Science Weblecture for Unit 23
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The human nervous system differs from other body systems in several ways. There is only one real "organ", the brain, but it doesn't differ in tissue kind from the body's nerves except in density and number of connections. Rather than growing tissues and repairing themselves readily, most nerve cells are fully grown by the time a child reaches seven years old. If damaged, they may take months to years to recover. Nerve cells generate both electronic signals and chemicals that alter the state of other cells, causing motion, sleep, changes in heartbeat, and even emotions. Nerve cells can also detect signals that come from sensory receptors like the light cones in the eye or the inner channels of the ear. This is their primary function: to respond to stimuli and communicate something about that response to other systems.
The human nervous system consists of the central nervous system (CNS), which includes the brain and spinal cord, and the peripheral nervous system (PNS), which consists of nerves connecting the CNS with all the other parts of the body. All parts consist of cells called neurons.
An electric signal triggers the neuron to "fire" and transmit the information-bearing signal or impulse to another cell by releasing chemicals called neurotransmitters from the synaptic nodes into the synaptic gap or space between the releasing cell and the next cell. There are different kinds of neurotransmitters, which affect other nerve cells and muscle cells in different ways. Neurotransmitters can be picked up by the dendrites of another nerve cell, which then carries the impulse a bit further. Sensory neurons pick up signals from muscles and sense organs and transmit them to association neurons in the spinal cord and brain for interpretation. The association neurons also carry signals from the brain back to motor neurons, which stimulate muscles to move at a particular rate in in a particular way.
Some signals and responses are preprogrammed: we call these reflexes. Human reflexes are unlearned responses that usually protect us in some way, like pulling back from a source of high heat or pain, and blinking or contracting the iris in the eye in response to a bright light. The stimulus that results in a reflex never reaches the brain: instead, the information is processed in the spinal cord and the response returned. This shortens the amount of time involved for signal transmission and analysis.
Drugs like caffeine, alcohol, and epinephrine can change the way that neurons send out their stimulants or receive signals from other cells. Some forms of neurological diseases do not involve damage to the cell at all, but only corruption or low levels of neurotransmitters. When this condition is corrected, memory and thought may be fully recovered.
The brain is divided into three major areas. The medulla handles the automatic systems like heartbeat, digestion, sleeping, and breathing rates. The cerebellum coordinates movements in response to sound, balance, and visual information: its what makes it possible for you to catch a baseball or duck when you hear someone sneaking up behind you. The cerebrum is the largest area and controls thought processes, memory and learning, and directs most voluntary muscle movements.
Use the Interactive Brain Anatomy page at the the National Academy of Sciences to explore the parts of the brain.
The ancient world identified five senses:
Modern biologists the perception of location body parts as a sixth type of sense. This sense helps you understand where your hand is in relation to your face, even in the dark. Another major sensation is your sense of balance or equilibrioception.
There are lots of names of specific parts of the inner ear in this short article; don't worry about learning them! Hairs lining the three ducts (which go in different directions) and in the utricle and saccule can detect pressure. As you move your head, liquid sloshing around in the saccule and ducts bends the hairs, which helps you figure out which way is up.
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