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Medical Study Center https://www.facebook.com/Medicalstudycenter2012 https://www.facebook.com/Medicalstudycenter2012 Notes On CNS Physiology Gray Matter of Spinal Cord: in the spinal cord gray matter is in the form of H shaped pillars which can be divided into three types of columns i.e. anterior horn or ventral horn, posterior horn or dorsal horn and in segments from T1 to L2 there is lateral horn. Neurons in these horns are Ventral Horn: Two groups of neurons Alpha motor neurons which are large multi polar neurons and their nerve fibers are alpha efferents which innervate skeletal muscle. Gamma neurons present in ventral horn are small and multi polar neurons and there nerve fibers are gamma efferents which innervate the intrafusal fibers of the muscle spindles. Both these alpha and gamma efferents come out of the spinal cord through ventral root of spinal nerves. Dorsal Horn: there are 4 groups of neurons 1. Substantia gelatinosa: this group of neurons is present at the apex of the posterior gray column. These neurons receive afferent nerve fibers carrying impulses of pain, temperature and crude touch. 2. Nucleus Proprius: this group is located anterior to the first group. These neurons receive afferent nerve fibers carrying impulses of proprioception and two point tactile discrimination. 3. Clarke’s column or nucleus dorsalis: this group is present at the base of the posterior gray column. These neurons are present in segments from T1 to L3, 4. These neurons are part of spinocerebellar tract and these receive afferent nerve fibers from spinocerebellar tract. 4. Visceral Afferent Nucleus: Present at the base of the posterior horn lateral to the clarke’s column. It is also present in segments only from T1 to L3, 4. It receives afferent nerve fibers carrying impulsesMedical from various viscera. Study Center Lateral Horn: in this horn, there are Autonomic preganglionic neurons. Preganglionic sympathetic in T1 to L2. Similar neurons are also present in sacral segments which are parasympathetic preganglionic. Nowadays we divide the gray matter of spinal cord into 9 laminae and in the sensory tract we use these laminae to describe the synapses. Shared By: Ussama Maqbool(N61) https://www.facebook.com/Medicalstudycenter2012 Medical Study Center Shared By: Ussama Maqbool(N61) https://www.facebook.com/Medicalstudycenter2012 FUNCTIONS OF SPINAL CORD: 1. It is a part of central nervous system and it contains ascending and descending tracts. 2. In the spinal cord there are neuronal circuits and centers for various reflexes and these reflexes are involved in posture maintenance, movements, and withdrawal of parts, local regulation of blood flow, regulation of GIT motility and secretions, micturition, defecation and other reflexes. 3. Initial integration of various sensory informations. REFLEX ACTION: an automatic response to a stimulus without involvement of will or consciousness. The purpose of this is to have quick response and to save time because this action has a protective value. For the reflex action a specific organization is required known as reflex arc. The reflex arc consists of 5 subcomponents. 1. Receptor: it responds to the stimulus and generates impulses 2. Sensory or afferent neuron: it carries impulses from the receptor to the centre. 3. Centre: it is always present in CNS and this is integrating station. In the centre there is one or more synapse between the afferent and efferent neuron. 4. Efferent neuron or motor neuron: it carries impulses from the centre to the effector or target tissue. 5. Effector or target tissue: it is a responding tissue. These include skeletal muscle, smooth muscle, cardiac muscle and glands. Medical Study Center For the reflex action, it is essential to have integrity of reflex arc. We can classify reflexes in different ways. Shared By: Ussama Maqbool(N61) https://www.facebook.com/Medicalstudycenter2012 Depending upon the number of synapse in the centre i.e. monosynaptic, disynaptic and multi or poly synaptic. The other way of classifying is unconditioned or inborn reflexes and acquired or conditioned reflexes Unconditioned Reflexes: Salivation, micturition, light reflex etc Conditioned reflex: these are acquired in response to condition stimuli with the help of unconditioned stimulus. Pavlov’s experiment and salivary secretion. 05 August, 2010 Reflexes can be clinically classified into four types i.e. 1. Superficial: receptors are present in skin or mucus membrane like conjunctival, corneal, plantar, cremesteric etc 2. Deep: receptors are present in deep tissues i.e. muscles or tendons like biceps, triceps jerk. 3. Visceral: receptors are present in the viscera i.e. light reflex, baro receptor, bain bridge reflex 4. Pathological: present in diseased state i.e. babinski’s sign and clonus PROPERTIES OF REFLEX ACTION: In reflex arc there is one or more than one synapse, so most of the properties of reflex arc are properties of synapses.- 1. Central Delay: delay in the centre of reflex arc. If it is monosynaptic reflex arc then delay is 0.5 milli sec that is the time taken by one synapse to conduct impulse. By finding out central delay we can say that the reflex arc is monosynaptic, disynaptic or polysynaptic. Central delay can be calculated by Central Delay = Reaction time (time in which the response occurs after application of stimulus) – peripheral time (Time taken by the impulses to be conducted by afferent and efferent neurons) 2. Summation: adding up of the effect of stimuli especially if the stimuli are subthreshhold. It is of two types i.e. a. SpatialMedical Summation: Impulses areStudy conducted along increased Center number of synaptic knobs or afferent nerve fibers to the post synaptic neurons simultaneously. b. Temporal Summation: impulses are conducted along the same number of presynaptic knobs or afferent nerve fibers repeatedly to the post synaptic neurons and this result into the adding up of the effect of repeated stimuli leading to excitation of postsynaptic neuron. The rate of repetition of stimuli should be such that the effects of previous stimulus is there when the second stimulus comes. 3. Facilitation: when response of post synaptic neuron or motor neuron is more than expected. Suppose there are two afferent nerve fibers i.e. A and B. Afferent A and B makes synapse with Shared By: Ussama Maqbool(N61) https://www.facebook.com/Medicalstudycenter2012 two types of motor neurons or post synaptic neurons i.e. subliminal fringe and discharge zone or excited zone neurons. In facilitation the affect of subliminal fringe neurons is added to the affect of discharge zone neurons when both A and B neurons are stimulated simultaneously. When each motor neuron is excited in the muscle one G tension is produced. 4. Occlusion: Response of the post synaptic or motor neurons is less than expected. Mechanism is partial sharing of motor neurons by both the afferents. 5. Recruitment and after discharge: Recruitment means more and more motor neurons become excited upon continuous repeated stimulation in same muscle. After discharge is that the response of postsynaptic neurons continues after the stoppage of stimulus. After discharge is shown by the poly synaptic reflexes only. After discharge is because of two mechanisms i.e. a. Long or Delayed Path: because of presence of more inter neurons. b. Presence of Reverberating Circuits 6. Irradiation: Excitation spreads from motor neurons of one muscle to the motor neurons of the other muscles. Example is that of flexor withdrawal reflex. With stronger stimulus there is more stimulation. Medical Study Center Shared By: Ussama Maqbool(N61) https://www.facebook.com/Medicalstudycenter2012 Medical Study Center 7. Reciprocal Inhibition: For a smooth reflex action, when there is contraction of agonist there must be relaxation of antagonist. This is because of reciprocal innervations. Shared By: Ussama Maqbool(N61) https://www.facebook.com/Medicalstudycenter2012 8. Fatigue: if a reflex is elicited repeatedly, after sometimes the response decreases which is because of fatigue of synaptic transmission. 9. RENSHAW CELLS: these are the inhibitory inter neurons Medical Study Center MONOSYNAPTIC REFLEXES STRETCH REFLEX: Most vastly studied reflex. Whenever a skeletal muscle is stretched, it contracts. Example is that of knee jerk. Shared By: Ussama Maqbool(N61) https://www.facebook.com/Medicalstudycenter2012 Medical Study Center Receptors for the stretch reflex are muscle spindles. Stretch reflex controls the increase in the length. Shared By: Ussama Maqbool(N61) https://www.facebook.com/Medicalstudycenter2012 Muscle Spindles are present in between the muscle fibers. Length is 3-10 mm. it is along the long axis of muscle. Ends are fused with the sides of the muscle fiber. So whenever there is change in the length of muscle, same change occurs in muscle spindle. Each muscle spindle is encapsulated and contains modified muscle fibers which are called intrafusal muscle fibers. Number of muscle spindles in the muscle varies. There is greater number of muscle spindles in the muscles concerned with fine skilled movements like those of hand. There are 4-12 intrafusal fusal fibers in each muscle spindle. There are 2 types of intrafusal fibers i.e. Nuclear bag fibers and nuclear chain fibers. 1. Nuclear Bag fibers: number is 1-3. it has a central dilated portion which contains the nuclei. 2. Nuclear Chain fibers : shorter, number 3-9 and these contain nuclei in the form of a chain throughout the length of fiber. Ends are attached to the sides of nuclear bag fibers. These fibers has got two portions i.e. end portion which is contractile and contains actin myosin filaments. The central portion is without actin myosin filaments and this is the receptor portion. Muscle spindles are excitedMedical whenever the central portion Study of intrafusal fibers areCenter stretched. Nerve Supply of Muscle Spindle: There are two types of nerve endings present along the intrafusal fibers 1. Primary or annulospiral: these are wrapped around the central portion of both nuclear bag and chain fibers.
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