Introduction to the Nervous System

• Central Nervous System (CNS) consists of:

  • Spinal Cord

    • Integrates and processes information.

    • Can function with or independently of the brain.

  • Brain

    • Integrates and processes information.

    • Can function with or independently of the spinal cord.

Gross Anatomy of the Spinal Cord

• Features of the Spinal Cord:

  • Consists of cervical enlargement, lumbosacral enlargement, conus medullaris, filum terminale, cauda equina, posterior median sulcus, and anterior median fissure.

  • Contains 31 spinal segments, each with:

    • Dorsal roots

    • Dorsal root ganglia

    • Ventral root

    • Mixed spinal nerves (afferent and efferent fibers).

Spinal Meninges

• Function:

  • Provide protection, physical stability, and shock absorption.

  • Continuous with cranial meninges.

• Layers:

  • Dura Mater:

    • Tough, fibrous outermost layer.

  • Arachnoid Mater:

    • Middle layer, separated from pia mater by the subarachnoid space (contains cerebrospinal fluid).

  • Pia Mater:

    • Innermost layer, directly adjacent to spinal and brain tissue; contains blood vessels.

Structure of Spinal Cord

• Gray Matter:

  • Surrounds a central canal and is composed of somas (cell bodies) and glial cells.

• White Matter:

  • Composed of axons arranged in tracts or columns (ascending and descending tracts).

Spinal Nerves

• Total of 31 pairs:

  • 8 Cervical Nerves

  • 12 Thoracic Nerves

  • 5 Lumbar Nerves

  • 5 Sacral Nerves

  • 1 Coccygeal Nerve.

• Branches of Spinal Nerves:

  • Each nerve has a dorsal (posterior) and ventral (anterior) ramus; additional branches include white and gray rami communicantes, which are essential for autonomic functions.

Nerve Plexuses

• Types of Plexuses:

  • Cervical Plexus (C1-C5): Innervates neck and diaphragm.

  • Brachial Plexus (C5-T1): Innervates upper limbs.

  • Lumbar Plexus (T12-L4): Innervates pelvic girdle and anterior thigh.

  • Sacral Plexus (L4-S4): Innervates lower limbs and pelvic structures.

Nerve Contributions

• Cervical Plexus:

  • Supplies nerves to the neck, chest, and diaphragm (e.g., phrenic nerve).

• Brachial Plexus:

  • Supplies nerves like the radial, median, and ulnar, which control muscles of the upper limb.

• Lumbar Plexus:

  • Includes nerves like the femoral nerve, innervating the thigh muscles; and obturator nerve, involved in adductor muscle control.

• Sacral Plexus:

  • Major nerves include the sciatic nerve, which innervates posterior thigh and lower leg.

Reflexes

• Definition:

  • Immediate involuntary motor response to stimuli.

• Reflex Arc Components:

  1. Activation of a sensory receptor.

  2. Relay of information to the CNS.

  3. Information processing.

  4. Activation of a motor neuron.

  5. Response from the effector.

• Classification of Reflexes:

  • By Development: Innate or acquired.

  • By Processing Site: Spinal or cranial.

  • By Motor Response Nature: Somatic, visceral, or autonomic.

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

Sensory and Motor Tracts in the Spinal Cord

• Sensory Tracts:

  • Ascending Tracts: Carry sensory information to the brain (e.g., spinothalamic tracts for pain).

  • Major sensory tracts include: posterior columns, spinothalamic tract, and spinocerebellar tract.

• Motor Tracts:

  • Descending Tracts: Carry motor commands from the CNS to body.

  • Involves somatic and autonomic nervous systems.

  • Major pathways include:

    • Corticospinal Tracts (conscious control).

    • Subconscious Motor Pathways (e.g., vestibulospinal tracts for balance).

Summary of Major Pathways

• Corticobulbar Tracts: Control facial muscles.

• Lateral Corticospinal Tracts: Control limbs.

• 1Anterior Corticospinal Tracts: Control axial muscles.