Human Physiology

Lesson 41 : Membrane Potentials

Membrane Potentials of Nerve Cell

Functional nerve cells perform their activity by altering the electrical potentials across their cell membrane. These changed electrical potentials traverses along the nerve in the form of nerve impulse and reach their destination in same manner as that of electrical current along a wire. To understand this phenomenon it is pertinent to know the electrical property of nerve cells, in particular the cell membrane called neurolemma. Basically the membrane potentials are categorized as resting membrane potential, action potential threshold potential and hyperpolarization potentials.

1. Resting membrane potential (RMP): The concept of RMP implies to the potential difference across the cell membrane during resting state. In resting state, the nerve cell membrane is electrically polar to a magnitude of -75 millivolts (mV) which means inner side of membrane 75 mV more electronegative than outer side. Reasons for this polarity are:
1. Surplus of negatively charged ions inside the cell.
2. Presence of more non-diffusible protein ions inside the cell.
3. Selective permeability of cell membrane only to K+ at resting state.
4. Due to above reasons, few potassium ions leak out of the cell membrane, making it more electronegative from inside as compared to outside.
2. Action Porential (AP): The term AP pertains to membrane potentials during activity of the cell. When a nerve cell membrane is excited (through application of stimulus or action of a neurotransmitter), there is rapid increase in membrane permeability resulting in influx (inward movement) of sodium ions into the cell. This sodium ion results in loss of polarity (called depolarization) of cell membrane from -75mv to +40mv for few (1-2) milliseconds (ms) only. This depolarization is immediately followed by repolarization (i.e. return of polarity) during which membrane potentials returns to resting level. The repolarization is achieved by outflow (efflux) of potassium ions from the cell. Thus after stimulation, the cell membrane undergoes depolarization followed by repolarization which collectively called as Action Potential (Fig 41a). AP so generated is propagated by itself along the nerve fiber as nerve impulse until it reach nerve ending (i.e. synapse).
3. Threshold Potential (TP): When a stimulus is applied to a nerve cell, then AP results as shown above. Sometimes the stimulus applied is weak and unable to depolarize the membrane to +20mv level. If the stimulus applied is able to cause depolarization beyond -50mV, then full AP (i.e. +40mV) takes place, otherwise no AP results (-75mV). This limit of -50mV is termed as TP i.e. the membrane potential beyond which full AP occur and below which no action potential can occur. This is stated as all or none law of nerve potentials.
4. Hyperpolarization potential: Hyper mean more, therefore HP is used for excessively polarized state of nerve cell membrane (below -80mV). This potential is achieved due to inflow of negatively charged chloride ions into the cell, making the cell more electronegative. Such cell membrane becomes difficult to stimulate by usual neurotransmitters. This type of membrane potential is developed in the nerve cells during inhibitory functions i.e. when nerve signals are stopped from transmission.