sensory homunculus returns!

CNS/PNS components, functional relationships

CNS includes: cerebral hemispheres, diencephalon, cerebellum, brainstem, and spinal cord (everything in bony structures)
- analyzes and integrates sensory and motor information

PNS includes:
- sensory components: sensory ganglia and nerves, sensory receptors
- motor components: visceral motor system (autonomic ganglia and nerves), somatic motor system (motor nerves)

CNS communicates to the motor components of PNS, which can then communicate to effectors (smooth muscles, cardiac muscles, glands, skeletal muscles).

Sensory components of PNS communicates sensory info from the internal or external environment to the CNS.

Somatosensation

Many sensations: touch, pressure, vibration, limb position, heat, cold, itch, pain

Transduced by receptors in skin/muscles → CNS

different circuits for different sensations (yay!)

Somatosensory System Overview

1. Cutaneous mechanoreceptors

   • fine touch, vibration, pressure

2. Specialized receptors associated with muscles, tendons, and joints

   • Proprioception - ability to sense the position of our own limbs in space

3. Free nerve endings

   • Pain, temperature, coarse touch

Somatosensory afferents convey information from the skin surface to central circuits

• recall: afferent - towards CNS, efferent - away from CNS

Dermatomes

Each spinal nerve innervates a certain area of the body. Each of these areas is a dermatome.

Response Properties of Somatic Afferents

• Ia, Ib, II afferents: largest and fastest. They supply sensory receptors to muscles for proprioception

• Aβ afferents: smaller. convey touch

• Aδ and C afferents: small and slow, pain and temperature

Receptive Fields

Receptive fields are the area of skin surface over which stimulation results in a significant change in the rate of action potentials (the area a receptor responds to)

Two-point discrimination threshold: the minimum distance two points have to be on a certain area of the body to be differentiated between. More sensitive areas (fingers, face, etc) have a lower threshold.