NEURON AND THE PHYSIOLOGY OF NERVE IMPULSE The nervous system is made of millions of structural and functional units called nerve cell or neurons and a number of supporting cells called glial cells. The function of the nerve cell is to receive and transmit impulses or messages and integrate the entire body as a single unit. The neurons can be classified into three types – 1. Unipolar neuron- it has a cell body or cyton from which arises a single short branch called neurite that immediately divides into the dendritic or receiving branch and axonic or transmitting branch. 2. Bipolar neuron- The neuron has the cell body from which arises two neurites called the dendron and the axon respectively. 3. Multipolar neuron- From the cell body arise many dendrites and one long axon. Neurons can also be classified into 1) myelinated when myelin sheath surrounds the axon and 2) non-myelinated neurons when the axon is naked and not covered by the myelin sheath. 1 Structure of a myelinated neuron/nerve cell The neuron can be divided into three parts-cell body or cyton from which arises two types of branches, i.e., many branched processes called dendrons and a long unbranched process called axon. The cyton or cell body is filled with the cytoplasm also called neuroplasm that is limited by the plasma membrane. It contains the nucleus, mitochondria, endoplasmic reticulum, golgi bodies, neurofibrils (filaments that act as a cytoskeleton) and special dense RNA rich granules called Nissl’s granules (help in protein synthesis). The dendrons terminate in slender processes called dendrites. The axon can be as small as 1 cm to 1m long. The axon is a long thin process that arises from the cyton called axon hillock. There is a trigger zone at the initial region of the axon hillock where the action potential or nerve impulse arises. The axon contains the axoplasm which is covered by a thin plasma membrane. It is enclosed within a fatty myelin sheath in myelinated neurons which is in turn surrounded by the neurilemma of the Schwann cell with the nucleus of Schwann cell. The myelin sheath is interrupted at regular intervals called nodes of Ranvier. The myelin sheath is protective, insulates the axon and also speeds up the transmission of nerve impulse. The axon terminates in fine branches called axon terminals that end in bulb-shaped synaptic end bulbs or buttons that contain many small vesicles containing neurotransmitters and many mitochondria. The neurons are not physically joined but there is a definite space or gap between axon terminal of one neuron and dendrites of the next neuron called the synaptic gap or cleft and the neurotransmitters help to transfer the nerve impulse from one neuron to the next across the synaptic gap. 2 Physiology of nerve impulse The message transmitted by an axon in the form of the nerve impulse is an electrical phenomenon and it is first necessary to understand membrane potential or electrical potential. All cells maintain an electrical potential across their membrane which is called irritability. This is caused due to the difference in the concentration of ions on either side of the membrane.. A nerve impulse has three phases – polarized state, depolarized state and repolarized state. Resting potential: It has been found that when the axon is in the resting state, that is when it is not transmitting an impulse, a potential difference is maintained between the inside and outside of the axon. Therefore the resting membrane potential (RMP) is due to the build-up of the negative charges inside the membrane and positive charges on the outside of the membrane. There is more K+ ions inside the cell and other negatively charged anions like amino acids, proteins and Cl- ions make the internal environment of the cell more electronegative. There are more Na+ ions are concentrated more on the outside (extracellular) of the cell thus rendering it more electropositive. This difference in charged ions across the membrane is maintained by the Na-K pump. The RMP is about -70 mv and the neuron having a resting potential is in the polarized state. That is, the neuron is capable of maintaining a different electrical charge on its two sides. At this state the membrane is less permeable to Na+ 3 Action potential: If the stimulus is strong enough to generate the threshold potential of -55mv it will generate an action potential in the nerve fibre or axon as it is easily irritable or excitable. The action potential is generated due to the temporary reversal of charges across the membrane which now becomes more permeable to Na+ as the voltage gated channels for Na+ open. The inside becomes now positively charged due to the accumulation of Na+ and the outside becomes negatively charged due to the presence of K+. This causes a change in the membrane potential which now becomes +30 mv which is known as the action potential. This process is called depolarization and the neuron is in the depolarized state. The action potential is extremely short lived, it lasts for only about a millisecond after which the original resting potential is restored once again or it becomes repolarized. Now the voltage gated channels for K+ open and for Na+ close. This brings back the neuron to the original membrane potential and the process is called repolarization. The nerve impulse is thus a propagated negative charge on the outside of the membrane caused by a wave of depolarization which passes along the axon. The changes in the membrane potential when stimulated can be plotted on a graph by taking time in millisecond on the X-axis and the membrane potential in millivolts on the Y axis. The graph shows the following features At first there is a resting potential of -70mv, this is called the resting potential and the neruron is in the polarized state (voltage gated Na ion channels are closed) When stimulated the potential changes to -50 mv called the threshold potential (Na ion channels are open).There is a reversal of charges and the potential reaches +30 mv. This is the action potential and the neuron is in the depolarized state. After it reaches the peak it immediately comes back to 0 mv and then to the resting potential of -70 mv and is known as the repolarized state. 4 As the channels for K ions are still open the membrane potential dips below -70 mv which is called hyperpolarization. Refractory period: The period of inexcitability that accompanies the recovery phase of the axon is called the refractory period and typically lasts for about 3 millisec. It can be divided into the i) absolute refractory period and ii) relative refractory period. i) Absolute refractory period is a time during which the axon is completely incapable of transmitting another impulse irrespective of the strength of the stimulus being higher than the threshold potential. ii) Relative refractory period during which it is possible to generate an impulse provided that the stimulus is stronger than the usual. The importance of the refractory period is that, together with transmission speed, it determines the frequency at which an axon can transmit an impulse. Axonal conduction 1. Continuous conduction It takes place in non myelinated neurons. The depolarization that takes place athe the trigger zone move along in one direction like a self propagating wave till it reaches the end of the axon. The action potential is conducted by a process of depolarization and then repolarization of the membrane along the axon. 5 2. Saltatory conduction This is seen in myelinated neurons. The presence of the myelin sheath aids in faster conduction of nerve impulse as it acts as insulation. The impulse jumps from one node to the next as there are numerous voltage gated Na ion channels at these regions where there is the flow of current. Synaptic transmission The nerve impulse is transmitted from one neuron to the next through a gap or cleft called a synaptic gap or cleft or a synapse by a chemical process. Synapses are specialized junctions through which cells of the nervous system communicate to one another and also non-neuronal cells such as muscles and glands. Structure of a synapse The synapse has a pre-synaptic neuron and a post-synaptic neuron. The pre-synaptic terminals or axon terminals ends in small synaptic knobs or bulbs. It contains a number of vesicles that contain the neurotransmitters and a number of mitochondria. The pre-synaptic membrane also has a number of voltage gated Ca ion channels which open as the nerve impulse reaches the 6 knob. As Ca+ enters the knob the vesicles become attached to the docking proteins on the inside of the pre-synaptic membrane and they burst open releasing the neurotransmitters into the synaptic cleft or gag. These neurotransmitters are chemicals like acetylcholine, epinephrine, nor epinephrine, histamine, serotonin etc. The post-synaptic membrane of the succeeding neuron has specific protein receptors to which the neurotransmitters are attached and they excite, inhibit or modify the post- synaptic neuron. The changes in the potential of the postsynaptic cell are called post- synaptic potential. 7 .