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Part I - The Nervous System Function: • To coordinate the actions of your body • To ensure effective behavior • To maintain the internal environment within safe limits (homeostasis) 17-1 Workings of the Nervous System Messages are relayed throughout the body via electrochemical messages from the brain or through chemical messengers – hormones There are more nerve cells in the body than there are visible stars in the Milky Way! 1 cm3 of brain tissue houses several million neurons with each connecting with several thousand others 17-2 Nervous Tissue The nervous system is divided into a Central nervous system =brain & spinal cord and a Peripheral nervous system = nerves carrying sensory & motor information between the CNS & muscles & glands. 17-3 Central Nervous System 17-4 Peripheral Nervous System 17-5 17-6 Neuron Structure -A nerve cell is called a neuron -Neurons are composed of dendrites that receive signals, a cell body with a nucleus, and an axon that conducts a nerve impulse away. Sensory neurons take information to the CNS. Motor neurons take information from the CNS to muscles 17-7 Neurons 17-8 Parts of a Neuron dendrites – receive information conducting towards the cell body (~200 dendrites/cell body) cell body – location of the nucleus, high metabolic rate (so contains mitochondria) axon– may be 1m long, very thin, conducts the impulse towards other neurons 17-9 Sensory and Motor Neurons 17-10 Myelin Sheath nodes of Ranvier– the unmyelinated sections of a myelinated neuron, impulses “jump” between the nodes of Ranvier 17-11 The Nerve Impulse- Action Potential The nervous system uses the nerve impulse to convey information. Voltage (in millivolts, mV) measures the electrical potential difference between the inside and outside of the axon. 17-12 The Action Potential 17-13 Membrane Polarization (Resting Potential) When an axon is not conducting a nerve impulse, the inside of an axon is negative (-70mV) compared to the outside (+40mV); -This is the resting potential. To establish the –70mV potential in the cell: • Na+ is actively pumped out of the cell • K+ is actively pumped into the cell Sodium pump 17-14 The Action Potential 17-15 Membrane Depolarization When the nerve cell is excited, the membrane DEPOLARIZES (Action Potential) The membrane’s polarity changes: – Na+ channels open, Na+ rushes in, K+ gates close The positive ions flowing in causes a charge reversal to +40 mV inside the neuron 17-16 Depolarization- Action Potential 17-17 Membrane Repolarization Once the charge becomes positive, the Na+ gates close, K+ gates open, eventually restoring the charge inside the neuron to –70 mV (but the Na+ excess is inside and K+ excess is outside!) The Na/K Pump restores the ion concentrations inside and outside the cell 17-18 Membrane Repolarization During the repolarization, the nerve cannot be reactivated – this is called the refractory period (1 to 10 ms) and is a recovery time for the neuron The pump requires ATP in order to operate 17-19 Refractory Period 17-20 Fig. 48-13 Schwann cell Depolarized region (node of Ranvier) Cell body Myelin sheath Axon 17-21 Transmission Across a Synapse • The junction between neurons or neurons & effectors is called the synapse. • Transmission of a nerve impulse takes place when a neurotransmitter molecule stored in synaptic vesicles in the axon bulb is released into a synaptic cleft between the axon and the receiving neuron. 17-22 Synapse structure and function 17-23 Neurotransmitter Molecules Acetylcholine (ACh) Norepinephrine (NE) Neurotransmitters are removed from the synapse by the enzyme acetylcholinesterase (AChE) that breaks down acetylcholine. This prevents continuous stimulation or inhibition 17-24 Other Neurotransmitters • • • • Serotonin Dopamine GABA Glutamate • *see table of neurotransmitters and their functions 17-25 Reflex Arc Diagram 17-26