Chapter 41 The Nervous System

The Neuron The use of electrical signals to send messages rapidly through the body Is a nerve cell specialized in doing this All sensory info is processed via neurons (called nerve impulses). Structure of a neuron Most unusual looking of all cells dendrites: the “antennae” , receive signals from other cells Cell body, much like that of a normal cell Axon: long membrane-covered extension of cytoplasm that conducts the nerve impulse. The ends of axons are axon terminals, this is where they communicate with each other

The insulated part… Myelin sheath: is a layer of insulation around the axon, allows impulses to move faster. Nodes of Ranvier: are gaps (where the myelin sheath is interrupted) where the axon is exposed to surrounding fluid. Nerve Function There is a different “charge” on the inner surface of the cell and the fluid outside the cell. Membrane potential: is the difference in electrical charge across the cell membrane. It is expressed in voltage, just like a battery, but is very, very, small. Everything operates on ion channels, remember those proteins that allow ions to pass into or out of a cell, via the membrane? Even the slightest change/movement of ions affects the charge, and thus the signal.

Resting Potential When a neuron is not conducting an impulse (said to be at rest). Roughly -70 millivolts (mV) The inside of the cell is negatively charged with respect to the outside. There is a greater concentration of Na+ ions outside than there are K+ ions inside. Remember the sodium potassium pump? This is very important in nerve impulses.

Action potential A neuron is conducting an impulse A change from the negative charge at RP, to a more positive inside than outside. It moves very quickly down the axon. How does it work? A signal is received, which changes the membrane potential to become more positive The causes ion gates/channesl to open and sodium ions rapidly move into the axon. Briefly it gets up to +40 mV This sudden reversal of polarity begins a chain reaction cause the gates to open down the entire length of the axon. The gates close immediately after the action potential has passed This allows Na+ to flow back out, returning to a negative charge.

Communication synapse: the junction between two neurons Usually do not touch Gap is called a synaptic cleft Presynaptic neuron (sending signal), postsynaptic cell (receiving signal) As a signal arrives at the terminal, it can not cross the gap Impulse triggers the release of signal molecules, neurotransmitters, into the synaptic cleft. They are stored in vesicles, and there are many different types (example: dopamine)

Releases of neurotransmitters Released via exocytosis NT’s bind with post. Syn. Cell. They may open ion channels, or close them This may excite or inhibit the activity of the post. Syn cell Very small amounts can cause changes in our moods, from ecstasy to depression. (norepinephrine and dopamine involve feelings, to low, we feel depressed)

Structures of the Nervous System CNS: central nervous system, brain and spinal cord, control center of the body, interprets and responds. PNS: peripheral nervous system, sensory and motor neurons, everything outside of spine and brain. The brain (see senses power point) PNS Connects brain and spinal cord to rest of the body Two divisions: sensory division and motor division Sensory div. directs sensory info to the CNS Motor div. carries out the responses to sensory information (and is also divided into two independent systems)

Motor divisions two parts Somatic More of a “conscious” control Skeletal muscles Some is involuntary, ie. Reflexes. (a self protective motor response) do not usually involve the brain Autonomic Smooth muscle (not conscious) Heart rate, blood flow, digestion, etc… Also divided into two division

Two divisions of the Autonomic nervous system (help maintain balance) Parasympathetic Most active under normal conditions Keeps body functioning even during non activity (ex. Continuing to breath while you are asleep) Sympathetic Dominates during physical or emotional stress “fight or flight” response Nervousness Can increase heart rate, blood flow, and breathing rate See table 955 for a cool comparison of body reactions by each of the above.