Nervous System Overview

Principal Divisions of the Nervous System

The nervous system is divided into two main parts: the central nervous system (CNS) and the peripheral nervous system (PNS). Each division has distinct structures and functions essential for body regulation and response to stimuli.

• CNS (Central Nervous System): Consists of the brain and spinal cord. It integrates and coordinates incoming sensory information and issues instructions. The CNS is connected to sensory receptors, muscles, and glands via the PNS.

• PNS (Peripheral Nervous System): Composed of cranial and spinal nerves that emerge from the CNS. These nerves carry impulses to and from the CNS, connecting it to the rest of the body. The PNS includes sensory (afferent) and motor (efferent) neurons.

PNS Subdivisions:

• Somatic Nervous System: Controls voluntary movements of skeletal muscles.

• Autonomic Nervous System (ANS): Regulates involuntary functions in smooth muscle, cardiac muscle, and glands. The ANS is further divided into:

  • Sympathetic Division: Prepares the body for 'fight or flight' responses.

  • Parasympathetic Division: Promotes 'rest and digest' activities.

Example: The somatic system allows you to move your arm, while the autonomic system controls your heart rate.

Basic Components of the Peripheral Nervous System

Structure and Function

The PNS links the CNS to the external environment, enabling the body to receive information and carry out decisions. It includes all neural structures outside the brain and spinal cord, such as sensory receptors, nerves, ganglia, and motor endings.

Sensory Receptors

Classification by Location, Stimulus Detected, and Structure

Sensory receptors are specialized structures that detect changes in the environment (stimuli) and trigger nerve impulses along afferent fibers to the CNS.

• Location:

  • Exteroceptors: Respond to stimuli outside the body (e.g., touch, pressure, pain, temperature).

  • Interoceptors: Respond to stimuli within the body (e.g., chemical changes, tissue stretch).

  • Proprioceptors: Respond to internal stimuli in muscles, tendons, joints, and ligaments; monitor body position and movement.

• Stimulus Detected:

  • Mechanoreceptors: Detect touch, pressure, vibration, stretch.

  • Thermoreceptors: Detect temperature changes.

  • Photoreceptors: Detect light (in the eye).

  • Chemoreceptors: Detect chemicals in solution (e.g., taste, smell, blood chemistry).

  • Nociceptors: Detect potentially damaging stimuli (pain).

• Structural Complexity:

  • Simple Receptors: General senses (touch, pressure, pain, temperature, proprioception).

  • Complex Receptors: Special sense organs (vision, hearing, smell, taste).

Anatomy of General Sensory Receptors

• Free Nerve Endings: Abundant in epithelial and connective tissues; respond to pain, temperature, and some touch.

• Encapsulated Nerve Endings: Terminal fibers enclosed in connective tissue capsules; include mechanoreceptors such as Meissner's corpuscles (touch), Pacinian corpuscles (deep pressure), muscle spindles (muscle stretch), and Golgi tendon organs (tendon stretch).

Nerves and Nerve Structure

Organization of Nerves

Nerves are bundles of peripheral axons enclosed by connective tissue. Each nerve contains parallel bundles of fibers, blood vessels, and lymphatics. The connective tissue layers are:

• Endoneurium: Surrounds individual axons.

• Perineurium: Surrounds bundles of axons (fascicles).

• Epineurium: Surrounds the entire nerve.

Nerves are classified as:

• Mixed Nerves: Contain both sensory and motor fibers.

• Sensory (Afferent) Nerves: Carry impulses toward the CNS.

• Motor (Efferent) Nerves: Carry impulses away from the CNS.

Fibers can also be classified by region: somatic afferent/efferent (body surface/skeletal muscle) and visceral afferent/efferent (internal organs).

Regeneration of Nerve Fibers

Process and Limitations

Damage to nervous tissue is serious because mature neurons do not divide. If the damage is severe, the entire neuron may die. Peripheral axons can regenerate if the cell body is intact, but CNS axons do not regenerate.

• After injury, Schwann cells proliferate and form a regeneration tube to guide axon regrowth.

• Axons regenerate at about 1-5 mm/day. Greater distance between severed ends reduces the chance of full recovery.

• Within the CNS, regeneration is extremely limited; damage is usually permanent.

Example: Peripheral nerve injuries (e.g., in the arm) may recover function, but spinal cord injuries typically do not.

Motor Endings

Somatic and Autonomic Motor Endings

Motor endings are structures that activate effectors (muscles and glands). Somatic motor endings form neuromuscular junctions with skeletal muscle fibers, while autonomic motor endings form simpler junctions with smooth muscle, cardiac muscle, and glands.

• Somatic Motor Endings: Release neurotransmitter (acetylcholine) to stimulate muscle contraction.

• Autonomic Motor Endings: Release neurotransmitters at varicosities along the axon.

Cranial Nerves

Number, Function, and Classification

There are twelve pairs of cranial nerves, each with specific sensory and/or motor functions. They are numbered I-XII from anterior to posterior.

Additional info: Cranial nerves I, II, and VIII are purely sensory; the rest are mixed or primarily motor.

Spinal Nerves

Number and Distribution

There are 31 pairs of spinal nerves, each formed by the fusion of dorsal and ventral roots from the spinal cord. All are mixed nerves (sensory and motor fibers).

The first 7 cervical nerves exit above their corresponding vertebrae; C8 exits below C7. All others exit below their corresponding vertebrae.

Plexuses

Plexuses are networks of nerves formed by the ventral rami of spinal nerves (except T2-T12). Major plexuses include:

• Cervical Plexus: Supplies the neck and diaphragm (phrenic nerve).

• Brachial Plexus: Supplies the upper limb (axillary, musculocutaneous, radial, median, ulnar nerves).

• Lumbar Plexus: Supplies the anterior and medial thigh (femoral, obturator nerves).

• Sacral Plexus: Supplies the posterior thigh, leg, and foot (sciatic nerve).

Example: Injury to the phrenic nerve can cause paralysis of the diaphragm, affecting breathing.

Summary Table: Sensory Receptor Classification

Additional info: This guide covers the essential structure and function of the nervous system, including sensory and motor pathways, nerve regeneration, and the organization of cranial and spinal nerves.