What is a nerve cell ?
A nerve cell or neuron is a particular type of cell in the body, belong to the most important elements of the nervous system.
A human being has approximately 100 billion neurons. By far the largest part of it is located in the central nervous system (brains and spinal cord). Nerve cells are the information and signal processors of the body. A specific feature of nerve cells is that they are irritable; they can receive and transmit signals without loss of signal strength. In the brains there are circuits of neurons that regulate many of the body and are also responsible for our mind.
Morphology
On the basis of their shape are nerve cells to be classified as follows:
- unipolar; These nerve cells have but one offshoot; the axon. Ramifications of these structures serve as host, but there are no separate dendrites. These nerve cells are mainly found in invertebrates.
- bipolar; Bipolar neurons have two suckers; an axon and a dendrite. The dendrite receives information from the peripheral nervous system, the axon sends this information to the central nervous system.
- multipolar; Most nerve cells in the nervous system of vertebrates are multi-polar. They have one axon and multiple dendritic branches, which sprout from different parts of the cell body.
Anatomy
A neuron has a cell body or soma, and a number of long thin offshoots. Generally speaking, there are two types of suckers: axons and dendrites. Axons conduct away from the cell, dendrites (usually) go. The axon is often very long (up to about a meter), much thicker (about 25 micrometers) than dendrites and unbranched, except at the end; the dendrites are thin and highly branched. A Purkinje cell in the cerebral cortex may be connected by means of dendrites with thousands of other cells.
The boundaries of a nerve cell are determined by the external cell membrane, also known as plasma lemma. The inside of a nerve cell is composed of cytoplasm, which in turn consists of the cytosol, preventing the watery part of the cytoplasm in which few proteins, and organelles with their own membrane, such as;
- rough endoplasmic reticulum
- smooth endoplasmic reticulum
- the Golgi apparatus
- lysosomes
- a variety of transport vesicles
- mitochondria
- peroxisomes.
Physiology
Along a nerve fiber impulses are passed on, by changing the electrical potential across the cell membrane, the so-called action potential. Although this is an electrical phenomenon, it does not involve simple conduction of electricity, there occurs a change of concentration gradients of ions across the cell membrane so that the potential difference between the inside and the outside of the cell is changing and expanding this change along the foothills of a nerve cell from, somewhat similar to the move of the flame front in a piece of wick. The velocity of a pulse which is passed through a nerve is therefore many times smaller than that of an electrical pulse in a copper wire. In fast fibers is that, with increased speed (up to between 70 and 120 meters per second) than slow fibers. The fastest nerves are motor nerves that go to the large skeletal muscles. The speed at which the signal propagates is dependent on both the thickness of the myelin sheath and of the diameter of the nerve. At the 'slowest' nerve fibers is the propagation speed between 1 and 2 m/s.
Depolarisatieproces of the cell membrane
The membrane of most of the animal cells is an electrically insulating medium in itself, consisting of a double layer of phospholipids in which a large number of proteins are located. Ions (charged particles in the solution) can not normally spontaneously move from one side to the other: they are or actively pumped through the cell to the other side, or make use of very selective 'channels' that can open the cell and close set. Between the inside and moving to the outside of the membrane of a neuron is a certain potential difference of several tens of millivolts which is held in an active position by the cell by ions in and out of the cell. The inside of the cell is approximately 70 milli volts negative with respect to the outside. If the nerve cell now stimulated, then occurs after a short latent period of approximately 1 millisecond in which nothing seems to happen an increase in depolarization of the cell membrane, which even passes through the neutral point, and after about 1 ms a positive potential reaches about 35 millivolts. After this, the voltage falls back to -70 millivolts, and even slightly more than that for approximately 50 milliseconds, after which the initial state is achieved again.
Mechanism
On either side of the membrane there is a difference between the concentrations of the major ions of sodium and potassium: within in the cell is much more potassium, outside it a lot more sodium. After challenge arises suddenly a much greater permeability to positively charged sodium ions, which begin to flow in the cell. As a result, the membrane potential is reversed. These elevated sodium permeability is of short duration. A little later will sweep through an increased permeability for potassium ions, which will leave the cell and thus counteract the change in potential. This phase lasts a little longer. For example, the action potential is formed.
After a few milliseconds, is the permeability of the cell membrane as well as before, and that entered the sodium ions and the potassium ions are flowed out slowly through the cell pumped back to the place where they belong. These changes in permeability occur in response to a stimulus; these can be of different nature, electrically, mechanically, or chemically. If the 'provocation' once exceeded the depolarization continues. An adjacent piece of membrane is then through the depolarization in turn depolarized and thus the plant pulse produced along the cell membrane, and along an axon or dendrite.
The synapse
At the end of the path of a nerve impulse by a neuron, the pulse must be used to excite a subsequent nerve. If not, then the information contained in the pulse will be lost. Neurons can transmit impulses to each other through synapses. A synapse is a connection between two neurons through which an impulse can be transmitted. At a synapse is contact between two neurons. The signal can there be transferred as an electrical signal, but it is usually transferred by chemical substances called neurotransmitters. These neurotransmitters are usually produced at the end of an axon and transferred to a dendrite of another neuron, but there are also axons and synapses between the cell body or dendrites between themselves.
Most synapses are chemical, often between the end of an axon (the presynaptic neuron) and the (postsynaptic) neuron. Upon the arrival of a pulse at the end of the calcium ports are opened through which calcium ions flow into the presynaptic neuron. Which calcium ions cause with neurotransmitter-filled vesicles (called vesicles) fuse with the cell membrane of the axon. The neurotransmitters are then placed in the narrow space between axoneinde and postsynaptic neuron (the so-called synaptic cleft). They stabbing the gap over and attach to the other side on sodium gates to let sodium ions. In the idle state, the gate is closed but if there is the appropriate neurotransmitter perching on the outside of the gate, it is open for some time. In the foregoing section on impulse conduction can see that after the opening of sodium gates, the sodium ions across the cell membrane Retraction and depolarizing the cell membrane. This could be the beginning of a pulse which is going to propagate through to the next neuron.
There are many types of neurotransmitters. Some precautions as indicated above a reduction in the potential difference across the membrane of the receiving neuron. These neurotransmitters are called 'exciting'. Other neurotransmitters ensure correct for an increase in the membrane stress these works 'inhibitory'. There are usually several types of neurotransmitters of different neurons at a receiving neuron to. If the exciting signals together (one piece) are stronger than the inhibitory affects the excited neuron: the membrane is polarized, and the receiving neuron will send a pulse by its axon. Well-known neurotransmitters are as follows: acetylcholine (the most well known because it was first discovered) and GABA, which plays a role in the toxicity of alcohol.
New theory
Recently, researchers at the University of Copenhagen invented a new theory that explains many problems. With the "old" theory, namely, for example, the effect of anesthetic agents are not explained. The new theory is not based on an electric signal as a transfer agent, but goes out of solitons - a sort of sound waves. Solitons are formed only under the proper fluidity of the cell membrane. Anesthetic agents make sure that the melting point of the lipids in the cell membrane bearing comes to lie - below body temperature - so that no more solitons can be formed, which results in it not feel pain.
Research into nerve cells
Particularly by animal studies we know a lot about nerve cells. In the 60s of the twentieth century, much research done through lesion studies. It is made a cut in a nerve cell and examined the effect of this. Often, this led to neuronal degeneration, the degradation of these nerve cell. In the seventies, a new technique: retrograde and anterograde tracing tracing. By means of these techniques, a nerve cell may be tinted, so that the position thereof can be examined. An even more sophisticated technique is that of the virus tracing. Hereby, the connections and nerve paths can be tracked. The problem here is that a nerve cell may have very many connections.