Peripheral Nervous System

    1. The Peripheral Nervous System

      Overview

      • Comprises all neural structures outside the brain and spinal cord.

      • Links the body to the brain and spinal cord.

      • Consists of:

        • Cranial Nerves: 12 pairs originating from the brain.

        • Spinal Nerves: 31 pairs originating from the spinal cord.

      • Contains bundles of sensory (afferent) and motor (efferent) neurons bound together by connective tissue.

      Functional Components of the PNS

      • Sensory Inputs and Motor Outputs

        • Classified as somatic or visceral.

        • Sensory inputs further classified as general (e.g., pain, temperature) or special (e.g., sight, taste).

      Nervous System Structure

      Central Nervous System (CNS)

      • Composed of the brain and spinal cord.

      • Integrates and processes sensory data and motor commands.

      Peripheral Nervous System (PNS)

      • Divisions of PNS:

        • Autonomic Nervous System (ANS): Controls involuntary functions (heart rate, digestion).

          • Divided into sympathetic (arousing) and parasympathetic (calming).

        • Somatic Nervous System (SNS): Controls voluntary movements by communicating with skeletal muscles.

      Neural Tissue Organization

      • Ganglia: Clusters of nerve cell bodies outside CNS.

      • Nerves: Bundles of axons supported by connective tissue, including:

        • Spinal nerves (to/from spinal cord)

        • Cranial nerves (to/from brain).

      Neurons

      Structure of Neurons

      • Basic unit of the nervous system: specialized for communication.

      • Divided into four anatomical regions:

        1. Cell Body (Soma): Contains the nucleus; responsible for basic cell functions.

        2. Dendrites: Highly branched areas that receive signals from other neurons.

    2. Axon: Long fiber that transmits signals away from the cell body.

    3. Axon Terminals: Endpoints of axons that release neurotransmitters.

Axonal Transport

  • Neurons as secretory cells; transport occurs along microtubules using motors (dynein, kinesin).

    • Anterograde transport: From cell body to axon terminals.

    • Retrograde transport: From axon terminals to cell body; involved in recycling synaptic vesicles and signaling molecules.

Classification of Neurons

Structural Classification

  • Neurons categorized by the number of processes:

    • Unipolar Neurons: One process (servo one segment for reception & transmission).

    • Bipolar Neurons: Two processes; specialized sensory functions.

    • Multipolar Neurons: One axon and multiple dendrites (most common type).

Functional Classification

  • Afferent Neurons: Convey sensory information to the CNS.

  • Efferent Neurons: Carry motor commands from CNS to muscles/glands.

  • Interneurons: Integrate signals within the CNS, accounting for 99% of neurons.

Glial Cells

Role of Glial Cells

  • Comprise approximately 90% of CNS; provide support, maintenance, and protection for neurons.

  • Functions include:

    • Maintaining ionic balance around neurons.

    • Modulating nerve signal propagation and synaptic functions.

    • Providing structural support during neural development.

Types of Glial Cells

  1. Microglia: Act as immune cells in the CNS.

  2. Astrocytes: Support neurons, regulate blood-brain barrier, involved in neurotransmitter uptake.

  3. Oligodendrocytes: Myelinate CNS axons.

  4. Schwann Cells: Myelinate PNS axons.

  5. Ependymal Cells: Line cavities of the brain; involved in CSF production.

Nerve Fibers and Conduction Velocity

  • Classifications based on conduction velocity connected to fiber diameter and myelination:

    • Larger and myelinated fibers conduct signals faster.

    • Nerve fibers categorized as:

      • A fibers: Large, myelinated, fast (e.g., motor neurons).

      • B fibers: Intermediate size and speed (autonomic functions).

      • C fibers: Small, unmyelinated, slow (pain signals).

Cranial Nerves

  • 12 pairs, primarily serve head and neck structures; vagus nerve extends into the abdomen.

  • Functions and locations include:

    • CN I: Olfactory, sensory (smell).

    • CN II: Optic, sensory (vision).

    • CN III: Oculomotor, motor (eye movements).

    • CN V: Trigeminal, mixed (facial sensation).

    • CN X: Vagus, mixed (autonomic control).

Spinal Cord Structure and Function

  • Extends from skull to first lumbar vertebra, contains 31 pairs of spinal nerves.

  • Functions include:

    1. Sensory and motor innervation.

    2. Two-way conduction pathway between body and brain.

    3. Major reflex center.

Spinal Nerve Organization

  • Sensory axons enter via dorsal roots; motor axons exit via ventral roots.

  • Contains sensory receptors from the skin, joints, and muscles.