Chapter 38 BSC 1005 TidwellThe Nervous System- The nervous system has two principal cell types:o Neurons often called nerve cells, receive, process, and transmit information Receives information from the environment Process the information and produce electrical signals Conduct electrical signals over distances to a junction where it meets another cell Transmit information to other neurons, muscle, or glandso Glia assist neuronal function by: Providing nutrients Regulating the composition of extracellular fluid in the brain and spinal cord Modulating communication between neurons Speeding up the movement of electrical signals within neurons- Typical neurons have dendrites (receive), cell body (process), axon (conduct), and synaptic terminals (transmit).o Nerve impulses follow this path:Dendrite →Cell Body → Axon → Synaptic Terminal- Dendrites respond to stimuli.o Dendrites are branched tendrils protruding from the cell body that perform the “receive information” function. Their branches provide a large surface area for receiving signals, either from the environment or from other neurons. Dendrites of sensory neurons respond to specific stimuli, such as pressure, odor, light, body temperature, blood pH, or the position of a joint. Dendrites of neurons in the brain and spinal cord usually respond to chemicals, called neurotransmitters, that are released by other neurons.- The cell body processes signals from the dendrites.o Electrical signals travel down the dendrites and converge on the neuron’s cell body, which integrates incoming information, performing the “process information” function. If incoming signals are positive enough, a large, rapid electrical signal called an action potential is produced.o The cell body also contains other organelles such as the nucleus, endoplasmic reticulum, and Golgi apparatus that are typical of other cells, synthesizing complex molecules and coordinating the cell’s metabolism.- The axon conducts action potentials along long distances.Page 1 of 8Chapter 38 BSC 1005 Tidwello In a typical neuron, a long, thin strand called an axon extends outward from the cell body and conducts action potentials from the cell body to synaptic terminals at the axon’s end. Single axons may stretch from our spinal cord to our toes, a distance of about three feet. Axons are typically bundled together to form nerves, much like wires are bundled within an electrical cable.- At synapses, signals are transmitted from one cell to another.o The site where a neuron communicates with another cell is called a synapse.o A typical synapse consists of: The synaptic terminal which is a swelling at the end of an axon of the “sending” neuron. A dendrite or cell body of a “receiving” neuron, muscle, or gland cell. A small gap separating the two cells.o Most synaptic terminals contain neurotransmitters that are released in response to an action potential reaching the terminal.o The plasma membrane of the receiving neuron bears receptors that bind the neurotransmitters and stimulate a response in this cell.o At a synapse, the output of the first cell becomes the input to the second cell.- Nerve – bundle of axonso Sensory o Motor o Mixed - Types of neurons:o Sensory o Motor o Interneurons - Resting Potential - neuron is not conducting an impulseo Neuron is electrically charged at resto Outside is positively chargedo Inside is negatively charged- If the membrane potential becomes less negative, it reaches a level called threshold andtriggers an action potential.o Voltage gated Na+ channels openo During an action potential, the membrane potential rises rapidly to +50 mV inside the cell, then returns to resting potential.o The action potential signal flows down the axon to the synaptic terminals with no change in voltage from the cell body to the synaptic terminals.Page 2 of 8Chapter 38 BSC 1005 Tidwell- Stimulation – Na+ channels open- Depolarization – Na+ ions flood in- Recovery – K+ channels open- Potassium (K+) can travel in and out of cells through leak channels.- Myelin speed up the conduction of action potentials.o The thicker an axon, the faster the action potential moves.o In addition to variable thickness, neurons can increase the rate of action potential conduction by covering portions of the axon with a fatty insulation called myelin. Myelin is formed by glial cells that wrap themselves around the axon, leaving naked nodes in between the segments of myelin. In myelinated neurons, action potentials “jump” rapidly from node to node, traveling at a rate of 10 to 330 feet per second compared to unmyelinated neurons in which the action potential travels continuously at about 3 to 6 feet per second.- Although there are electrical synapses where electrical activity can pass directly from neuron to neuron through gap junctions connecting the insides of the cells, most synapses use chemicals to transmit information from one neuron to another.o A synapse is where the synaptic terminal of one neuron meets the dendrite or cell body of another.o The two neurons do not actually touch at a synapse. A tiny gap, synaptic cleft, separates the first or presynaptic neuron, from the second, postsynaptic neuron. The presynaptic neuron sends neurotransmitter chemicals across the gap to the postsynaptic neuron.o There are many types of neurotransmitters. A synaptic terminal contains scores of vesicles, each full of neurotransmitter molecules. When an action potential is initiated, it travels down an axon until it reaches its synaptic terminal.- Synapses produce excitatory or inhibitory postsynaptic potentials.o At most synapses, the binding of neurotransmitter molecules to receptors on a postsynaptic neuron opens ion channels in the neuron’s plasma membrane.o Depending on which channels are associated with the receptors, ions such as Na+, K+, Ca2+, or Cl- may move through these channels causing a small, brief change in voltage, called a postsynaptic potential or PSP.Page 3 of 8Chapter 38 BSC 1005 Tidwello If the postsynaptic neuron becomes more negative, its resting potential moves farther away from threshold, reducing the likelihood or firing an action potential. This change in voltage is called an inhibitory postsynaptic potential (IPSP).o If the postsynaptic neuron becomes less negative, then its resting potential will move closer to