Neurology => Neurophysiology
Neurophysiology, the study of how nerve cells, or neurons, receive and transmit information.
In the nervous system there are two main types of cell, the neuron and the neuroglia. Neurons relay information to and from the central nervous system (the brain and spinal cord) in the form of electrical impulses and are specialized to perform the functions of the nervous system. They sense changes occurring inside and outside the body, control thinking and feeling, and control muscles and glands in the body. Neuroglia do not usually convey electrical signals but lie close to the neurons and serve to support and protect them, especially in the central nervous system.
A neuron is a cell with a thick central area containing the nucleus (the cell body or perikaryon). The cell body usually has one long process called an axon that in the human body can be as short as 1 mm or as long as 1 m, and one or more short, bushy processes called dendrites. Axons conduct impulses away from the cell body to stimulate other cells, for example, another neuron, a gland cell, or a muscle. Dendrites, on the other hand, conduct impulses initially generated at another site in the body towards the cell body of the neuron.
The axon contains a number of neurofibrils, which are fibrils running the length of the axon and made of long, thin filaments called microtubules. The cytoplasm of the axon, the axoplasm, is surrounded by a membrane known as the axolemma. At the end of the axons are bulbous structures known as end feet (or synaptic boutons), which are important in the conduction of impulses from the neuron to a neighbouring cell (another neuron, a gland, or a muscle). Dendrites receive impulses from other neurons. The exceptions are sensory neurons, such as those that transmit information about temperature or touch-for example, from the skin to the brain-in which the signal is generated by specialized receptors in the skin at the end of the dendrite.
° Myelinated Axons
Many of the axons of larger neurons are encapsulated by a multi-layered, white, segmented sheath called a myelin sheath; they are therefore said to be "myelinated". Axons lacking such a covering are said to be "unmyelinated". Flattened neuroglia cells called Schwann cells situated along the length of axons in the peripheral nervous system (nerves outside the brain and spinal cord) produce the myelin sheath by wrapping themselves many times around axons as they develop in the embryo and after birth.
As this occurs, the cytoplasm and nucleus are pushed to the outside and the layers of Schwann cell membrane that remain constitute the sheath. This structure insulates and protects the axon, and increases the speed of nerve impulse conduction. The sheath is not continuous along the entire length but is rather segmented into sections, each section comprising one Schwann cell. Between these are short stretches of unmyelinated axon that are encapsulated by Schwann cells but without the multiple layers forming the sheath. These gaps in the sheath are known as the nodes of Ranvier.
Myelinated axons are also to be found within the central nervous system, but here the myelin sheath is formed by another type of neuroglial cell, the oligodendrocyte. These axons also have nodes of Ranvier, though in lower numbers than for nerves in the peripheral nervous system.
° Types of Neuron
There are many different types of neuron in the human body. On the basis of structure, they can be classified according to the number of processes extending from the cell body of the neuron. Multipolar neurons have several dendrites and one axon; bipolar neurons have one dendrite and one axon; unipolar neurons have only one process extending from the cell body, which usually divides into a central branch that functions as an axon and a peripheral branch that serves as a dendrite.
On the basis of their function, neurons can be classified as afferent or efferent neurons. Afferent neurons convey information (impulses) towards the central nervous system. This information is mainly about sensations in parts of the body such as the skin, eyes, nose, mouth, and muscles. Efferent neurons transmit impulses from the brain and spinal cord to more peripheral parts of the body to induce a change in their activity-for example, to make a muscle contract or a gland secrete.
Nerves, as opposed to nerve cells, are bundles of processes, usually in the form of axons, from different neurons collected together. These are also sometimes termed nerve fibres.
CONDUCTION OF A NERVE IMPULSE
A characteristic feature of the neuron is its ability to produce and transmit nerve impulses, which are electrical messages. The cell membrane of a resting neuron-one not carrying an impulse-is polarized due to its negative electrical charge inside, and net positive charge outside, the cell membrane. This difference in electrical charge is called the potential difference and is due largely to unequal distribution of sodium (Na+) and potassium (K+) ions between the inside and the outside of the neuron. There are more K+ ions inside than outside the cell and more Na+ ions outside than inside the cell.
The unequal distribution is largely achieved by an energy-consuming pump in the membrane that ensures both that Na+ ions are pumped outside and K+ ions inside the cell (the Na+/K+ pump). Furthermore, large negatively charged molecules (usually proteins) are retained within the neuron. These do not diffuse freely across the membrane and add to the negative charge inside the cell. As Na+ ions are positively charged and actively transported outside the neuron a positive charge develops outside the cell membrane.
There are insufficient K+ ions being pumped inside to neutralize the negatively charged ions inside the cell, thereby yielding a net negative charge inside the neuron. This establishes a concentration gradient across the membrane with K+ ions tending to leak out and Na+ ions tending to be drawn inside the cell. This diffusion occurs through potassium and sodium channels in the membrane. As the membrane is many times more permeable to K+ than Na+, the Na+/K+ pump has to work hard to maintain the correct concentration of these ions (and hence net charge) either side of the membrane. The difference in charge on either side of the resting neuron's membrane is called the resting membrane potential and the membrane is said to be polarized.
All cells have this charge difference, but when a stimulus of adequate strength is applied to a neuron, a unique event occurs. First, the properties of the membrane change and the cell membrane becomes less permeable to K+ and many times more permeable to Na+, which rapidly enters the cell and induces a net positive charge inside the neuron. This causes the electrical potential of the membrane to change and it is said to be depolarized. Once sufficient Na+ ions have entered the cell to completely reverse the potential, so that the cell has a net positive instead of negative charge inside, the cell is said to have initiated an action potential (or nerve impulse).
Depolarization of one part of the membrane sends an electrical current to neighbouring unstimulated membrane, the outer surface of which is still positively charged. This local current stimulates the adjacent portion of the neuron's membrane to depolarize in a similar fashion. This event repeats itself along the membrane of the cell, thereby conveying the nerve impulse along the neuron.
The size of the action potential is self-limiting, because the membrane becomes less permeable to Na+ again. The membrane then becomes more permeable to K+, which rapidly leaves the cell. The neuron is then said to be repolarized. The whole process takes less than one thousandth of a second. After a very brief period, called the refractory period, the neuron can repeat this process. By then, the Na+/K+ pump has re-established the necessary concentration differences across the membrane.
This is the way that impulses pass down unmyelinated nerves and is termed continuous conduction. In myelinated nerves, however, the myelin sheath around the nerve does not conduct an electric current and forms an insulatory layer around the axon. However, depolarization can occur at the short sections of non-myelination along myelinated nerves, the nodes of Ranvier. In such nerves, the impulse is conducted by jumping sequentially from one node to another along the nerve. This form of impulse conduction is usually quicker than continuous conduction and is known as saltatory conduction.
The speed at which an impulse moves along a nerve depends upon the strength of the stimulus and the properties of the nerve itself. Once the threshold for the stimulus is reached (the point where it is strong enough to stimulate an impulse), the actual speed at which conduction is conveyed along the nerve is determined by such factors as the diameter and degree of myelination of the nerve fibre. Nerves with a large diameter and a myelin sheath conduct impulses faster than smaller, unmyelinated ones.
CHEMICAL TRANSMISSION OF AN IMPULSE:
THE SYNAPSE At the tip of the axon, the signal is chemically transmitted to an adjacent neuron; this junction is known as a synapse. The tiny space between the cells is the synaptic cleft. When the impulse is conducted from a neuron to a muscle or gland cell, it is known as a neuromuscular or a neuroglandular junction.
When the electrical signal reaches the tip of an axon (synaptic bouton), it stimulates small vesicles known as synaptic vesicles in the cell. These small, membrane-enclosed sacs contain chemicals called neurotransmitters, which are made by the neuron. When the impulse reaches the knob it causes calcium ions to move inside the nerve ending. This stimulates the movement of the vesicles towards, and their fusion with, the cell membrane of the nerve. The neurotransmitters in the vesicle are then released into the synaptic cleft where they attach to specialized receptors on the surface of the adjacent neuron. This stimulus causes the adjacent cell to depolarize and propagate an action potential of its own, or to trigger a muscle contraction or glandular secretion.
At the synapse, the impulse can only move in one direction: towards the cell to be stimulated, not back to the neuron. Occasionally, the effect of the neurotransmitter(s) released by the pre-synaptic nerve is to inhibit the post-synaptic nerve rather than stimulate it. Whereas the excitatory transmitter-receptor interactions make the resting membrane potential of the post-synaptic neuron less negative, inhibitory transmitter-receptor interactions make this more negative. This is referred to as hyperpolarization and makes it more difficult for the post-synaptic neuron to generate an action potential.
Examples of neurotransmitters that are usually excitatory are acetylcholine, noradrenaline, serotonin (sometimes known as 5-hydroxytryptamine, or 5-HT), and glutamate. Gamma aminobutyric acid, the opioids B-endorphin and met-enkephalin, and dopamine are neurotransmitters that tend to inhibit neuronal activity in the brain.
One or a number of neurons can synapse with each neuron. The net response of a post-synaptic neuron is determined by the sum of all the excitatory and inhibitory signals it receives.
The duration of a stimulus from a neurotransmitter is limited by the breakdown of these chemicals by enzymes in the synaptic cleft or their re-uptake by the neuron that produced them. Formerly, each neuron was thought to make only one transmitter, but recent studies have shown that some cells make two or more.