Nervous System: Afferent and Efferent Divisions

Nervous System Divisions

• The nervous system can be divided into afferent (sensory) and efferent (motor) divisions.

Afferent Division

• Brings sensory information from the outside world into the brain.
• Involves special senses (vision, hearing, taste, smell) and general somatic senses (somatosensory system).
• The somatosensory system is involved in the sense of touch, proprioception, pain, and temperature.

Sensory Neurons

• Found all over the body.
• Have receptors classified according to the stimulus they respond to:
  • Mechanoreceptors for touch and proprioception.
  • Nociceptors for pain.
  • Thermoreceptors for temperature.

Neuron Structure

• Composed of a cell body (containing organelles) and nerve fibers (projections extending from the neuron cell body).
• Nerve fibers are either:
  • Dendrites (receive signals from other neurons).
  • Axons (send signals to other neurons).

Synapse

• Where two neurons come together.
• One end of an axon sends neurotransmitters to the dendrites or directly to the cell body of the next neuron in the series.

Somatosensory Pathways

• Made up of a relay of four neurons:
  • First order neuron (sensory neuron) with sensory receptors that convert stimuli into an impulse.
  • Second order neuron with its cell body in the spinal cord or brainstem.
  • Third order neuron with its cell body in the thalamus.
  • Fourth order neuron with its cell body in the sensory cortex of the brain.

First Order Neurons

• Also called pseudounipolar neurons.
• Have only one axon that extends out from the cell body, with two branches:
  • Peripheral branch (goes from the cell body to peripheral tissues).
  • Central branch (goes from the peripheral branch to the spinal cord).

Receptive Fields

• The area that a first order neuron receives sensory input from.
• Vary in size, with smaller receptive fields having higher resolution (allowing stimuli to be localized more precisely).
• Example: smaller receptive fields in the fingertips than at the back, allowing for high resolution in reading Braille.

Action Potential

• Triggered when a first order neuron reaches a certain threshold of depolarization.
• Sent through the peripheral branch and back to the central branch, and from there to the spinal cord.

Two-Point Discrimination

• Requires receptive fields of two sensory neurons to be separated by at least one neuron's receptive field.
• The third neuron sends a negative signal between the two positive ones, allowing two-point discrimination.