Chapter 13: The Spinal Cord, Spinal Nerves, and Spinal Reflexes – Study Guide

Overview of the Nervous System

Structural and Organizational Characteristics

The nervous system is divided into two main parts: the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS consists of the brain and spinal cord, serving as processing centers, while the PNS includes cranial and spinal nerves, responsible for sensory input and motor response pathways. Reflexes are quick, automatic responses to stimuli, with spinal reflexes controlled by the spinal cord alone.

• Central Nervous System (CNS): Brain and spinal cord; processing and integration centers.

• Peripheral Nervous System (PNS): Cranial nerves and spinal nerves; pathways for sensory input and motor output.

• Reflexes: Automatic responses; spinal reflexes do not require brain input.

Structure and Function of the Spinal Cord

Gross Anatomy

The spinal cord is an organ made of nervous tissue, protected by meninges and the vertebral column. It carries sensory and motor information between the brain and most parts of the body and is the origin of spinal nerves.

• Length: About 45 cm (18 in.) long; maximum width 14 mm (0.55 in.).

• Regions: Cervical, thoracic, lumbar, and sacral.

• Gray Matter: Neuronal cell bodies; deep in the cord.

• White Matter: Myelinated axons; superficial in the cord.

Spinal Cord Segments and Enlargements

The spinal cord has 31 segments, each giving rise to a pair of spinal nerves. Areas supplying the limbs have more gray matter and are visibly wider (cervical and lumbosacral enlargements).

• Cervical Enlargement: Supplies nerves to shoulder and upper limb.

• Lumbosacral Enlargement: Supplies nerves to pelvis and lower limb.

Distal End and Spinal Roots

• Conus Medullaris: Tapered end below lumbar enlargement.

• Cauda Equina: Nerve roots extending below conus medullaris.

• Filum Terminale: Thin thread of fibrous tissue attaching to coccygeal ligament.

Spinal Roots and Ganglia

Spinal nerves attach to the spinal cord via two roots: anterior (motor) and posterior (sensory). Spinal ganglia (dorsal root ganglia) contain cell bodies of sensory neurons.

• Anterior Root: Motor neuron axons.

• Posterior Root: Sensory neuron axons.

• Spinal Ganglia: Cell bodies of sensory neurons.

Spinal Nerves and Branches

Spinal nerves are mixed nerves, containing both sensory and motor fibers. They branch into rami, which innervate different regions.

• White Ramus Communicans: Myelinated axons to glands and smooth muscle.

• Gray Ramus Communicans: Unmyelinated fibers to glands and smooth muscle.

• Posterior Ramus: Skin/muscles of the back.

• Anterior Ramus: Ventrolateral trunk and limbs.

Spinal Meninges

Three Layers

Specialized membranes surround the spinal cord, providing stability, shock absorption, and a pathway for blood vessels.

• Dura Mater: Tough, outer layer; dense collagen fibers; continuous with cranial dura mater.

• Arachnoid Mater: Middle layer; weblike membrane and trabeculae.

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

Spaces Associated with Meninges

• Epidural Space: Between vertebrae and dura mater; contains loose connective and adipose tissue.

• Subdural Space: Potential space deep to dura mater.

• Subarachnoid Space: Between arachnoid and pia mater; filled with cerebrospinal fluid (CSF).

Gray Matter and White Matter

Roles in Processing and Relaying Information

• Gray Matter: Cell bodies, neuroglia, unmyelinated axons; integrates information and initiates commands.

• White Matter: Myelinated and unmyelinated axons; carries information between regions.

Organization of Gray Matter

• Nuclei: Masses of gray matter; organized into horns.

• Posterior Horns: Somatic and visceral sensory nuclei.

• Anterior Horns: Somatic motor nuclei.

• Lateral Horns: Visceral motor nuclei (thoracic/lumbar).

• Gray Commissure: Band around central canal.

Organization of White Matter

• Columns: Posterior, anterior, and lateral white columns.

• Tracts: Bundles of axons; ascending (sensory) and descending (motor).

Spinal Nerves and Plexuses

Connective Tissue Layers

Spinal nerves are surrounded by three connective tissue layers: epineurium, perineurium, and endoneurium.

• Epineurium: Outermost; collagen fibers.

• Perineurium: Middle; separates nerve into fascicles.

• Endoneurium: Innermost; surrounds individual axons.

Dermatomes and Peripheral Neuropathies

• Dermatome: Region of skin monitored by a single pair of spinal nerves.

• Peripheral Neuropathies: Regional loss of sensory/motor function.

• Shingles: Varicella-zoster virus attacks spinal nerves, causing rash along dermatomes.

Nerve Plexuses

Nerve plexuses are complex networks formed from the anterior rami of adjacent spinal nerves, allowing multiple nerves to supply the same structures. Four major plexuses: cervical, brachial, lumbar, and sacral.

• Cervical Plexus: C1–C5; innervates neck, scalp, diaphragm.

• Brachial Plexus: C5–T1; innervates pectoral girdle, upper limb.

• Lumbar Plexus: T12–L4; innervates abdominal muscles, thigh.

• Sacral Plexus: L4–S4; innervates gluteal region, leg, foot.

Neuronal Pools and Neural Circuits

Functional Organization

Neuronal pools are groups of interconnected interneurons with limited input and output sources. They may stimulate or depress parts of the CNS.

• Sensory Neurons: Bring information to CNS.

• Motor Neurons: Send commands from CNS to effectors.

• Interneurons: Interpret, plan, and coordinate signals.

Patterns of Neural Circuits

• Divergence: One neuron spreads information to many; common in sensory pathways.

• Convergence: Several neurons synapse on a single neuron; e.g., control of diaphragm.

• Serial Processing: Information moves sequentially; e.g., pain signals.

• Parallel Processing: Several neurons process same information simultaneously.

• Reverberation: Positive feedback loop; maintains consciousness, breathing, coordination.

Reflexes and Reflex Arcs

Neural Reflexes

Reflexes are rapid, automatic responses to stimuli, helping preserve homeostasis. The reflex arc is the route nerve impulses follow to produce a reflex.

• Steps in Reflex Arc:

  1. Stimulus activates receptor.

  2. Activation of sensory neuron.

  3. Information processing in CNS.

  4. Activation of motor neuron.

  5. Response by effector.

Classification of Reflexes

• Development: Innate (inborn) or acquired (learned).

• Motor Response: Somatic (muscular) or visceral (autonomic).

• Complexity: Monosynaptic (single synapse) or polysynaptic (multiple synapses).

• Site of Processing: Spinal or cranial.

Spinal Reflexes

Monosynaptic Reflexes

• Stretch Reflex: Regulates skeletal muscle length; very rapid; example: patellar reflex.

• Muscle Spindle: Sensory receptor involved in stretch reflex; detects muscle stretch.

Polysynaptic Reflexes

• Tendon Reflex: Prevents excessive tension; involves Golgi tendon organs.

• Withdrawal Reflex: Moves body part away from stimulus; involves reciprocal inhibition.

• Crossed Extensor Reflex: Contralateral response; supports body weight during withdrawal.

Higher Brain Centers and Reflex Control

Modification of Reflex Responses

Higher brain centers can facilitate or inhibit spinal reflexes, adjusting sensitivity and motor patterns.

• Reinforcement: Enhances reflexes via excitatory interneurons.

• Inhibition: Suppresses reflexes via inhibitory interneurons.

Plantar and Babinski Reflexes

• Plantar Reflex: Toe-curling; normal in adults.

• Babinski Reflex: Toe-fanning; normal in infants, pathological in adults.

Summary Table: Spinal Cord Regions and Functions

Region	Main Function
Cervical	Neck, shoulders, upper limbs
Thoracic	Chest, abdominal muscles
Lumbar	Lower back, legs
Sacral	Pelvis, lower limbs

Key Equations and Concepts

• Action Potential Generation: $\text{Threshold stimulus} \rightarrow \text{Depolarization} \rightarrow \text{Action potential}$

• Reflex Arc Pathway: $\text{Receptor} \rightarrow \text{Sensory neuron} \rightarrow \text{Interneuron} \rightarrow \text{Motor neuron} \rightarrow \text{Effector}$

Additional info:

• Clinical notes on shingles and carpal tunnel syndrome highlight the importance of spinal nerves in disease.

• Mapping dermatomes and peripheral nerve function is essential for diagnosing nerve damage.