The peripheral nervous system connects your brain and spinal cord to the rest of your body. It's made up of nerves, ganglia, and receptors that work together to send and receive signals, allowing you to move, feel, and react to your environment.

This complex network is divided into somatic and autonomic systems. The somatic system controls voluntary movements, while the autonomic system manages involuntary functions like heart rate and digestion. Understanding these components helps explain how your body responds to various stimuli.

Peripheral Nervous System

Structures of peripheral nervous system

• Nerves consist of bundles of axons wrapped in connective tissue (epineurium, perineurium, endoneurium)
  • Cranial nerves originate from the brain and include olfactory, optic, and vagus nerves
  • Spinal nerves originate from the spinal cord and include cervical, thoracic, lumbar, and sacral nerves
  • Peripheral nerves are the extensions of the nervous system outside the brain and spinal cord
• Ganglia are clusters of neuron cell bodies located outside the central nervous system
  • Sensory ganglia contain cell bodies of sensory neurons (dorsal root ganglia, trigeminal ganglia)
  • Autonomic ganglia contain cell bodies of autonomic neurons (sympathetic chain ganglia, ciliary ganglia)
• Receptors are specialized structures that detect stimuli and generate nerve impulses
  • Sensory receptors in the skin detect touch, pressure, temperature, and pain (Meissner's corpuscles, Pacinian corpuscles)
  • Proprioceptors in muscles and tendons detect body position and movement (muscle spindles, Golgi tendon organs)
• Effectors are structures that respond to nerve impulses
  • Muscles contract in response to motor neuron stimulation (skeletal, smooth, cardiac muscle)
  • Glands secrete substances in response to autonomic neuron stimulation (salivary glands, sweat glands)

Somatic vs autonomic nervous systems

• Somatic nervous system provides voluntary control of skeletal muscles
  • Afferent (sensory) components transmit information from receptors to the central nervous system
  • Efferent (motor) components transmit signals from the central nervous system to skeletal muscles
  • Enables conscious control of movement (walking, grasping) and sensation (touch, pain)
• Autonomic nervous system provides involuntary control of smooth muscle, cardiac muscle, and glands
  • Sympathetic division activates "fight or flight" response (increases heart rate, dilates pupils)
  • Parasympathetic division promotes "rest and digest" functions (slows heart rate, constricts pupils)
  • Most organs receive dual innervation from both sympathetic and parasympathetic divisions
• Enteric nervous system is a subset of the autonomic nervous system that controls gastrointestinal functions
  • Regulates motility (peristalsis), secretion (digestive enzymes), and blood flow in the gut
  • Can function independently of the central nervous system due to intrinsic nerve plexuses (myenteric, submucosal)

Cranial nerves and functions

1. Olfactory (I) detects odors and transmits smell information to the brain
2. Optic (II) transmits visual information from the retina to the brain
3. Oculomotor (III) controls eye movement, pupillary constriction, and lens accommodation
4. Trochlear (IV) controls eye movement by innervating the superior oblique muscle
5. Trigeminal (V) provides sensation from the face and motor control of jaw muscles (chewing)
6. Abducens (VI) controls eye movement by innervating the lateral rectus muscle
7. Facial (VII) controls facial expression, taste, salivation, and lacrimation (tear production)
8. Vestibulocochlear (VIII) transmits auditory and vestibular (balance) information to the brain
9. Glossopharyngeal (IX) mediates taste, swallowing, and salivation
10. Vagus (X) provides parasympathetic control of thoracic and abdominal organs, swallowing, and speech
11. Accessory (XI) controls motor function of neck and shoulder muscles (turning head, shrugging)
12. Hypoglossal (XII) controls motor function of tongue muscles (speech, swallowing)

Composition of spinal nerves

• 31 pairs of mixed nerves (sensory and motor) arise from the spinal cord
  • 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal
  • Named and numbered according to the vertebral level of origin (C1-C8, T1-T12, L1-L5, S1-S5, Co1)
• Each spinal nerve has dorsal (sensory) and ventral (motor) roots that combine to form the nerve
  • Dorsal root contains afferent (sensory) fibers and the dorsal root ganglion
  • Ventral root contains efferent (motor) fibers
• Spinal nerves form plexuses, networks of interconnected nerves that distribute fibers to specific regions
  • Cervical plexus innervates the neck and upper shoulder region
  • Brachial plexus innervates the upper limb (roots C5-T1)
  • Lumbar plexus innervates the lower abdomen, anterior thigh, and medial leg (roots L1-L4)
  • Sacral plexus innervates the posterior thigh, most of the lower leg, and the foot (roots L4-S4)

Nerve conduction and synaptic transmission

• Schwann cells wrap around axons to form myelin sheaths, which insulate and speed up nerve conduction
• Nodes of Ranvier are gaps between myelin segments that allow for saltatory conduction of action potentials
• Neurotransmitters are chemical messengers released at synapses to transmit signals between neurons
• The neuromuscular junction is a specialized synapse where motor neurons communicate with muscle fibers
• Reflex arcs are neural pathways that produce rapid, automatic responses to stimuli without conscious thought

Disorders of the peripheral nervous system

• Neuropathy refers to damage or dysfunction of peripheral nerves, which can cause pain, numbness, or weakness