Peripheral Nervous System and Related Concepts
Peripheral Nervous System (PNS)
Definition: All neural structures outside the brain and spinal cord.
Structural Organization of the Nervous System
Central Nervous System (CNS)
Peripheral Nervous System (PNS)
Divisions of PNS:
Sensory (Afferent) Division: Transmits sensory information to CNS.
Motor (Efferent) Division: Carries commands from CNS to effectors.
Somatic Nervous System: Controls voluntary movements.
Autonomic Nervous System (ANS):
Sympathetic Division: Prepares the body for stressful or emergency situations.
Parasympathetic Division: Promotes relaxation and conserves energy.
Classification of Receptors
Based on Stimulus:
Mechanoreceptors: Respond to mechanical pressure or distortion.
Thermoreceptors: Detect temperature changes.
Photoreceptors: Sensitive to light; found in the retina.
Chemoreceptors: Respond to chemical stimuli.
Nocioreceptors: Detect pain.
Based on Location:
Exteroreceptors: Respond to stimuli from outside the body.
Interoreceptors: Respond to stimuli within the body.
Proprioreceptors: Provide information about body position and movement.
Types of Receptors
Free Nerve Endings: Unencapsulated receptors for pain and temperature.
Tactile Discs: Also known as Merkel cells; respond to light touch.
Hair Follicle Receptors: Respond to hair movement.
Tactile (Meissner’s) Corpuscles: Sensitive to light touch and vibration; found in dermal papillae.
Lamellar (Pacinian) Corpuscles: Detect deep pressure and vibration; located deeper in the dermis.
Bulbous (Ruffini’s) Corpuscles: Respond to skin stretch and are located in the dermis.
Muscle Spindles: Detect muscle stretch; found within skeletal muscles.
Golgi Tendon Organs: Monitor tension within tendons.
Joint Kinesthetic Receptors: Detect joint position and movement.
Somatosensory System
Function: Receives input from various receptors to perceive stimuli.
Sensation: Awareness of changes in the environment.
Perception: Interpretation of sensory input.
Levels of Integration:
Receptor Level: Sensory reception and transmission to CNS.
Circuit Level: Processing in ascending neural pathways.
Perceptual Level: Processing in cortical sensory centers (includes motor cortex, somatosensory cortex, thalamus, reticular formation, cerebellum, pons, medulla, and spinal cord).
Nerve Structure
Endoneurium: Delicate connective tissue surrounding individual nerve fibers.
Fascicle: A group of nerve fibers.
Perineurium: Connective tissue wrapping that bundles groups of fibers (fascicles).
Epineurium: Fibrous sheath that surrounds the entire nerve.
Types of Nerves
Sensory Nerves: Carry sensory information to the CNS.
Motor Nerves: Transmit motor commands from the CNS.
Mixed Nerves: Contain both sensory and motor fibers.
Ganglia
Wallerian Degeneration: A process that occurs when a nerve fiber is damaged and the distal part degenerates.
Cranial Nerves
List of Cranial Nerves:
Olfactory (I)
Optic (II)
Oculomotor (III)
Trochlear (IV)
Trigeminal (V)
Abducens (VI)
Facial (VII)
Vestibulocochlear (VIII)
Glossopharyngeal (IX)
Vagus (X)
Accessory (XI)
Hypoglossal (XII)
Function and Location of Cranial Nerves:
Olfactory Nerve (I): Related to the sense of smell.
Optic Nerve (II): Relays visual information.
Oculomotor Nerve (III): Controls eyelid and eye movement.
Trochlear Nerve (IV): Innervates superior oblique muscle of the eye.
Trigeminal Nerve (V): Provides sensation to the face; motor functions for mastication.
Abducens Nerve (VI): Controls lateral rectus muscle for eye movement.
Facial Nerve (VII): Controls facial expressions; taste sensations from the anterior two-thirds of the tongue.
Vestibulocochlear Nerve (VIII): Involved in hearing and balance.
Glossopharyngeal Nerve (IX): Taste and gag reflex; sensory from the oropharynx.
Vagus Nerve (X): Involved in autonomic control of the heart, lungs, and digestive tract.
Accessory Nerve (XI): Controls shoulder and neck muscles.
Hypoglossal Nerve (XII): Controls tongue movements.
Spinal Nerves
Total: 31 pairs.
Components:
Ventral Roots: Carry motor fibers away from the spinal cord.
Dorsal Roots: Carry sensory fibers toward the spinal cord.
Dorsal Ramus: Supplies muscles and skin of the posterior trunk.
Ventral Ramus: Supplies muscles and skin of the anterior trunk and limbs.
Meningeal Branch: Supplies the meninges.
Regions:
Cervical Plexus: C1-C4.
Brachial Plexus: C5-T1.
Lumbar Plexus: L1-L4.
Sacral Plexus: L4-S4.
Dermatome
Definition: Area of skin innervated by the cutaneous branches of a single spinal nerve.
Clinical Significance:
Neurological Assessment: Important for diagnosing and localizing spinal nerve damage or spinal cord lesions.
Herniated Discs: Pain or sensory loss in a specific dermatome can indicate compression of a particular spinal nerve by a herniated disc.
Shingles (Herpes Zoster): The characteristic rash typically appears in a dermatomal pattern because the virus reactivates in a single dorsal root ganglion and spreads along its associated spinal nerve.
Mapping: Dermatomes are systematically mapped around the body. While generally consistent, there can be some individual variation and overlap between adjacent dermatomes. Cranial nerves (specifically the trigeminal nerve) innervate the skin of the face, performing a similar function to spinal nerve dermatomes in the body but are not typically referred to as dermatomes in the same context.
C2: Posterior half of the skull cap.
C3: Entire neck and posterior cheek areas.
C4: Lower neck and upper shoulder region.
C5: Deltoid area and lateral arm to the elbow.
C6: Lateral forearms, thumb, and index finger.
C7: Middle finger and central palm.
C8: Medial forearm, ring, and little fingers.
T1: Axilla and medial arm.
T2: Axilla and upper inner arm.
T4: Nipple line.
T6: Xiphoid process level.
T10: Umbilicus (navel) level.
T12: Suprapubic region.
L1: Inguinal region.
L2: Anterior thigh.
L3: Medial knee.
L4: Medial malleolus and great toe.
L5: Dorsum of the foot, digits 2-4.
S1: Lateral malleolus, little toe, and plantar surface of the foot.
S2: Posterior thigh.
S3: Perianal area.
S4-S5: Perianal area and buttocks.
Features:
Each spinal nerve, except C1, has a corresponding dermatome.
Dermatomes are not precisely delineated and often show considerable overlap, meaning an area of skin often receives sensory innervation from more than one spinal nerve. This overlap provides a protective mechanism; if one spinal nerve is damaged, sensation may still be partially preserved in its dermatome due to innervation from adjacent nerves.
Motor Endings
Neuromuscular Junction: Site of skeletal muscle innervation.
Visceral Muscle and Gland: Synapses en passant, varicosities serve to innervate.
Motor Integration:
Segmental: Involves spinal cord circuits.
Projection: Involves upper motor neurons.
Precommand: Involves cerebellum and basal nuclei.
Reflex Arc
Components:
Receptor: Detects a stimulus.
Sensory Neuron: Transmits information to CNS.
Integration Center: Processes the information (either in spinal cord or higher centers).
Motor Neuron: Sends commands from CNS to effector.
Effector: Muscle or gland that responds.
Types of Reflexes
Spinal Reflexes: Can occur with or without higher brain involvement.
Stretch Reflexes: Initiated by muscle spindles; monitor muscle length; accompanied by gamma motor neuron activity.
Golgi Tendon Reflex: Prevents muscle damage from excessive tension; activated by contraction of muscle and relaxation of its antagonistic muscle.
Flexor Reflex: Initiated by painful stimuli; typically polysynaptic.
Crossed Extensor Reflex: Involves coordination between both sides of the body in response to a stimulus.
Superficial Reflexes: Reflexes elicited by gentle cutaneous stimulation, such as the plantar and abdominal reflexes.