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The Nervous System • Neuron – Cell body; Dendrites; Axon • Three general groups of neurons – Sensory neurons (afferent or receptor) • Receive the initial stimulus – Motor neurons (efferent) • Stimulate effectors, target cells that produce some kind of response. – Interneurons (connector or association) • Located in the spinal cord and brain • Receive impulses from sensory neurons and send impulses to motor neurons. The Nervous System • Transmission of nerve impulse – Chemical changes across the membrane of neuron. – Membrane of a unstimulated neuron is polarized. • Difference in electrical charges between the outside and inside of the membrane. • Inside is negative; outside is positive. – Polarization • Excess sodium ions (Na+) on the outside • Excess potassium ions (K+) on the inside • Leakage of Na+ and K+ ions membranes, but Na+/K+ pumps in the membrane actively restore the ions to the appropriate side. • Other ions, such as large negatively charged proteins and nucleic acids, reside inside the cell, creating the negatively charged interior. The Nervous System • Transmission of a nerve impulse – – – – – – – Resting potential Threshold potential Depolarization Action potential Repolarization Refractory period Hyperpolarization The Nervous System • Myelin sheath – Schwann cells • Insultors – Nodes of Ranvier – Action potentials jump from node to node, thereby speeding the propagation of the impulse. The Nervous System • Synapse or Synaptic cleft – The gap that separates adjacent neurons – Transmission of impulse across the synapse • Presynaptic cell to postsynaptic cell • Electrical or Chemical – Most synaptic clefts are traversed by chemicals The Nervous System • Chemical synapse transmision – Calcium (Ca++) gates open • When the action potential reaches the end of an axon, the depolarization of the membrane causes gated channels to open and allow Ca++ to enter the cell. – Synaptic vesicles release neurotransmitter • Influx of Ca++ into the terminal end of the axon causes synaptic vesicles to release molecules called neurotransmitters into the synaptic cleft. The Nervous System • Chemical synapse transmision – Neurotransmitter binds with postsynaptic receptors • Neurotransmitter diffuses across the synaptic cleft and binds with proteins on the postsynaptic membrane. – The postsynaptic membrane is excited or inhibited. • Depending upon the kind of neurotransmitter and the kind of membrane receptors, there are two possible outcomes for postsynaptic membrane – Excitatory postsynaptic potential • Na+ gates open; membrane becomes depolarized; action potential is generated. – Inhibitory postsynaptic potential • K+ gates open; membrane becomes more polarized (hyperpolarized); inhibits action potential The Nervous System • Chemical synapse transmision – The neurotransmitter is degraded and recycled. • After the neurotransmitter binds to the postsynaptic membrane receptors, it is broken down by enzymes in the synaptic cleft. • Example – Acetylcholine is broken down by cholinesterase. – Degraded neurotransmitters are recycled by the presynaptic cell. The Nervous System Neurotransmitters • Acetylcholine – Commonly secreted at neuromuscular junctions (between motor neurons and muscle cells). – Stimulates muscles to contract – At other junctions, it produces a inhibitory postsynaptic potential. The Nervous System Neurotransmitters • Epinephrine; norepinephrine; dopamine; and serotonin. – Dervived from amino acids – Secreted between neurons of the CNS • Gamma aminobutyric acid (GABA) – Usually an inhibitory neurotransmitter among brain neurons The Nervous System of Humans and other Vertebrates • Central nervous system – Brain and spinal cord • Peripheral nervous system – Consists of: • Sensory neurons – Transmit impulses to the CNS • Motor neurons – Transmits from the CNS to effectors. The Nervous System of Humans and other Vertebrates • Motor neuron system (two groups) – Somatic nervous system • Directs the contraction of skeletal muscles – Autonomic nervous system • Controls the activities of organs and various involuntary muscles (cardiac and smooth) The Nervous System of Humans and other Vertebrates • Two divisions of the Autonomic N.S – Sympathetic nervous system • Stimulates the activities of the body for action – Increasing heart rate – Increasing the release of sugar from the liver into the blood – “fight or flight” response – Parasympathetic nervous system • Activates activities of relaxation – Stimulating the secretion of saliva or digestive enzymes