Peripheral Nervous System: Structure, Function, and Major Nerves

Peripheral Nervous System (PNS)

Overview of the PNS

The Peripheral Nervous System (PNS) includes all nerves and neurons outside the brain and spinal cord. It connects the Central Nervous System (CNS) to limbs and organs, facilitating communication between the CNS and the rest of the body.

• Motor (efferent) neurons: Transmit action potentials away from the CNS to effectors (muscles and glands).

• Sensory (afferent) neurons: Transmit action potentials towards the CNS from sensory receptors.

• Both divisions can be voluntary (somatic) or involuntary (autonomic).

Divisions of the PNS

• Somatic Nervous System (SNS): Controls voluntary movements and transmits sensory information from skin, muscles, and joints.

• Autonomic Nervous System (ANS): Regulates involuntary functions (e.g., heart rate, digestion).

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

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

Classification of Sensory Receptors

By Stimulus Type

Sensory receptors detect specific types of stimuli and convert them into nerve impulses.

• Mechanoreceptors: Respond to mechanical forces (touch, pressure, vibration, stretch).

• Thermoreceptors: Detect changes in temperature.

• Photoreceptors: Respond to light (e.g., in the retina of the eye).

• Chemoreceptors: Respond to chemical changes (e.g., taste, smell, blood chemistry).

• Nociceptors: Respond to pain (e.g., extreme heat, cold, pressure).

By Location

• Exteroceptors: Detect stimuli from outside the body (touch, temperature, pain).

• Interoceptors: Detect stimuli from within the body (internal organs, blood vessels).

• Proprioceptors: Detect position and movement of the body (muscles, tendons, joints).

By Structural Complexity

• Complex receptors: Sense organs (e.g., eyes, ears) with specialized cells.

• Simple receptors: Modified dendritic endings of sensory neurons.

  • Encapsulated dendritic endings: Surrounded by connective tissue (e.g., Meissner's corpuscles, Pacinian corpuscles, Ruffini endings).

  • Nonencapsulated dendritic endings: Free nerve endings (e.g., pain receptors, Merkel discs).

Major Sensory Receptors

• Meissner's corpuscles: Detect light touch, located in dermal papillae.

• Pacinian corpuscles: Detect deep pressure and vibration, found in deeper dermis and subcutaneous tissue.

• Ruffini endings: Detect stretch, located in dermis.

• Muscle spindles: Monitor muscle stretch and length.

• Golgi tendon organs: Monitor tension in tendons.

Sensory Perception

Main Aspects of Sensory Perception

• Perceptual detection: Awareness of a stimulus.

• Magnitude estimation: Ability to detect stimulus intensity.

• Spatial discrimination: Ability to identify the site or pattern of stimulation.

• Quality discrimination: Ability to differentiate submodalities (e.g., sweet vs. bitter).

• Pattern recognition: Ability to recognize familiar patterns.

Nerves

Structure and Types

• Nerve: A bundle of axons in the PNS surrounded by connective tissue.

• Mixed nerves: Contain both sensory (afferent) and motor (efferent) fibers.

• Classification by direction:

  • Sensory nerves: Carry impulses toward the CNS.

  • Motor nerves: Carry impulses away from the CNS.

Regeneration of Nerves

Peripheral nerves can regenerate if the soma (cell body) remains intact. Most CNS fibers do not regenerate. Axon regeneration occurs at a rate of 1-5 mm/day. Schwann cells play a key role in regeneration by releasing growth factors and guiding axon regrowth.

Cranial Nerves

Overview

• 12 pairs of cranial nerves arise from the brain.

• Each nerve has specific sensory, motor, or mixed functions.

Major Cranial Nerves and Functions

• Olfactory (I): Smell.

• Optic (II): Vision.

• Oculomotor (III): Eye movement, pupil constriction.

• Trochlear (IV): Eye movement (superior oblique muscle).

• Trigeminal (V): Sensation from face, chewing muscles.

• Abducens (VI): Eye movement (lateral rectus muscle).

• Facial (VII): Facial expression, taste (anterior tongue).

• Vestibulocochlear (VIII): Hearing and balance.

• Glossopharyngeal (IX): Taste (posterior tongue), swallowing.

• Vagus (X): Parasympathetic control of heart, lungs, digestive tract.

• Accessory (XI): Neck and shoulder movement.

• Hypoglossal (XII): Tongue movement.

Spinal Nerves

Overview

• 31 pairs supply all body regions except the head.

• Classified by region:

  • 8 pairs of cervical nerves (C1–C8)

  • 12 pairs of thoracic nerves (T1–T12)

  • 5 pairs of lumbar nerves (L1–L5)

  • 5 pairs of sacral nerves (S1–S5)

  • 1 pair of coccygeal nerves (Co1)

Spinal Nerve Plexuses

• Cervical plexus: Innervates neck muscles and skin.

  • Phrenic nerve (C3–C5): Innervates diaphragm, essential for breathing.

• Brachial plexus: Innervates shoulder, arm, and hand.

  • Ulnar nerve: Supplies medial forearm and hand.

  • Radial nerve: Supplies posterior arm and forearm.

  • Median nerve: Supplies anterior forearm and hand.

  • Musculocutaneous nerve: Supplies anterior arm muscles.

  • Axillary nerve: Supplies shoulder muscles.

• Lumbar plexus: Innervates abdominal wall and anterior thigh.

  • Femoral nerve: Largest nerve of the plexus, supplies anterior thigh.

  • Obturator nerve: Supplies adductor muscles of thigh.

• Sacral plexus: Innervates lower limb.

  • Sciatic nerve: Largest nerve in the body, supplies posterior thigh and leg.

  • Tibial nerve: Supplies calf and sole of foot.

  • Peroneal (fibular) nerve: Supplies lateral leg and foot.

Table: Major Cranial Nerves and Their Functions

Nerve	Number	Main Function
Olfactory	I	Smell
Optic	II	Vision
Oculomotor	III	Eye movement, pupil constriction
Trochlear	IV	Eye movement (superior oblique)
Trigeminal	V	Facial sensation, chewing
Abducens	VI	Eye movement (lateral rectus)
Facial	VII	Facial expression, taste
Vestibulocochlear	VIII	Hearing, balance
Glossopharyngeal	IX	Taste, swallowing
Vagus	X	Parasympathetic control
Accessory	XI	Neck, shoulder movement
Hypoglossal	XII	Tongue movement

Additional info:

• Regeneration in the CNS is limited due to inhibitory factors and lack of supportive cells like Schwann cells.

• Damage to specific nerves can result in characteristic deficits (e.g., facial paralysis, loss of sensation, muscle weakness).