Neurons: Their Structure and Function
Neurons: Their Structure and Function
The human central nervous system contains a vast complex of information processing circuits formed by interconnecting networks of nerve cells. Together the brain and spinal cord constitute the central nervous system, which is kept in contact with the receptors and effectors of the rest of the body by the peripheral nervous system. The nerves of the peripheral nervous system convey messages to and from the spinal cord. Afferent nerves run towards the spinal cord and have a sensory function, whereas efferent nerves run away from it and have a motor function. If for example you pricked your finger with a pin, the afferent nerves would carry this information about the sensation to the spinal cord, while the efferent nerves would bring about the movements of muscle groups, thereby bringing the limb into action and to withdraw. Another component of the nervous system is the autonomic nervous system, which is concerned with controlling the body's involuntary activities. These include such functions as the beating of the heart, movements of the gastrointestinal tract, and the secretion of sweat.
The person most directly responsible for the acceptance of the neuron constituting an integral part of the nervous system was Ramon y Cajal (Jones, 1981). Ramon y Cajal was able to describe the shapes and distributions of individual neurons in many different parts of the nervous system. Nerve cells, or neurons, are cells surrounded by a thin plasma membrane and filled with cytoplasm containing various organelles such as mitochondria and the nucleus. Nerve cells share many properties with cells from other types of tissue but are distinguished by a particularly complicated geometric form that is related to neuronal function (Carterette & Friedman, 1973).
A neuron usually consists of a cell body and one or more slender branches that grow out from it. The cell body, which contains the cell nucleus, is often called the soma and the branches leading off of it are termed neurites. In most neurons there is one long neurite called an axon and several shorter neurites called dendrites. An axon is functionally defined as a neurite that conveys information away from the cell body and a dendrite toward the cell body (Delcomyn, 1998).
Not all neurons have the same structure and a way to classify neurons is by their shape or general appearance, particularly by the number of neurites that branch from the soma. Using this type of classification system, four main types of neurons are identified. Multipolar neurons are the most frequently encountered and are characterized by the possession of a single axon and many dendrites extending from the cell body. Bipolar neurons have two neurites, one an axon and the other a dendrite, extending roughly from the opposite sides of the cell body. Monopolar neurons are characterized by a single neurite extending from the cell body that typically branches into an axon. And finally neurons lacking any neurite extension at all are called apolar neurons.
But what makes neurons the unique cell types that they are is their ability to communicate between other neurons. Neurons can communicate in two ways. In the process called chemical transmission, communication takes place via a chemical intermediary called a neurotransmitter which is released by one neuron and influences another. In the process known as electrical transmission, communication takes place by the flow of electrical current directly from one neuron to another. Chemical synapses differ from electrical synapses both structurally and functionally. Structurally, chemical synapses have a small gap, called the synaptic cleft, between the communicating neurons. There ...
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But what makes neurons the unique cell types that they are is their ability to communicate between other neurons. Neurons can communicate in two ways. In the process called chemical transmission, communication takes place via a chemical intermediary called a neurotransmitter which is released by one neuron and influences another. In the process known as electrical transmission, communication takes place by the flow of electrical current directly from one neuron to another. Chemical synapses differ from electrical synapses both structurally and functionally. Structurally, chemical synapses have a small gap, called the synaptic cleft, between the communicating neurons. There is no synaptic cleft in electrical synapses, since the communicating neurons touch one another. The main functional difference, apart from the defined chemical and electrical differences is that the chemical synapses are polarized, meaning that communication can take place in only one direction (Grayson, 1981). However, communication across electrical synapses may flow in either direction.
Bond & McConkey (2000), state that there are three structures of the neuron that are critical to its function as an information-processing unit: the neuronal membrane, the axon and the synapse. The neuronal membrane separates the cell from its environment. All the information that the neuron receives from other cells and the information transmitted from the neuron to other cells must pass through this membrane. The membrane is made up of elements called phospholipids, or fatty acids. The phospholipid components of cell membranes consist of molecules with a head that seeks for water, (known as hydrophilic), and two water avoiding tails (known as hydrophobic). Since both the inside and the outside of the cell contain water the phospholipids arrange themselves in two layers. Because of this double layered structure, a membrane is referred to as a lipid bilayer. The protein components of membranes are embedded in the lipid bilayer. These protein molecules constitute more than 20% of the membrane of a typical neuron and present on the neuron many of the characteristics that distinguish it from other types of cells and also other neurons (Alberts & Raff, 1994).
Different proteins can have different properties, and because of the presence of particular types of proteins with specific characteristics it brings about certain neurons having a particular functional feature (Brodal, 1992). Despite there being a wide array of proteins they can be grouped into three categories based on characteristics, capabilities and function. The first being transport proteins. Transport proteins mediate the transfer of ions across the cell membrane. Within the category of transport proteins there are proteins known as ion channels that by the process of diffusion allow the passive flow of ions through the membrane. Another is the ion pump protein that expends energy to move one or more ions across the membrane against a concentration gradient in a process known as active transport. The second type of protein is the signalling protein. Signalling proteins are necessary for the transporting of information from one cell to another. An important type of signalling protein is the receptor, which has a high binding ability for other molecules and is used to respond to messages. A further type of signalling protein is the guanine nucleotide protein that initiates biochemical reactions that lead to a neuron's response to a signalling molecule. Both of these signalling proteins that are important in responding to a message are enzymes. These enzymes help catalyse one or more chemical reactions to begin the response of the neuron to the message. The third type of protein is the binding protein. There are two types of binding proteins the first being adhesion proteins and they help bind one cell to another. The second are the anchor proteins that face into the cell and fasten the membrane to the internal group of proteins that make up the inner wall of the cell membrane. This gives the cell structural integrity as well as dictating its specific form.
While all cells in the brain have the same organelles as the soma, only neurons have axons. Like other cells, neurons must ensure that organelles and molecules are properly distributed throughout the cell. In neurons this transport system is known as axonal transport (Nauta & Feirtag, 1986). Axonal transport plays a fundamental role in the life of a neuron. Any function, such as growth, development or repair, that requires the movement of molecules or organelles from one part of the neuron to another will depend on axonal transport. At the end of the axon is a region known as the axon terminal. Running the length of the axon are microtubules and microfilaments for carrying substances from the cell body to the axon terminal and back from the terminal to the cell body.
In 1948 Paul Weiss and H.B. Hiscoe discovered that there are several types of axonal transport (Delcomyn, 1998). Anterograde transport is the movement of materials from the cell body toward the axon terminal. There are certain rates of anterograde transport ranging from a low of 0.5 mm/day to a high of 400 mm/day. The other type of axonal transport is retrograde transport, which is movement of materials from the axon terminal towards the cell body.
The axon terminal is where the neuron communicates with other neurons. Communication among neurons takes place across the synapse. There are three components of the synapse: the presynaptic membrane, the postsynaptic membranes and the synaptic cleft. The presynaptic membrane is typically the axon terminal because it is at the end of the axon. The postsynaptic membrane may be the surface of another cell body or dendrite of another neuron. Separating the presynaptic and postsynaptic membranes is the synaptic cleft. Synaptic transmission occurs across this synaptic cleft.
This unique structure of the neuron now allows it to conduct and transmit information through electrochemical signals, or also known as the nerve impulse. The fluid inside and outside a neuron contains electrically charged ions, such as sodium and potassium. When the neuron is at a resting state the distribution of ions inside and outside of the cell are equal and this has approximately been measured at -70 millivolts (Bond & McConkey, 2000). When a neuron is stimulated it causes the entry of sodium ions into and exit of potassium ions out of the neuron resulting in an action potential, which has been measured to as high as +40 millivolts (Beatty, 1975). The initial axon potential occurs at the axon hillock and is passed down the axon where a succession of action potentials is created at the nodes of Ranvier. The nodes of Ranvier are separated sections of the axon that are covered by a myelin sheath. The nodes of Ranvier and the myelin sheath help to increase the speed of the action potential. The axon potential then arrives at the axon terminal which results in the release of neurotransmitters. Neurotransmitters then cross the synaptic cleft and activate receptors on the membrane of the postsynaptic neuron. This can then cause the postsynaptic neuron to fire a nerve impulse. After the neuron has fired it then returns to its resting value of -70 millivolts. In electrical transmission, the current flow generated by an impulse in the presynaptic neuron spreads directly into the next neuron. This may result in the initiation of an impulse in the postsynaptic neuron. The most notable feature with the electrical synapse is its speed compared to the chemical transmission. There are several types of neurons in which action potentials are not generated. These cells operate on a passive potential and are known as non-spiking neurons (the word spike is another name for action potential). These types of non-spiking neurons can be found in the sensory neurons that operate over short distances, such as the ones found in the visual systems (Young, 1989).
The ability of the neuron to transmit information is the functional basis of all the nervous system. The minute building block of the neuron that leads to the nervous system and eventually to human consciousness is extremely complex. Its unique structure and function enable the human mind to think, feel see and act.
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