Somatic Nervous System Notes

Somatic Nervous System

Subdivision of PNS

  • The peripheral nervous system (PNS) is subdivided with components in both the brain and spinal cord.
  • Somatic Nervous System:
    • Upper motor neurons in the primary motor cortex.
    • Somatic motor nuclei of the brain stem and spinal cord.
    • Connected to skeletal muscle.
  • Autonomic Nervous System:
    • Visceral motor nuclei in the hypothalamus and autonomic nuclei in the brain.
    • Preganglionic neurons in the brain and spinal cord connect to autonomic ganglia.
    • Ganglionic neurons connect to visceral effectors (smooth muscle, cardiac muscle, glands, adipocytes).

Autonomic vs. Somatic Pathways

  • (a) Autonomic Pathway:
    • Sensory neuron transmits information to the spinal cord.
    • Interneurons process the information.
    • Preganglionic fiber from the spinal cord extends to an autonomic ganglion.
    • Postganglionic fiber from the autonomic ganglion innervates a viscera.
  • (b) Somatic Pathway:
    • Sensory neuron transmits information to the spinal cord.
    • Somatic motor neuron extends from the spinal cord to a skeletal muscle.

Sensory Receptors

  • Specialized cell or cell process that monitors specific conditions in the body or external environment.
  • General Senses:
    • Distributed throughout the body.
    • Simple in structure.
    • Include somatic senses (tactile, thermal, pain, proprioception) and visceral senses.
  • Somatic Senses:
    • Tactile sensation: touch, pressure, vibration.
    • Thermal sensation: warm & cold.
    • Pain sensation.
    • Proprioception
  • Visceral sensation:
    • Internal organs

General Sensory Receptor

  • Sensory input is received by a sensory neuron's dendrites, which lead to the cell body and axon.
  • The receptor, often a free nerve ending, initiates the sensory process.

Special Senses

  • Elaborate structures located in sense organs.
  • Receptors are protected by surrounding tissue.
  • Include:
    • Olfaction (smell).
    • Vision (sight).
    • Gustation (taste).
    • Equilibrium (balance).
    • Hearing.

Detection of Stimuli

  1. Stimulation of the sensory receptor:
    • An appropriate stimulus occurs within the receptor’s receptive field.
  2. Transduction or translation of stimulus:
    • Energy is converted to a graded potential.
    • The receptor exhibits stimulus selectivity.
  3. Generation of impulses:
    • Involves a first-order neuron.
  4. Integration of sensory input:
    • The CNS receives and integrates the sensory nerve impulse.

Tonic vs. Phasic Receptors

  • Tonic receptors:
    • Always active.
    • Always “record” background levels of stimulation.
  • Phasic receptors:
    • Normally inactive.
    • Active for a short period of time.
    • Provide information about the intensity and rate of change of a stimulus.

Adaptation

  • A reduction in sensitivity in the presence of a constant stimulus.
    • Fast-adaptors (characteristic of phasic receptors):
      • Specialized for signaling changes in stimulus.
      • Examples: Touch, pressure, smell.
    • Slow-adaptors (characteristic of tonic receptors):
      • Continue to trigger nerve impulses as long as the stimulus persists.
      • Examples: Pain, body position, chemical composition of the blood.

Receptors for General Senses Classified by Stimulus Type

  • Nociceptors: Pain.
  • Thermoreceptors: Temperature.
  • Mechanoreceptors: Physical changes in position, touch, etc.
  • Chemoreceptors: Chemical composition of body fluids.

Nociceptors

  • Free nerve endings with large receptive fields.
  • Tonic receptors.
  • Found in every tissue of the body except the brain.
  • Sensitive to:
    • Extremes of temperature.
    • Mechanical damage.
    • Dissolved chemicals.
  • Two types of axons:
    • Type A (fast pain - acute, sharp; e.g., needle prick).
    • Type C (slow pain - excruciating, throbbing).

Thermoreceptors

  • Temperature receptors are free nerve endings.
  • Located in the dermis of the skin, skeletal muscle, liver, and hypothalamus.
  • Cold receptors are more numerous than warm receptors.
  • Phasic receptors and highly adaptable.

Mechanoreceptors

  • Sensitive to stimuli that distort their cell membranes.
  • Contain mechanically regulated ion channels.
    • Tactile receptors: Provide sensation of touch, pressure, and vibration.
    • Baroreceptors: Detect changes in pressure in walls of blood vessels & portions of the digestive, reproductive, and urinary tracts.
    • Proprioceptors: Position of joints and muscles.

Tactile Receptors

  • Mediate crude vs. fine touch & pressure, distortion & movement
  • Specific examples include those for fine touch, vibration, stretch, and pressure.

Chemoreceptors

  • Detect small changes in the concentration of specific chemicals or compounds.
  • Sensory information is routed to brain centers that deal with the autonomic control of respiratory and cardiovascular function.
  • Located in:
    • Respiratory centers of the medulla oblongata.
    • Carotid bodies.
    • Aortic bodies.

Organization of Sensory Pathways

  • First-order neurons: Sensory neuron delivers sensations to the CNS.
  • Second-order neurons: Axon of the sensory neuron synapses on an interneuron in the CNS.
  • Third-order neurons: If the sensation is to reach our awareness, the second-order neuron synapses on a third-order neuron in the thalamus.

3 Major Somatic Sensory Pathways

  • Posterior column pathway
  • Spinocerebellar pathway
  • Spinothalamic pathway

Posterior Column Pathway

  • Carries sensations of highly localized (