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Neurophysiology

Neurophysiology. Functions How a Nerve Impulse Occurs The synapse. Neurophysiology – Major functions. Irritability (or excitable) : nerve cells respond to a stimulus and convert it into a nerve impulse Conductivity : nerve cells transmit impulse to other nerves, muscles, or glands.

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Neurophysiology

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  1. Neurophysiology Functions How a Nerve Impulse Occurs The synapse

  2. Neurophysiology – Major functions • Irritability (or excitable): nerve cells respond to a stimulus and convert it into a nerve impulse • Conductivity: nerve cells transmit impulse to other nerves, muscles, or glands

  3. How a Nervous Impulse Occurs • Resting potential of the membrane: polarized and unstimulated • POLARIZED : The electrical charge on the outside of the membrane is positive while the electrical charge on the inside of the membrane is negative. • The outside of the cell contains excess sodium ions (Na+); the inside of the cell contains excess potassium ions (K+). This arrangement of Na+ and K+ is maintained by the Sodium-Potassium Pump and protein channels in the membrane.

  4. How a Nervous Impulse Occurs • A stimulus occurs (ex. Light, sound, touch) cause depolarization • DEPOLARIZATION: The inside of the cell membrane becomes more positive • Na+ channels open and Na+ ions diffuse into the cell along the concentration gradient making the cell’s interior more positive • If enough Na+ enters the cell, then more Na+ channels open. Once the charge reversal occurs, the Na+ channels close and the K+ channels open.

  5. How a Nervous Impulse Occurs 3. K+ leaves through open K+ channels causing repolarization • REPOLARIZATION: Another change that returns the cell membrane to its resting potential – a polarized state • The K+ carries positive (+) charges out of the cell making the cell’s interior more negative (-) • Neuron can not conduct another impulse until repolarization occurs

  6. How a Nervous Impulse Occurs • Depolarization and repolarization constitutes an action potential • ACTION POTENTIAL: change in charge that is transmitted along a nerve. • To go from resting potential to action potential takes 1 or 2 milliseconds • It is an “all or nothing event”- • Stimuli/neurotransmitters arrive and open some of the chemically-gated Na+ channels • If stimuli reach the threshold level  depolarization occurs • Voltage-gated Na+ channels open • An action potential is generated • If stimuli do not reach the threshold level  nothing happens

  7. How a Nervous Impulse Occurs • Sodium-Potassium Pump restores balance of resting state. • Restores sodium and potassium by pumping 3 Na+ ions out of the cell and 2 K+ ions into the cell • concentration gradient drives Na+ to go into the cell • concentration gradient drives K+ to go out of the cell • Occurs against a concentration gradient, so ATP needed (active transport) Occurs extremely fast to allow for quick nerve impulse reactions

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  9. How a Nervous Impulse Occurs • Remember: the impulse doesn’t jump from one neuron to the next. A new impulse is started when the neurotransmitter diffuses across the synapse to the next neuron. • Saltatory Conduction: faster conduction of action potential along myelinated nerves. > Energy efficient – the membrane only has to depolarize and repolarize at the nodes > Less Na+/K+ATPase activity is required, therefore, less energy is required

  10. Saltatory Conduction • Not a continuous process of region to region depolarization • A “jumping” depolarization • Myelinated axons transmit an action potential differently • The myelin sheath acts as an insulator preventing ion flow in, and out of, the membrane • Neurofibril nodes (nodes of Ranvier) interrupt the myelin sheath and permit ion flows at the exposed locations on the axon membrane; nodes make it unnecessary for action potentials to travel the entire cell membrane.

  11. The Synapse • Function • There must be a means of communication between each neuron and the next target cell • The synapse is the connection • Organization • Presynaptic neuron • Synaptic cleft • Postsynaptic neuron

  12. Action of a Chemical Synapse • Presynaptic Events • An action potential reaches the presynaptic terminal and depolarizes the axonal terminal. • Ca2+ channels open…Ca2+ ions enter the axoplasm • Neurotransmitters are released from their vesicles by exocytosis • Neurotransmitter diffuses across the cleft and bind to specific receptor molecules on the postsynaptic membrane.

  13. Action of a Chemical Synapse • Postsynaptic Events • Ion channels (Na+, K+, or Cl-) on the postsynaptic membrane open or close as a result of the neurotransmitter binding to their specific receptors • An action potential on the postsynaptic membrane is either stimulated or inhibited. • Neurotransmitter molecules are removed quickly • degraded by enzymes in the synapse, with the products returned to the presynaptic terminal, or • diffused away from the synapse to the blood circulation

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