sensory receptors

Physiology

Lec.(2)
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Sensory Receptors :-
Sensations are mostly due to stimulation of cutaneous receptors and sensory nerves carry impulses to spinal cord. the nerve fibers in the spinal cord are organized to form the ascending tracts
there are five basic types of sensory receptors:
(1)mechanoreceptors
which detect mechanical compression or stretching of the receptor or of tissues adjacent to the receptor.
(2) thermo receptors
which detect changes in temperature, some receptors detecting cold and others warmth.
(3) nociceptors,(pain receptors)
which detect damage occurring in the tissues, whether physical damage or chemical damage.
(4)electromagnetic receptors
which detect light on the retina of the eye.
(5) chemoreceptor
which detect taste in the mouth, smell in the nose, oxygen level in the arterial blood, osmolality of the body fluids, carbon dioxide concentration.

Properties of receptors:-

1-Generator potential
this term relates to the electrical events in the receptor, it is anon-propagated electrical potential developed in the receptor after it is stimulated .when it reaches the threshold level it gives rise to the propagated action potential
2-Adaptation
When a receptor is continuously stimulated, the frequency of action potential declines over a period of time. this is called as adaptation. Touch receptors adapt quickly and they are known as aphasic receptors while muscle spindles do not adapt themselves to the stimulus and they are called tonic receptors
3-Sensory units and recruitment
a single sensory axon with its branches in the periphery is called a sensory unit. The area covered by it, is known as receptive field .When the intensity of the stimulus is increased ,larger number of sensory units are excited, this is termed as recruitment of sensory units
4-intensity discrimination:-it depends upon the frequency of firing of the action potentials, which is proportional to the intensity of the stimulus by a power function


Modality of SensationThe Labeled Line Principle
Each of the principal types of sensation that we can experiencepain, touch, sight, sound, and so forthis called a modality of sensation. if a pain fiber is stimulated, the person perceives pain regardless of what type of stimulus excites the fiber. The stimulus can be electricity, overheating of the fiber, crushing of the fiber, once a receptor initiates action potentials in its associated sensory fiber, which then conveys these impulses into the spinal cord in the form of labeled line in peripheral nerve. these impulses or action potentials are similar in all sensory fibers. they may exhibit qualitative differences in amplitude or frequency, but an action potential initiated by painful stimulus is not perceived as uniquely distinguishable from an action potential initiated by any other receptor or sensory modality. What does allow us to differentiate one type of sensation from another is the location in the nervous system where the fiber leads or terminates. each fiber or collection of neurons linked by related sensory fibers is referred to as a "labeled line". action potentials for example traveling along the fibers and neurons that compose the "anterolateral system"(spinothalamic tract)are perceived as pain, whereas action potentials carried over the dorsal column-medial lemniscal system are perceived as touch or pressure.

General Classification of Nerve Fiber

The fibers are divided into two types A and C, and the type A fibers are further subdivided into alpha(α,) beta(β), gamma,( γ) and delta((δ) fibers. Type A fibers are the typical large and medium-sized myelinated fibers of spinal nerves. Type C fibers are the small unmyelinated nerve fibers that conduct impulses at low velocities. The C fibers constitute more than one half of the sensory fibers in most peripheral nerves as well as all the postganglionic autonomic fibers. a few large myelinated fibers can transmit impulses at velocities as great as 120 m/sec, a distance in 1 second that is longer than a football field.
Conversely, the smallest fibers transmit impulses as slowly as 0.5 m/sec, requiring about 2 seconds to go from the big toe to the spinal cord.

Alternative Classification Used by Sensory Physiologist

Group Ia
Fibers from the annulospiral endings of muscle spindles (average about 17 microns in diameter; these are a-type A fibers in the general classification).
Group Ib
Fibers from the Golgi tendon organs (average about 16 micrometers in diameter; these also are a-type A fibers).
Group II
Fibers from most discrete cutaneous tactile receptors and from the flower-spray endings of the muscle spindles (average about 8 micrometers in diameter; these are beta- and gamma-type A fibers in the general classification).
Group III
Fibers carrying temperature, crude touch, and pricking pain sensations (average about 3 micrometers in diameter; they are delta-type A fibers in the general classification).
Group IV
Unmyelinated fibers carrying pain, itch, temperature, and crude touch sensations (0.5 to 2 micrometers in diameter; they are type C fibers in the general classification).
CLASSIFICATION OF SOMATIC SENSES
The somatic senses can be classified into three physiologic types:
(1) the mechanoreceptive somatic senses, which include both tactile and position sensations
(2) the thermoreceptive senses, which detect heat and cold;
(3) the pain sense, which is activated by any factor that damages the tissues.


Other Classifications of Somatic Sensation:-
Exteroreceptive sensations are those from the surface of the body. Proprioceptive sensations are those having to do with the physical state of the body, including position sensations, tendon and muscle sensations, pressure sensations from the bottom of the feet, and even the sensation of equilibrium
Visceral sensations are those from the viscera of the body sensations from the internal organs.
Deep sensations are those that come from deep tissues, such as from fasciae, muscles, and bone. These include mainly deep pressure, pain, and vibration.

Tactile Receptors. There are at least six entirely different types of tactile receptors

First,free nerve endings, which are found everywhere in the skin and in many other tissues, can detect touch and pressure. For instance, even light contact with the cornea of the eye, which contains no other type of nerve ending besides free nerve endings.
Second, a touch receptor with great sensitivity is the Meissners corpuscle , an elongated encapsulated nerve ending of a large (type Abeta) myelinated sensory nerve fiber. Inside the capsulation are many branching terminal nerve filaments.
These corpuscles are present in the nonhairy parts of the skin and are particularly abundant in the fingertips, lips, and other areas of the skin . Meissners corpuscles adapt in a fraction of a second after they are stimulated, which means that they are particularly sensitive to movement of objects over the surface of the skin as well as to low frequency vibration.
Third, Merkels discs,. The hairy parts of the skin also contain moderate numbers of expanded tip receptors, These receptors differ from Meissners corpuscles in that they transmit an initially strong but partially adapting signal and then a continuing weaker signal that adapts only slowly. Merkels discs are often grouped together in a receptor organ called the Iggo dome receptor, which projects upward against the underside of the epithelium of the skin, a This causes the epithelium at this point to protrude outward, thus creating a dome and constituting an extremely sensitive receptor. the entire group of Merkels discs is innervated by a single large myelinated nerve fiber (type Abeta). These receptors, , play extremely important roles in localizing touch sensations to specific surface areas of the body and in determining the texture of what is felt.

Fourth, hair end-organ, are also a touch receptor. This receptor adapts readily and, like Meissners corpuscles, detects mainly (a) movement of objects on the surface of the body or (b) initial contact with the body.
Fifth, Ruffinis end-organs located in the deeper layers of the skin and also in still deeper internal tissues , which are multibranched, encapsulated endings, .These endings adapt very slowly and, therefore, are important for signaling continuous states of deformation of the tissues, such as heavy prolonged touch and pressure signals. They are also found in joint capsules and help to signal the degree of joint rotation.
Mechanoreceptors in the periodontal ligament are Ruffini-type endings supplied by large diameter mylinated fibers ,there is considerable variability in their morphology
Sixth, pacinian corpuscles lie both immediately beneath the skin and deep in the fascial tissues of the body. They are stimulated only by rapid local compression of the tissues because they adapt in a few hundredths of a second. Therefore, they are particularly important for detecting tissue vibration or other rapid changes in the mechanical state of the tissues.



The sense of atickle or an itch is related to very sensitive, rapidly adapting free nerve endings in the superficial layers of the skin that mainly transmit over type c-fibers ,the function of this sensory modality is presumably to call attention to light skin irritations that can be relieved by movement or scratching, stimulus that appears to override the itch signals











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