The Nervous System and Senses |
Whenever I decide to cook a hearty meal, I tend to want to use the oven. But, I try to make sure that my thirteen-month-old daughter steers clear of the kitchen, because she is so curious of her surroundings and trouble seems to draw her in. When she is in the kitchen and starts barreling towards the stove, I stop her in her tracks by exclaiming, “That’s hot!” She usually looks at me with concern, then, repeats my warning, and trails off to find something else to get into. My daughter understands that when she touches something hot that it hurts. And, the reasons for that are due to the nervous system and the senses that human beings contain within their bodies. The nervous system is an extremely complex system that alerts humans of the internal and external environments through its specific functions and the five senses.
As stated earlier, the nervous system is a very multifaceted arrangement that alerts humans of the internal and external environments. It performs three basic functions that help the body maintain homeostasis. The first is sensory output. This is when the body gathers information about the conditions that arise within the inside or outside of the body. Next, the information that was gathered by sensory input is processed and analyzed by the spinal chord and brain. And the last function is motor output. After the brain processes the information provided, it initiates a response by causing the body muscles to move or glands to secrete. In order for these functions to occur, they need to be carried out by the cells of the nervous system.The cells of the nervous system are called neurons. Neurons have three main parts. The first part of a neuron is the cell body. The cell body contains the nucleus, where it helps maintain function of the cell. The second part is the dendrites. Dendrites are the spider-like extensions from the nerve cell that, “…receive signals from sensory receptors or other neurons” (Mader 249). And, the last part, the axon, is the area of the cell where nerve impulses are transmitted. Although all the neuron cells in the body contain this type of structure, they can differ in outer appearance due to the different types of neurons.
Figure 1: The physical appearance of a neuron.
One type of neuron is called a sensory neuron. Sensory neurons “…send information from sensory receptors…toward the central nervous system” (faculty.washington.edu/chuder/cells.html). Therefore, they are located within the skin, eyes, nose, tongue, and ears. Another type of neuron is motor neurons. Motor neurons take information away from the central nervous system to muscle fiber or gland in the body. In order for each type of neuron to transmit information, they follow a particular process called action potential. Action potential, which will be discussed in more detail later, gives the ability of neurons to carry on ion diffusion through a rapid process of voltage change along the cell membrane. Along with the process of action potential, is the significance of the myelin sheath in neurons.
The myelin sheath is a protective barrier that covers the axon of a neuron. This sheath is formed by Schwann cells that contain the fatty substance of myelin in their plasma membranes. These cells wrap around the axon several times until the myelin sheath has formed, but does not cover the entire axon of a neuron. Instead, there are gaps that are created in between each sheath that is formed called nodes of Ranvier. The function of the myelin sheath aids the neuron in the process of action potential by allowing the action potential to jump to the nodes of Ranvier, which is called saltatory conduct. It also helps save the energy of the cell and speeds up the process of action potential. In accordance with the myelin sheath, the action of nerve impulse occurs.
The nerve impulse, “…convey(s) information within the nervous system” (Mader 250). And, it occurs throughout two different procedures. One of the procedures is called resting potential. During resting potential, the axon of the neuron is not administering an impulse. And, resting potential maintains its status for all neurons and muscle cells, so when they need to, they can send an impulse. This occurs from the application of the sodium-potassium pump. The sodium-potassium pump, “…actively transports
Na+ out of and K+ into the axon” (Mader 250). When a voltmeter is applied to a neuron that is at resting potential, it displays -70 mV, which is due to the unequal distribution of the sodium and potassium ions from the pump. Another procedure that conveys information within the nervous system is action potential. Action potential is a quick change of nerve impulses across the axon of a neuron. This occurs when the cell membrane is depolarized and then repolarized by the opening and closing of the sodium and potassium gates. As the voltage within the cell increases to -40 mV, “…sodium channels open and sodium ions flood inside” (web.lemoyne.edu). The flooding of the sodium ions causes a change in the mV inside the cell, making it go from a negative to a positive. This creates the depolarization of the cell. As the voltage of the cell increases to +50 mV, “…sodium channels close and potassium channels open so that potassium ions flood outside” (web.lemoyne.edu). The flooding of the potassium ions causes a decrease in the mV, returning the cell back to its negative charge, or resting potential. This creates the depolarization of the cell. As action potential nears the end of an axon, it it is accepted by a synapse.
A synapse is a nearby section of a neuron that can carry on the nerve impulse of action potential. In order for a neuron to receive a synapse, the arriving action potential from the axon releases calcium ions into the membrane of the cell to create attachment. Then, the receiving neuron secretes neurotransmitters and binds the two neurons together. Then, sodium ions diffuse into the neurotransmitters and the process of action potential maintains. As the information is transmitted throughout the neurons, the body responds with motor output.The part of the body that proceeds with motor output is the spinal chord. And it is from the spinal chord that particular reflexes, such as the reflex arc, occur. The reflex arc is put into action from the information provided by the sensory and motor neurons. The sensory neurons provide information of sensory input to the spinal chord
Figure 2: The process of the reflex arc that occurs in the spinal chord.
through the process of action potential. And, motor neurons provide information of motor output away from the spinal chord, also through action potential. Then, the body either moves the muscle or secretion demanded. For example, if a person touches something that is hot, the sensory receptors in the skin send the message of the changes of the external environment. Then, the reaction, set forth by the motor neurons, occurs usually by the person wincing and expressing pain. In relation to the reflex arc, the activation of the different senses occurs.
The occurrence of sensation is provided by sensory receptors. These are specialized parts of the neurons that respond to external and internal stimuli and begin the process of action potential within the body. There are two types of sensory receptors. The first is proprioceptors. These sensory receptors are involved in information of sensing muscle tension and providing the muscles with position and posture. The second is cutaneous receptors. Cutaneous receptors are reflex actions by bringing information of pain, pressure, temperature, and touch. As these sensory receptors bring the information to the specialized neurons, and eventually to the brain, the brain organizes the information into different sensory fields in order to understand and respond to the messages received. The visual cortex of the brain forms visual information of the external stimuli. And the sensory cortex of the brain, detects the sensation of touch from the entire surface of the body. Along with the special sensory receptors, are specialized senses located in the head.There are five senses that are located within the head that provide the understanding of the body’s outer surroundings. The first two senses are taste and smell. Both of these senses are called, “…chemical senses because their receptors are sensitive to molecules in the food we eat and the air we breathe” (Mader 278). Taste is achieved by the taste buds located on the tongue. They are separated into four major groups of salty, sweet, sour and bitter and are located in particular regions of the tongue. The brain receives the information of taste by nerve impulses within the pores of the taste buds. Then, they reach the taste cortex of the brain and are deciphered as the major groups of taste. Smell is achieved by the olfactory cells located within the uppermost area of the nose. The brain receives the information of smell by
Figure 3: The five senses: taste, smell, vision, hearing, and equilibrium.
nerve fibers in the olfactory cells. The smell of a scent is broken down into what type of molecule that is being provided, so the brain can understand what odor is being stimulated. Then, the neurons that were stimulated by the olfactory cells transmit the message to the olfactory areas of the cerebral cortex, and the information is analyzed. Lastly, the brain sends messages back to the nasal cavity to identify what has been smelled. The third sense is vision. The eye consists of three layers. The first is the sclera, which is the outer layer, which supports and protects the eye. Second, is the choroid, which is the middle layer, absorbs light rays that have strayed from its original formation. And, the third is the retina, which is the inner layer, which has the sensory receptors for color, dull, and bright light. The outside of the eye draws the light rays into the retina, where the sensory receptors create the visual information that is provided. Then, nerve impulses transmit the information to the occipital lobe of the brain, where it is interpreted and applied. And the last two senses are hearing and equilibrium. The three parts of the ear attain hearing. First, the outer ear leads the external noise into the middle ear. Then, the middle ear amplifies the noise provided. Last, the inner ear receives the noise through its sensory receptors. The sensory receptors, which are tiny hair cells, provide nerve impulses to the temporal lobe of the cerebral cortex for interpretation and application of the noise. The sense of equilibrium is also achieved from the sensory receptors in the inner ear. They detect rotational and gravitational equilibrium. As each type of equilibrium occurs, membranes within the inner ear are displaced. The sensory receptors pick up the changes and send impulses to the brain.
Whenever I use the oven for cooking, I warn my daughter that it is hot. She responds to my warning by retreating her curiosity, because her nervous system and senses have provided past information that touching something hot hurts. The nervous system is a very complex arrangement that contains basic functions. These basic functions are performed by different neurons that transmit messages by processes such as, resting and action potential, and synapse. Also, once the information has reached the spinal chord and brain, it can be deciphered and acted upon through the reflex arc. The senses also provide an understanding of the body’s external environment from particular sensory receptors like, cutaneous receptors and proprioceptors that are sent to different sensory fields in the brain. And, the head has five special senses: taste, smell, vision, hearing, and equilibrium, that allows for the body to respond to different stimuli. The nervous system and the senses are very vital to survival, because without them, living things would cease to exist.
Sources:
Pictures:
1. www.bcm.edu/cain_foundation/noframes/html/pag...
2. academic.kellogg.cc.mi.us/herbrandsonc/bio201...
3. imagecache2.allposters.com/images/pic/JAG/03-...
Works:
Human Biology, 10e. Mader, Sylvia s.
faculty.washington.edu/chudler/cells.html
web.lemoyne.edu/~hevern/psy340/lectures/psy340.02.2a.neur.impulse.html
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