. Thus, a spinal nerve contains both motor axons (from the anterior root) and sensory axons (from the posterior root).

Spinal Nerve Distribution

• After leaving the intervertebral foramen, a typical spinal nerve almost immediately splits into branches, termed rami.

• The posterior (dorsal ) ramus is the smaller of the two main branches. It innervates the deep muscles of the back and the skin of the back. The anterior (ventral ) ramus is the larger of the two main branches. 

• The anterior ramus splits into multiple other branches, which innervate the anterior and lateral portions of the trunk, the upper limbs, and the lower limbs. Many of the anterior rami go on to form nerve plexuses.

• Additional rami, called the rami communicantes, are also associated with spinal nerves. These rami contain axons associated with the autonomic nervous system. Each set of rami communicantes extends between the spinal nerve and a ball-like structure called the sympathetic trunk ganglion. These ganglia are interconnected and form a beaded necklace–like structure called the sympathetic trunk.

Dermatome

• A dermatome is a specific segment of skin supplied by a single spinal nerve.

Nerve Plexus

• A nerve plexus is a network of interweaving anterior rami of spinal nerves.

• The principal plexuses are:

  • Intercostal Nerves

    • The anterior rami of spinal nerves T1–T11 are called intercostal nerves because they travel in the intercostal space sandwiched between two adjacent ribs.

  • Cervical Plexuses

    • The left and right cervical plexuses are located deep on each side of the neck, immediately lateral to cervical vertebrae C1–C4. 

    • They are formed primarily by the anterior rami of spinal nerves C1–C4. 

    • The fifth cervical spinal nerve is not considered part of the cervical plexus, although it contributes some axons to one of the plexus branches. 

    • Branches of the cervical plexuses innervate anterior neck muscles as well as the skin of the neck and portions of the head and shoulders.

    • One important branch of the cervical plexus is the phrenic nerve, which is formed primarily Brachial Plexuses

    • from the C4 nerve and some contributing axons from C3 and C5.

  • Brachial Plexuses

    • The left and right brachial plexuses are networks of nerves that supply the upper limb. Each brachial plexus is formed by the anterior rami of spinal nerves C5–T1. 

    • The components of the brachial plexus extend laterally from the neck, pass superior to the first rib, and then continue into the axilla.

    • Each brachial plexus innervates the pectoral girdle and the entire upper limb of one side.

    • Five major terminal branches emerge from the three cords: the axillary, median, musculocutaneous, radial, and ulnar nerves.

  • Lumbar Plexuses

    • The left and right lumbar plexuses are formed from the anterior rami of spinal nerves L1–L4 located lateral to the L1–L4 vertebrae and along the psoas major muscle in the posterior abdominal wall. 

    • The lumbar plexus is structurally less complex than the brachial plexus.

  • Sacral Plexuses

    • The left and right sacral plexuses are formed from the anterior rami of spinal nerves L4–S4 and are located immediately inferior to the lumbar plexuses.

    • The nerves emerging from a sacral plexus innervate the gluteal region, pelvis, perineum, posterior thigh, and almost all of the leg and foot.

Reflexes are rapid, automatic, involuntary reactions of muscles or glands to a stimulus. All reflexes have similar properties:

• A stimulus is required to initiate a response to sensory input.

• A rapid response requires that few neurons be involved and synaptic delay be minimal.

• An automatic response occurs the same way every time.

• An involuntary response requires no intent or pre-awareness of the reflex activity. Thus, reflexes are usually not suppressed. Awareness of the stimulus occurs after the reflex action has been completed, in time to correct or avoid a potentially dangerous situation.

Components of a Reflex Arc

• A reflex arc is the neural “wiring” of a single reflex. It always begins at a receptor in the PNS, communicates with the CNS, and ends at a peripheral effector, such as a muscle or gland cell.

• The number of intermediate steps varies, depending on the complexity of the reflex. Generally, five steps are involved in a simple reflex arc:

  • Stimulus activates receptors

  • Nerve impulse travels through sensory neurons to the CNS.

  • Information from nerve impulse is processed in the integration center by interneurons.

  • Motor neurons transmits nerve impulse to effector.

  • Effector responds to nerve impulse from motor neuron.

• Reflex arcs may be ipsilateral or contralateral. 

  • A reflex arc is termed ipsilateral when both the receptor and effector organs of the reflex are on the same side of the spinal cord.

  • A reflex arc is contralateral when the sensory impulses from a receptor organ cross over through the spinal cord to activate effector organs in the opposite limb.

• Reflexes may also be monosynaptic or polysynaptic. 

  • A monosynaptic reflex is the simplest of all reflexes. 

    • The sensory axons synapse directly on the motor neurons, whose axons project to the effector. Interneurons are not involved in processing this reflex. 

    • Very minor synaptic delay is incurred in the single synapse of this reflex arc, resulting in a very prompt reflex response.

  • Polysynaptic reflexes have more complex neural pathways that exhibit a number of synapses involving interneurons within the reflex arc. 

    • Because this reflex arc has more components, there is a more prolonged delay between stimulus and response.