Consider the sciatic nerve soleus gastrocnemius preparation

flow. tI may be recalled that because of the flow of prolem by axonal flow, the regeneration of a nerve fiber is possible Centripetal or retrograde flow, that is, flow from the axoplasm to the soma also occurs. Movements of the tetanus toxin occurs in this way. The tetanus bacteria having gained entry into the body remain at the local site, only their toxin, called exotoxin, moves towards the soma by retrograde flow and does the damage to produce the signs and symptoms of tetanus. Nerve growth factor, NGF (chap I, sec XA) can also travel centripetally. NEUROGLIA 1. Introduction 2. Classification 3. Probable junctions Introduction In the nervous system, two kinds of cells are found (i)the neuron, and the (ii) neuroglia. The neuroglial cells occur in plentiful number, and yet so little about the functions of the neuroglial cells are know. Classsification The neuroglial cells, traditionally, are divided into three classes: (i) astrocyles, (n) microglia, and(iii) the otigodendrocytes. Probable functions (I) Astrocytes, the star shaped cells, invest the neuron, they may be intermediate agents for transferring substances from the blood to the neuron, and may be an anatomical basis of the 'blood brain barrier' BBB. For details of BBB, see chap 7, sec XD. In tissue culture, only those neurons that are invested by the astrocytes, survive, others do not, (n) Microgha acts as phagocytes within the brain. (iii) Oligodendrocytes lay down the myelin sheath around the nerve fibers within the brain, i e they act as counterparts of the Schwann cells. The neuroglial cells do not conduct impulses. Unlike the neurons, they retain the ability to divide and multiply throughout the life. SUMMARY & HIGHLIGHTS Nerve fibers are of 2 types, (i) myelinated, (ii) non myelinated. In mammals majority of the fibers are myelinated (pain carrying fibers called, C fibers, post ganglionic sympathetic fibers etc. are non myelinated) Myelinated fibers have faster conductivity and hence the swiftness of the mammalian movements (compared to non vertebrates). The CNS nerve fibers have no neurilemma Neurons cannot divide, hence death of a neuron is an irreplacable damage, ie, no new neuron can replace the dead neuron (the dead neurons are replaced by neuroglial cells) If the fiber is damaged, the damaged part of the fiber can be removed by a process called Wallerian degeneration. The re-moved portion can be replaced by growth from the undamaged (and not removed) part of the fiber, by a process called, Wallerian regeneration, provided, there is neurilemma. Hence, regeneration not possible in CNS. Also, retrograde degeneration occurs in the CNS, killing Ihe nerve cell soma. Many chemicals, notably the neurotransmitters (eg, ACh), are synthesized in the soma and Iravel down to reach the termination of the axon, where they are stored as vesicles, by a process called, axonal flow. PROPERTIES OF NERVE FIBRES SYNAPSE INTRODUCTION The two basic properties of a nerve fiber are, (i) excitability and (ii) conductivity, that is, a nerve fiber, when stimulated adequately, is excited and the wave of excitation propagates onwards, with undiminished strength. These points will now be discussed in greater details. EXCITABILITY When a nerve fiber is stimulated adequately, the fiber is excited. If the strength of the stimulus is below a minimal level, the fiber is not excited, there is thus a 'threshold' (which is the minimum strength of stimulus which can cause excitation) intensity of stimulus. In chap. 2, sec I (fig. 1. 2. 5), it was said that when a cell is stimulated, there occurs a very gradual voltage drop across the cell membrane until the membrane potential reaches a critical value, (say, -60 mv), when there is sudden development of (fig. 1. 2. 5) action potential (AP). A threshold potential is of such intensity that it is just sufficient to produce the critical level of the membrane potential. In short, a threshold stimulus is that stimulus which can cause a sufficient voltage drop of a resting cell membrane so that it (= vollage of the cell membrane) can reach a critical value (which is usually -60 mV). Once this critical value is reached the action potential develops. (A stimulus whose intensity is more than the threshold value, will of course excite the nerve]. How to know whether a nerve fiber is excited or not ? In the yester years of physiology, the method was to lake a nerve muscle preparation (e.g sciatic soleus - gastrocnemius preparation) stimulate the nerve if the muscle contracts, it means the nerve developed excitation. Now, instead, the AP can be recorded from the nerve it self (by cathode ray oscilloscope) when it is excited, and there is thus no need to look for contraction of the muscle any more. Nerve fibers of bigger diameters are more easily excitable (i.e the threshold value of stimulation, for them, is low) whereas those with smaller diameters ('fine nerve fibers') are less easily excitable. However, in a given nerve fiber, the amplitude of AP does not increase if (other things remaining constant) the stimulus intensity is increased. This is called, all or none law. Maximal and supramaximal stimulus. Consider the sciatic nerve soleus gastrocnemius preparation. The sciatic nerve contains different types of nerve fibers, some thick and some thin. A low intensity stimulus will be threshold for the thick fibers but will remain subthreshold for the fine fibers. Consequently with a low intensity stimulus, some, (but not ail), molor units will contract and the muscle will contract no doubt, but weakly. By gradually increasing the strength of the stimulus, a point can just be reached, when all fibers (thick and thin) will be stimulated, a stimulus of this intensity is called 'maximal' stimulus and causes a powerful contraction. Still more intense stimuli are called 'supramaximal' stimuli, but they do not make stronger contraction than the