Peripheral Nervous System and Reflex Activity Notes

Peripheral Nervous System (PNS)

  • The PNS consists of the voluntary and autonomic nervous systems.

  • The nervous system is divided into:

    • Central Nervous System (CNS)

    • Peripheral Nervous System (PNS)

      • Sensory (afferent) division

      • Motor (efferent) division

        • Somatic nervous system

        • Autonomic nervous system (ANS)

          • Sympathetic division

          • Parasympathetic division

Sensory Receptors

  • Sensory (input) cells can be unencapsulated or encapsulated.

Unencapsulated Sensory Receptors

  • Free nerve endings of sensory neurons

    • Location: Exteroceptors, interoceptors, and proprioceptors

    • Stimulus: Thermoreceptors (warm and cool), chemoreceptors (itch, pH, etc.), mechanoreceptors (pressure), nociceptors (pain, hot, cold, pinch, and chemicals)

    • Body Location: Most body tissues; most dense in connective tissues (ligaments, tendons, dermis, joint capsules, periostea) and epithelia (epidermis, cornea, mucosae, and glands)

  • Tactile discs (Merkel discs)

    • Location: Exteroceptors

    • Stimulus: Mechanoreceptors (light pressure); slowly adapting

    • Body Location: Basal layer of epidermis

  • Hair follicle receptors

    • Location: Exteroceptors

    • Stimulus: Mechanoreceptors (hair deflection); rapidly adapting

    • Body Location: In and surrounding hair follicles

Encapsulated Sensory Receptors

  • Meissner's corpuscles (tactile corpuscles)

    • Location: Exteroceptors

    • Stimulus: Mechanoreceptors (light pressure, discriminative touch, vibration of low frequency); rapidly adapting

    • Body Location: Dermal papillae of hairless skin, particularly nipples, external genitalia, fingertips, soles of feet, eyelids

  • Pacinian corpuscles (lamellated corpuscles)

    • Location: Exteroceptors, interoceptors, and some proprioceptors

    • Stimulus: Mechanoreceptors (deep pressure, stretch, vibration of high frequency); rapidly adapting

    • Body Location: Dermis and hypodermis; periostea, mesentery, tendons, ligaments, joint capsules; most abundant on fingers, soles of feet, external genitalia, nipples

  • Ruffini endings

    • Location: Exteroceptors and proprioceptors

    • Stimulus: Mechanoreceptors (deep pressure and stretch); slowly or non-adapting

    • Body Location: Deep in dermis, hypodermis, and joint capsules

  • Muscle spindles

    • Location: Proprioceptors

    • Stimulus: Mechanoreceptors (muscle stretch, length)

    • Body Location: Skeletal muscles, particularly those of the extremities

  • Golgi tendon organs

    • Location: Proprioceptors

    • Stimulus: Mechanoreceptors (tendon stretch, tension)

    • Body Location: Tendons

  • Joint kinesthetic receptors

    • Location: Proprioceptors

    • Stimulus: Mechanoreceptors and nociceptors

    • Body Location: Joint capsules of synovial joints

Nerve Structure

  • Epineurium: Encloses the entire nerve bundle.

  • Perineurium: Encloses fascicles.

  • Endoneurium: Surrounds each individual nerve cell.

  • Myelin sheath: Encases the axon.

  • Neuron cell membrane: Plasma membrane of a neuron.

  • Ganglia: A collection of nerve cell bodies in the PNS.

  • Nuclei: A collection of nerve cell bodies in the CNS.

Regeneration of Damaged PNS Fibers

  1. The axon becomes fragmented at the injury site.

  2. Macrophages clean out the dead axon distal to the injury.

  3. Schwann cells align and form a regeneration tube. They release growth factors and express cell adhesion molecules (CAMs) that encourage axon growth. Fine axon sprouts or filaments grow through the regeneration tube.

  4. The axon regenerates, and a new myelin sheath forms.

  • Schwann cells are necessary for neuronal recovery.

Cranial Nerves

  • The 12 cranial nerves exit the brain and project to their targets.

  • All cranial nerves leave the brain directly without entering the spinal cord.

  • Cranial nerves I and II (olfactory and optic) leave the cerebrum.

  • Cranial nerves III – XII leave from the brainstem.

  • The job of the cranial nerves is to carry incoming sensory information as well as output motor information.

Cranial Nerve I: Olfactory Nerve

  • Olfactory receptors (sensory) enter the cranial cavity via the olfactory foramina.

  • They contact the olfactory bulb, which projects back to the brain.

  • This entire track, from olfactory receptors to the point where the track enters the brain, is cranial nerve I.

  • It is a sensory nerve tract.

Cranial Nerve II: Optic Nerve

  • Carries information from the retina to the brain.

  • Enters the brain through foramina in the skull and makes contact with neurons in the cerebrum.

  • It is a sensory nerve tract.

Visual Pathway

  • Retina → Optic nerve → Optic chiasm → Optic tract → Visual area of the thalamus → Visual cortex

  • Stereopsis (ranging) can happen where the field of view overlaps.

Clinical Significance

  • Understanding the functions of the cranial nerves allows for the identification of lesions in the brainstem by observing clinical manifestations.

Mnemonic

  • Mnemonic for remembering cranial nerves:

    • "On Old Olympus’s Towering Top A Friendly Viking Grew Vines And Hops"

  • Mnemonic for remembering sensory, motor, or both:

    • "Some Say Marry Money But My Brother Says Big Brains Matter More"

    • S = sensory, M = motor, B = both

Cranial Nerve III: Oculomotor

  • Damage results in an inability to move the eye up, down, or inward. At rest, the eye rotates laterally (external strabismus).

  • The upper eyelid droops (ptosis).

  • The person experiences double vision and difficulty focusing on close objects.

  • Motor ('marry')

Cranial Nerve IV: Trochlear

  • Damage results in double vision and reduced ability to rotate the eye inferolaterally.

  • Motor ('money')

  • Only innervates the superior oblique muscle.

    • When the eye is pointing laterally, contraction of the superior oblique moves the direction of the eye upwards.

    • When the eye is pointed medially, contraction of the superior oblique contorts the eye (rotates the eye on the central axis).

  • Symptoms of a patient with a defective trochlear nerve:

    1. The patient exhibits double vision when looking downwards (e.g., difficulty going down stairs and when reading).

    2. The patient’s head is tilted to one side.

    3. When asked to follow a horizontal pointer, the defective side eye would drift upwards.

Cranial Nerve V: Trigeminal

  • Carries taste information of sweet, sour, and bitter.

  • Damage causes loss of control of facial muscles and excruciating pain.

  • Pain medications are ineffective.

  • Both ('but')

Cranial Nerve VI: Abducens

  • Damage results in inability to move the eye laterally. At rest, the eye rotates medially (internal strabismus).

  • Motor ('my')

  • A functioning abducens moves the eye laterally.

  • A malfunctioning abducens makes the patient unable to move the eye laterally, and the patient will complain of double vision only when looking laterally on the side on which the malfunctioning abducens is present.

General Principle

  • A lesion in a cranial nerve will result in the absence of that nerve’s normal motor function.

  • An overactive cranial nerve sensory pathway can lead to pain.

Cranial Nerve VII: Facial Nerve

  • Affliction of the facial nerve

  • Condition may disappear without treatment.

  • Possible symptoms:

    • Droopy eyelid, dry eye, or excessive tears

    • Facial paralysis, twitching, or weakness

    • Drooping corner of mouth, dry mouth, impaired taste

  • Related to Bell's Palsy

  • Both ('brother')

Cranial Nerve VIII: Vestibulocochlear

  • Damage results dizziness and involuntary rapid eye movements.

  • Sensory ('says')

Cranial Nerve IX: Glossopharyngeal

  • Damage results in an inability to swallow and taste.

  • Both ('big')

Cranial Nerve X: Vagus

  • Damage results in loss of voice or hoarseness, difficulty swallowing, and impaired digestive mobility.

  • Both ('brains')

Cranial Nerve XI: Accessory

  • Damage results in inability to shrug shoulders due to loss of motor input to Trapezius.

  • Motor ('matter')

Cranial Nerve XII: Hypoglossal

  • Damage results in difficulty in speech and swallowing.

  • Bilateral damage results in inability to protrude tongue.

  • Unilateral damage results in tongue pointing to the side of damage.

  • Motor ('more')

Spinal Nerves

  • All spinal nerves carry both motor and sensory information.

  • Include the cervical plexus, brachial plexus, intercostal nerves, lumbar plexus, sacral plexus , and coccygeal nerve.

Brachial Plexus

  • Roots (ventral rami): C5-T1

  • Nerves include axillary, musculocutaneous, radial, median, and ulnar.

Lumbar Plexus

  • Ventral rami: L1-L5

  • Nerves include iliohypogastric, ilioinguinal, femoral, lateral femoral cutaneous, obturator, genitofemoral, anterior femoral cutaneous and saphenous.

Sacral Plexus

  • Ventral rami: L4-S4

  • Nerves include superior gluteal, inferior gluteal, pudendal, sciatic, posterior femoral cutaneous, common fibular, and tibial.

Reflex Arc

  • A stimulus is detected, initiating an action potential in a sensory axon that projects to the dorsal horn of the spinal cord.

  • The sensory axon stimulates, via a synapse, an interneuron, which then stimulates a motor neuron connected to a muscle that withdraws the finger.

  • The brain is not involved, which speeds up the reaction.

  • Example: The reflex arc that keeps you standing upright.

Muscle Spindle

  • Sends action potentials to the CNS whenever a muscle is stretched.

  • Stretch receptors fire more action potentials when stretched.