Chapter 38 BSC 1005 Tidwell The 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 Conduct electrical signals over distances to a junction where it meets Process the information and produce electrical signals another cell Transmit information to other neurons muscle or glands assist neuronal function by o Glia 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 receive information function are branched tendrils protruding from the cell body that perform the 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 8 Chapter 38 BSC 1005 Tidwell o 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 axons o Sensory o Motor o Mixed Types of neurons o Sensory o Motor o Interneurons Resting Potential neuron is not conducting an impulse o Neuron is electrically charged at rest o Outside is positively charged o Inside is negatively charged If the membrane potential becomes less negative it reaches a level called threshold and triggers an action potential o Voltage gated Na channels open o 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 8 Chapter 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 is formed by glial cells that wrap themselves around the axon potential conduction by covering portions of the axon with a fatty insulation called myelin Myelin 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 When an action potential is initiated it travels down an axon until it neurotransmitter molecules 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 8 Chapter 38 BSC 1005 Tidwell o 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 threshold and it will be more likely to fire an action potential This voltage change is called an excitatory postsynaptic potential EPSP Neurotransmitter action is usually brief o Some neurotransmitters are rapidly broken down by enzymes in the synaptic o Many other neurotransmitters are transported