physiology of sensory system

SENSORY PHYSIOLOGY
OBJECTIVES
After studying this lecture, you should be able to:
• Define receptor potential.
• Explain the differences between pain and nociception, first and second pain, acute and chronic pain, hyperalgesia and allodynia.
• Describe and explain visceral and referred pain.
• Compare the pathway that mediates sensory input from touch, proprioceptive, and vibratory senses to that mediating information from nociceptors and thermoreceptors.
• Describe processes involved in modulation of transmission in pain pathways.
Sensory physiology
The afferent division of the peripheral nervous system transmits information detected by sensory receptors that respond to specific types of stimuli from the periphery to the central nervous system (CNS). Whereas some of these receptors detect stimuli from the skin like touch and pain (called cutaneous receptors), others, called visceral receptors, detect stimuli that arise within the body like baroreceptor and chemoreceptors. Following table (1) show the classification of sensory receptors:-




Type of sensation Receptor
A- Mechanoreceptors
I. Free nerve ending
II. Merkel s discs
III. Ruffini s endings
IV. Meissner s corpuscles
V. Hair end-organs
VI. Pacinian corpuscles
Muscle receptors
I. Muscle spindles
II. Golgi tendon receptors
Hearing
Sound receptors of cochlea
Equilibrium
Vestibular receptors
Arterial pressure
Baroreceptors of carotid sinuses and aorta.
B- Thermoreceptors I. Cold receptors
II. Warm receptors
C- Nociceptors Free nerve endings
D- Electromagnetic receptors Rods Cones
E- Chemoreceptors
I. Taste-Receptors of taste buds
II. Smell-Receptors of olfactory epithelium
III. Arterial oxygen-Receptors of aortic and carotid bodies
IV. Osmolality- supraoptic nuclei
V. Blood CO2-Receptors in medulla and in aortic and carotid bodies
VI. Blood glucose, amino acids, fatty acids-Receptors in hypothalamus

table (1) showing the classification of sensory receptors.

Receptor Physiology
Sensory receptors are specialized structures that detect a specific form of energy in the external environment. Each of the principal types of sensation that we can experience like pain, touch, sight, sound, and so forth-is called a modality of sensation. Each receptor is sensitive and respond to one modality ex. nociceptors respond only to painful stimuli and will not be stimulated by pressure, but if pressure become so intense and causes damage to the tissue, it will activate the pain receptors and perceived as painful stimulus.
Receptor potentials
When a small amount of pressure is applied to a sensory receptor like Pacinian corpuscle, a non-propagated depolarizing potential resembling an excitatory postsynaptic potential (EPSP) is recorded in the receptor. This is called the receptor potential. This potential results from converting some form of energy like mechanical or thermal energy into an electrical response (change in membrane potential), the magnitude of which is proportional to the intensity of the stimulus. As the pressure is increased, the magnitude of the receptor potential is increased. Thus, the responses are described as graded potentials rather than all-or-none as is the case for an action potential.
The intensity of sensation is determined by the amplitude of the stimulus applied to the receptor. As a greater pressure is applied to the skin, the receptor potential in the mechanoreceptor increases, and the frequency of the action potentials in a single axon is also increased, activation of receptors with higher threshold, because of