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, which serve as processing centers. The PNS includes cranial and spinal nerves, responsible for sensory input and motor response pathways.

• Central Nervous System (CNS): Brain and spinal cord; processes information.

• Peripheral Nervous System (PNS): Cranial nerves and spinal nerves; transmits sensory and motor signals.

• Reflexes: Quick, automatic responses to stimuli; spinal reflexes are controlled by the spinal cord without brain input.

The Spinal Cord

Structure and Functions

The spinal cord is a nervous tissue organ housed within protective membranes (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.

• Regions: Cervical, thoracic, lumbar, sacral.

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

• Gray Matter: Contains neuronal cell bodies; integrates information.

• White Matter: Contains myelinated axons; transmits information.

• Central Canal: Contains cerebrospinal fluid (CSF).

Enlargements and Distal End

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

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

• Conus Medullaris: Tapered end below lumbar enlargement.

• Cauda Equina: Nerve roots extending below conus medullaris.

• Filum Terminale: Fibrous tissue thread attaching to coccygeal ligament.

Spinal Roots and Ganglia

• Anterior (Ventral) Root: Motor neuron axons.

• Posterior (Dorsal) Root: Sensory neuron axons.

• Spinal Ganglia: Cell bodies of sensory neurons.

Spinal Nerves

• Formed by union of anterior and posterior roots.

• Mixed nerves: contain both afferent (sensory) and efferent (motor) fibers.

• Branches: white ramus communicans (myelinated), gray ramus communicans (unmyelinated), posterior ramus (back), anterior ramus (trunk and limbs).

Spinal Meninges

Specialized membranes surrounding 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; includes arachnoid membrane and trabeculae.

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

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

Gray Matter and White Matter

Roles in Processing and Relaying Information

Gray matter integrates information and initiates commands, while white matter carries information between different regions.

• Gray Matter: Cell bodies, neuroglia, unmyelinated axons; deep in spinal cord.

• White Matter: Myelinated and unmyelinated axons; superficial in spinal cord.

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, lateral.

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

Spinal Nerves and Plexuses

Components and Distribution

Spinal nerves are surrounded by three connective tissue layers and branch to form peripheral nerves. Nerve plexuses are networks formed from the blending of anterior rami of adjacent spinal nerves.

• Epineurium: Outermost layer; collagen fibers.

• Perineurium: Middle layer; 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 losses of function due to trauma or compression.

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

Nerve Plexuses

• Cervical Plexus: Innervates neck, scalp, diaphragm.

• Brachial Plexus: Innervates pectoral girdle, upper back, upper limbs.

• Lumbar Plexus: Innervates abdominal muscles, thigh, genitalia.

• Sacral Plexus: Innervates gluteal muscles, posterior thigh, leg, perineum.

Neuronal Pools

Patterns of Interaction

Neuronal pools are functional groups of interconnected interneurons. They process and coordinate signals from and to the periphery, using various circuit patterns.

• Divergence: Spreads information from one neuron to many.

• Convergence: Several neurons synapse on a single neuron.

• Serial Processing: Information moves sequentially through neurons.

• Parallel Processing: Several neurons process the same information simultaneously.

• Reverberation: Positive feedback loop maintains activity.

Reflexes

Steps and Classification

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

• Stimulus activates a receptor.

• Activation of a sensory neuron.

• Information processing in CNS (interneurons).

• Activation of a motor neuron.

• Response by a peripheral effector.

Classification of Reflexes

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

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

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

• Processing Site: Spinal or cranial.

Spinal Reflexes

Types and Motor Responses

Spinal reflexes can be monosynaptic or polysynaptic, producing various motor responses. Reflexes interact to produce complex behaviors.

• Monosynaptic Reflexes: Fast, single synapse; e.g., stretch reflex (patellar reflex).

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

• Postural Reflexes: Maintain upright posture; involve multiple muscle groups.

• Polysynaptic Reflexes: More complex; involve interneurons and multiple muscle groups.

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

• Withdrawal Reflexes: Move body part away from stimulus; reciprocal inhibition ensures antagonistic muscles relax.

• Crossed Extensor Reflexes: Contralateral reflex arcs; coordinate with flexor reflexes for balance.

Brain Control of Spinal Reflexes

Modification by Higher Centers

Although reflex behaviors are automatic, higher brain centers can facilitate, inhibit, or fine-tune spinal reflex motor patterns. This allows voluntary movements to activate reflex motor patterns and adjust their sensitivity.

• Reinforcement: Excitatory synapses enhance reflexes.

• Inhibition: Inhibitory neurons suppress reflexes.

• Plantar Reflex: Normal toe-curling in adults.

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

Summary Table: Classification of Reflexes