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