Revision:Anatomy and physiology chapter 5

Nervous sytem basics

The four charecteristics of the nervous system: system of communication, mechanism by which sensations are receives, interpretation of these senses through memory, system by which actions are carried out. The nervous system is grouped into the central nervous sytem and the peripheral nervous system or CNS and PNS. It is composed of neurones and neuroglia. Nerve cells form the grey matter of the brain and spinal cord. Nerve processes or fibres form the white matter of the brain and spinal cord. A neurone with many processes arising from the cell body is termed multipolar. A neurone with one process is termed unipolar. The thin fatty sheath of myelin covering the axons and dendrites is called sarcolemma. The nodes of ranvier allow exchange of nutrients. A major feature of multiple sclerosis is the degeneration of sarcolemma in the spinal cord, optic nerve and brain. This results in muscular weakness, double vision, and an unsteady gait. Non-myelinated fibres are found in the autonomic nervous system and in certain parts of the brain and spinal cord. A synapse is a point of communication between one neurone and the next. End feet allow the passage of a nerve impulse in one direction only. The potential differnce (p.d) across membranes is -70mV. Depolarisation: occurs when an electrical impulse arrives. when this happens, Na+ ions enter the cell and reverse the p.d to a slightly positive potential. Action potentials obey the all or nothing law. The refractory period is 0.5ms. The CNS consists of the brain and spinal cord. the PNS consists of cranial and spinal nerves and the autonomic nervous system.

The brain is subdivided into the forebrain, midbrain and hindbrain. In the centre, the hemispheres are joined by the corpus callosum. The outer layer of the cerbrum is called the cerebral cortex. It is composed of grey matter in gyri and supported by white matter in sulci. Three main sulci divide each hemisphere into four lobes. These are the cental sulcus, lateral sulcus and parieto-occipital sulcus. The lobes are the frontal lobe, the parietal lobe, the occipital lobe and the temporal lobe. The area lying immediately in front of the central sulcus is called the precentral gyrus. Cerebral nuclei are ares of grey matter in areas of white matter. They are involved in movement and posture. The cavities within the brain are called ventricles. They are filled with cerobrospinal fluid.

Hideous hideous anatomy

The midbrain consists of two stalk-like bands of white matter called the cerebral peduncles, which convey impulses passing to and from the brain and spinal cord, and four small prominences called the quadreginal bodies which are concerned with sight and hearing reflexes. The pineal body lies between the two upper quadrigenal bodies. Th hindbrain has three parts. They are the pons, which contains fibres, the medulla oblangata, which contains cardiac and respiratory centres, and the cerebellum, which co-ordinates muscular activity, controls muscle tone, and maintains posture. The spinal cord tapers off into the conus medullaris at its lower end. From the end of this, tapers off the filum terminale, which descends to the coccyx, surrounded by nerve roots called the cauda equina. The cord contains nerve fibres. The cranial nerves constitute 12 pairs. The spinal nerves constitute 31 pairs. The autonomic nervous sytem is concerned wuth the control of internal organs. The motor system is concerned with the movement of various parts of the body.In front of the motor area, which lies in the precentral gyrus, lies the pre-motor area. Beginning from cells in the motor area, the corticospinal fibres pass downwards in a fan shape and then pass through the internal capsule., where all the motor fibres serving one side of the body are gathered. The diadvantage of this: injury there will cause paralysis in the affected side(hemiplegia). The fibres pass through the pons to the medulla oblangata, where they form pyramids. Here, most of the fibres will cross over to the other side at the dicussation of the pyramids. The fibres then pass down the spinal cord as the lateral corticospinal tract. The fibres that did not cross over pass down the spinal cord as the anterior corticospinal tract. The fibres pass to the anteroir horn of the spinal cord, where they form a synapse with the cell bodies ssituated there, and then emerge as the anterrior root and join the posterior root to form a mixed spinal nerve. As peripheral nerves, these end in branches to various areas, including muscles. The motor fibres divide into brances and each branch ends in a motor end plate attatched to an individual muscle fibre. Sensory fibres have their cells in the posterior root ganglion on the spinal nerves and have endings of various types within the muscles. The motor area receives impulses from many other parts of the brain, including the sensory area. From the cortex, impulses are sent to the spinal cord, the motor nuclei in the brain stem, the basal ganglia, the cerebellum and the pons. Through the various nerve tracts, stimuli are passed through the peripheral nerves to the skeletal muscles, which are kept in a state of tension called muscle tone. An upper motor neurone decribes the motor fibre within the CNS as far as its synapse with an anterior horn cell. A lower motor neurone is an anterior horn cell and its fibre. Damage in an upper neurone may lead to paralysis. Damage in a lower neurone reults in flaccid paralysis, whereby the muscles atrophy. The term extrapyrimidal system describes all motor sytems other than the corticospinal and corticonuclear tracts. The sensory system is concerned with interpreting the impulses which are constantly stimulating it. Sensory impulses are transmitted to the CNS from the special sense organs, the skin, and from deeper parts of the body.