Chapter 13: The Peripheral Nervous System

Overview of the Peripheral Nervous System (PNS)

The Peripheral Nervous System (PNS) is a major division of the nervous system that connects the central nervous system (CNS) to limbs and organs. It is essential for transmitting sensory and motor information between the body and the CNS.

• Definition: The PNS consists of all neural structures outside the brain and spinal cord, including nerves and ganglia.

• Main Function: Relays information to and from the CNS, enabling sensation, movement, and autonomic functions.

Divisions of the Peripheral Nervous System

The PNS is divided into two main functional components, each with distinct roles in the body.

• Sensory (Afferent) Division: Transmits sensory information from receptors to the CNS.

• Motor (Efferent) Division: Carries motor commands from the CNS to effectors (muscles and glands).

Motor (Efferent) Division Subdivisions

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

• Autonomic Nervous System (ANS): Regulates involuntary functions, such as heart rate and digestion.

Autonomic Nervous System Divisions

• Sympathetic Division: Responsible for fight-or-flight responses (e.g., increased heart rate, pupil dilation).

• Parasympathetic Division: Responsible for rest-and-digest activities (e.g., decreased heart rate, increased digestion).

Nerves: Structure and Classification

Nerves are bundles of axons in the PNS that transmit electrical impulses. They are classified based on the direction of impulse transmission.

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

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

• Mixed Nerves: Contain both sensory and motor fibers, transmitting impulses in both directions.

Cranial and Spinal Nerves

• Cranial Nerves: Originate from the brain; primarily serve the head and neck.

• Spinal Nerves: Originate from the spinal cord; serve the rest of the body.

Connective Tissue Layers of Spinal Nerves

Spinal nerves are protected and organized by three connective tissue layers:

• Epineurium: Outermost layer, surrounds the entire nerve.

• Perineurium: Surrounds bundles of axons (fascicles).

• Endoneurium: Surrounds individual axons.

Cranial Nerve Functions and Classification

Each cranial nerve has a specific function and can be classified as sensory, motor, or mixed.

• Example: The Optic nerve (II) is sensory and transmits visual information from the retina to the brain.

• Example: The Oculomotor nerve (III) is motor and controls most eye movements.

• Example: The Trigeminal nerve (V) is mixed, providing sensory input from the face and motor control for mastication.

Nerve Roots

Nerve roots are the initial segments of spinal nerves emerging from the spinal cord.

• Dorsal (Posterior) Root: Contains sensory neurons.

• Ventral (Anterior) Root: Contains motor neurons.

Nerve Plexuses

A plexus is a network of intersecting nerves. Major plexuses serve different regions of the body.

• Cervical Plexus: Includes the phrenic nerve (innervates the diaphragm).

• Lumbar Plexus: Includes the femoral nerve (innervates anterior thigh muscles).

• Sacral Plexus: Includes the sciatic nerve (innervates posterior thigh and lower leg).

Dermatomes and Clinical Significance

A dermatome is an area of skin innervated by sensory fibers from a single spinal nerve root.

• Clinical Significance: Dermatomes are used to diagnose nerve injuries and neurological disorders.

• Example: Loss of sensation in a specific dermatome can indicate damage to the corresponding spinal nerve.

Sensory Receptors: Exteroreceptors and Interoreceptors

Sensory receptors detect changes in the environment and transmit signals to the CNS.

• Exteroreceptors: Respond to stimuli from outside the body (e.g., touch, temperature).

• Interoreceptors: Respond to stimuli from within the body (e.g., blood pressure, internal pain).

Types of Sensory Receptors and Detected Stimuli

Sensory receptors are classified by the type of stimulus they detect.

• Mechanoreceptors: Detect mechanical forces (touch, pressure, vibration).

• Thermoreceptors: Detect temperature changes.

• Photoreceptors: Detect light (vision).

• Chemoreceptors: Detect chemical changes (taste, smell, blood chemistry).

• Nociceptors: Detect pain (tissue damage).

Reflexes and Reflex Arcs

A reflex is a rapid, automatic response to a stimulus, often protecting the body from harm.

• Reflex Arc: The neural pathway that mediates a reflex action.

Components of a Reflex Arc:

1. Receptor: Detects the stimulus.

2. Sensory Neuron: Transmits impulse to the CNS.

3. Integration Center: Processes information (may be a single synapse or multiple).

4. Motor Neuron: Carries impulse from CNS to effector.

5. Effector: Muscle or gland that responds to the stimulus.

Example: The knee-jerk (patellar) reflex is a classic example of a simple reflex arc.