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NERVOUS SYSTEM READING The nervous system of many animals consists of the brain, the spinal cord, and nerves. This system allows animals to obtain quick feedback about their surroundings and to react immediately. The nervous system can be separated into two divisions, the central nervous system which includes the brain and spinal cord and the peripheral nervous system which includes all of the nerves that branch off to various parts of the body. Complete questions 1-4 on the worksheet. Be extremely careful during this portion of the dissection as the brain tissue is very fragile. You will examine the brain and the spinal cord that is connected at the posterior end of the brain. To do this, CAREFULLY cut into the cranium between the eyes. Cut about the cranium in pieces (if need be) being very careful not to cut too deep. Keep removing pieces of skull until you have cut past each ear to the back of the head or top of the neck. http://www.k-state.edu/organismic/rat_dissection.htm April 23, 2008 Identify the parts of the rat’s brain: olfactory bulbs, cerebral cortex, and cerebellum. The rat brain is different from the human brain in several ways. First, the human brain has many folds called gyri; the rat’s brain is smooth in appearance. Second, the olfactory bulbs (for smelling) of the rat brain are proportionately much larger than in the human brain. Third, the cerebral cortex (where higher level thinking takes place) and the corpus callosum (where the left and right halves of the brain connect to exchange information) are proportionately much smaller in the rat’s brain than in the human. The cerebellum (where balance and coordination are controlled) and the brain stem (where all of the necessary vital functions are controlled) are proportionately similar in the human and rat. Complete questions 5-7 on the worksheet. Continue removing the rest of the cranium until you come to the location where the spinal cord descends into the cervical (neck) vertebrae. Working down the vertebrae, remove the muscle tissue until you have at least 5 or 6 vertebrae that are revealed. Look for the intervertebral disks that act as shock absorbers for the spine. Then look for the smaller peripheral nerves that are exiting (or entering) the spinal cord. With a sharp scalpel, try to carefully break through the vertebrae to reveal the spinal cord inside OR make a cut in the spinal cord after the 6th vertebrae and try sliding the vertebrae off of the spinal cord one by one. Identify the peripheral nerves that must be cut as you go. http://idiolect.org.uk/notes/?cat=5&paged=2 After exposing part of the spinal cord, carefully remove the top of the spinal cord and brain together. Examine the end of the spinal cord. Make a clean cut through it if need be. You should notice 2 different areas within the spinal cord. This is the grey matter and the white matter of the spinal cord. The white matter, towards the outside of the spinal cord, is the nerve fibers that are sending messages to and from the brain. http://www.infovisual.info/03/024_en.html The grey matter, inside the white matter, is the cells that support the nerve fibers, making sure that the cells of the white matter receive the energy that they need to function properly. The human spinal cord is very similar to this. The brain, however, is reversed. The grey matter is on the outside and the white matter is on the inside. http://www.brainexplorer.org/glossary/grey_matter.shtml Complete questions 8-10 on the worksheet. The cells of the nervous system are called neurons. Neurons carry messages to and from the brain by electrical charges that run the length of the neuron and by chemicals that trigger the next neuron or suppress the next neuron from firing. Obtain a prepared slide of neurons. A typical neuron looks something like the drawing below, but there are other neuron types. Sources: http://www.epilepsyfoundation.org/about/science/functions.cfm http://lpmpjogja.diknas.go.id/kc/b/brain/brain.htm April 25, 2008 Neurons can be classified into three types depending on direction of the impulse. Sensory neurons send an impulse from the sense organs to the spinal cord and brain. Motor neurons send an impulse from the brain and spinal cord to muscles or glands. Interneurons interconnect sensory and motor neurons. The largest part of a typical neuron is the cell body. The cell body contains the nucleus and much of the cytoplasm. Most of the metabolic activity of the neuron takes place in the cell body. Spreading out from the cell body are short, branched extensions called dendrites. Dendrites carry impulses from the environment or other neurons to the cell body. The long fiber that carries impulses away from the cell body is called the axon. The axon ends in a series of swellings called axon terminals. The axon terminals are where neurons link with other cells. In most animals, axons and dendrites are bundled in fibers called nerves. In some neurons, the axon is insulated by a membrane called the myelin sheath. The myelin sheath leaves many gaps, called nodes, where the axon is exposed. As an impulse travels along the axon, it leaps from node to node, speeding up the impulse. Complete questions 11-13 on the worksheet. The synapse is the location where an impulse is transferred to another neuron. The axon terminal of the first neuron fits very closely with the dendrite on the second neuron, leaving a very small gap. The axon terminal contains tiny vesicles filled with chemicals called neurotransmitters. The vesicles move to the edge of the axon terminal and release their neurotransmitters into the gap by the process of exocytosis. The neurotransmitters cross the gap by the process of diffusion. Receptors on the dendrite across the gap receive the neurotransmitters. Complete question 14 on the worksheet. http://www.coolschool.ca/lor/BI12/unit12/U12L04.htm