Peripheral Nervous System

The Peripheral Nervous System

• The peripheral nervous system consist of nervous system structures outside of the brain and spinal cord
• Its structure contains white and grey matter
• White matter consists of nerves (bundles of axons)
• Grey matter consists of ganglia (clusters of cell bodies)
• The peripheral nervous system is connected to the CNS via 31 pairs of spinal nerves and 12 pairs of cranial nerves
• The main functions are to deliver sensory information from receptors in the periphery to the CNS, and deliver motor commands from the CNS to muscles and glands (both involuntary and voluntary)

Cells of the PNS

• Neurons are either sensory (afferent) or motor (efferent)
• Glia primarily consist of Schwann cells, and are responsible for myelinating and non-myelinating

Structure of a Peripheral Nerve

• Axons group together and travel in peripheral nerves
• Each peripheral nerve is made of several bundles of axons called fascicles
• Three levels of connective tissue wrap around the nerve bundle hierarchy
  • Endoneurium wraps around axons
  • Perineurium wraps around fascicles
  • Epineurium wraps around the entire nerve
• In a transverse section of a peripheral nerve, myelin stains blue/turquoise and connective tissue stains pink/purple

Organization of the PNS

• The PNS separates into sensory and motor components
• Sensory (afferent) neurons categories
  • Visceral relates to organs
  • Special category includes cranial nerves
  • Somatic includes everything else
• Motor (efferent) neurons categories:
• Somatic relates to skeletal muscle
• Visceral (autonomic) nerves relate to organs and glands

The Peripheral Sensory System

• Somatic sensory components include information from skin, skeletal muscle, tendons, joints, connective tissue and meninges
• Exteroceptors relate to the skin, and include sensations like touch, pressure, thermal and pain
• Proprioceptors relate to muscles, tendons, and joints, and provide information about the body's location in space
• Interoceptors (visceral sensory receptors) relate to thermal, pain, changes in blood composition and pressure in internal organs in the thoracic and abdominal cavities
• Primary sensory neurons have their cell body in ganglia and deliver information from the receptors into the CNS

Peripheral Motor System (Somatic)

• Consists of somatic motor neurons that control skeletal muscle (lower motor neurons)
• Cell bodies are located in the CNS
• Motor axons exit the spinal cord via the ventral roots, spinal nerves, and peripheral nerves
• Axons can also exit the brainstem via cranial nerves and travel to the muscle
• Somatic motor system allows for voluntary movements and somatic reflexes
• The synapse between the axon terminal of the lower motor neuron and the skeletal muscle refers to the neuromuscular junction

Peripheral Motor System (Visceral)

• Visceral motor control equates to the autonomic nervous system
• It regulates cardiovascular, respiratory, digestive, urinary, reproductive functions, and internal environment composition
• It is mostly involuntary and subconscious, and controls smooth/cardiac muscles and glands
• It has 2 divisions, sympathetic and parasympathetic
  • CNS → effectors = 2 neurons
    • The first neuron is located in the spinal cord and brainstem, with its axon exiting the CNS via selected spinal nerves and cranial nerves
  • The second neuron is found entirely in the PNS, with the cell body in autonomic ganglia

Spinal Nerves

• A spinal nerve directly interacts with the spinal cord
• Spinal nerves deliver sensory and motor information from a similar area - Sensory information travels via the dorsal root (cell bodies located in the dorsal root ganglion) - Motor information travels via the ventral root (cell bodies in the ventral horn or autonomic ganglion)
• Nerve branches form dorsal (posterior) and ventral (anterior) rami, carrying both sensory and motor information

Segmental Organization of Spinal Cord and Nerves

• 31 spinal nerve pairs is mapped to approximately 31 dermatomes

Spinal Nerve Plexuses

• Several ventral rami of spinal nerves innervating the neck, upper, and lower limbs reorganize to form interwoven networks referred to as plexuses
• Cervical plexus - Provides motor innervation to some muscles of the neck and diaphragm - Provides sensory innervation to the head, neck, and chest
• Brachial plexus
  • Provides sensory and motor information from the arm and hand
  • Damage can result in sensory loss and upper limb paralysis
• Lumbar plexus innervates the muscles on the lower abdominal, genital areas, and thighs
• Sacral plexus innervates the skin and muscles of the thigh, lower leg, and foot
• overlap can occur at T4, so often grouped as lumbosacral plexus

Plexus Organization

• The ventral rami combine to form superior, middle, and inferior trunks → anterior and posterior divisions (innervate ventral and dorsal upper limbs) → converge to form posterior, medial, and lateral cords → splitting into five terminal branches: the axillary, median, musculocutaneous, radial, and ulnar nerves

Cranial Nerves

• The 12 pairs of nerves that directly interact with the brain
• These nerves carry sensory and motor information from head and neck structures (with the exception of CNX)
• These nerves carry special sensory information such as taste, smell, vision, hearing and balance
• Cranial nerves project to - Cortex- I - Diencephalon-II - Midbrain-III and IV - Pons- V - Pons-Medulla border- VI, VII and VIII - Medulla- IX, X, XI and XII
  • Nerves III, VII, IX and X are parasympathetic (autonomic visceral motor control)

Cranial Nerve Mnemonic

• I - Olfactory "Only"
• II - Optic "One"
• III - Oculomotor "Of"
• IV - Trochlear "The"
• V - Trigeminal "Two"
• VI - Abducens "Athletes"
• VII - Facial "Felt"
• VIII - Vestibulocochlear "Very"
• IX - Glossopharyngeal "Good"
• X - Vagus "Victorious"
• XI - Accessory "And"
• XII - Hypoglossal "Healthy"

Cranial Nerves – Type of Info

• I - Olfactory "Some" - Sensory
• II - Optic "Say" - Sensory
• III - Oculomotor "Marry" - Motor
• IV - Trochlear "Money" - Motor
• V - Trigeminal "But" - Both
• VI - Abducens "My" - Motor
• VII - Facial "Brother" - Both
• VIII - Vestibulocochlear "Says" - Sensory
• IX - Glossopharyngeal "Big" - Both
• X - Vagus "Brains" - Both
• XI - Accessory "Matter" - Motor
• XII - Hypoglossal "More" - Motor

CN I- Olfactory

• A special sensory nerve involved in the sense of smell
• Receptors are located in the olfactory epithelium in the nasal cavity
  • 1° neurons (detect stimuli) located in the olfactory bulb above the cribriform plate with axons traveling through the bone
  • 2° neurons (relay stimuli) form the olfactory tract into the cerebrum

CN II- Optic Nerve

• Main function is special sensory for vision
• This nerve carry visual information from the retina of the eye
• Optic nerves merge in the optic chiasm where fibers cross the midline before entering the CNS via the optic tract
• Nerve destination is the diencephalon (thalamus, LGN)

CN III, IV, VI - Muscles of the Eye

• Motor information included
• One of 4 extrinsic eye muscles moves eyeball up, medially, down and rotation, as well as eyelid closure
• Visceral motor control includes pupil constriction and lens accommodation
• Trochlear is an extrinsic eye muscle
• Abducens controls movement of the extrinsic eye muscle-lateral

CN V- Trigeminal

• Function as mixed sensory and motor nreve
• It is major somatosensory nerve for the face, top of scalp, nasal cavity and oral cavity
• Nerve is attached laterally in the pons and then branches into three divisions- ophthalmic (top), maxillary (middle) and mandibular (lower)
  • Trigeminal ganglion contains the cell bodies of the primary sensory neurons (equivalent to the dorsal root ganglia)
  • Axons of LMN for muscles of mastication travel in the mandibular branch (equivalent to the ventral root) from cell bodies in the brainstem (pons)

CN VII- Facial

• Emerges from the medulla-pons border
• Functions as a mixed sensory and motor nerve
  • Somatic motor innervates the muscles of facial expressions
  • Visceral carries motor signal for innervation of saliva and tear glands
  • Special sensory component taste sensations from the anterior two thirds of the tongue

CN VIII - Vestibulocochlear

• Only carries sensory information related to hearing and balance
• Enters the brainstem at the medulla-pons border
• Contains two branches originating in the inner ear
  • Vestibular branch innervates the vestibular apparatus and carries information regarding head movements to vestibular nuclei
  • Cochlear branch innervates the cochlea and carries audio information to cochlear nuclei

CN IX- Glossopharyngeal

• Carries both sensory and motor information
• Emerges from the medulla lateral to the olives
  • Somatosensory sensations from the back of the oral cavity and upper pharynx
  • Visceral sensory sensation from chemoreceptors (CO2 and pH) and baroreceptor (blood pressure) in the carotid body and sinus
  • Special taste sensations from the back third of the tongue
  • Visceral motor function, and is associated with parotid gland (saliva)

CN X- Vagus

• Functions as a mixed motor and sensory nerve
• CN X nerve emerge from the brainstem close to each other on the lateral side of the olives in the medulla, and is the only CN that is distributed below the head and neck - Major visceral functions include both motor and sensory innervation, the thoracic and abdominal regions - Most of the Vagus fibres carry sensory information from visceral tissue in the thoracic and abdominal regions, as well as chemoreceptors and stretch receptors in the aortic arch, letting the brain know about the internal environment of the body General and somatosensory component is associated from the pharynx, larynx and regions of the meninges - Motor signals regulate airways, digestive and cardiovascular functions. - Innervates the muscles of the pharynx and larynx, soft palate and epiglottis

CN XI- Spinal Accessory

• A motor nerve that innervates two muscles: - The trapezius, which tilts the head back and to the side and elevates the shoulders - Thee sternocleidomastoid, and rotates the head and jaw to the opposite side The cell bodies of the axons in the nerve are found in the ventral horn of the cervical spine, and exit through ventral roots. They travel up in the subarachnoid space, through the foramen magnum and then out through the cranium to travel to muscles

CN XII- Hypoglossal

• The rootlets of the hypoglossal emerge between the pyramids and the olives of the medulla
• All fibers are somatic motor, innervating muscles of the tongue (all but one, which is innervated by the vagus)