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Nervous system

الكلية كلية طب الاسنان     القسم  العلوم الاساسية     المرحلة 2
أستاذ المادة حيدر حميد عباس الحيدري       09/09/2020 23:43:30
Motor functions of the CNS
The motor functions of the CNS can be divided into reflexes and the higher motor control functions. The basic unit of integrated neural activity is the reflex arc. The arc consists of a sense organ (receptor), an afferent neuron, one or more synapses in a central integrating station or sympathetic ganglion, an efferent neuron, and an effector. The connection between the afferent and efferent neurons is generally in the brain or spinal cord. The afferent neurons enter via the dorsal roots or cranial nerves and have their cell bodies in the dorsal root ganglia or in the homologous ganglia on the cranial nerves. The efferent fibers leave via the ventral roots or corresponding motor cranial nerve. The connection between the afferent and the efferent neurons is usually in the central nervous system and activity in the reflex arc is modified by multiple input converging on them from higher motor control centers.

The spinal cord reflexes: Sensory signals enter the cord through the sensory roots. After entering the cord, every sensory signal travels to two separate destinations:
1- The sensory nerve or its collateral terminate in the gray matter of the cord and elicit local segmental motor responses.
2- The signals travel to higher levels of the cord itself or to the brain stem or even to the cerebral cortex.

Each segment of the spinal cord has several millions neurons in its gray matter and these are:
A- The sensory neurons that we discussed previously.
B- The anterior motor neurons: These gives rise to the nerve fibers that leave the cord via the anterior roots and innervate the effector such as skeletal muscle fibers. The cells of the anterior horn of spinal cord or motor cranial nuclei and their efferent fibers that run to motor units are also called the lower motor neurons to distinguish them from the upper motor neurons of the higher motor control centers. Thus the lower motor neuron is the final common path for all efferent impulses directed at the muscle.
The anterior motor neurons are of two types:
1. The alpha motor neurons: Which give off large nerve fibers (type A alpha nerve fiber) that innervate the large skeletal muscle fibers forming the motor units.
2. The gamma motor neurons or gamma efferent neurons: Which give off nerves fibers (type A gamma nerve fiber) that innervate very small special skeletal muscle fibers called intrafusal fibers which are part of the muscle spindle.
C- The interneurons: These are small neurons that have many interconnections one with the other. Most of the incoming sensory signals from the spinal nerves are transmitted first through interneurons where they are appropriately processed and then terminate on the anterior motor neurons.
Some of the anterior motor neurons immediately after the motor axons leave the soma give collateral branches to innervate an adjacent interneurons called Renshaw cells which are located in the ventral horn of the spinal cord. These cells in turn are inhibitory cells that transmit inhibitory signals to the same motoneuron (recurrent inhibition) and to the nearby motor neurons (lateral inhibition).
? The recurrent inhibition is important to allow only the initial impulses arriving at a motoneuron to pass through easily while the late impulses will find the anterior horn cells partially inhibited and will therefore produce a smaller motor discharge than the initial excitatory impulses.
? The lateral inhibition is to focus or sharpen the signals that is to allow transmission of the primary signal while suppressing the tendency for signals to spread to adjacent neurons.
? The recurrent inhibition is important to allow only the initial impulses arriving at a motor neuron to pass through easily while the late impulses will find the anterior horn cells partially inhibited and will therefore produce a smaller motor discharge than the initial excitatory impulses.
? The lateral inhibition is to focus or sharpen the signals that is to allow transmission of the primary signal while suppressing the tendency for signals to spread to adjacent neurons.

The muscle receptors and their roles in muscle control: Proper control of muscle requires not only excitation of the muscle by the anterior motor neurons but also continuous feedback of information from each muscle to the nervous system which is achieved by two special types of sensory receptors and these are:
1— Muscle spindles: Which are distributed throughout the belly of muscle and which send information to the NS about the muscle length and the rate of change of its length.
2— Golgi tendon organs: Which are located among the fascicles of a tendon between it and the muscle itself and which send information about tension or rate of change of tension.


المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .