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Epilepsy Awareness Program - Structure of Nervous System

 

Nervous System Cells

Overview


The nervous system is the most complicated and highly organized of the various systems which make up the human body. It is the mechanism concerned with the correlation and integration of various bodily processes and the reactions and adjustments of the organism to its environment. In addition the cerebral cortex is concerned with conscious life. It may be divided into two parts, central and peripheral.
The central nervous system consists of the encephalon or brain, contained within the cranium, and the medulla spinalis or spinal cord, lodged in the vertebral canal; the two portions are continuous with one another at the level of the upper border of the atlas vertebra.
The peripheral nervous system consists of a series of nerves by which the central nervous system is connected with the various tissues of the body. For descriptive purposes these nerves may be arranged in two groups, cerebrospinal and sympathetic, the arrangement, however, being an arbitrary one, since the two groups are intimately connected and closely intermingled. Both the cerebrospinal and sympathetic nerves have nuclei of origin (the somatic efferent and sympathetic efferent) as well as nuclei of termination (somatic afferent and sympathetic afferent) in the central nervous system. The cerebrospinal nerves are forty-three in number on either side—twelve cranial, attached to the brain, and thirty-one spinal, to the medulla spinalis. They are associated with the functions of the special and general senses and with the voluntary movements of the body. The sympathetic nerves transmit the impulses which regulate the movements of the viscera, determine the caliber of the bloodvessels, and control the phenomena of secretion. In relation with them are two rows of central ganglia, situated one on either side of the middle line in front of the vertebral column; these ganglia are intimately connected with the medulla spinalis and the spinal nerves, and are also joined to each other by vertical strands of nerve fibers so as to constitute a pair of knotted cords, the sympathetic trunks, which reach from the base of the skull to the coccyx. The sympathetic nerves issuing from the ganglia form three great prevertebral plexuses which supply the thoracic, abdominal, and pelvic viscera; in relation to the walls of these viscera intricate nerve plexuses and numerous peripheral ganglia are found.

 


Source of Video: Brightstorm



Sensory input

When your eyes see something or your hands or touch a warm surface, the sensory cells, also known as Neorons, send a message straight to your brain. This action of getting information from your surrounding environment is called sensory input because your putting things in your brain by way of your senses.

Integration

Integration is best known as the interpretation of things you have felt, tasted, and touched with your sensory cells, also known as neurons, into responses that the body recognizes. This process is all accomplished in the brain where many, many neurons work together to understand the environment.

Motor Output

Once your brain has interpreted all that you have learned, either by touching, tasting, or using any other sense, then your brain sends a message through neurons to effecter cells, muscle or gland cells, which actually work to perform your requests and act upon your environment. The word motor output is easily remembered if one should think that your putting something out into the environment through the use of a motor, like a muscle which does the work for our body.

Nerve Cells (Neuron)

 

The nervous system is composed of specialized cells, termed nerve cells or neurons, that communicate with each other and with other cells in the body. A neuron has three parts:

1- The cell body, containing the nucleus.

2- Dendrites, hair-like structures surrounding the cell body, which conduct incoming signals.

3- The axon (or nerve fiber), varying in length from a millimeter to a meter, which conduct outgoing signals emitted by the neuron. Axons are encased in a fat-like sheath, called myelin, which acts like an insulator and, along with the Nodes of Ranvier, speeds impulse transmission.

Typically a given neuron is connected to many thousands of neurons. The specific point of contact between the axon of one cell and a dendrite of another is called a synapse. Messages passed to and from the brain take the form of electrical impulses, or action potentials, produced by a chemical change that progresses along the axon. At the synapse, the impulse causes the release of neurotransmitters (like acetylcholine or dopamine) and this, in turn, drives the impulse to the next neuron. These impulses travel very fast along these chain of neurons -- up to 250 miles per hour. This contrasts with other systems, such as the endocrine system, which may take many hours to respond with hormones.

 

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Neuronal Function


The basis of neuronal electrochemical signals lies in the neuron membrane. Like all cell membranes, the neuron membrane is a phospholipid bilayer. In other words, the membrane is a "fat sandwich", with fatty acids between two slices of polar (phosphate) "bread". The membrane is pierced with proteins that serve as channels for ions to "flow" though (like straws stuck in a sandwich).
The two ion-channels in the axon (both voltage-gated) allow passage of positive ions of sodium (Na+) and potassium (K+). A pump in the cell membrane (the "sodium pump") actively transports sodium ions out of the cell, and potassium ions into the cell, in a ratio of 3 sodium per 2 potassium. This creates a voltage (potential) difference across the membrane of -70 millivolts. Lowering this potential increases the probability that ion-channels will open. Once some ion channels begin opening, the voltage drops further, causing more channels to open until the membrane depolarizes. Sodium channels are more sensitive to voltage change than potassium channels are, and open more rapidly. Thus, in a depolarization, the sodium ions will rush into the axon faster than the potassium ions will rush out. This sudden depolarization (called an action potential) will briefly result in a +30 millivolt potential difference. Once the slowly-opening voltage-gated potassium ion-channels have opened and allowed potassium to flow out, the action potential is ended. Thus, the sodium ion-channels initiate the action potential, and the potassium ion-channels terminate it. The channels then close, and the sodium pump can restore the resting potential of -70 millivolts. But the action potential will propagate down the axon toward the synapse like a line of falling dominos.


Dendrite

Dendrites are short, thick branched extensions which extend like the roots of a tree over other neurons or body cells. The dendrites all branch off dendritic spines, which in turn branch of the cell body. Dendrites are the receptive sites of the neurons. Here, the neurons receive electric messages from other neurons or body cells. The site where one dendrite meets another neuron's impulse is called the synapse. Usually, neurons have hundreds of dendrite extensions. These extensions are spread over a large area, giving the neuron better reception of signals. Some dendrites are specialized for the accumulation of information. These cells are finer than other dendrites and found near the brain.


Cell Body

Also called the perikaryon-sound or soma-sound, the cell body contains a spherical nucleus with a nucleolus and lots of cytoplasm. Like many cells, the neuron cell body of the neuron contains the usual cellular particles or organells-sound, except centrioles-sound. Centrioles are the basis by which cells are able to divide and form new cells. Because the neurons lack centrioles , they are unable to divide and reproduce themselves. Therefore, if one should damage nerves, then they are not able to be replaced. Nevertheless, neurons do have specialized hardworking endoplasmic reticulum-sound (ER), which help transport proteins and molecules at high speeds due to the fact that neurons work at lightning speeds. Also, the neurofibrils, bundles of micro filaments and micro tubules, which are important in intracellular transport, are seen through the body. A pigment called lipofuscin, which is yellow-brown, is one of the many pigments believed to be in the neuron.


Axon

The axon is a long cylindrical tube, with the same consistent diameter, which runs through the body for long or short lengths. For example, the axon of your neuron controlling your toe, extends all the way from the lumbar back area. The axon branches off a cone shaped region of the cell body called the axon hillock-sound Axons diameters differ in many parts of the body, but the ruel is the thicker the axon, the more message it transmits through the neurons. The main purpose of the axon is to send impulses away from the cell body to neuron dendrite or other body cells called effecter cells-sound. A nerve impulse travels from a dendrite, to the cell body, and down the axon to thousands of branches called telondria which connect at a synapse to dendrites from other neurons. Once the impulse reaches the synapse, neurotransmitters, chemicals, which excite or calm effecter or neurons, diffuse into the extra cellular space and reach the dendrite, once again turning into an impulse. Protecting and insulating electric fibers from one another is the myelin sheath. It is a whitish, fatty, segmented sheath which covers the majority of nerve fibers and helps transmit nerve impulses faster. Throught the axon of the neuron, cells which protect the neuron envelope . These cells forms slope like structures with indents in between them called a Node of Ranvier-sound. The myelin sheath is exceedingly important because one can lose control of your muscles due to the uncoordinated fibers of an axon without myelin sheath.


Impulse movement

Neurons communicate or send impulses through an action potential., This takes place from the dendrite and all the way to the axon ends. An action potential is a change of voltage within the axon. In other words, the negative state off the inner axon turns positive when the impulses comes by. This happens by the use of a sodium and potassium pump. Sodium [Na] surrounds the axon with a positive charge, while the potassium [K] is within the axon. As an impulse enters at the axon hillock, the sodium, potassium pump puts positive sodium into the axon while it puts negative potassium out of the axon. As more sodium enters the potential of the impulse changes from -70 mV to +30 mV, (a difference of 100 mV) This change is called an action potential. The sodium, potassium pump works furiously to pass the impulse through the axon. As the impulse leaves the axon, it is reverted to a normal state which is called the resting potential. At the end of the axon, the impulse or stimulus enters the synapses and is called a post synaptic potential. From here, the impulse is transferred into neurotransmitters, some of which are chemicals called epinephrine and dopamine. These neurotransmitters flow into the fluid filled gap called the synaptic cleft and enter the dendrites. And, again, the process is repeated.

Supporting cells

Nine times more numerous than neurons, supporting cells assist, segregate and insulate neurons. Each supporting cell has its specific function and location. The majority of the central nervous system (CNS) support cells are referred to as glial cells-sound which wrap around the nerves of the space to protect them. Other specific support cells are also very recognizable. Astrocyles are specific supporting cells which anchor neurons to capillaries for energy and regulate the ions around the nerve. Microgelia are a specific macrophage-hyp immune which help protect the central nervous system by engulfing invading microorganisms and dead nerve tissue. Ependymal cells form cerebral spinal fluid and help circulate the fluid by the beating of their cilia. Scwhuan cells are massive cells which cover the mylein sheath and protect neurons by acting as a phagocyte-hyp immune to clean damaged nerves. Satellite cells help control the chemical environment around nerve cells.

 

 

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