Lab Exercise The Nervous System Objectives -You should be able to recognize a neuron and identify its components. - Be able to identify the principal components of the brain and be able to name at least one function of each. - Be able to identify the principal components of the spinal cord and their functions. Introduction Neural impulses travel in two directions. Messages that originate in the brain travel in motor neurons through the spinal cord to peripheral nerves and affect a response. Sensory receptors respond to various types of stimuli and carry information through sensory neurons back to the brain for processing. Both sensory and motor neurons have the same basic structure. One end of the neuron is highly branched and receives impulses. The branches are called dendrites. The impulse then passes through the cell body that contains the nucleus of the neuron. The impulse exits the neuron through the axon. Sensory neurons deliver information to the central nervous system. The central nervous system is composed of the spinal cord and brain. The spinal cord is responsible for delivering nervous impulses to and from the brain and reflex responses. The brain is the principal site of information processing in mammals. Review the organization of the nervous system as illustrated in the Nervous System simulation in the BiologyOne DVD. 1
Activity.1 Neurons Activity.2 CNS: The Brain Nervous tissue (found in the brain, spinal cord and peripheral nerves) is composed of cells called neurons. Examine the illustrations and micrographs of neurons in the Nervous System simulation of the BiologyOne DVD. The neuron has three main parts. Locate the cell body that contains the nucleus, the branched dendrites that receive the stimuli or impulse from another neuron, and axons that transmit the impulse away from the cell body. Neurons are generally the largest cells found in the human body. They can range in length from less than a millimeter to as long as a meter. Examine the photographs of a preserved sheep s brain here and of a human brain in the Nervous System simulation in the BiologyOne DVD. Notice how the brain is divided into symmetrical right and left halves. The large convoluted region of the brain is known as the cerebrum. This is where conscious thought occurs. Through research, scientists have been able to map the areas of the cerebrum and determine sites controlling motor function, sensation, and activities such as speech and perception. Notice how the cerebrum makes up about 80% of the total weight of the brain. After studying these, label the illustration of a neuron in the Results Section. Underneath the cerebrum, there is a smaller mass just superior and dorsal to the spinal cord. This mass is also convoluted and is known as the cerebellum. The primary function of the cerebellum is to regulate skeletal muscle activity. Micrograph of Neuron Examine the inferior portions of the brain. There are 12 pairs of cranial nerves that join the central nervous system in this area. See if you can find some of them. The easiest to find might be the olfactory and the optic nerves. These are the first two and are usually quite prominent. The second pair of nerves (the optic nerves) partially crosses from the left hemisphere of the cerebrum to the right hemisphere. This is known as the optic chiasma and is very easy to find. After locating the optic chiasma, you may be able to see the pituitary gland. The pituitary is known as the master gland. It produces several hormones and releasing factors that regulate body function. The most anterior enlargement of the brain stem is called the pons. The pons primarily contains conduction fibers carrying messages from higher regions of the central nervous system (CNS) to lower regions, or vice-versa. Below the pons is the medulla oblongata, a smaller bulge of the brain stem. The medulla oblongata is important in regulating heart rate, blood pressure, respiration, and those activities that can prove to be so socially embarrassing (such as vomiting, hiccupping, and sneezing.) Below the medulla oblongata is the spinal cord. 2
View the sagittal section of the brain. You should be able to locate the corpus callosum. It is the main link between the right and left halves of the brain. This structure insures that the brain performs as a coordinated whole. The thalamus and the hypothalamus should be visible with the brain sectioned in halves. Though these structures are not distinctly outlined in the brain, it is easy to tell the area where they are located. The thalamus is sometimes called Grand Central Station. Almost all neural impulses travel through the thalamus and are channeled to their appropriate point. A smaller structure beneath the thalamus (the hypothalamus) could be referred to as the thermostat of the body. It is important in maintaining homeostasis. Temperature, glucose level, water balance, hormones levels, your biological clock and many emotions are regulated by the tiny hypothalamus. You should be able to see the tiny pineal gland dorsal to the thalamus. All of the functions of the pineal are not understood. It is sometimes called the third eye and is a light detector in some animals. In humans it secretes melatonin and seems to play a role in your body s day- night cycle. The thalamus, hypothalamus, and pineal body collectively make up the diencephalon. After completing this review of the brain structure, label the illustration of the brain in the Results Section. Sheep's Brain cerebrum optic chiasma pituitary pons pineal body cerebrum cerebellum thalamus corpus callosum hypothalamus 3
Activity.3 CNS: The Spinal Cord The spinal cord provides for transmission of impulses to and from the brain. It is protected, much like the brain, in three ways; the bony encasement of the vertebrae, the protective meninges, and by cerebrospinal fluid. The meninges (made of three layers) form a protective covering for both the brain and the spinal cord. Between two layers of the meninges is found the cerebrospinal fluid that cushions these delicate organs. The spinal cord has two distinct regions: a region of white matter and one of gray matter. The gray matter is composed of unmyelinated neurons and their processes; the white matter contains both myelinated and unmyelinated neurons. Myelin is a fatty insulating sheath and it causes the neurons to appear white in cross section. Most of the myelinated tissues are axons running up and down the spinal cord. The right and left sides of the spinal column are mirror images. On each side, the gray matter is divided into three regions: the posterior (dorsal) horn, the lateral horn, and the anterior (ventral) horn. The anterior horns are made up of neurons traveling to skeletal muscle (somatic motor neurons) and the posterior horns are made up of neurons that form the bridge between sensory and motor neurons (interneurons). Sensory neurons enter the spinal cord through the dorsal roots, entering the posterior horns. The cell bodies of these neurons are located just outside the spinal cord in a swelling called the dorsal root ganglion. Motor neurons leave the spinal cord through the ventral roots. The cell bodies of the motor neurons are located in the gray matter. The white matter is also separated into three regions on each side: the posterior funiculus, the anterior funiculus, and the lateral funiculus. There is no functional difference between the fibers lying in these three regions. Examine the illustrations and micrographs of the spinal cord here and in the Nervous System simulation of the BiologyOne DVD. Locate the white matter and the gray matter. Notice the H or butterfly shape of the gray matter. The connecting area (the cross-bar of the H shape) is the gray commissure. This encloses the central canal of the spinal cord. In order to differentiate the posterior (dorsal) from the anterior (ventral), you should look for the anterior median fissure. A fissure is a deep groove or cleft found in the brain and spinal cord. By locating the anterior media fissure, you can tell the anterior from the posterior and then identify all the parts of the cross section. There is a groove on the posterior side (the posterior median sulcus), but it is less deep than the fissure. When you have finished your study of the spinal cord, you should be able to identify the structures indicated on the diagram in the Results Section. Micrograph of Spinal Cord gray matter white matter 4
Lab Exercise Name Results Section Activity.1 Neurons 3. nucleus 1. axon hillock 2. (entire portion of cell) 4. (gaps) 5. (cells) 6. Describe the differences you observe between the dendrites and axons of neurons. 5
Activity.2 CNS: The Brain 1. 4. 2. 3. 5. 6. 7. Activity.3 CNS: The Spinal Cord 5. 1. 6. 2. 3. 4. 6