Chapter 48 Overview Lines of Communication The nervous system functions to 1 Respond rapidly to changes in the environment inside and outside the body 2 Coordinates body activates 3 Store knowledge memories Neurons are nerve cells that transfer information with in the body 2 signals to communicate 1 Electrical Long distance 2 Chemical Short Distance Ganglia simple cluster of neurons Brain complex organization of neurons Introduction to information processing Nervous systems process information in 3 stages sensory input integration and motor output Sensors detect external stimuli and internal conditions and transmit information along sensory neurons Sensory information is sent to the brain or ganglia where interneurons integrate the information Motor output leaves the brain or ganglia via motor neurons which triggers a muscle or gland Many animals have complex nervous systems Central Nervous System CNS where integration takes place this includes the brain or spinal cord Peripheral Nervous System PNS Which carries info in and out of CNS neurons in PNS when bundled together form nerves Neuron Structure and function Most of neurons organelles are in the cell body Most neurons have dendrites highly branched extension that receive signals from other neurons or sensory cells The Axon is typically a much longer extension that transmits signals to other cells at synapses The cone shaped base of an axon is called the axon hillock Connects the axon to the soma and is the area where electrical signals called action potentials are stored The Synaptic Axon terminal of one axon passes information across the synapse in the form of chemical messengers called neurotransmitters A synapse is a junction between an axon and another cell Neurotransmitters are released from the axon terminal to signal the target cell muscle cells glands or other neurons Information is transmitted from a presynaptic cell neuron to a postsynaptic cell neuron muscle or gland potential cell Transmission of information is usually in one direction from the presynaptic cell to the postsynaptic cell Most neurons are nourished or insulated by cells called glia Glial cells Glial cells create myelin sheath around axon Concept 48 2 Ion pumps and ion channels establish the resting potential of a neuron Every cell has a voltage across the plasma membrane called a membrane Message are transmitted as changes in this membrane potential Resting Potential membrane of a neuron not sending signals Formation of a resting potential At resting potential K is highest in the cell and Na is highest outside the Sodium potassium pumps Use ATP to maintain K and Na gradients These concentration gradients represent chemical potential energy Opening of Ion Channels in the plasma membrane converts the chemical potential to electrical potential fewer open Na ion channels source of membrane potential A neuron at resting potential contains many open K ion channels and Resulting build up of negative charge within the neuron is the major Anions such as protein contribute to the charge of cells Concept 48 3 Action potentials are signals conducted by Axons Changes in membrane potential occur because neurons contain gated ion channels that open or close in response to stimuli 1 Ligand Gated Responds to chemical stimuli 2 Voltage Gated Responds to a change in membrane potential 3 Mechanically Gated Responds to physical stimuli Hyperpolarization and Depolarization When gated K channels open K diffuses out making the inside of the This is hyperpolarization an increase in magnitude of membrane cell more negative potential becomes Opening other types of ion channels such as Na channels triggers depolarization a reduction in the magnitude of the membrane potential Na flows in Graded Potentials and Action Potentials Graded Potentials are changes in polarization where the magnitude of the change varies with the strength of the stimulus More channels that become open cause a greater change in membrane potential If a depolarization shifts the membrane potential sufficiently it results in a massive change in membrane voltage called an action potential Action potentials are all or none and transmit over long distances They arise because some ion channels are voltage gated opening or closing when the membrane potential reaches a certain level Generation of an Action Potential An action potential as a series of steps At resting potential 1 Most voltage gated sodium Na and Potassium K channels are closed When an Action potential is generated 2 Voltage gated Na channels open first sodium flows into the cell 3 During the rising depolarization phase the threshold is crossed and the membrane potential increases as more voltage gated sodium channels are opening during the depolarization phase 4 During the falling repolarization phase voltage gated sodium channels close voltage gated potassium channels open and potassium flows out of the cell 5 During undershoot Hyperpolarization membrane permeability to potassium is at first higher than the rest then voltage gated potassium channels close and resting potential is restored A Neuron can produce hundreds of action potentials per second 1 2 msecs each During refractory period after an action potential a second action potential can not be initiated Due to temporary inactivation of sodium channels Conduction of Action Potentials At the site where the action potential is generated usually the axon hillock an electrical current depolarizes the neighboring region of the axon membrane Action potentials travel in one direction toward synaptic axon terminals Action potential can regenerate itself by depolarizing the region of the axon membrane Inactivated sodium channels behind the zone of depolarization prevents the action potential from going backwards Evolutionary Adaption of Axon Structure Speed of an action potential increases with Axon s diameter Myelin Sheath speeds up action potential even through it is made mostly of lipids which are poor conductors of electricity Made by glia oligodenrocytes in the CNS and Schwann cells in the Action potentials are formed only at nodes of Ranvier gaps in the myelin sheath where voltage gated sodium channels found Action potentials in myelinated axons jump between the nodes of Ranvier in a process called saltatory conduction Concept 48 4 Neurons communicate with other cells at synapses At Electrical Synapses the electrical current flows from one neuron to PNS another These types of synapses are
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