Ionic Basis of Membrane Potential

Membrane Potential at Rest

Like other excitable tissues in the body, the electrical potential of a neurone in the resting state is more negative on the inside of the cell compared with the outside. This polarity is maintained by the active transport of Na+ ions out of the cell, together with the active transport of K+ ions into the cell. However, there is a tendency for both ions to diffuse passively down their concentration gradient through leaky ion channels. During its resting state, the membrane is more permeable to K+ than to Na+ ion, thus more K+ ions leak out of the neurone than Na+ ions enter the cell. At the same time, the resting membrane is not permeable to anions. The result is that the interior of the neurone is more electronegative (-70 mV) than the outside, this is the resting membrane potential.

Characteristics of the Action Potential

Neurones respond to a stimulus by transiently producing changes in ion permeability or conductance in the cell membrane. Ion conductance is defined as the reciprocal of electrical resistance of the membrane to a given ion and, therefore, reflects permeability. When the stimulus is below the threshold potential, the changes produced remain localized. However, when the stimulus reaches the threshold, the membrane becomes depolarized. When this depolarization is propagated along the axon, it gives rise to an action potential. The latter can be divided into a number of phases (Figure NE.3).

At the beginning of an action potential, the rate of depolarization increases so that the inside of the cell becomes increasingly positive until it rises to a peak, then falls when repolarization begins. This sharp rise and decline is called the spike potential. The sharp rise is due to an increase in Na+ conductance, so that Na+ ions diffuse down their electrical and concentration gradients. However, there are three factors which limit the depolarization process: first, the Na+ channels open only very transiently; second, as the inside of the cell becomes increasingly more electropositive, the initial gradients which facilitate Na+ influx disappear; and finally K+ conductance also increases. The rate of repolarization slows down when the process is about 70% complete; this phase is known as after-depolarization. During the final recovery phase, there is a slight but prolonged overshoot after the resting potential is reached. This is due to the slow return of K+ conductance to normal (Figure NE.4). This phase is called the after-hyperpolarization,

A neurone is said to be in the absolute refractory period when it is totally unresponsive to any stimulus regardless of its strength. This corresponds to the period between the threshold being reached and when repolarization is one third completed. The relative refractory period starts at this point until the beginning of after-depolarization. During this period, a stronger than normal stimulus may lead to excitation (Figure NE.4).

Figure NE.2 A typical neurone
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Figure NE.3 Different phases of an action potential

Figure NE.4 Changes in Na+ and K+ conductance during the course of an action potential

Synaptic Transmission

Nerve impulses are transmitted from one neurone to another through junctions known as synapses. Synapses are formed between the terminal buttons of a neurone and the cell body or axon of another neurone. The number of terminal buttons forming synapses with a neurone varies from one to several thousand. Synapses almost invariably allow unidirectional impulse conduction, i.e. from pre-synaptic to the post synaptic neurone. This ensures nerve impulses are transmitted in an orderly fashion. Most synaptic transmissions are chemical in nature, others are either electrical or mixed. In electrical synapses, the membranes between the pre-synaptic and post synaptic neurones meet to form gap junctions, which contain channels that facilitate diffusion of ions.

Structure of a Synapse

Chemical synapses consist of a small gap known as synaptic cleft. The synaptic cleft measures about 20 nm wide and contains extracellular fluid across which the neurotransmitters diffuse. There are three essential structures that can be found in the cytoplasm of the terminal button: synaptic vesicles, mitochondria and endoplasmic reticulum (Figure NE.5). The synaptic vesicles, containing the neurotransmitters, are usually found in large concentrations in the release zone adjacent to the synaptic cleft. The endoplasmic reticulum is responsible for the production of new, and recycling the used vesicles. Mitochondria provide the energy required for chemical transmission and the formation of synaptic vesicles by the endoplasmic reticulum.

Synaptic Mechanism

When an action potential is transmitted down an axon, the de-polarization opens the voltage-gated calcium channels, allowing an influx of Ca2+ ions into the terminal button. In the release zone the Ca2+ ions bind with groups of protein molecules in synaptic vesicle membranes. These protein molecules spread apart, allowing fusion between vesicle and terminal button membranes. This releases neurotransmitter into the synaptic cleft. The amount of neurotransmitter released is directly proportional to the Ca2+ influx.

The post synaptic receptors are activated by the binding of neurotransmitters which lead to the opening of ion channels, resulting in post synaptic potentials. Post synaptic potentials are transient in action because of two mechanisms: first and predominantly, by transmitter re-uptake and, second, by enzymatic de-activation. The terminal buttons at the end of the transmission rapidly and actively takes up neurotransmitters. Figure NE.6 summarizes the different types of neurotransmitter found in the CNS.

Figure NE.5 Structure of a chemical synapse

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Figure NE.5 Structure of a chemical synapse

DIFFERENT TYPES OF NEUROTRANSMITTER, LOCATION AND FUNCTION

Neurotransmitter

Location

Function

Acetylcholine

Cerebral cortex, thalamus, limbic system

Likely to be involved in memory, perception, cognition, attention and arousal functions

Noradrenaline

Locus ceruleus

Cerebellum

Hypothalamus

Descending pain pathway Inhibits Purkinje cells Regulates secretion of anterior pituitary hormones

Adrenaline

Medulla

Functions uncertain

Dopamine

Substantia nigra Hypothalamus

Control of motor functions Regulates prolactin secretion

Serotonin

Neocortex and limbic system

Alters mood and behaviour

Hypothalamus Nucleus raphe magnus and spinal cord

Increases prolactin secretion

Pain modulation

Figure NE.6

Figure NE.6

The events following the generation of post synaptic potentials depends mainly on two conditions: first, the amount of neurotransmitter released and, second, the type of ion channel that is being opened. Not all post synaptic potential changes are propagated as action potentials in the post synaptic neurone. When an insufficient amount of neurotransmitter becomes bound to the post synaptic receptor, the change in membrane potential may not reach the firing threshold of the neurone, thus only giving rise to local potential changes. When Na+ channels are open, there is a sudden influx of Na+ ions down its concentration and electrical gradients producing an excitatory post synaptic potential (EPSP). However, when K+ or Cl- channels are open, K+ and Cl- ions move down their concentration gradients making the inside of the neurone more electronegative with respect to the outside of the neurone, i.e. hyperpolarized, resulting in an inhibitory post synaptic potential (IPSP) (Figure NE.7). When the post synaptic potential reaches threshold an action potential occurs.

Figure NE.7 Synaptic transmission

Sensory Receptors

Sensory receptors are specialized structures that receive and transmit information from the external and internal environment to the CNS. A sensory receptor may be part of a neurone such as nerve endings or a separate structure that is capable of generating and transmitting action potentials to a neurone. They are essentially transducers that respond to different forms of energy, such as mechanical or thermal energy, and convert them into electrical signals. Special sense organs such as the eye are a collection of sensory receptors supported by highly organized structural and connective tissue.

Sensory receptors may be classified according to whether they perceive visceral or somatic sensory changes. Visceral receptors are primarily concerned with perceiving changes in the internal environment; such information does not usually reach consciousness. These include chemoreceptors which are sensitive to changes in glucose level, oxygen tension, osmolality and acidity in the plasma. Stretch receptors in the lungs and pressure receptors in the carotid sinus are other examples of visceral receptors.

Somatic receptors are sensory receptors that respond to external stimuli such as temperature, light touch and pressure. Pain is initiated by noxious or potentially damaging stimuli; pain receptors are, therefore, also known as nociceptors. Information from the somatic receptors usually reaches consciousness and are represented at the cerebral level, giving rise to a variety of sensations.

Sensory pathways are multisynaptic involving a peripheral first-order neurone (e.g. dorsal root ganglion), which synapses with a central chain of second-order (e.g. posterior column nuclei) and third-order neurones (e.g. thalamic nuclei) (Figure NE.8).

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