Pain some times occurs in the absence of nociceptor stimulation. this is most likely to happen after damage to peripheral nerves or to part of CNS involved in transmitting nociceptive information. examples of pain secondary to neural damage are phantom limb pain and thalamic pain
Phantom pain follows amputation in some individuals and is clearly not caused by activation of nociceptors
Inhibition of painNeurons of the dorsal horn in the spinal cord contain enkephalins, which inhibit the release of substance P .and reduce the intensity of pain, certain neuropeptides known as Enkephalins and Endorphins are present in the brain, they bind themselves to the opioid receptors and inhibit the intensity of pain perception. in addition to these ,there is also the gate control mechanism present in the dorsal horn of the spinal cord. The arrival of impulses through a large mylinated nerve fiber at the dorsal horn inhibits the transmission of pain impulses through the smaller unmylinated C nerve fibers.
Thermal sensations1-pain receptors are stimulated by extreme degree of cold or warmth
2-Specific warmth receptors they are simply regarded as free nerve endings .warmth signals transmitted over type C-sensory fibers
3-The cold receptors has been identified as a small nerve endings, with tips that protrude in to the basal aspect of basal epidermal cells. signals from these receptors are transmitted over A delta type. sensory fibers there are (3) to(10)times as many cold receptors as warmth receptors and their density varies from (15) to(25)per square centimeter on lips to3 to5receptors persquare meters on fingers, temperature below(7Ċ)and above(50)ċ activate pain receptors ,both of these extremes are perceived similarly as very painful and not cold or warm. the peak temperature for activation of cold receptors is about (24ċ)whereas the warmth receptors are maximally active at about (45Ċ).both cold and warm receptors can be stimulated with temp.in the range of (31Ċ)to(43Ċ).
Organization of the Spinal Cord for Motor FunctionsThe cord gray matter is the integrative area for the cord reflexes. Figure shows the typical organization of the cord gray matter in a single cord segment. Sensory signals enter the cord almost entirely through the sensory (posterior) roots. After entering the cord, every sensory signal travels to two separate destinations:
(1) One branch of the sensory nerve terminates almost immediately in the gray matter of the cord and elicits local segmental cord reflexes and other local effects.
(2) Another branch transmits signals to higher levels of the nervous systemto higher levels in the cord itself, to the brain stem, or even to the cerebral cortex, Each segment of the spinal cord (at the level of each spinal nerve) has several million neurons in its gray matter. Aside from the sensory relay neurons, the other neurons are of two types:
(1) anterior motor neurons and
Anterior Motor Neurons:
Located in each segment of the anterior horns of the cord gray matter are several thousand neurons that are 50 to 100 per cent larger than most of the others and are called anterior motor neurons. They give rise to the nerve fibers that leave the cord by way of the anterior roots and directly innervate the skeletal muscle fibers. The neurons are of two types, alpha motor neurons and gamma motor neurons.
Alpha Motor Neurons:
The alpha motor neurons give rise to large type A alpha motor nerve fibers, averaging 14 micrometers in diameter; these fibers branch many times after they enter the muscle and innervate the large skeletal muscle fibers. Stimulation of a single alpha nerve fiber excites anywhere from three to several hundred skeletal muscle fibers, which are collectively called the motor unit. Transmission of nerve impulses into skeletal muscles and their stimulation of the muscle motor units.
Gamma Motor Neurons:
Along with the alpha motor neurons, which excite contraction of the skeletal muscle fibers, about one half as many much smaller gamma motor neurons are located in the spinal cord anterior horns. These gamma motor neurons transmit impulses through much smaller type A gamma motor nerve fibers, averaging 5 micrometers in diameter, which go to small, special skeletal muscle fibers called intrafusal fibers, shown in figure These fibers constitute the middle of the muscle spindle,
Interneurons are present in all areas of the cord gray matterin the dorsal horns, the anterior horns, and the intermediate areas between them, as shown in Figure. These cells are about 30 times as numerous as the anterior motor neurons. They are small and highly excitable, often exhibiting spontaneous activity and capable of firing as rapidly as 1500
times per second.
Renshaw Cell Inhibitory System:Also located in the anterior horns of the spinal cord, in close association with the motor neurons, are a large number of small neurons called Renshaw cells. Almost immediately after the anterior motor neuron axon leaves the body of the neuron, collateral branches from the axon pass to adjacent Renshaw cells.These are inhibitory cells that transmit inhibitory signals to the surrounding motor neurons. Thus, stimulation of each motor neuron tends to inhibit adjacent motor neurons, an effect called latera linhibition. This effect is important for the following major reason: The motor system uses this lateral inhibition to focus, or sharpen, its signals in the same way that the sensory system.
Structure and Motor Innervation of the Muscle Spindle.The organization of the muscle spindle is shown in Figure. Each spindle is 3 to 10 millimeters long. It is built around 3 to 12 very small intrafusal muscle fibers that are pointed at their ends and attached to the glycocalyx of the surrounding large extrafusal skeletal muscle fibers.Each intrafusal muscle fiber is a very small skeletal muscle fiber. However, the central region of each of these fibersthat is, the area midway between its two endshas few or no actin and myosin filaments. Therefore, this central portion does not contract when the ends do. Instead, it functions as a sensory
receptor,. The end portions that do contract are excited by small gamma motor nerve fibers that originate from small type A gamma motor neurons in the anterior horns of the spinal cord, as described earlier. These gamma motor nerve fibers are also called gamma efferent fibers, in contradistinction to the large alpha efferent fibers (type A alpha nerve fibers) that innervate the extrafusal skeletal muscle.
Sensory Innervation of the Muscle Spindle:
The receptor portion of the muscle spindle is its central portion. In this area, the intrafusal muscle fibers do not have myosin and actin contractile element. They are stimulated by stretching of this midportion of the spindle.
Usually one but sometimes two smaller sensory nerve fiberstype II fibers with an average diameter of 8 micrometersinnervate the receptor region on one or both sides of the primary ending, . This sensory ending is called the secondary ending; sometimes it encircles the intrafusal fibers in the same way that the type Ia fiber does, but often it spreads like branches on a bush. There are also two types of muscle spindle intrafusal fibers: (1) nuclear bag muscle fibers (one to three in each spindle), in which several muscle fiber nuclei are congregated in expanded bags in the central portion of the receptor area, as shown by the top fiber and (2) nuclear chain fibers (three to nine), which are about half as large in diameter and half as long as the nuclear bag fibers and have nuclei aligned in a chain throughout the receptor area,