Nervous system

The nervous system is a biological system that regulates the body's responses to internal and external stimuli by carrying information to and from all parts of the body through a vast network of nerve cells. Nervous tissue is found in the brain, spinal cord, and nerves. It is responsible for coordinating and controlling many body activities. It stimulates muscle contraction, creates an awareness of the environment, and plays a major role in emotions, memory, and reasoning. To do all these things, cells in nervous tissue need to be able to communicate with each other by way of electrical nerve impulses.

The cells in nervous tissue that generate and conduct impulses are called neurons or nerve cells. Neurons consist of a large cell body and elongated extensions called axons used for sending impulses, and usually dendrites for receiving impulses. Normally, nerves transmit impulses electrically in one direction, from the impulse-sending axon of one nerve cell to the impulse-receiving dendrites of the next nerve cell. The information the dendrites received can cause the cell body to generate a nerve impulse. Axon terminal is a swelling at the tip of each of the tiny nerve endings. Where an axon terminal comes close to another cell, the membranes of both cells are modified to from a synapse.(Purse, p846) The axon secretes tiny amounts of chemical messengers called neurotransmitters. Neurotransmitters trigger the receptors on the next nerve cell's dendrites to start up a new electrical current. Different types of nerves use different neurotransmitters to transmit impulses across the synapses.

Nervous tissue also includes cells that do not transmit impulses, but instead support the activities of the neurons. These are the glial cells (neuroglial cells), together termed the neuroglia. Supporting, or glia, cells bind neurons together and insulate the neurons. Some are phagocytic and protect against bacterial invasion, while others provide nutrients by binding blood vessels to the neurons. The nervous system has two basic parts: the central nervous system (CNS) and the peripheral nervous system. The CNS includes the brain and spinal cord, and the peripheral nervous system includes the nerves outside the CNS (i.e. motor and sensory system functions).

Neurons


Nerve cells, called neurons, are specialized to receive the information and encode it, and transmit the information to other cells. Nervous systems consist of neurons and their specialized supportive cells called glial cells.(Purves, p844)

Although nervous systems are capricious in structure and function, neurons function similarly in animals as different as squids and humans. Their plasma membranes generate electric signals-called nerve impulses or action potentials-and conduct these signals from one location on a neuron to the most distant reaches of that cell.(Purves, p845) Most of the neurons have four regions a cell body, dendrites, an axon, and axon terminals. The cell body contains the nucleus and most of the cell's organelles. Also, many projections may sprout from the cell body. Most of the projections are dendrites, which bring information from other neurons or sensory cells to the cell body. In neurons, there is one projection called axon, it is much longer than the others, and it carries away the information from the cell body. The length of the axon are as different as the types of neurons, such as those that run from the spinal cord to the toes. These are remarkably long.(Purves, p845-846)

To transmit these messages, charged particles (primarily sodium ions), jet across a nerve cell membrane, creating an electrical impulse that speeds down the axon. When the electrical impulse reaches the end of the axon, it triggers the neuron to release a chemical messenger (called a neurotransmitter) that passes the signal to a neighboring nerve cell. This continues until the message reaches its destination, usually in the brain, spinal cord, or muscle.


 * Axon terminal is a swelling at the tip of each nerve ending, and comes very close to the target cell. A synapse is where the axon terminal comes close to the other, and modifies the membranes of both cells. Axon terminals synapse with a target cell.


 * Neurotransmitters are stored in the axon terminal and released when a nerve impulse arriving at an axon terminal, then it diffuses across the space and binds to the receptors on the plasma membrane of the target cell.

Glial cells
Glial cells are the nonconducting cells that support cells in the nervous system and help protect neurons, and they also help the neurons make the right contacts during embryonic development, others insulate axons.(Purve, p846)

In the peripheral nervous system there are a group of cells called the Schwann cells. They wrap around the axons of neurons and covering them with concentric layers of insulating plasma membrane. Other glial cells which are called oligodendrocytes, perform a similar function in the central nervous system. The Schwann cells and the oligodendrocytes are covered by the Myelin that gives many parts of the nervous system a glistening white appearance.(Purve, p846)

Glial cells are well known for their important supportive roles they play in the nervous systems. Some of them supply the neurons with nutrients and others consume foreign particles and cell debris. They also help keep the proper ionic environment around the neurons. Some of the glial cells communicate with one another electrically through gap junctions which is a special type of connection that can let the ions flow between cells, although they have no axons and do not generate or conduct nerve impulses.(Purves, p847)

One of the glial cells called the astrocytes contribute to the blood-brain barrier. They protect the brain from toxic chemicals in the blood. The blood vessels throughout the body are very permeable to many chemicals including the toxic. If the blood-brain barrier didn't work they would reach our brain and we probably would get hurt from it. Astrocytes help us from the blood-brain barrier by surrounding the smallest and most permeable blood vessels in the brain. However, the barrier is not perfect. Because it consists of plasma membranes and it is permeable to fat-soluble substances such as anesthetics and alcohol. That's why when we do an operation we need anesthetics to be anesthetized and we would became drunk after drinking a lot of wine.(Purves, p847)

Neurons

 * Main Article: Neuron

The simplest neural network consists of three cells: a sensory neuron connected to a motor neuron connected to a muscle cell. The human brain contains nearly 1011 neurons, and most of those neurons receives information from a thousand or more synapses. There are probably 1014 synapses in the human brain. That's why our brain can process information with those incredible abilities. Neurons and synapses are divided into thousands of distinct but interacting networks similar functions. The properties of individual neurons allow them to generate and conduct nerve impulses.(Purves, p847)

Simple electrical concepts underlie neuronal function. Voltage is the tendency for electrically charged particles like electrons or ions to move between two points. As we know, voltage is to the flow of electrically charged particles, if the negative and the positive poles of a battery by a copper wire, then the electrons will flow from the negative to positive because there is a voltage difference between them.

Electrons carry the electric current in wires, but in solutions and across cell membranes the electron are carried by charged ions. The major ones which carry electric charges across the plasma membranes of neurons are sodium, chloride, potassium, and calcium.(Purves,p847-848)

Nerve impulse
The inside of the cells are electrically negative in comparison to the outside. The membrane potential is the difference in the voltage which is across the plasma membrane of a cell. And in an unstimulated neuron, this voltage difference is called a resting potential. Electrodes can measures the membrane potentials. If a pair of electrodes is placed one on each side of the plasma membrane of a resting axon, they may measure a voltage difference of about 60 millivolts.

The resting potential may change when a neuron is stimulus, because neurons are sensitive to any chemical or physical factor. And the most extreme change in membrane potential is an action potential, which is a sudden and rapid reversal in the voltage across a portion of the plasma membrane. For 1 or 2 milliseconds, a bioelectric current crosses the membrane and the inside of the cell will become more positive than the outside. The action potentials that moves along axons called nerve impulses.

Ion pumps use energy to move ions or other molecules against their concentration gradients. Sodium-potassium pump is the major ion pump in the plasma membranes of neurons. The action of this pump is that it expels sodium ions from inside the cell and exchange them for potassium ions from outside the cell. This pump keeps the concentration of potassium ions inside the cell greater than the extracellular fluid and the concentration of sodium ions inside the cell less than that of the extracellular fluid. The concentration differences established by the pump mean that the sodium ions and the potassium would diffuse out and in the lipid bilayer.(Purves, p848)

Diseases and Disorders
The nervous system is an very complex communication system, which can send and receive voluminous amounts of information at one time. However, the system is vulnerable to diseases and injuries. For example, nerves can degenerate, causing Alzheimer's disease or Parkinson's disease. Bacteria or viruses can infect the brain or spinal cord, causing encephalitis or meningitis. A blockage in the blood supply to the brain can cause a stroke. Injuries or tumors can cause structural damage to the brain or spinal cord.