Peripheral Nervous System

Peripheral Nervous System

Place of the PNS in the Structural Organization of the Nervous System

• The peripheral nervous system (PNS) provides links to the outside world.
• It consists of neural structures outside the brain and spinal cord.
• The PNS is divided into four parts:
  • Sensory Receptors
  • Transmission Lines: Nerves and Their Structure and Repair
  • Motor Endings and Motor Activity
  • Reflex Activity

Part 1: Sensory Receptors and Sensation

• Sensory receptors are activated by changes in the internal or external environment.
• Learning Outcomes:
  • Classify general sensory receptors by stimulus detected, body location, and structure.
  • Outline the events that lead to sensation and perception.
  • Describe receptor and generator potentials and sensory adaptation.
  • Describe the main aspects of sensory perception.

13.1 Sensory Receptors

• Sensory receptors are specialized to respond to changes in the environment (stimuli).
• Sensation is the awareness of a stimulus.
• Classification of sensory receptors is based on:
  • Type of stimulus
  • Body location
  • Structural complexity

Classification by Stimulus Type

• Mechanoreceptors: Respond to touch, pressure, vibration, and stretch.
• 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.
  • Sometimes cause discomfort but usually the person is unaware of their workings.
• 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.

Structural Complexity

• Complex Receptors (Special Senses):
  • Vision: Receptors in the retina of the eye.
  • Olfaction: Receptors in the olfactory epithelium.
  • Taste: Receptors in the tongue papillae.
  • Hearing and Balance:
    • Hearing receptors are in the cochlea (spiral organ).
    • Balance receptors are in the crista ampullaris and vestibule maculae.
• Simple Receptors:
  • Modified dendritic endings of sensory neurons that monitor general senses.
  • Tactile: Touch, pressure, stretch, vibration.
  • Others: Temperature, pain, and muscle sense.
  • Found throughout the body.
  • Receptors respond to multiple stimuli.
  • Have either nonencapsulated (free) nerve endings or encapsulated nerve endings.

13.2 Sensory Processing: Somatosensory System

• Survival depends upon:
  • Sensation: Awareness of changes in the internal and external environment.
  • Perception: Conscious interpretation of those stimuli.
• Somatosensory system: Part of the sensory system serving the body wall and limbs.
  • Receives inputs from exteroceptors, proprioceptors, and interoceptors.
  • Input is relayed toward the head but processed along the way.
• Levels of neural integration in sensory systems:
  • Receptor level: Sensory receptors.
  • Circuit level: Processing in ascending pathways.
  • Perceptual level: Processing in cortical sensory areas.

Receptor Level - Generating a Signal

• Stimulus energy must match receptor specificity.
  • Example: Touch receptors respond to pressure but not to light.
• Stimulus must be applied within the receptive field.
  • Small receptive field = better tactile localization.
• Transduction must occur.
  • Energy of stimulus is converted into a graded potential:
    • Generator potential in general receptors.
    • Receptor potential in special sense receptors.
• Graded potentials must reach threshold, leading to an action potential (AP).

Receptors Can Adapt

• Adaptation: Change in sensitivity in the presence of a constant stimulus.
  • Receptor membranes become less responsive.
  • Action potentials decline in frequency or stop.
• Phasic receptors are fast-adapting.
  • Send signals at the beginning or end of a stimulus.
  • Examples: Receptors for pressure, touch, and smell.
• Tonic receptors adapt slowly or not at all.
  • Examples: Nociceptors and most proprioceptors.

Processing at the Circuit Level

• Ascending pathways conduct sensory impulses from receptors to the cerebral cortex.
  • Dorsal column-medial lemniscal pathway.
  • Spinothalamic pathway.
  • Spinocerebellar pathway (only 2 neurons).
• First-order sensory neurons:
  • From receptor level to spinal reflexes or second-order neurons in the CNS.
• Second-order sensory neurons:
  • Impulses to third-order sensory neurons in the thalamus or cerebellum.
• Third-order sensory neurons:
  • Impulses from the thalamus to the somatosensory cortex.

Perceptual Level: Somatosensory Cortex

• Aspects of sensory perception:
  • Perceptual detection: Ability to detect a stimulus; requires stimulation of multiple receptors.
  • Magnitude estimation: Intensity coded in the frequency of impulses; increase in frequency = increase in intensity.
  • Spatial discrimination: Identifying the site or pattern of the stimulus (studied by the two-point discrimination test).
    • Varies depending on how sensitive an area is.
  • Feature abstraction: Identification of more complex aspects and several stimulus properties.
  • Quality discrimination: Ability to identify submodalities of a sensation.
    • Example: Sweet or sour tastes.
  • Pattern recognition: Recognition of familiar or significant patterns in stimuli.
    • Example: Melody in a piece of music.

Perception of Pain

• Pain warns of actual or impending tissue damage so protective action can be taken.
• Stimuli include extreme pressure and temperature.
• Impulses travel on fibers that release certain neurotransmitters.
  • Example: Glutamate.
• Some pain impulses are blocked by inhibitory endogenous opioids (example: endorphins).
• Pain tolerance:
  • All perceive pain at the same stimulus intensity.
  • Pain tolerance varies.
  • "Sensitive to pain" means low pain tolerance, not low pain threshold.

Clinical – Homeostatic Imbalance 13.1

• Hyperalgesia (pain amplification):
  • Caused by long-lasting or intense pain.
  • Early pain management is critical to prevent after amputation.
• Phantom limb pain: Pain felt in a limb that has been amputated.
  • Epidural anesthesia is now used during surgery to reduce phantom pain.

Part 2: Transmission Lines: Nerve Structure and Repair

Learning Outcomes

• Describe the general structure of a nerve.
• Define ganglion and indicate the general body location of ganglia.
• Follow the process of nerve regeneration.

Structure of a Nerve

• Nerve: Cord-like organ of the PNS.
• Bundle of myelinated and nonmyelinated peripheral axons enclosed by connective tissue.
• Connective tissue coverings include:
  • Endoneurium: Loose connective tissue that encloses axons and their myelin sheaths (Schwann cells).
  • Perineurium: Coarse connective tissue that bundles fibers into fascicles.
  • Epineurium: Tough fibrous sheath around all fascicles to form the nerve.

Classification of Nerves

• Most nerves are mixtures of afferent and efferent fibers and somatic and autonomic (visceral) fibers.
• Mixed nerves transmit impulses in both directions:
  • Somatic afferent (sensory from muscle to brain).
  • Somatic efferent (motor from brain to muscle).
  • Visceral afferent (sensory from organs to brain).
  • Visceral efferent (motor from brain to organs).
• Sensory (afferent) nerves: Impulses only toward the CNS.
• Motor (efferent) nerves: Impulses only away from the CNS.
• Pure sensory (afferent) or pure motor (efferent) nerves are rare; most nerves are mixed.

Ganglia

• Contain neuron cell bodies associated with nerves in the PNS.
• Ganglia associated with afferent nerve fibers contain cell bodies of sensory neurons.
  • Dorsal root ganglia (sensory, somatic).
• Ganglia associated with efferent nerve fibers contain autonomic motor neurons.
  • Autonomic ganglia (motor, visceral) (Chapter 14).

Regeneration of Nerve Fibers

• Mature neurons are amitotic.
• PNS axons can regenerate if damage is not severe.
  • If the soma (cell body) of the damaged nerve is intact.
  • The regenerated axon is not the same as before the injury.
  • New nerves need re-training.
• CNS axons rarely regenerate.
  • Neuroglia associated with CNS neurons block inhibit repair.
  • Oligodendrocytes release growth-inhibiting proteins that prevent CNS fiber regeneration.
  • Astrocytes at the injury site form scar tissue.

Regeneration of a Nerve Fiber in a Peripheral Nerve

1. Axon fragments and myelin sheaths distal to injury degenerate (Wallerian degeneration); degeneration spreads down the axon.
2. Macrophages clean dead axon debris; Schwann cells are stimulated to divide.
3. Axon filaments grow through the regeneration tube.
4. Axon regenerates, and a new myelin sheath forms.

13.4 Cranial Nerves

Learning Outcome

• Name the 12 pairs of cranial nerves; indicate the body region and structures innervated by each.
• 12 pairs of cranial nerves are associated with the brain.
  • Two attach to the forebrain, the rest with the brain stem.
  • Most are mixed nerves, but two pairs are purely sensory: Olfactory & Optic (I & II).
  • Each is numbered (I through XII) and named from rostral to caudal.
• Mnemonic phrases to remember the names and order:
  • "On occasion, our trusty truck acts funny—very good vehicle anyhow"
  • "Oh once one takes the anatomy final, very good vacations are heavenly"
• The 12 cranial nerves are:
  • I: Olfactory—smell
  • II: Optic—vision
  • III: Oculomotor—"eye mover"
  • IV: Trochlear—innervates Superior Oblique eye muscle
  • V: Trigeminal—three branches, chewing muscles
  • VI: Abducens—abducts the eyeball
  • VII: Facial—facial expression
  • VIII: Vestibulocochlear—Hearing and balance (aka auditory nerve)
  • IX: Glossopharyngeal—"tongue and pharynx"
  • X: Vagus—"wanderer", the only cranial nerve that goes beyond the head and neck
  • XI: Accessory—part of the vagus nerve
  • XII: Hypoglossal—under the tongue

13.5 Spinal Nerves

Learning Outcomes

• Describe the general structure of a spinal nerve and the general distribution of its rami.
• Define plexus.

Spinal Nerves and Peripheral Motor Endings

• 13.5: 31 pairs of spinal nerves innervate the body.
• 13.6: Peripheral motor endings connect nerves to their effectors.
• All are mixed nerves named for their point of exit from the spinal cord.
• Supply all body parts except the head and part of the neck.
• 31 pairs of spinal nerves:
  • 8 pairs of cervical nerves (C1–C8)
  • 12 pairs of thoracic nerves (T1–T12)
  • 5 pairs of lumbar nerves (L1–L5)
  • 5 pairs of sacral nerves (S1–S5)
  • 1 pair of tiny coccygeal nerves (Co1)

Names of Spinal Nerves

• 7 cervical vertebrae give rise to 8 pairs of cervical spinal nerves because:
  • Each of the first 7 pairs (C1 to C7) exits the vertebral canal superior to the vertebra for which it is named.
  • The last spinal nerve (C8) exits the canal inferior to C7.
  • So vertebra C7 has a nerve that leaves above it and one that leaves below it.
  • Each of the other spinal nerves exits inferior to the vertebra for which it is named.

Roots and Rootlets

• Each spinal nerve is connected to the spinal cord via two roots:
  • Ventral roots:
    • Contain motor (efferent) fibers from the ventral horn motor neurons that innervate skeletal muscles.
  • Dorsal roots:
    • Contain sensory (afferent) fibers from sensory neurons in the dorsal root ganglia that conduct impulses from peripheral receptors.
  • Both ventral and dorsal roots are branched medially as rootlets that then join laterally to form the spinal nerve.

Spinal Nerve Branches

• Spinal nerves are quite short (~1–2 cm).
• Spinal nerves divide into three branches:
  • Dorsal ramus: Smaller branch.
  • Ventral ramus: Larger branch.
  • Meningeal branch: A tiny branch that reenters the vertebral canal to innervate meninges and blood vessels.
  • Rami communicantes contain autonomic nerve fibers that join ventral rami in the thoracic region.

Innervation of Specific Body Regions

• Spinal nerve rami and their main branches supply the entire somatic region of the body from the neck down.
  • Dorsal rami supply the posterior body trunk.
  • Ventral rami supply the rest of the trunk and limbs.
• Difference between roots and rami:
  • Each root is purely sensory or motor.
  • Rami are lateral branches of spinal nerves and can carry both sensory and motor signals.

Nerve Plexuses

• All ventral rami except T2–T12 form interlacing nerve networks called nerve plexuses.
  • Found in the cervical, brachial, lumbar, and sacral areas.
  • Only ventral rami form plexuses.
• Within a plexus, fibers crisscross so that:
  • Each branch contains fibers from several different spinal nerves.
  • Fibers from a ventral ramus go to the body periphery via several routes.
  • Means each limb muscle is innervated by more than one spinal nerve, so damage to one does not cause paralysis.

Part 3: Motor Endings and Motor Activity

13.6 Peripheral Motor Endings

Learning Outcome

• Compare and contrast the motor endings of somatic and autonomic nerve fibers.

• PNS elements that activate effectors by releasing neurotransmitters.

• These elements innervate skeletal muscle.

  • Takes place at the neuromuscular junction

• Also innervate visceral muscle and glands.

• Autonomic motor endings and visceral effectors are simpler than somatic junctions.

  • Branches form synapses en passant ("synapses in passing") with effector cells via varicosities (knob-like swellings).
  • Acetylcholine and norepinephrine act indirectly via second messengers.
  • Visceral motor responses are slower than somatic responses.

13.7 Levels of Motor Control

Learning Outcome

• Outline the three levels of the motor hierarchy.
• Compare the roles of the cerebellum and basal nuclei in controlling motor activity.
• The cerebellum and basal nuclei are the ultimate planners and coordinators of complex motor activities.
• Complex motor behavior depends on complex patterns of control.
  • Segmental level.
  • Projection level.
  • Precommand level.

Part 4: Reflex Activity

13.8 The Reflex Arc

• Enables rapid and predictable responses.

13.9 Spinal Reflexes

• Somatic reflexes mediated by the spinal cord.

Learning Outcomes

• Name the components of a reflex arc and distinguish between autonomic and somatic reflexes.
• Compare and contrast stretch, flexor, crossed-extensor, and tendon reflexes.
• Describe two superficial reflexes.

13.8 The Reflex Activity

• Inborn (intrinsic) reflex: Rapid, involuntary, predictable motor response to a stimulus.
  • Examples: Maintain posture, control visceral activities.
  • Can be modified by learning and conscious effort.
• Learned (acquired) reflexes result from practice or repetition.
  • Example: Driving skills.

Components of a Reflex Arc

• Receptor: Site of stimulus action.
• Sensory neuron: Transmits afferent impulses to the CNS.
• Integration center: Either monosynaptic or polysynaptic region within the CNS.
• Motor neuron: Conducts efferent impulses from the integration center to the effector organ.
• Effector: Muscle fiber or gland cell that responds to efferent impulses by contracting or secreting.

Types of Reflex Arcs

• Reflexes are classified functionally as:
  • Somatic reflexes: Activate skeletal muscle.
  • Autonomic (visceral) reflexes: Activate visceral effectors (smooth or cardiac muscle or glands).

13.9 Spinal Reflexes

• Are Somatic.
• Occur without direct involvement of higher brain centers.
  • The Brain is still advised of spinal reflex activity and may have an effect on the reflex.
• Testing of somatic reflexes is clinically important to assess the condition of the nervous system.
  • If exaggerated, distorted, or absent, it may indicate degeneration or pathology of specific nervous system regions.
  • Most commonly assessed reflexes are stretch, flexor, and superficial reflexes.

Initiating Stretch and Tendon Reflexes

• To smoothly coordinate skeletal muscle, the nervous system must receive proprioceptor input regarding:
  • Length of muscle: Information sent from muscle spindles.
  • Amount of tension in muscle: Information sent from tendon organs.

Stretch Reflexes

• The brain sets the muscle’s length via the stretch reflex.
• Example: The knee-jerk reflex is a stretch reflex that keeps knees from buckling when you stand upright.
• Stretch reflexes maintain muscle tone in large postural muscles and adjust it reflexively.
  • Causes muscle contraction on the side of the spine in response to increased muscle length (stretch) on the other side of the spine.

Tendon Reflex

• Involves polysynaptic reflexes.
• Helps prevent damage due to excessive stretch.
• Important for smooth onset and termination of muscle contraction.
• Produces muscle relaxation (lengthening) in response to tension.
• Contraction or passive stretch activates the tendon reflex.
• Afferent impulses transmitted to the spinal cord.
  • Contracting muscle relaxes; antagonist contracts (reciprocal activation).
  • Information transmitted simultaneously to the cerebellum and used to adjust muscle tension.

The Flexor Reflexes

• The flexor (withdrawal) reflex is initiated by a painful stimulus.
• Causes automatic withdrawal of the threatened body part.
  • Ipsilateral and polysynaptic.
  • Many different muscles may be called into play, so it needs to be polysynaptic.
  • Protective and important for survival.
  • The Brain can override.
  • Example: Knowing a finger stick for a blood test is coming, the brain overrides pulling arm away.

The Crossed-Extensor Reflex

• Occurs with flexor reflexes in weight-bearing limbs to maintain balance.
• Consists of an ipsilateral withdrawal reflex and a contralateral extensor reflex.
  • Stimulated side withdrawn (flexed).
  • Contralateral side extended.
• Examples:
  • Stepping barefoot on broken glass causes damaged leg to withdraw, and the opposite leg extends to support weight shift.
  • Someone grabbing your arm causes that arm to flex, and the opposite arm extends to pull the body away.

Superficial Reflexes

• Are elicited by gentle cutaneous stimulation of an area.
• Clinically important reflexes signal problems in upper motor pathways or cord-level reflex arcs.
• Plantar reflex: Tests integrity of the cord from L4 to S2.
  • Downward flexion of toes when the lateral sole of the foot is stroked.
  • Damage to the motor cortex or corticospinal tracts causes an abnormal response known as Babinski’s sign.
    • The hallux dorsiflexes; smaller toes fan laterally.
    • Normal in infancy to the age of ~1 year.
• Abdominal reflex: Tests integrity of the cord from T8 to T12.
  • Contraction of abdominal muscles when stroking the skin of the lateral abdomen above, below, or to the side of the umbilicus.
  • Movement of the umbilicus toward the stimulus (tickle reflex).