Peripheral Nervous System

Exam Information

  • Exam 4 (Final Exam)
    • Not comprehensive.
    • Location: This room.
    • Covers the current chapter and the muscle chapter.
    • Hardest exam of the semester, so study accordingly.

Review of Neuron Structure

  • Neuron: Cell body, axon, presynaptic terminals.
    • Releases neurotransmitter onto receptors of another cell (neuron or muscle cell).
    • Synapse: The junction between neurons or between a neuron and a muscle cell.

Nerves vs. Tracts vs. Ganglia vs. Nucleus

  • Nerve: Bundle of axons in the peripheral nervous system (PNS).
    • Analogy: Like a rope made of smaller strings (axons).
  • Tract: Bundle of axons in the central nervous system (CNS).
  • Ganglion: Group of cell bodies in the PNS.
  • Nucleus: Group of cell bodies in the CNS.

Peripheral Nervous System Overview

  • Green areas in the diagram represent the PNS.
  • Receptors: Detect stimuli.
  • Sensory Nerves: Carry information from receptors to the CNS (brain, spinal cord).
    • Sensory Input: Information to the CNS.
    • Information Conduction: Via action potentials along axons (saltatory or continuous).
  • Motor Nerves: Carry information from the CNS to effectors.
    • Motor Output: Information away from the CNS.
  • Effectors: Structures (muscles, glands) that respond to nerve stimulation.

Divisions of the Peripheral Nervous System

  • Somatic Nervous System: Voluntary control (e.g., skeletal muscles).
  • Autonomic Nervous System: Involuntary control (e.g., heart, smooth muscle).
    • Subdivisions:
      • Sympathetic Nervous System
      • Parasympathetic Nervous System
      • Enteric Nervous System: Controls the digestive system -- will be discussed in the second semester.

Sensory Receptors

  • Somatic Receptors: Located in skin, muscles, joints, nose.
    • Exteroceptors: Detect external stimuli (touch, pressure, pain, smell).
      • Examples: Merkel's disc, Pacinian corpuscle (detect pressure), free nerve endings (pain receptors).
    • Proprioceptors: Detect body position and movement (muscle spindles, Golgi tendon apparatus).
      • Stretch reflex example: Knee-jerk reflex.
  • Autonomic Receptors: Located in viscera (organs, blood vessels).
    • Detect internal stimuli.
      • Baroreceptors: Detect blood pressure.
      • Mechanoreceptors: Detect stretch (e.g., in the stomach, bladder).
      • Chemoreceptors: Detect chemicals (H+ ions, pHpH, NaHCO3NaHCO_3).
      • Osmoreceptors: Detect osmolarity (concentration of fluids).

Receptor Function

  • All receptors function similarly: Stimulus → electrical signal (membrane potential change) → action potential frequency.

  • Diagram Explanation

    • Receptor: Detects a specific stimulus.
    • Sensory Axon: Carries information via action potentials to the CNS.
    • Baseline Stimulus: Initial stimulus level.
      • Baseline Action Potential Frequency.
    • Increased Stimulus:
      • Depolarizes the receptor and axon.
      • Increases action potential frequency.
    • Decreased Stimulus:
      • Still depolarizes to the threshold.
      • Decreases action potential frequency.
    • Stimulus↑Stimulus \uparrow → Depolarization↑Depolarization \uparrow → AP Frequency↑AP\ Frequency \uparrow
    • Stimulus↓Stimulus \downarrow → Depolarization↓Depolarization \downarrow → AP Frequency↓AP\ Frequency \downarrow

Cholinergic and Adrenergic Terminology

  • Cholinergic: Anything associated with acetylcholine (ACh).
    • Cholinergic Neuron: A neuron that releases acetylcholine.
    • Cholinergic Receptor: Receptor that binds acetylcholine.
      • Two Types: Muscarinic and Nicotinic.
  • Adrenergic: Anything associated with epinephrine and norepinephrine.
    • Adrenergic Neuron: A neuron that releases norepinephrine.
    • Adrenergic Receptor: Receptor that binds norepinephrine.
      • Two Types: Alpha ($\alpha$) and Beta ($\beta$).

Somatic Motor Output

  • Voluntary control of skeletal muscles.
  • Central Nervous System: Brain stem and spinal cord.
  • Motor Neurons: Myelinated.
  • Effectors: Skeletal Muscles.
  • Neurotransmitter: Acetylcholine.
    • Motor neurons of the somatic nervous system are cholinergic because they release acetylcholine.
  • Receptor on skeletal muscle: Nicotinic.
  • Process:
    • Higher parts of the brain tell motor neurons to release acetylcholine.
    • That release causes muscles to contract so you can move.

Autonomic Motor Output

  • Controls visceral cells, smooth muscle, and cardiac muscle.
  • Two Motor Neurons:
    • Preganglionic Neuron: From CNS, myelinated.
    • Postganglionic Neuron: To effector, not myelinated.
  • Effectors:
    • Cardiac muscle.
    • Smooth muscle: Wrapped around hollow organs (stomach, intestines, bladder, uterus, blood vessels).
    • Visceral cells: Cells of glands and other organs that release hormones.
  • Ganglia: Cell bodies of postganglionic neurons.
  • Preganglionic Neuron: Arises from the brainstem, thoracic, lumbar, or sacral region of spinal cord, always before.
  • Post ganglionic Neurons: Synapses with preganglionic neurons, located after the ganglion.