The Nervous System: Structural and Organizational Characteristics

Central and Peripheral Nervous System

The nervous system is divided into two main structural components: the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS consists of the brain and spinal cord, which serve as processing centers for sensory input and motor output. The PNS includes cranial nerves and spinal nerves, responsible for transmitting sensory information to the CNS and carrying motor commands to effectors throughout the body.

• CNS: Brain and spinal cord; processing centers.

• PNS: Cranial nerves and spinal nerves; sensory input and motor response pathways.

• Reflexes: Quick, automatic responses triggered by specific stimuli. Spinal reflexes are controlled by the spinal cord alone, without input from the brain.

The Spinal Cord: Structure and Functions

Gross Anatomy and Regions

The spinal cord is an organ made up of nervous tissue, housed within protective membranes called meninges and the vertebral column. It carries sensory and motor information between the brain and most other parts of the body and is the origin of spinal nerves.

• Length: ~45 cm (18 in.), width: ~14 mm (0.55 in.)

• Extends from the brain to vertebrae L1 and L2

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

• Four regions: Cervical, Thoracic, Lumbar, Sacral

Spinal Cord Segments and Naming

The spinal cord is divided into 31 segments, each giving rise to a pair of spinal nerves. Spinal nerves are named according to their vertebral region and number.

• C1 runs above the first cervical vertebra; C8 is below the seventh cervical vertebra.

• All other spinal nerves are named for the vertebra above.

• Grooves: Posterior median sulcus (posterior side), Anterior median fissure (anterior groove)

• Central canal: Internal passageway containing cerebrospinal fluid (CSF)

Enlargements and Distal End

Areas of the spinal cord that supply the limbs have more gray matter and are visibly wider, known as enlargements. The distal end of the spinal cord features the conus medullaris, cauda equina, and filum terminale.

• 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 below lumbar enlargement.

• Cauda equina: Nerve roots extending below conus medullaris.

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

Spinal Roots, Ganglia, and Nerves

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

• Spinal nerve roots divide into rootlets before entering/leaving the spinal cord.

• Spinal nerves are mixed nerves (contain both afferent and efferent fibers).

• Branches: White ramus communicans (myelinated axons), Gray ramus communicans (unmyelinated fibers), Posterior ramus (skin/muscles of back), Anterior ramus (trunk and limbs).

Spinal Meninges: Protective Membranes

Three Layers of Meninges

The spinal cord is surrounded by three specialized membranes called meninges, which provide physical stability, shock absorption, and a pathway for blood vessels.

• Dura mater: Tough, outer fibrous layer; continuous with cranial dura mater.

• Arachnoid mater: Middle layer; includes arachnoid membrane and trabeculae.

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

• Meningitis: Infection of the meninges (viral or bacterial).

Gray Matter and White Matter: Roles and Organization

Gray Matter

Gray matter consists of cell bodies of neurons, neuroglia, and unmyelinated axons. It integrates information and initiates commands, and is located deep within the spinal cord.

• Organized into nuclei and horns (posterior, anterior, lateral).

• Posterior horns: Somatic and visceral sensory nuclei.

• Anterior horns: Somatic motor nuclei.

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

• Gray commissure: Narrow band around central canal.

White Matter

White matter consists of myelinated and unmyelinated axons, carrying information from place to place. It is organized into three columns: posterior, anterior, and lateral.

• Tract: Bundle of axons in the CNS, relaying the same type of information in the same direction.

• Ascending tracts: Sensory information toward the brain.

• Descending tracts: Motor commands down to the spinal cord.

Spinal Nerves and Plexuses

Connective Tissue Layers

Spinal nerves are surrounded by three connective tissue layers: epineurium (outermost), perineurium (middle, separates nerve into fascicles), and endoneurium (innermost, surrounds individual axons).

Peripheral Distribution and Dermatomes

Each spinal nerve monitors a specific bilateral region of skin called a dermatome. Damage or infection of a spinal nerve or ganglion produces loss of sensation in the corresponding dermatome.

• Peripheral neuropathies: Regional losses of sensory and motor function, often from trauma or compression.

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

Nerve Plexuses

A nerve plexus is a complex, interwoven network of nerve fibers formed from blended fibers of the anterior rami of adjacent spinal nerves. Four major nerve plexuses supply the body:

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

• Brachial plexus: C5–T1; innervates pectoral girdle, upper back, upper limbs.

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

• Sacral plexus: L4–S4; innervates gluteal region, posterior thigh, leg, foot.

Neuronal Pools and Neural Circuits

Patterns of Interaction

Neuronal pools are functional groups of interconnected interneurons. They may stimulate or depress parts of the CNS and are organized into specific patterns:

• Divergence: Spreads information from one neuron to many (common in sensory pathways).

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

• Serial processing: Information moves sequentially through neurons (e.g., pain signals).

• Parallel processing: Several neurons process the same information simultaneously (e.g., stepping on a sharp object).

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

Reflexes: Steps and Classification

Reflex Arc

A neural reflex is a rapid, automatic response to specific stimuli, helping preserve homeostasis. The reflex arc is the route followed by nerve impulses to produce a reflex, usually operating by negative feedback.

• Steps: 1) Stimulus activates receptor; 2) Sensory neuron activation; 3) CNS processing; 4) Motor neuron activation; 5) Response by effector.

Classification of Reflexes

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

• Motor response: Somatic (muscular system) or visceral (autonomic, internal organs).

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

• Site of processing: Spinal (spinal cord) or cranial (brain).

Spinal Reflexes: Types and Interactions

Monosynaptic Spinal Reflexes

The stretch reflex regulates skeletal muscle length and is very rapid. The muscle spindle is the sensory receptor involved, made of intrafusal muscle fibers and surrounded by extrafusal fibers.

• Example: Patellar reflex (knee-jerk)

• Gamma motor neurons adjust spindle sensitivity

Polysynaptic Reflexes

Polysynaptic reflexes allow for more complex responses, involving interneurons that control multiple muscle groups. Examples include tendon reflex, withdrawal reflexes, and crossed-extensor reflexes.

• Tendon reflex: Prevents excessive tension and tendon damage (Golgi tendon organs).

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

• Crossed extensor reflex: Coordinates with flexor reflexes to maintain balance.

Characteristics of Polysynaptic Reflexes

• Involve pools of interneurons (excitation/inhibition)

• Involve more than one spinal segment

• Involve reciprocal inhibition

• Have reverberating circuits

• Several reflexes may cooperate for coordinated response

Brain Influence on Spinal Reflexes

Integration and Control

Higher brain centers can facilitate or inhibit spinal reflex motor patterns. Voluntary movements and reflex motor patterns can be activated or fine-tuned by the brain through descending pathways.

• Reinforcement: Excitatory synapses enhance reflexes.

• Inhibition: Inhibitory neurons suppress reflexes.

Plantar and Babinski Reflexes

The plantar reflex (toe-curling) is normal in adults, while the Babinski reflex (toe fanning) is normal in infants but pathological in adults, indicating CNS damage.

Summary Table: Spinal Cord Regions and Functions

Additional info: The notes above expand on brief points from the original slides, providing definitions, examples, and context for each major topic. Images are included only when directly relevant to the explanation, reinforcing anatomical and physiological concepts.