THE NERVOUS SYSTEM  NERVOUS SYSTEM: Receives & relays info. About activities w/in the body & monitors & responds to internal & external changes.  NEURONS: Cells that carry messages throughout the NS.  Impulses- Messages taking the form of electrical signals  Sensory- from sense organs to brain & spinal cord  Motor- from brain & spinal cord to muscles or glands  Interneurons- connect sensory & motor neurons  Parts: Cell body-Lgst. Part w/ nucleus & cytoplasm; ATP  Dendrites-Short branched extensions carrying impulses from environment or other neurons toward cell body.  Axon- Long fiber carrying impulses away from cell body  Axon terminals- series of small swellings at the end of the axon; located some distance from cell body. (fig. 37-3)

THE NERVE IMPULSE-1  Luigi Galvani: Found that nerves work with electrical charges along the cell membrane due to mov’t of ions across membranes.  RESTING POTENTIAL (Figure 37-4)  Nerve cells have an electrical potential of 70 mV. Due to the difference of + & - charged ions on each side of the cell mem.  Na + -K + pumps move Na + ions out of cell and actively pump K + ions into the cell. This Active transport causes the cytoplasm to have more K + ions and fewer Na + ions than the surrounding medium.  The cytoplasm has negatively charged protein molecules & ions. The K + ions leak back out more easily than Na + ions leak in, and the – charged molecules don’t leak in or out.  The net result is a – charge on the inside of the cell membrane. This is known as resting potential. Causing it to be polarized. (fig. 37-4) (- chgd. On the inside & + chgd. On the outside)

THE NERVE IMPULSE 2  The moving impulse (like the mov’t. of water rippling in a pond)  Cell membrane has “gates” that allow Na & K to go through  At impulse, gates open so Na + flow inside the membrane causing the cell membrane to be depolarized. As impulse passes, K + gates open allowing + chgd. K + ions to flow out. (repolarized)  Action Potential: The depolarization and repolarization of a membrane.  Several impt. Facts:  Not an electric current; it’s a wave of depolarization and repolarization passing along a neuron.  Much slower than an electric current. (10cm to 1m/sec.)  The strength of an impulse is always the same.

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THE SYNAPSE  Receptor: Sensory neurons in sense organs that receive stimuli from their external environment.  Effector: Muscles or glands that bring about a response.  Synapse: Points of contact at which impulses are passed from one cell to another.  Neurotransmitters: substance used by neurons to signal each other located within tiny vesicles at axon terminals.  Impulse moves down axon & vesicles discharge the neurotransmitters into gap between which diffuse into special receptors on next neuron.  Permeability of the membrane changes, Na + ions diffuse through membrane which can depolarize the neuron. Threshold is reached and an impulse begins in 2 nd cell.  Neurotransmitter detaches, is remove or destroyed halting the effect.

THE NERVE IMPULSE-3  Propagation of the impulse  An impulse at one point of the membrane causes the impulse at the next point of the membrane  Unlike dominoes, it can restore itself. The mov’t. is only in one direction because Na gates will not open.  The Role of Myelin  Made of lipids & proteins, it forms an insulated sheath, wrapping around the the axon.  Myelin has small nodes, gaps, between adjacent sheaths along axons. This allows the impulse to jump from node to node greatly increasing the speed of the impulse.  The Threshold: Minimal level of a stimulus that is required to activate a neuron. (All or none; dominos)  Not restricted to impulses as they travel along neurons. It also occurs when they move from one neuron to another.