Peripheral Nervous System Overview

Peripheral Nervous System (PNS)

• The PNS branches from the brain and spinal cord
• It includes all nervous tissue outside the CNS
• Components: spinal nerves, cranial nerves, autonomic nervous system (ANS)
• Spinal nerves: 31 pairs
• Cranial nerves: 12 pairs
• ANS: The autonomic nervous system is not under conscious control and is regulated by the hypothalamus and brainstem

Spinal Nerves

• Spinal nerves and their branches connect to the spinal cord through posterior and anterior roots
• They connect the CNS to parts of the body, including sensory receptors, muscles, and glands.
• 31 pairs of spinal nerves are named and numbered based on their location in the vertebral column
• Each nerve has 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal spinal nerves
  • Cervical nerves (C1-C8)
  • Thoracic nerves (T1-T12)
  • Lumbar nerves (L1-L5)
  • Sacral nerves (S1-S5)
  • Coccygeal nerve (Co1)

Spinal Nerves: Structure & Function

• Spinal nerve has two connections: posterior root (sensory axon) and anterior root (motor axons).
• The posterior and anterior roots unite to form a spinal nerve (a mixed nerve)
• The posterior root ganglion contains the cell bodies of sensory neurons.

Spinal Nerves: Protective Covering

• Spinal nerves have protective connective tissue coverings
• Endoneurium: wraps a single axon
• Perineurium: wraps a fascicle (bundle of axons)
• Epineurium: the outer covering of a nerve
• Blood vessels are within the perineurium and epineurium, nourishing the nerve tissues.

Spinal Nerves: Plexuses

• Spinal nerves branch after passing through intervertebral foramina.
• In cervical, lumbar, and sacral regions, spinal nerves join with branches from neighboring nerves, forming plexuses.
• Thoracic nerves do not form plexuses
• 5 main plexuses: cervical, brachial, lumbar, sacral, coccygeal

Cervical Plexus

• Supplies posterior head, neck, shoulders, and diaphragm (including the phrenic nerve, crucial for diaphragm function).
• Damage can lead to respiratory failure.

Brachial Plexus

• Supplies upper limbs and some neck and shoulder muscles
• Includes important nerves like radial, ulnar, and median nerves.

Lumbar Plexus

• Supplies abdominal wall, external genitalia, and parts of the lower limbs
• Includes the femoral nerve.

Sacral Plexus

• Supplies buttocks, perineum, and most parts of the lower limbs
• Includes the sciatic nerve, which is the longest and largest nerve in the human body.

Cranial Nerves

• 12 pairs of cranial nerves
• Designated with Roman numerals (I-XII) representing their order as they emerge from the brain
• Cranial nerves I, II, and VIII: sensory nerves
• Cranial nerves III, IV, VI, XI, and XII: motor nerves
• Cranial nerves V, VII, IX, and X: mixed nerves

Cranial Nerves (Functions)

• Olfactory (I): Smell
• Optic (II): Vision
• Oculomotor (III): Eye movement and pupil dilation
• Trochlear (IV): Eye movement
• Trigeminal (V): Somatosensation from face and head; chewing
• Abducens (VI): Eye movement
• Facial (VII): Taste (anterior 2/3 of tongue) and facial muscles
• Vestibulocochlear (VIII): Hearing and balance
• Glossopharyngeal (IX): Taste (posterior 1/3 of tongue) and swallowing muscles
• Vagus (X): Sensory, motor, and autonomic functions for viscera and swallowing
• Accessory (XI): Head movement muscles
• Hypoglossal (XII): Tongue movement

Autonomic Nervous System (ANS)

• The ANS is an important component of the peripheral nervous system (PNS)
• It is not under conscious control and regulates involuntary functions like digestion, heart rate, blood pressure, and other body functions
• Two divisions: sympathetic and parasympathetic
• ANS pathways have two neurons: preganglionic neuron (first neuron), postganglionic neuron (second neuron)
• Preganglionic fiber leaves the CNS and synapses with the second neuron in an autonomic ganglion
• The postganglionic fiber from the ganglion goes to the effector organ.
• The second neuron's cell body is in an autonomic ganglion.
• ANS motor pathways have two neurons

ANS Motor Pathways

• Sympathetic pathways release norepinephrine (NE), and parasympathetic pathways release acetylcholine (ACh).
• ANS regulates glands, smooth muscle, and the heart.
• Examples: Heart rate, breathing rate, blood flow, and digestion

Sympathetic Division (ANS)

• Part of the autonomic nervous system which triggers fight-or-flight responses
• Stimulated during stressful or emergency situations (e.g., exercise, excitement, emergency, embarrassment)
• Causes increased heart rate, breathing rate to skeletal muscles, and decreases activity in the digestive system.
• Longer-lasting effects and more widespread response than parasympathetic
• Release of hormones adrenaline (epinephrine) and noradrenaline into the bloodstream

Parasympathetic Division (ANS)

• Activated when at rest
• Stimulates the rest-and-digest process, decreasing functions like heart rate and increasing functions like digestion
• Effects are less widespread, shorter-lasting than sympathetic responses

Dermatomes

• The skin regions supplied by specific spinal nerves are called dermatomes
• Each spinal nerve (except C1) has a specific cutaneous sensory distribution.

Clinical Disorders

• Traumatic brain injuries (concussion, contusion)
• Strokes
• Alzheimer's disease
• Neural tube defects (e.g., anencephaly, spina bifida)
• Paraplegia
• Quadriplegia
• Myelitis
• Cerebral palsy
• Microcephaly