Sensory Receptors and Nerves: Comprehensive Details

Sensory Receptors and Sensory Nerves

Sensory Receptors

Information from the environment and internal organs is detected by sensory receptors. These receptors can be:

• Surrounded by cells (not neurons)
• Connected with a neuron (sensory nerve) in which they will generate action potentials
• Specialized to respond to one particular form of energy

There are many different types of sensory receptors:

• Mechanoreceptors - detect touch and pressure
• Thermoreceptors - detect cold and warmth
• Nociceptors - detect pain
• Electromagnetic receptors - detect light
• Chemoreceptors - detect taste, smell, blood O₂ and CO₂ concentration, blood glucose, amino acids, and fatty acids

A receptor can be excited by:

• Mechanical deformation - stretches the receptor, opening ion channels
• Application of a chemical to the membrane
• Change of the temperature of the membrane, altering its permeability
• Electromagnetic radiation

Receptor Potential

A receptor potential is an impulse applied to a receptor that changes the membrane's electrical potential. The magnitude of the electric response is proportional to the intensity of the stimulus. When the receptor potential rises above the threshold, it triggers an action potential in the nerve fiber attached to the receptor.

Action Potentials

Action potentials are generated as long as the receptor potential is above the threshold. Their frequency declines over time if a stimulus is maintained at a constant intensity. This is known as adaptation.

• Rapidly adapting receptors: Stimulated only when the stimulus intensity changes (e.g., receptors for light)
• Slowly adapting receptors: Transmit impulses to the brain as long as the stimulus is present (minutes, hours). Examples include nociceptors, which provide a warning, and muscle spindle receptors, which maintain posture.

Sensory Nerves (Afferent Fibers)

Nerve Fibers

General classification of nerve fibers includes types A, B, and C. Sensory nerve classification includes:

• Ia (Aα) (d = 17 μm): From the anulo-spiral endings of muscle spindles
• Ib (Aα) (d = 16 μm): From the Golgi tendon organs
• II (Aβ, Aγ) (d = 8 μm):
  • From discrete cutaneous tactile receptors
  • From the flower-spray endings of the muscle spindles
• III (Aδ) (d = 3 μm): Temperature, crude touch, pricking pain
• IV (C, unmyelinated) (d = 0.5 - 2 μm): Pain, itch, temperature, crude touch sensations

The impulse from the receptor is transmitted through the afferent fiber to the integration center.

Afferent Fiber

• For spinal cord reflexes, the afferent fiber goes through the posterior root ganglion.
• One branch terminates in the grey matter of the spinal cord and produces a local segmental cord reflex.
• Another branch transmits signals to higher levels: spinal cord, brain stem, cerebral cortex.
• Increasing signal strength is transmitted by spatial summation (using a progressively greater number of fibers) or by temporal summation (increasing the frequency of impulses along a single fiber).