Analysing The Nervous System Biology Essay


The two main types of cells found in the nervous system are neurons and neuroglia. The nervous system is composed of billions of cells, the most essential being the neurons.  There are estimated to be as many as 100 billion neurons in our nervous system.

A typical neuron has all the parts that any cell would have, and a few specialized structures that set it apart.  The main portion of the cell is called the soma or cell body.  It contains the nucleus, which in turn contains the genetic material in the form of chromosomes. Neurons have a large number of extensions called dendrites.  They often look likes branches or spikes extending out from the cell body.  It is primarily the surfaces of the dendrites that receive chemical messages from other neurons. One extension is different from all the others, and is called the axon.  In some neurons, it is hard to distinguish from the dendrites, in others it is easily distinguished by its length.  The purpose of the axon is to transmit an electro-chemical signal to other neurons, sometimes over a considerable distance.  In the neurons that make up the nerves running from the spinal cord to your toes, the axons can be as long as three feet. Longer axons are usually covered with a myelin sheath, a series of fatty cells which have wrapped around an axon many times.  These make the axon look like a necklace of sausage-shaped beads.  They serve a similar function as the insulation around electrical wire. At the very end of the axon is the axon ending, which goes by a variety of names such as the bouton, the synaptic knob and the axon foot. It is there that the electro-chemical signal that has travelled the length of the axon is converted into a chemical message that travels to the next neuron. Between the axon ending and the dendrite of the next neuron is a very tiny gap called the synapse. For every neuron, there are between 1000 and 10,000 synapses. Neuroglia cells, the other major cell type in neural tissue, provide structural integrity to the nervous system and functional support that enables neurons to perform. The word 'neuroglia' (nerve glue) was introduced by Rudolf Virchow about 1854. Neuroglias do not typically have synapses at their surface.  Classically neuroglia cells are described as existing only in the central nervous system where there are three kinds: astrocyte, oligodendrocyte, and microglia. Astrocytes, known for the many processes attached to their cell body, provide structural support and their processes often have 'end feet' that lie alongside the basal lamina around the capillary endothelium or line the exterior surface of the CNS, where they contribute to the pial-glial external limiting membrane.  Cell bodies of astrocytes are among the largest for the glia, but only overlap the lower end for size of neurons.  Oligodendrocytes form myelin sheaths around axons in the CNS.  One oligodendrocyte can form myelin sheaths along more than one internodes of more than one axon.  They have smaller cell bodies than astrocytes and relatively fewer processes leaving the cell body. Microglia are the main phagocytic cell and antigen-presenting cells in the CNS.  They have the smallest cell bodies among the neuroglia.

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There are two types of cells in the peripheral nervous system. These cells carry information to (sensory nervous cells) and from (motor nervous cells) the central nervous system (CNS). Cells of the sensory nervous system send information to the CNS from internal organs or from external stimuli. Motor nervous system cells carry information from the CNS to organs, muscles, and glands. The motor nervous system is divided into the somatic nervous system and the autonomic nervous system. The somatic nervous system controls skeletal muscle as well as external sensory organs such as the skin. This system is said to be voluntary because the responses can be controlled consciously. Reflex reactions of skeletal muscle however are an exception. These are involuntary reactions to external stimuli. The autonomic nervous system controls involuntary muscles, such as smooth and cardiac muscle. This system is also called the involuntary nervous system. The autonomic nervous system can further be divided into the parasympathetic and sympathetic divisions. The parasympathetic division controls various functions which include inhibiting heart rate, constricting pupils, and contracting the bladder. The nerves of the sympathetic division often have an opposite effect when they are located within the same organs as parasympathetic nerves. Nerves of the sympathetic division speed up heart rate, dilate pupils, and relax the bladder. The sympathetic system is also involved in the flight or fight response. This is a response to potential danger that results in accelerated heart rate and an increase in metabolic rate.

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The brain and spinal cord are the organs of the central nervous system. Because they are so vitally important, the brain and spinal cord, located in the dorsal body cavity, are encased in bone for protection. The brain is in the cranial vault, and the spinal cord is in the vertebral canal of the vertebral column. Although considered to be two separate organs, the brain and spinal cord are continuous at the foramen magnum. In addition to bone, the central nerves system is surrounded by connective tissue membranes, called meninges, and by cerebrospinal fluid. There are three layers of meninges around the brain and spinal cord. The outer layer, the dura mater, is tough white fibrous connective tissue. The middle layer of meninges is arachnoid, which resembles a cobweb in appearance, is a thin layer with numerous threadlike strands that attach it to the innermost layer. The space under the arachnoid, the subarachnoid space, is filled with cerebrospinal fluid and contains blood vessels. The pia mater is the innermost layer of meninges. This thin, delicate membrane is tightly bound to the surface of the brain and spinal cord and cannot be dissected away without damaging the surface.

The brain is divided into the cerebrum, diencephalons, brain stem, and cerebellum. The largest and most obvious portion of the brain is the cerebrum, which is divided by a deep longitudinal fissure into two cerebral hemispheres. The two hemispheres are two separate entities but are connected by an arching band of white fibers, called the corpus callosum that provides a communication pathway between the two halves. Each cerebral hemisphere is divided into five lobes, four of which have the same name as the bone over them: the frontal lobe, the parietal lobe, the occipital lobe, and the temporal lobe. A fifth lobe, the insula or Island of Reil, lies deep within the lateral sulcus. The diencephalon is centrally located and is nearly surrounded by the cerebral hemispheres. It includes the thalamus, hypothalamus, and epithalamus. The thalamus, about 80 percent of the diencephalon, consists of two oval masses of gray matter that serve as relay stations for sensory impulses, except for the sense of smell, going to the cerebral cortex. The hypothalamus is a small region below the thalamus, which plays a key role in maintaining homeostasis because it regulates many visceral activities. The epithalamus is the most dorsal portion of the diencephalon. This small gland is involved with the onset of puberty and rhythmic cycles in the body. It is like a biological clock. The brain stem is the region between the diencephalon and the spinal cord. It consists of three parts: midbrain, pons, and medulla oblongata. The midbrain is the most superior portion of the brain stem. The pons is the bulging middle portion of the brain stem. This region primarily consists of nerve fibers that form conduction tracts between the higher brain centers and spinal cord. The medulla oblongata, or simply medulla, extends inferiorly from the pons. It is continuous with the spinal cord at the foramen magnum. All the ascending (sensory) and descending (motor) nerve fibers connecting the brain and spinal cord pass through the medulla. The cerebellum, the second largest portion of the brain, is located below the occipital lobes of the cerebrum. Three paired bundles of myelinated nerve fibers, called cerebellar peduncles, form communication pathways between the cerebellum and other parts of the central nervous system.

10 pathological conditions affecting the nervous system are:

Bell's palsy is a form of Neuritis that involves paralysis of the facial nerve causing weakness of the muscles of one side of the face and an inability to close the eye. The cause is unknown but recovery may be spontaneous. The effects are: paralysis of the facial nerve;

weakness of the muscles of one side of the face; may result in inability to close the eye. Cerebral Palsy is a nonprogressive disorder of movement resulting from damage to the brain before, during, or immediately after birth. The cause of Cerebral Palsy is attributed to damage to the brain, generally occurring before, during, or immediately after birth. It is often associated with other neurological and mental problems. The effects are: spastic paralysis,

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sensation is often affected, leading to a lack of balance, and intelligence, posture and speech are frequently impaired. Contractures of the limbs may cause fixed abnormalities.

Other associated features include epilepsy, visual impairment, squint, reduced hearing, and behavioral problems. Motor Neuron Disease is a progressive degenerative disease of the motor system occurring in middle age and causing muscle weakness and wasting. Some forms of Motor Neuron Disease are inherited. Motor Neuron disease primarily affects the cells of the anterior horn of the spinal cord, the motor nuclei in the brainstem, and the corticospinal fibres. Sciatica is a common condition arising from compression of, or damage to, a nerve or nerve root. It is usually caused by degeneration of an intervertebral disc, which protrudes laterally to compress a lower lumbar or an upper sacral spinal nerve root. The onset may be sudden, brought on by an awkward lifting or twisting movement. Pain felt down the back and outer side of the thigh, leg, and foot. The back is stiff and painful. There may be numbness and weakness in the leg. Neuritis is a disease of the peripheral nerves showing the pathological changes of inflammation. The effect is Inflammation of the nerves, which may be painful. Myalgic Encephalomyelitis (ME) is a condition characterized by extreme disabling fatigue that has lasted for at least six months, is made worse by physical or mental exertion, does not resolve with bed rest, and cannot be attributed to other disorders. The cause is unknown but often occurs as a sequel to such viral infections as glandular fever. The effects are extreme disabling fatigue that has lasted for at least six months. Multiple Sclerosis is a chronic disease of the nervous system that can affect young and middle-aged adults. The cause is the myelin sheaths surrounding nerves in the brain and spinal cord are damaged, which affects the function of the nerves involved. Multiple Sclerosis affects different parts of the brain and spinal cord, resulting in typically scattered symptoms. Hyperesthesia is an abnormal acuteness to sensation--an increased sensitivity. Paresthesia is an abnormal spontaneous sensation such as tingling, prickling, or burning. Diabetes usually causes a symmetrical "glove and stocking" distribution in the hands and feet. Neuralgia consists of severe spasms of throbbing or stabbing pain along the pathway of a nerve