Chapter 13: The Spinal Cord, Spinal Nerves, and Spinal Reflexes

Overview

This chapter explores the structure and function of the spinal cord, spinal nerves, and the mechanisms underlying spinal reflexes. It also covers the organization of the nervous system, the roles of white and gray matter, and the integration of reflexes by higher brain centers.

The Nervous System

Structural and Organizational Characteristics

• Central Nervous System (CNS): Consists of the brain and spinal cord. Acts as the main processing center for the entire nervous system.

• Peripheral Nervous System (PNS): Composed of cranial nerves and spinal nerves. Responsible for transmitting sensory input to the CNS and carrying out motor responses.

Reflexes are quick, automatic responses to specific stimuli. Spinal reflexes are controlled by the spinal cord, often without input from the brain.

The Spinal Cord

Structure and Function

• The spinal cord is an organ made of nervous tissue, protected by membranes called meninges and the vertebral column.

• It serves as a conduit for sensory and motor information between the brain and the rest of the body.

• The spinal cord is the origin of spinal nerves.

Gross Anatomy

• Length: Approximately 18 inches (45 cm); maximum width: 0.55 inches (14 mm).

• Extends from the brain to the level of vertebrae L1 and L2.

• Contains gray matter (neuronal cell bodies) and white matter (myelinated axons).

• Divided into four regions: cervical, thoracic, lumbar, and sacral.

• 31 segments, each giving rise to a pair of spinal nerves.

• Features bilateral symmetry, with grooves dividing the cord into left and right halves: the posterior median sulcus (posterior) and anterior median fissure (anterior).

• The central canal runs longitudinally, containing cerebrospinal fluid (CSF).

Enlargements and Terminal Structures

• Cervical enlargement: Supplies nerves to the shoulder and upper limb.

• Lumbosacral enlargement: Supplies nerves to the pelvis and lower limb.

• Conus medullaris: Tapered, conical end of the spinal cord below the lumbar enlargement.

• Cauda equina: Bundle of nerve roots extending below the conus medullaris, resembling a horse's tail.

• Filum terminale: Thin thread of fibrous tissue anchoring the spinal cord to the coccyx.

Spinal Roots and Ganglia

• Each spinal nerve is attached to the spinal cord by two roots:

  • Anterior (ventral) root: Contains axons of motor neurons.

  • Posterior (dorsal) root: Contains axons of sensory neurons.

• Spinal ganglia (dorsal root ganglia): Contain cell bodies of sensory neurons, located between adjacent vertebrae.

Spinal Nerves

• Formed by the union of anterior and posterior roots; all are mixed nerves (contain both sensory and motor fibers).

• Branches include:

  • White ramus communicans: Myelinated axons to glands and smooth muscle.

  • Gray ramus communicans: Unmyelinated fibers to glands and smooth muscle.

  • Posterior ramus: Innervates skin/muscles of the back.

  • Anterior ramus: Innervates ventrolateral trunk and limbs.

Naming Spinal Nerves

• Named according to their vertebral region and number (e.g., C1 above the first cervical vertebra, T1 below the first thoracic vertebra).

Spinal Meninges

• Specialized membranes providing physical stability, shock absorption, and a pathway for blood vessels.

• Three layers:

  • Dura mater: Tough, outer fibrous layer; continuous with cranial dura mater and fuses with the periosteum of the occipital bone.

  • Arachnoid mater: Middle layer; includes a weblike membrane and trabeculae (collagen/elastic fibers).

  • Pia mater: Innermost layer; mesh of collagen and elastic fibers, firmly attached to neural tissue.

• Epidural space: Between vertebrae and dura mater; contains loose connective and adipose tissue. Site for epidural anesthesia.

• Subarachnoid space: Between arachnoid and pia mater; filled with CSF. Site for lumbar puncture (spinal tap).

• Denticulate ligaments: Extensions of pia mater anchoring the spinal cord laterally.

• Meningitis: Inflammation of the meninges, often due to infection.

Gray Matter and White Matter

Roles in Processing and Relaying Information

• Gray matter: Contains neuron cell bodies, neuroglia, and unmyelinated axons. Integrates information and initiates commands. Located deep in the spinal cord.

• White matter: Contains myelinated and unmyelinated axons. Carries information from place to place. Located superficially in the spinal cord.

Organization of Gray Matter

• Gray matter is organized into nuclei (masses of neuron cell bodies) and horns (regions):

  • Posterior horns: Somatic and visceral sensory nuclei.

  • Anterior horns: Somatic motor nuclei.

  • Lateral horns: Visceral motor nuclei (thoracic and lumbar segments).

• Gray commissure: Band of gray matter around the central canal.

Organization of White Matter

• Divided into columns:

  • Posterior white columns: Between posterior horns and posterior median sulcus.

  • Anterior white columns: Between anterior horns and anterior median fissure.

  • Lateral white columns: On each side between anterior and posterior columns.

• Tracts: Bundles of axons relaying the same type of information in the same direction.

  • Ascending tracts: Carry sensory information to the brain.

  • Descending tracts: Carry motor commands from the brain.

Spinal Nerves and Plexuses

Structure and Distribution

• Each spinal nerve is surrounded by three connective tissue layers:

  • Epineurium: Outermost, dense collagen fibers.

  • Perineurium: Middle, separates nerve into fascicles (bundles of axons).

  • Endoneurium: Innermost, surrounds individual axons.

• Peripheral nerves are formed as spinal nerves branch and interconnect; all are mixed nerves.

Dermatomes and Clinical Relevance

• Dermatome: Specific bilateral region of skin supplied by a single pair of spinal nerves.

• Damage or infection of a spinal nerve or ganglion leads to loss of sensation in the corresponding dermatome.

• Peripheral neuropathies: Regional loss of sensory/motor function due to trauma or compression.

• Shingles: Reactivation of varicella-zoster virus affecting spinal nerves, causing a painful rash along dermatomes.

Nerve Plexuses

• Nerve plexus: Complex, interwoven network of nerve fibers formed from anterior rami of adjacent spinal nerves.

• Allows multiple spinal nerves to supply the same structures.

• Four major plexuses:

  1. Cervical plexus (C1–C5): Innervates neck, scalp, and diaphragm. Major nerve: Phrenic nerve (controls diaphragm).

  2. Brachial plexus (C5–T1): Innervates pectoral girdle and upper limbs. Major nerves: Musculocutaneous, Radial, Median, Ulnar.

  3. Lumbar plexus (T12–L4): Innervates abdominal wall, anterior and medial thigh. Major nerves: Femoral, Obturator.

  4. Sacral plexus (L4–S4): Innervates pelvis, posterior thigh, leg, and foot. Major nerve: Sciatic nerve (divides into tibial and fibular nerves).

Clinical Note: Carpal Tunnel Syndrome

• Compression of the median nerve in the carpal tunnel (anterior wrist) leads to sensory and motor deficits in the hand.

Neuronal Pools

Functional Organization

• Neuronal pool: Functional group of interconnected interneurons with limited input and output destinations.

• May stimulate or depress parts of the CNS.

Patterns of Neural Circuits

• Divergence: One neuron sends information to multiple neurons/pools (common in sensory pathways).

• Convergence: Multiple neurons synapse on a single neuron (e.g., control of diaphragm by both conscious and subconscious pathways).

• Serial processing: Information passes sequentially through a series of neurons/pools (e.g., pain pathways).

• Parallel processing: Several neurons/pools process the same information simultaneously (e.g., withdrawal reflexes).

• Reverberation: Collateral axon branches stimulate presynaptic neurons, forming a positive feedback loop (e.g., maintaining muscle tone).

Reflexes

Neural Reflexes and Reflex Arcs

• Reflex: Rapid, automatic response to a specific stimulus, helping maintain homeostasis.

• Reflex arc: The pathway followed by nerve impulses to produce a reflex, typically involving negative feedback.

Steps in a Reflex Arc

1. Stimulus activates a receptor.

2. Activation of a sensory neuron (action potential generated).

3. Information processing in the CNS (usually via interneurons).

4. Activation of a motor neuron.

5. Response by a peripheral effector (muscle or gland).

Classification of Reflexes

• By development:

  • Innate reflexes: Genetically programmed, present at birth (e.g., withdrawal reflex).

  • Acquired reflexes: Learned through repetition (e.g., braking a car).

• By response type:

  • Somatic reflexes: Control skeletal muscles (e.g., stretch reflex).

  • Visceral (autonomic) reflexes: Control smooth/cardiac muscle or glands.

• By circuit complexity:

  • Monosynaptic: Single synapse (fastest, e.g., patellar reflex).

  • Polysynaptic: Multiple synapses (slower, more complex responses).

• By processing site:

  • Spinal reflexes: Processed in the spinal cord.

  • Cranial reflexes: Processed in the brain.

Spinal Reflexes

Monosynaptic Reflexes

• Stretch reflex: Regulates skeletal muscle length (e.g., patellar reflex).

• Muscle spindle: Sensory receptor detecting muscle stretch; composed of intrafusal fibers innervated by sensory and gamma motor neurons.

• Stretching increases action potential frequency, leading to muscle contraction to prevent overstretching.

• Gamma efferents: Adjust spindle sensitivity during voluntary movement.

Postural Reflexes

• Help maintain upright posture via stretch and complex polysynaptic reflexes.

• Involve multiple muscle groups and constant fine adjustments.

Polysynaptic Reflexes

• Allow for more complex responses, involving interneurons and multiple muscle groups.

• Examples:

  • Tendon reflex: Prevents excessive tension in tendons (involves Golgi tendon organs).

  • Withdrawal reflex: Moves body part away from painful stimulus (e.g., flexor reflex).

  • Crossed extensor reflex: Contralateral response to support body weight during withdrawal.

• Reciprocal inhibition: Inhibition of antagonistic muscles during reflex action.

• Reverberating circuits: Prolong reflexive responses.

Integration and Control by the Brain

Modification of Spinal Reflexes

• Higher brain centers can facilitate or inhibit spinal reflexes via descending pathways.

• Reinforcement: Excitatory input increases reflex sensitivity.

• Inhibition: Inhibitory input suppresses reflexes.

• Examples: Voluntary movements (walking, running) can activate or modulate spinal reflexes.

Clinical Reflexes

• Plantar reflex: Normal in adults; stroking the lateral sole causes toe curling.

• Babinski reflex: Normal in infants; stroking the lateral sole causes toe fanning. In adults, may indicate CNS damage.

Key Equations:

• Ohm's Law (for neural conduction):

• $V = IR$

• Where $V$ is voltage (potential difference), $I$ is current, and $R$ is resistance.

• Conduction velocity (approximate):

• $v = \sqrt{\frac{d}{\tau}}$

• Where $v$ is velocity, $d$ is axon diameter, and $\tau$ is time constant.

• Additional info: These equations are general principles relevant to nerve conduction and action potentials.