Spinal Cord, Nerves & Reflexes

Spinal Cord, Spinal Nerves, and Reflexes

• The chapter covers the spinal cord, spinal nerves, and reflexes.

Gray and White Matter

• The spinal cord contains both gray and white matter regions.
• The gray matter includes posterior (dorsal) horns and lateral horns.
• White matter includes posterior (dorsal) columns and lateral columns.
• The central canal is a feature of the spinal cord.
• Anterior (ventral) horns and columns are also present.

Basic Pattern of the Central Nervous System

• The brain consists of wrinkled, pinkish-gray tissue.
• The surface anatomy of the brain includes the cerebral hemispheres, cerebellum, and brain stem.
• The cerebellum contains gray matter in nuclei.
• In the cerebrum, nuclei and additional gray matter are located in the cortex.
• The spinal cord contains a central canal surrounded by a gray matter core.
• The external region of the spinal cord is composed of myelinated fiber tracts (white matter).

Meninges

• Meninges are protective layers surrounding the central nervous system, including the brain and spinal cord.

Dura Mater

• Dura Mater is a leathery and strong layer composed of two fibrous connective tissue layers.
• In certain areas, the two layers of the dura mater separate and form dural sinuses.
• It is the most superficial layer of the meninges.

Arachnoid Mater

• Arachnoid Mater is the middle layer that forms a loose brain covering.
• It is separated from the dura mater by the subdural space.
• Beneath the arachnoid mater is a wide subarachnoid space filled with cerebrospinal fluid (CSF).
• Arachnoid villi protrude superiorly and allow CSF to be absorbed into venous blood.

Pia Mater

• The deepest layer of the meninges is the Pia Mater, composed of delicate connective tissue that clings tightly to the brain.

Spinal Cord

• CNS tissue is enclosed within the vertebral column from the foramen magnum to L1.
• The spinal cord provides two-way communication to and from the brain.
• The spinal cord is protected by bone, meninges, and CSF.
• The epidural space is the area between the vertebrae and the dural mater, filled with fat and a network of veins.

Spinal Cord Structure

• Gray matter consists of soma, unmyelinated processes, and neuroglia.
• The gray commissure connects masses of gray matter and encloses the central canal.
• The dorsal half contains sensory roots and ganglia.
• The ventral half contains motor roots.
• Lateral horns contain sympathetic nerve fibers.
• Dorsal and ventral roots fuse laterally to form spinal nerves.

Gray Matter: Organization

• The dorsal horn contains sensory nerve roots and ganglia.
• The ventral horn contains motor nerve roots.
• Dorsal and ventral nerve roots merge laterally to form spinal nerves.
• The gray matter has four zones: somatic sensory (SS), visceral sensory (VS), visceral motor (VM), and somatic motor (SM).

White Matter in the Spinal Cord

• Fibers in the white matter run in three directions: ascending, descending, and transversely.
• The white matter is divided into three funiculi (columns): posterior, lateral, and anterior.
• Each funiculus contains several fiber tracts.
• Fiber tract names reveal their origin and destination.
• Fiber tracts are composed of axons with similar functions.

Cerebral Tracts

• White matter is on the interior of the cerebrum, below the cortex.
• Cerebrum contains projection, association, and commissural tracts.

Projection Tracts

• Projection tracts are extensions of tracts of spinal cord and brainstem (ascending & descending).
• An example of an ascending (sensory) tract is the spinothalamic tract.
• An example of a descending (motor) tract is the corticospinal tract.

Association Tracts

• Association tracts extend from one convolution to another convolution in the same hemisphere.
• Association tracts are most numerous.

Commissural Tracts

• Commissural tracts extend from one convolution to a corresponding convolution in the opposite hemisphere.
• These tracts compose the Corpus Callosum.

Spinal Nerves

• Spinal nerves extend from the spinal cord and emerge from the spinal cavity.
• There are 31 pairs of spinal nerves.
• These nerves are numbered according to the vertebral column level from which they emerge.
• Letters in the numbering indicate the spinal column region.
• Numbers indicate the nerve pair within that region.

Spinal Nerves Details

• C1-C8: Cervical Nerves
  • C1-C7 exit above each respective vertebra.
  • C8 exits below the C7 vertebra.
• T1-T12: Thoracic Nerves
  • All exit below each respective vertebra.
• L1-L5: Lumbar Nerves
  • All exit below each respective vertebrae.
• S1-S5: Sacral Nerves
  • Exit through openings in the sacrum.
• There is one pair of Coccygeal Nerves that passes over the coccyx.

Cauda Equina

• The spinal cord ends at the L1-L2 region.
• Lumbar, sacral, and coccygeal nerves "hang" in a bundle at the bottom of the spinal column before exiting at their respective locations.

Rami

• The large branches of a spinal nerve are referred to as rami.
• Each spinal nerve has two rami: dorsal and ventral.
  • Dorsal rami innervate muscles/skin of posterior surface of head/neck/trunk.
  • Ventral rami innervate muscles/glands in extremities and lateral/ventral portions of the head/neck.

Nerve Plexuses

• Nerve plexuses are complex networks of nerve fibers formed by most ventral rami (excluding T2-T12).
• These are formed when the Ventral Rami of several spinal nerves exchange fibers.
• Fibers recombine to form new nerves that supply specific areas

Cervical Plexus

• The cervical plexus is found deep within the neck.
• It is formed by ventral rami of C1-C4 and a branch of ventral ramus of C5.
• It supplies nerves to the neck, upper shoulders, and part of the head.
• The phrenic nerve exits this plexus innervates the diaphragm.

Brachial Plexus

• The Brachial Plexus is located deep within the shoulder.
• It is formed by the ventral rami of C5-T1.
• This provides nerve supply to the lower shoulders and arms.

Lumbosacral Plexus

• The Lumbosacral Plexus contains lumbar, sacral, and coccygeal plexuses and it provides nerve supply to the lower trunk and the legs.
• Lumbar Plexus
  • Formed by the ventral rami of L1-L4
  • Innervates thigh and leg
• Sacral Plexus
  • Formed by the ventral rami of L4, L5, & S1-S4
  • Sciatic nerve (largest in body) exits here
• Coccygeal Plexus
  • Formed by the ventral rami of S4, S5 and coccygeal nerves
  • Innervates skin over coccyx

Dermatome Map

• This information shows the levels of the spinal column to correlate with regions of the body.

Peripheral Nervous System

• The Peripheral Nervous System (PNS) includes all neural structures outside the brain and spinal cord.
• It contains sensory receptors, peripheral nerves, associated ganglia, and motor endings.
• Provides a link to the external environment

Structure of a Nerve

• Nerve is a cord-like organ of the PNS consisting of peripheral axons enclosed by connective tissue.
• Connective tissues incllude:
  • Endoneurium – loose connective tissue that surrounds axons
  • Perineurium - coarse connective tissue that bundles fibers into fascicles
  • Epineurium – tough fibrous sheath around a nerve

Classification of Nerves

• Sensory and motor divisions
• Sensory (afferent) – carries impulse to the CNS
• Motor (efferent) – carries impulses from CNS
• Mixed: sensory and motor fibers carry impulses to and from CNS

Peripheral Nerves

• Mixed nerves - carry somatic and autonomic (visceral) impulses.
• There are four types of mixed nerves: Somatic afferent, somatic efferent, Visceral afferent, and visceral efferent.
• Peripheral nerves originate from the brain or spinal column

Regeneration of Nerve Fibers

• Damage to nerve tissue is serious because mature neurons are amitotic and cannot replace themselves.
• Damage can be repaired if the soma of the damaged nerve remains intact.
• Regeneration process:
  • Schwann cells form a regeneration tube and secrete growth factors.
  • Macrophages remove debris.
• Most CNS fibers never regenerate, and damage is considered irreversible, due to the oligodendrocytes because they release growth-inhibiting proteins.

Reflexes

• A reflex is a rapid, predictable motor response to a stimulus.
• Reflexes may be inborn (intrinsic) or learned (acquired). -Relaxes may involve only peripheral nerves and the spinal cord, or higher brain centers

Reflex Arc Compenents

• Receptor: site of stimulus
• Sensory neuron transmits the afferent impulse to the CNS
• Integration center: monosynaptic or polysynaptic region within the CNS
• Motor neuron conducts efferent impulses from the integration center to an effector.
• Effector: muscle fiber or gland that responds to the efferent impulse

Stretch and Deep Tendon Reflexes

• The Golgi tendon organs (proprioceptors) inform the brain as to the state of the muscle to perform normally.
• Stretch reflexes are initiated by muscle spindles to maintain healthy muscle tone.