48 Nervous system 2009

48 Nervous system 2009 - Biol 61 How Neurons Function...

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Biol 61 – How Neurons Function (Chapter 48) 4/31/09-5/1/09 Most of the material we will be discussing is from Chapter 48, but read the first part of Chapter 49 as well (1065- 1067: reflex arc through glial cells). The nervous system is composed of excitable cells called neurons , or nerve cells. Neurons conduct electrical impulses and are helped by support cells called neuroglia, or glial cells . Glial cells act to help feed neurons, remove wastes, and make the cells work faster. Neurons communicate to one another via connections called synapses. The general function of the nervous system is to (Fig. 48.3): 1. collect information (from outside the body or within the body) and send it to the brain (sensory input). 2. process the information (in the brain or spinal cord). 3. respond to the information by doing (or not doing) something (often some kind of motor response). The nervous system contains 3 general types of neurons: - sensory neurons collect information for the brain and spinal cord (input) - interneurons are neurons that talk to other neurons (they are used for processing information) - motor neurons send signals out from the brain (output to smooth and skeletal muscles and glands) The nervous system itself is composed of the (Fig. 49.4 ): 1. central nervous system (CNS) – the brain and spinal cord. This is primarily filled with interneurons that take input, process it, and make decisions about things. 2. peripheral nervous system (PNS) – everything else. The PNS includes sensory neurons and motor neurons , but no interneurons. Reflex Arc (Figure 49.3 ) The simplest kind of neuronal connection, or circuit, is a reflex arc, and the one that most people are familiar with is the knee-jerk response . In this simple circuit, information is collected, sent to the spinal cord, and then a response occurs – the reflex initially bypasses the brain. It starts when a doctor (or someone) hits your patellar tendon with a hammer. This tendon is connected to your quadriceps muscle, and the tapping stretches the quadriceps enough to activate a sensory neuron in that muscle. The sensory neuron carries an electrical signal to two places in the spinal cord in response to the stimulus: 1) it synapses directly onto a motor neuron whose dendrites are in the spinal cord and stimulates the motor neuron - in response to this signal, the motor neuron then carries an electrical signal back to the quadriceps to stimulate the muscle to contract and pull the leg up; 2) the sensory neuron also synapses to an interneuron in the spinal cord, and this interneuron then acts to signal the motor neuron connected to the hamstring muscles which helps to keep the hamstring muscles relaxed (so they don’t fight against the contraction of the quadriceps muscles). All this is happening before your brain has even registered that your leg was tapped. Ultimately, of course, information about the tap and the muscle contraction will be sent up to your brain through
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48 Nervous system 2009 - Biol 61 How Neurons Function...

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