Spinal Reflexes Physiology PDF

IV. Spinal Reflexes (Spinal cord reflex function) Reminder on the anatomy of the spinal cord The spinal cord is shown in Figure 1. Cortex– white matter surrounds gray matter. Dorsal and ventral nerve roots of spinal nerves. Emerge between each pair of adjacent vertebrae The roots of the...

IV. Spinal Reflexes (Spinal cord reflex function) Reminder on the anatomy of the spinal cord The spinal cord is shown in Figure 1. Cortex– white matter surrounds gray matter. Dorsal and ventral nerve roots of spinal nerves. Emerge between each pair of adjacent vertebrae The roots of the dorsal nerve contain sensory fibers. Dorsal root ganglion. Ventral nerve roots contain motor fibers. Medulla– central part of the spinal cord (butterfly) composed of gray matter and a central canal – center of the cord. Figure1: Anatomy of the spinal cord The gray matter of the spinal cordof each horn contains (Fig. 2): Dorsal horns: Neurons transmit sensory (afferent) nerve impulses to the brain or other parts of the spinal cord Ventral horns: Neurons transmit motor (efferent) nerve impulses to the spinal nerves. Figure2: Neuronssensory (afferent) and motor neurons in the gray matter of the spinal cord (efferent) A. General organization of a medullary reflex Reflexes are rapid, predictable, automatic reactions to external changes, the purpose of which is to maintain homeostasis.(Fig. 3). 1  Figure3: Stages of a reflex reaction after stimulation There are different types of reflexes, including: (a) Somatic reflexesthat allow the animal to react to the external environment. An example of this is the palpebral reflex, where stimulation around the eyelids causes blinking to protect the eyeball. (b) Autonomic (or visceral) reflexeswhich allow the animal to respond to changes in its internal environment. An example of this is the increase in heart rate in response to reduced blood pressure. The main function of the spinal cord is to provide a center for integrating spinal reflexes. The brainstem is the center of integration of some reflexes that involve the cranial nerves, called cranial reflexes. The reflex arcinvolves the following components shown in Figure 4. Figure4: Schematic representation of a reflex arc Due to the decussation (crossing) of nerve fibers, reflex arcs can be: ipsilateral, contralateral B. Myotatic reflex 1. Definition:The myotatic reflex arc (stretching) is a characteristic of the muscles of the limbs and the trunk in particular. It is a monosynaptic reflex arc. Only 02 neurons are involved (one sensory and one 2 motor) and there is only one synapse in the nerve pathway. It is a major mechanism by which the animal maintains its posture and supports itself against gravity (Figure 5). Figure5: Stretch or myotatic reflex 2. Physiological significance The stretched neuromuscular spindle fibers send an afferent message to the spinal cord, via the Ia sensory fibers (Fig. 5). This afferent message will monosynaptically excite the alpha motor neurons which will allow the contraction of the muscle that has just been stretched. The explanation for this reflex is given by the percussion of the tendon or muscle stretching the muscle spindle (Figure 6). The IA afferent fibers are activated and synapse directly on the motor neurons of the muscle. The motor neuron discharges when a threshold of excitation is reached. The level of excitation of the motor neuron is linked to synapses from a variety of sources, as illustrated. The impulse travels down the axon (alpha efferent) to the neuromuscular junction, causing a release of acetylcholine. The acetylcholine binds to receptors on the muscle, causing depolarization and contraction of the muscle. The gamma neurons maintain tension on the muscle spindle regardless of the state of contraction of the muscle. Figure6: Myotatic (stretch) reflex in dogs. Posture and the myotatic reflexare represented by Figure 7. 3  Figure7: The myotatic reflex and postural support. C. Flexion reflex or flexor reflex 1. Definition The flexor reflex is a polysynaptic reflex of the spine that may be related to spinal locomotor centers.This reflex occurs following the application of a noxious stimulus to the skin surface that results in a generalized and powerful activation of the ipsilateral flexor muscles that typically produces a withdrawal of the limb away from the stimulus. 2. Mechanism Islandis more complex than the myotatic reflex. The response involves all the flexor muscles of the limb and therefore requires the activation of motor neurons in several segments of the spinal cord(See TD). The flexor reflex receptors are primarily free nerve endings in the skin and other tissues that respond to noxious stimuli, such as pressure, heat, and cold. A stimulus that produces a sensory discharge in these nerves ascends to the spinal cord through the dorsal root. The net result is withdrawal of the limb from the noxious stimulus. Inhibitory interneurons of the extensor motor neurons are also activated, resulting in decreased activity of the extensor muscles. Relaxation of the extensor muscles and contraction of the flexor muscles permit full flexion of the limb. The flexor reflex is a spinal reflex and does not require any brain activation. If an animal steps on a sharp piece of glass, it immediately withdraws its foot before consciously perceiving the pain. An example of this flexion reflex is illustrated in Figure 8. In A, the animal is positioned in a lateral recumbent position and a noxious stimulus is applied to one limb. The limb is immediately withdrawn. Sensory fibers enter the spinal cord through the dorsal root to synapse on interneurons. Flexor motoneurons are activated, causing flexion of the limb. Simultaneously, inhibitory interneurons cause relaxation of antagonistic extensor muscles. Other interneurons traverse the spinal cord to activate contralateral extensor muscles—the crossed extensor reflex. In B, the crossed extensor reflex is inhibited unless damage to the systemsupper motor neuronshave occurred. Sensory fibers also project to the brain, causing conscious awareness of pain and subsequently a behavioral response (A). The reflex does not depend on a behavioral response. The behavioral response may be absent if the sensory pathways are damaged. 4 UMN (upper motor neurons):systemsupper motor neurons Figure8: Reflexescrossed flexion and extension D. Medullary reflex control system 1. Recurrent inhibition of RENSHAW Renshaw cell: a small interneuron located in the anterior horn of the spinal cord. The Renshaw cell exerts potent inhibition on the agonist motor neuron and disinhibits antagonist motor neurons. The chemical neutralizer released is acetylcholine. excited in a monosynaptically mode by the recurrent collaterals of the neighboring motor neurons, Their inhibitory effect can be exerted both on motor neurons and on their inhibitory interneurons (so- called reciprocal inhibition of antagonist muscles) (Figure. Figure9:Recurrent inhibition of RENSHAW 2. Reversed myostatic reflex orreciprocal inhibition The reverse myotatic reflex of the Golgi tendon organ protects skeletal muscle from excessive contraction by causing reflex muscle relaxation (See TD). The Ia afferent fibers making direct synaptic contacts with the alpha motor neurons of the agonist muscle to cause the stretch reflex described above, can exert in parallel a disynaptic inhibitory action on the alpha motor neurons of the antagonist muscles. This second parallel command goes to 5 another neuron called inhibitory interneuron Ia intercalated between the Ia afferent fiber and the alpha motor neuron, and whose mission is to inhibit the motor neurons of the antagonist muscle and therefore cause its relaxation (Fig. 8). The function of this parallel organization at the spinal level is to allow muscular synergy during movement through active inhibition of the antagonist muscles. Figure10: Reverse stretch reflex 6