Chapter 41The 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 partsSomatic- 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.