Peripheral Nervous System: Structure, Function, and Sensation

Module 13.1: Overview of the Peripheral Nervous System (PNS)

Introduction to the Peripheral Nervous System

The Peripheral Nervous System (PNS) connects the central nervous system (CNS) to the body and the external environment. It is responsible for transmitting sensory information to the CNS and carrying out motor commands from the CNS to muscles and glands.

• Sensory Input: The PNS detects sensory stimuli and delivers the information to the CNS.

• Motor Output: The CNS processes the input and sends instructions through the PNS to muscles, glands, and organs.

Divisions of the PNS

The PNS is functionally divided into two main divisions: the sensory (afferent)motor (efferent)

• Sensory (Afferent) Division: Consists of neurons that detect and transmit sensory stimuli to the CNS. It is further subdivided into:

  • Somatic Sensory Division: Detects both internal and external stimuli, such as general senses from the skin.

  • Visceral Sensory Division: Relays information such as blood pressure from organs of the abdominopelvic and thoracic cavities.

• Motor (Efferent) Division: Consists of neurons that carry out motor functions. It is subdivided into:

  • Somatic Motor Division: Responsible for voluntary motor functions; composed of lower motor neurons (somatic motor neurons) that directly stimulate skeletal muscle contractions.

  • Visceral Motor Division (Autonomic Nervous System, ANS): Maintains many aspects of homeostasis by controlling involuntary motor functions. It innervates cardiac muscle, smooth muscle, and secretory glands. The ANS is further divided into sympathetic and parasympathetic nervous systems.

Module 13.2: The Cranial Nerves

Introduction to Cranial Nerves

Cranial nerves are twelve pairs of nerves that arise directly from the brain and primarily innervate structures in the head and neck. They are classified as sensory, motor, or mixed nerves.

• Sensory Cranial Nerves: Contain axons of only sensory neurons (e.g., olfactory (CN I), optic (CN II), vestibulocochlear (CN VIII)).

• Motor Cranial Nerves: Contain axons of motor neurons and are responsible for proprioception (e.g., oculomotor (CN III), trochlear (CN IV), abducens (CN VI), accessory (CN XI), hypoglossal (CN XII)).

• Mixed Cranial Nerves: Contain axons of both sensory and motor neurons (e.g., trigeminal (CN V), facial (CN VII), glossopharyngeal (CN IX), vagus (CN X)).

Module 13.3: The Spinal Nerves

Structure of Spinal Nerves and Spinal Nerve Plexuses

Spinal nerves are formed by the fusion of anterior and posterior roots and are classified as mixed nerves. The 31 pairs of spinal nerves can cluster to form networks called plexuses.

• Anterior Ramus: Travels to the anterior side of the body or an upper/lower limb.

• Posterior Ramus: Travels to the posterior side of the body.

• Plexuses: Networks of nerves formed by the fusion of spinal nerves, allowing for both somatic and sensory innervation.

Major Nerve Plexuses and Their Branches

• Cervical Plexus: Innervates the skin of the neck, head, chest, and shoulders. The phrenic nerve (C3–C5) innervates the diaphragm.

• Brachial Plexus: Innervates the upper limb and shoulder. Major branches include:

  • Axillary nerve: Deltoid and teres minor muscles, skin over the deltoid region.

  • Radial nerve: Triceps brachii, extensor muscles of the forearm, skin over the posterior thumb, second digit, third digit, and lateral half of the fourth digit.

  • Musculocutaneous nerve: Anterior arm muscles, skin covering the lateral forearm.

  • Median nerve: Most intrinsic hand muscles, skin over the anterior thumb, second and third digits, lateral half of the fourth digit.

  • Ulnar nerve: Extensor muscles in the forearm not innervated by the median nerve, most intrinsic hand muscles, skin of the fifth digit and medial side of the fourth digit.

• Lumbar Plexus: Innervates the pelvis and lower extremities. Major branches include:

  • Obturator nerve: Adductor muscles, hip joint, skin of the medial thigh.

  • Femoral nerve: Anterior thigh muscles, skin over the anterior and medial thigh, medial leg, and foot.

• Sacral Plexus: Innervates the pelvis, gluteal region, and lower extremities. Major branches include:

  • Sciatic nerve: Largest nerve, innervates the posterior thigh, leg, and foot.

  • Tibial nerve: Most hamstring muscles, posterior leg and foot.

  • Common fibular nerve: Anterior and lateral leg, dorsum of the foot.

Module 13.4: Sensation Part I: Role of the PNS in Sensation

From PNS to CNS: Sensory Reception and Sensory Integration

Sensory neurons transmit impulses to the CNS, where the stimulus is interpreted and integrated. Sensory transduction is the process by which a stimulus is converted into an electrical signal.

• Sensory Transduction: When a stimulus is detected by a sensory receptor, sodium ion channels open, allowing sodium ions to flow into the axoplasm, causing a temporary depolarization called a receptor potential. If enough sodium enters, the membrane potential may reach threshold, triggering an action potential that is propagated along the axon toward the CNS.

• Rapidly Adapting Receptors: Respond rapidly with high intensity but stop sending signals after a certain period (adaptation).

• Slowly Adapting Receptors: Respond to stimuli with constant action potentials that do not diminish over time.

Classification of Sensory Receptors

• Encapsulated Nerve Endings: Surrounded by specialized supportive cells.

• Free Nerve Endings: Lack supportive cells.

Location and Type of Stimuli Detected

• Exteroceptors: Located near the body’s surface; detect stimuli originating from outside the body.

• Interoceptors: Detect stimuli originating inside the body.

Key Terms and Definitions

• Peripheral Nervous System (PNS): The part of the nervous system outside the brain and spinal cord.

• Central Nervous System (CNS): The brain and spinal cord.

• Somatic Motor Neurons: Neurons that stimulate voluntary muscle contractions.

• Autonomic Nervous System (ANS): Controls involuntary bodily functions.

• Plexus: A network of intersecting nerves.

• Receptor Potential: Temporary depolarization in a sensory receptor cell in response to a stimulus.

• Adaptation: Decrease in response of a sensory receptor to a constant stimulus.

Table: Major Nerve Plexuses and Their Innervation

Plexus	Major Nerves	Innervated Regions
Cervical	Phrenic	Diaphragm
Brachial	Axillary, Radial, Musculocutaneous, Median, Ulnar	Shoulder, arm, forearm, hand
Lumbar	Obturator, Femoral	Pelvis, anterior thigh, medial leg
Sacral	Sciatic, Tibial, Common Fibular	Posterior thigh, leg, foot

Key Equations

• Action Potential Threshold:

$V_{m} \geq V_{threshold}$

• Propagation of Action Potential:

$\text{Action Potential} \rightarrow \text{Axon} \rightarrow \text{CNS}$

Example

• Example of Sensory Transduction: Touching a hot surface activates exteroceptors in the skin, which generate a receptor potential. If the stimulus is strong enough, an action potential is sent to the CNS, resulting in the perception of pain and a reflex withdrawal.