Peripheral Nervous System (PNS)

Overview of the Peripheral Nervous System (PNS)

• The PNS links the central nervous system (CNS) to the body and external environment.
• The PNS detects sensory stimuli and delivers information to the CNS as sensory input.
• The CNS processes input and transmits impulses through the PNS to muscle cells and glands as motor output.

Divisions of the PNS

• Functionally, the PNS is divided into the sensory and motor divisions.

Sensory division

• Consists of afferent neurons that detect and transmit sensory stimuli to the CNS, with anatomical subdivisions.
• The Somatic sensory division detects both internal and external stimuli.
• General sense receptors detect stimuli from the skin, muscles, joints, and bones.
• Special sensory receptors detect stimuli from special sense organs.
• The Visceral sensory division relays internal information (blood pressure) from organs of the abdominopelvic and thoracic cavities.

Motor division

• Consists of efferent neurons that carry out motor functions of the nervous system, with subdivisions based on the organs that neurons contact.
• The Somatic motor division has voluntary motor functions.
• Lower motor neurons (somatic motor neurons) directly trigger skeletal muscle contractions.
• The Visceral motor division, also known as the autonomic motor nervous system (ANS), maintains many aspects of homeostasis.
• The ANS controls involuntary motor functions in the body.
• Neurons innervate cardiac muscle cells, smooth muscle cells, and secretory cells of glands.

Motor division

• The ANS is further divided into sympathetic and parasympathetic nervous systems.
• The Sympathetic nervous system (fight or flight division) controls homeostasis activities surrounding physical work and visceral responses of emotions.
• The Parasympathetic system (rest and digest division) controls digestion and maintains the body’s homeostasis at rest.

Peripheral Nerves and Associated Ganglia

• Peripheral nerves are the main organs of the PNS.
• Axons of many neurons are bound together by connective tissue.
• Nerves of the PNS innervate the majority of structures in the body.
• Mixed nerves contain both sensory and motor neurons.
• Sensory nerves contain only sensory neurons
• Motor nerves contain mostly motor neurons, as well as some sensory neurons involved in muscle stretch and tension.

Spinal Nerves

• Originate from the spinal cord and innervate structures below the head and neck
• Includes anatomical structures

Axons of PNS & spinal cord

• Two collections connect the PNS with the spinal cord's gray matter.
• The Anterior root has motor neurons from the anterior horn.
• The Posterior root contains sensory neurons from the posterior horn.
• The Posterior root ganglion (dorsal root ganglion) is a swollen area in the posterior root that houses the cell bodies of sensory neurons.
• Posterior and anterior roots fuse to form the spinal nerve just lateral to the posterior root ganglion.
• All 31 pairs of spinal nerves are mixed nerves.

Structures and Spinal Nerves

• Epineurium is the outermost layer of connective tissue that holds motor and sensory axons together.
• Fascicles are small groups of bundled axons surrounded by connective tissue perineurium.
• Each individual axon within a fascicle is surrounded by its own connective tissue endoneurium.

Cranial Nerves

• Attach to the brain and mostly innervate structures in the head and neck.
• Cranial nerves are not formed by the fusion of sensory and motor roots, unlike spinal nerves
• Cranial nerves allow for purely sensory, mixed, and mostly motor nerves.

Functional Overview

• PNS functions are integrated with those of the CNS.
• Sensory neurons detect stimuli at the sensory receptors.
• Detected stimuli are transmitted along the sensory neuron (spinal or cranial) to the cerebral cortex.
• In the cortex, sensory information is interpreted and integrated, and an appropriate motor response is selected and initiated.
• Motor responses are initiated by commands from motor areas of the cerebral cortex.
• Impulses travel to the spinal cord where neurons synapse with lower motor neurons of the PNS.
• Lower motor neurons carry impulses to appropriate muscles via cranial or spinal nerves, triggering contractions

Sensory Cranial Nerves

• Three cranial nerves contain axons of only sensory neurons: olfactory (I), optic (II), and vestibulocochlear (VIII).

Motor Cranial Nerves

• Five cranial nerves primarily contain axons of motor neurons with associated sensory axons responsible for proprioception
• These nerves are oculomotor (III), trochlear (IV), abducens (VI), accessory (XI), and hypoglossal (XII).

Mixed Cranial Nerves

• Four cranial nerves contain axons of both sensory and motor neurons: trigeminal (V), facial (VII), glossopharyngeal (IX), and vagus (X).

Bell's Palsy

• Bell’s palsy is a common problem associated with the facial nerve.
• The nerve's motor root is impaired by a virus, tumor, trauma, or an unknown cause.
• Symptoms include weakness or complete paralysis of muscles of facial expression on the affected side only.
• Paralysis leads to problems with blinking, closing one eye, and making general facial expressions, such as smiling.
• Other structures innervated by the facial nerve, such as the lacrimal gland, salivary glands, and taste sensation from the anterior two-thirds of the tongue, may also be affected by Bells Palsy.
• Typically, individuals experience a rapid onset of symptoms
• Treatment includes anti-inflammatory medication, antiviral medication, physical therapy, and surgery.
• Even without treatment, many individuals recover function of the paralyzed muscles in about 3 weeks.

Remembering the Cranial Nerves

• Utilize the popular mnemonic: "Oh (I, Olfactory) Once (II, Optic) One (III, Oculomotor) Takes (IV, Trochlear) The (V, Trigeminal) Anatomy (VI, Abducens) Final (VII, Facial) Very (VIII, Vestibulocochlear) Good (IX, Glossopharyngeal) Vacations (X, Vagus) Are (XI, Accessory) Happening (XII, Hypoglossal)."
• You have one nose (I, olfactory) and two eyes (II, optic).

Remembering Cranial Nerves

• Utilize the popular mnemonic: "Some (I, Olfactory - Sensory) Say (II, Optic – Sensory) Money (III, Oculomotor – Motor) Matters (IV, Trochlear – Motor) But (V, Trigeminal – Both) My (VI, Abducens - Motor) Brother (VII, Facial – Both) Says (VIII, Vestibulocochlear – Sensory) Big (IX, Glossopharyngeal – Both) Brains (X, Vagus – Both) Matter (XI, Accessory – Motor) More (XII, Hypoglossal – Motor)"
• Connect them with word roots; for example, oculomotor, broken into its two components, oculo-, which means "eye,” and -motor, which means “movement”.