PNS and Sensory Receptors

Spinal Nerves and Peripheral Nervous System

Peripheral Nervous System (PNS)

• Includes all neural structures outside the brain and spinal cord.

• Composed of:

  • Sensory receptors

  • Peripheral nerves and associated ganglia

  • Motor endings

Sensory Receptors

• Specialized to respond to changes in their environment (stimuli).

• Activation leads to graded potentials that trigger nerve impulses.

• Sensation (awareness of stimulus) and perception (interpretation of the meaning of the stimulus) occur in the brain.

Classification of Receptors (Based On)

• Stimulus type

• Location

• Structural complexity

Classification by Stimulus Type

• Mechanoreceptors: Respond to touch, pressure, vibration, stretch, and itch.

• Thermoreceptors: Sensitive to changes in temperature.

• Photoreceptors: Respond to light energy (e.g., retina).

• Chemoreceptors: Respond to chemicals (e.g., smell, taste, changes in blood chemistry).

• Nociceptors: Sensitive to pain-causing stimuli (e.g., extreme heat or cold, excessive pressure, inflammatory chemicals).

Classification by Location

• Exteroceptors:

  • Respond to stimuli arising outside the body.

  • Receptors in the skin for touch, pressure, pain, and temperature.

  • Most special sense organs.

• Interoceptors (Visceroceptors):

  • Respond to stimuli arising in internal viscera and blood vessels.

  • Sensitive to chemical changes, tissue stretch, and temperature changes.

• Proprioceptors:

  • Respond to stretch in skeletal muscles, tendons, joints, ligaments, and connective tissue coverings of bones and muscles.

  • Inform the brain of one’s movements.

Classification by Structural Complexity

• Complex receptors (special sense organs):

  • Vision, hearing, equilibrium, smell, and taste.

• Simple receptors for general senses:

  • Tactile sensations (touch, pressure, stretch, vibration), temperature, pain, and muscle sense.

  • Unencapsulated (free) or encapsulated dendritic endings.

Unencapsulated Dendritic Endings

• Thermoreceptors:

  • Cold receptors ($10-40°C$) in superficial dermis.

  • Heat receptors ($32-48°C$) in deeper dermis.

• Nociceptors:

  • Respond to pinching, chemicals from damaged tissue and temperatures outside the range of thermoreceptors.

  • Capsaicin.

• Light touch receptors:

  • Tactile (Merkel) discs.

  • Hair follicle receptors.

Encapsulated Dendritic Endings

• All are mechanoreceptors.

• Meissner’s (tactile) corpuscles—discriminative touch.

• Pacinian (lamellated) corpuscles—deep pressure and vibration.

• Ruffini endings—deep continuous pressure.

• Muscle spindles—muscle stretch.

• Golgi tendon organs—stretch in tendons.

• Joint kinesthetic receptors—stretch in articular capsules.

Sensation and Perception

• Survival depends upon sensation and perception.

• Sensation: the awareness of changes in the internal and external environment.

• Perception: the conscious interpretation of those stimuli.

Sensory Integration

• Input comes from exteroceptors, proprioceptors, and interoceptors.

• Input is relayed toward the head, but is processed along the way.

Levels of Neural Integration in Sensory Systems

• Receptor level—the sensor receptors.

• Circuit level—ascending pathways.

• Perceptual level—neuronal circuits in the cerebral cortex.

Processing at the Receptor Level

• Receptors have specificity for stimulus energy.

• Stimulus must be applied in a receptive field.

• Transduction occurs: stimulus energy is converted into a graded potential called a receptor potential.

• In general sense receptors, the receptor potential and generator potential are the same thing: stimulus to receptor/generator potential in afferent neuron to action potential at first node of Ranvier.

• In special sense organs: stimulus to receptor potential in receptor cell to release of neurotransmitter to generator potential in first-order sensory neuron to action potentials (if threshold is reached).

Adaptation of Sensory Receptors

• Adaptation is a change in sensitivity in the presence of a constant stimulus.

• Receptor membranes become less responsive.

• Receptor potentials decline in frequency or stop.

• Phasic (fast-adapting) receptors signal the beginning or end of a stimulus (e.g., receptors for pressure, touch, and smell).

• Tonic receptors adapt slowly or not at all (e.g., nociceptors and most proprioceptors).

Processing at the Circuit Level

• Pathways of three neurons conduct sensory impulses upward to the appropriate brain regions.

  • First-order neurons: conduct impulses from the receptor level to the second-order neurons in the CNS.

  • Second-order neurons: transmit impulses to the thalamus or cerebellum.

  • Third-order neurons: conduct impulses from the thalamus to the somatosensory cortex (perceptual level).

Processing at the Perceptual Level

• Identification of the sensation depends on the specific location of the target neurons in the sensory cortex.

• Aspects of sensory perception:

  • Perceptual detection—ability to detect a stimulus (requires summation of impulses).

  • Magnitude estimation—intensity is coded in the frequency of impulses.

  • Spatial discrimination—identifying the site or pattern of the stimulus (studied by the two-point discrimination test).

  • Feature abstraction—identification of more complex aspects and several stimulus properties.

  • Quality discrimination—the ability to identify submodalities of a sensation (e.g., sweet or sour tastes).

  • Pattern recognition—recognition of familiar or significant patterns in stimuli (e.g., the melody in a piece of music).

Perception of Pain

• Warns of actual or impending tissue damage.

• Stimuli include extreme pressure and temperature, histamine, $K^+$, ATP, acids, and bradykinin.

• Impulses travel on fibers that release neurotransmitters glutamate and substance P.

• Some pain impulses are blocked by inhibitory endogenous opioids.

Structure of a Nerve

• Cordlike organ of the PNS.

• Bundle of myelinated and unmyelinated peripheral axons enclosed by connective tissue.

• Connective tissue coverings include:

  • Endoneurium: loose connective tissue that encloses axons and their myelin sheaths.

  • Perineurium: coarse connective tissue that bundles fibers into fascicles.

  • Epineurium: tough fibrous sheath around a nerve.

Classification of Nerves

• Most nerves are mixtures of afferent and efferent fibers and somatic and autonomic (visceral) fibers.

• Pure sensory (afferent) or motor (efferent) nerves are rare.

• Types of fibers in mixed nerves:

  • Somatic afferent and somatic efferent.

  • Visceral afferent and visceral efferent.

• Peripheral nerves classified as cranial or spinal nerves.

Ganglia

• Contain neuron cell bodies associated with nerves.

• Dorsal root ganglia (sensory, somatic).

• Autonomic ganglia (motor, visceral).

Regeneration of Nerve Fibers

• Mature neurons are amitotic.

• If the soma of a damaged nerve is intact, axon will regenerate.

• Involves coordinated activity among:

  • Macrophages—remove debris.

  • Schwann cells—form regeneration tube and secrete growth factors.

  • Axons—regenerate damaged part.

• CNS oligodendrocytes bear growth-inhibiting proteins that prevent CNS fiber regeneration.

Cranial Nerves

• Twelve pairs of nerves associated with the brain.

• Most are mixed in function; two pairs are purely sensory.

• Each nerve is identified by a number (I through XII) and a name.

I: The Olfactory Nerves

• Arise from the olfactory receptor cells of nasal cavity.

• Pass through the cribriform plate of the ethmoid bone.

• Fibers synapse in the olfactory bulbs.

• Pathway terminates in the primary olfactory cortex.

• Purely sensory (olfactory) function.

II: The Optic Nerves

• Arise from the retinas.

• Pass through the optic canals, converge and partially cross over at the optic chiasma.

• Optic tracts continue to the thalamus, where they synapse.

• Optic radiation fibers run to the occipital (visual) cortex.

• Purely sensory (visual) function.

III: The Oculomotor Nerves

• Fibers extend from the ventral midbrain through the superior orbital fissures to the extrinsic eye muscles.

• Functions in raising the eyelid, directing the eyeball, constricting the iris (parasympathetic), and controlling lens shape.

IV: The Trochlear Nerves

• Fibers from the dorsal midbrain enter the orbits via the superior orbital fissures to innervate the superior oblique muscle.

• Primarily a motor nerve that directs the eyeball.

V: The Trigeminal Nerves

• Largest cranial nerves; fibers extend from pons to face.

• Three divisions:

  • Ophthalmic (V1): passes through the superior orbital fissure.

  • Maxillary (V2): passes through the foramen rotundum.

  • Mandibular (V3): passes through the foramen ovale.

• Convey sensory impulses from various areas of the face (V1) and (V2), and supplies motor fibers (V3) for mastication.

VI: The Abducens Nerves

• Fibers from the inferior pons enter the orbits via the superior orbital fissures.

• Primarily a motor, innervating the lateral rectus muscle.

VII: The Facial Nerves

• Fibers from the pons travel through the internal acoustic meatuses, and emerge through the stylomastoid foramina to the lateral aspect of the face.

• Chief motor nerves of the face with 5 major branches.

• Motor functions include facial expression, parasympathetic impulses to lacrimal and salivary glands.

• Sensory function (taste) from the anterior two-thirds of the tongue.

VIII: The Vestibulocochlear Nerves

• Afferent fibers from the hearing receptors (cochlear division) and equilibrium receptors (vestibular division) pass from the inner ear through the internal acoustic meatuses, and enter the brain stem at the pons-medulla border.

• Mostly sensory function; small motor component for adjustment of sensitivity of receptors.

IX: The Glossopharyngeal Nerves

• Fibers from the medulla leave the skull via the jugular foramen and run to the throat.

• Motor functions: innervate part of the tongue and pharynx for swallowing, and provide parasympathetic fibers to the parotid salivary glands.

• Sensory functions: fibers conduct taste and general sensory impulses from the pharynx and posterior tongue, and impulses from carotid chemoreceptors and baroreceptors.

X: The Vagus Nerves

• The only cranial nerves that extend beyond the head and neck region.

• Fibers from the medulla exit the skull via the jugular foramen.

• Most motor fibers are parasympathetic fibers that help regulate the activities of the heart, lungs, and abdominal viscera.

• Sensory fibers carry impulses from thoracic and abdominal viscera, baroreceptors, chemoreceptors, and taste buds of posterior tongue and pharynx.

XI: The Accessory Nerves

• Formed from ventral rootlets from the C1–C5 region of the spinal cord (not the brain).

• Rootlets pass into the cranium via each foramen magnum.

• Accessory nerves exit the skull via the jugular foramina to innervate the trapezius and sternocleidomastoid muscles.

XII: The Hypoglossal Nerves

• Fibers from the medulla exit the skull via the hypoglossal canal.

• Innervate extrinsic and intrinsic muscles of the tongue that contribute to swallowing and speech.

Spinal Nerves

• 31 pairs of mixed nerves named according to their point of issue from the spinal cord:

  • 8 cervical (C1–C8)

  • 12 thoracic (T1–T12)

  • 5 Lumbar (L1–L5)

  • 5 Sacral (S1–S5)

  • 1 Coccygeal (C0)

Spinal Nerves: Roots

• Each spinal nerve connects to the spinal cord via two roots:

  • Ventral roots:

    • Contain motor (efferent) fibers from the ventral horn motor neurons.

    • Fibers innervate skeletal muscles.

  • Dorsal roots:

    • Contain sensory (afferent) fibers from sensory neurons in the dorsal root ganglia.

    • Conduct impulses from peripheral receptors.

• Dorsal and ventral roots unite to form spinal nerves, which then emerge from the vertebral column via the intervertebral foramina.

Cervical Plexus

• Formed by ventral rami of C1–C4.

• Innervates skin and muscles of the neck, ear, back of head, and shoulders.

• Phrenic nerve:

  • Major motor and sensory nerve of the diaphragm (receives fibers from C3–C5).

Brachial Plexus

• Formed by ventral rami of C5–C8 and T1 (and often C4 and T2).

• It gives rise to the nerves that innervate the upper limb.

• Major branches of this plexus:

  • Roots—five ventral rami (C5–T1)

  • Trunks—upper, middle, and lower

  • Divisions—anterior and posterior

  • Cords—lateral, medial, and posterior

Brachial Plexus: Nerves

• Axillary—innervates the deltoid, teres minor, and skin and joint capsule of the shoulder.

• Musculocutaneous—innervates the biceps brachii and brachialis and skin of lateral forearm.

• Median—innervates the skin, most flexors and pronators in the forearm, and some intrinsic muscles of the hand.

• Ulnar—supplies the flexor carpi ulnaris, part of the flexor digitorum profundus, most intrinsic muscles of the hand, and skin of medial aspect of hand.

• Radial—innervates essentially all extensor muscles, supinators, and posterior skin of limb.

Lumbar Plexus

• Arises from L1–L4.

• Innervates the thigh, abdominal wall, and psoas muscle.

• Femoral nerve—innervates quadriceps and skin of anterior thigh and medial surface of leg.

• Obturator nerve—passes through obturator foramen to innervate adductor muscles.

Sacral Plexus

• Arises from L4–S4.

• Serves the buttock, lower limb, pelvic structures, and perineum.

• Sciatic nerve:

  • Longest and thickest nerve of the body.

  • Innervates the hamstring muscles, adductor magnus, and most muscles in the leg and foot.

  • Composed of two nerves: tibial and common fibular.

Innervation of Skin

• Dermatome: the area of skin innervated by the cutaneous branches of a single spinal nerve.

• All spinal nerves except C1 participate in dermatomes.

• Most dermatomes overlap, so destruction of a single spinal nerve will not cause complete numbness.

Referred Pain

• Occurs when pain is perceived at a location other than the site of the painful stimulus.

• Pain of a heart attack is perceived as left shoulder and arm pain.

• This is because dermatomes for the shoulder and arm have cell bodies in the same dorsal root ganglia and synapse in the same second order neuron as general visceral sensory fibers to the heart.

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