Nerve Impulses

Neuron Physiology Action Potentials- nerve impulses which are sent by a change in electrical charge in the cell membrane. Depends on ions: Sodium (Na+) highly concentrated outside of cells Potassium (K+) highly concentrated inside cells Ion movement Ions move from high concentration to a low concentration passively Na+/K+ pumps move ions actively using ATP

Creating a Resting Potential Protein pumps open and close let ions through Active pumps Ions move against a gradient Use ATP to work Passive pumps Ions move with the gradient Ions move from high to low concentration Active transport Passive transport

Action Potential

Resting Membrane Potential (-70mV) Stage 1: Resting Potential

Depolarization and Threshold Potential Stage 2: Depolarizing neuron (-62mV) Sodium rushes in when channels open which are stimulated by neurotransmitters Stage 3: Threshold is reached once enough sodium rushes in (-55 mV)

Stage 4: Neuron continues to depolarize as sodium continues to rush in. Stage 5: Repolarization occurs as K+ channels open and K+ moves outward causing inside of membrane to become negative again. Na+ Na+ 0mV to +20mV K+ Potassium channels open -70mV

A Nerve Impulse- a series of action potentials

Refractory Period For a short period after the passage of an impulse, the threshold for stimulation is raised, so it limits the frequency of impulses and ensures uni-directional travel of impulse.

Action Potential

What happens when the nerve impulse reaches the end of the axon? Axon terminals Are found next to another neuron (as shown) or a muscle or gland The gap is called a synapse Synapse Action Neurotransmitters are released at the synapse to pass the message to the next neuron.

Synapse

Neurotransmitters released at synapse

Saltatory nerve impulse conduction The action potential jumps large distances from node to node, a process that is called saltatory propagation.

Myelinated vs. unmyelinated neurons

Speed of a Nerve impulse Temperature - higher the temperature= faster speed. Warm-blooded animals have faster responses than cold-blooded. Axon diameter - larger the diameter= faster speed. Marine invertebrates, (who live at temperatures close to 0°C), developed thick axons to speed up their responses. This explains why squid have their giant axons. Myelin sheath - Only vertebrates have a myelin sheath surrounding their neurons. The voltage-gated ion channels are found only at the nodes of Ranvier, and between nodes myelin sheath acts as a good electrical insulator. Increases the speed of propagation dramatically. unmyelinated neurons –travel at about of 1 meters/second myelinated neurons-travel at about 100 meters/second Depending on the type of fiber, modern measurements are from 6-122m/s

All or None Response The strength of a response of a nerve cell or muscle fiber is not dependent upon the strength of the stimulus. If a stimulus is above a certain threshold, a nerve or muscle fiber will fire. Full response or no response at all. "The all-or-none law guarantees that once an action potential is generated it is always full size, minimizing the possibility that information will be lost along the way."

Multiple Sclerosis Autoimmune disease-body’s natural defenses breaks down myelin in CNS Degeneration inhibits normal nerve impulse transmission, may slow or even stop in some cases.

Characterized by seizures Causes vary: Epilepsy Sudden disorderly discharge of brain neurons-impulses fire continuously. Characterized by seizures Causes vary: by injuries , infections, tumors, with drawl from drugs, etc.