*The surface of the cell membrane is usually electrically charged or polarized (with respect to the inside)

-The result is of unequal distribution of positive and negative ions between sides of the membrane

*Distribution of ions is determined by pores or channels in those membranes

-some channels are always open

-some can be opened or closed

-channels can be selective (will only allow one kid of ion to pass though)

*Potassium pass through more easily than sodium ions.

-This makes potassium ions a major contributor to membrane polarization

Greater concentration of sodium ions (Na+) outside

Greater concentration of potassium ions (K+) inside

Cytoplasm has many large, negatively charged particles, including phospate ions, sulfate ions, proteins (these are too large to pass through membrane) 

Because resting cell is more permeable to K+, K+ diffuse out of cell more rapidly. the potential difference between the inside and outside of the membrane. 

nerve cells are excitable

this means they can respond to changes in their surroundings

some detect changes in temperature

some detect changes in light

some detect changes in pressure

such changes (or stimuli) affect the resting potential

Membrane becomes depolarized (inside becomes less negative compared to outside)

Threshold the stimulation level that must be exceeded to elicit a nerve impulse or a muscle contraction

Action Potential the sequence of electrical charges when a nerve cell me is exposed to a stimulus that exceeds its threshold

At the threshold, permeability suddenly changes

sodium ions diffuse inwards 

Membrane loses it's negative electrical charge and becomes depolarized 

Almost the same time, potassium ions pas through and diffuse outward

Inside becomes negatively charged one more (repolarized)

Depolarization and Repolarization takes about one-thousandth of a second

At the threshold, permeability suddenly changes

sodium ions diffuse inwards 

Membrane loses it's negative electrical charge and becomes depolarized 

Almost the same time, potassium ions pas through and diffuse outward

Inside becomes negatively charged one more (repolarized)

Depolarization and Repolarization takes about one-thousandth of a second

The wave of action potential moves down the axon to the end= nerve impulse

If a neuron respons at all, it responds compltley 

A greater intensity of stimulation does not produce a stronger impuls but rather more impulses per second

Nerve Impulses travel from one neuro to another along nerve pathways

The junction between two communicating neurons is called the synaspse 

Two neurons don't directly touch there is a gap between the neurons called the synaptic cleft this impulse must cross the gap

axons  have several synaptic knobs at the distal ends, which dendrites lack. these knobs contain maany synaptic vesicles. these vesicles release nuro transmitters these diffuse across the cleft and react with specifi receptors on postsynaptic neuron

E-neurotransmitters that increase postsynaptic membrane permeability to sodium ions

I-neurotransmtters decrease permiability to sodium ions, thus making it less likely that threshold will be reached

Lessons the chance that a nerve impulse will occur

About 50 types of Neurotransmitters have been identified in the nervous system some neurons release only one type other neurons release two or three 

Acetylcholine: stimulates skeletal muscle contraction

Monoamines (epinephrine, norepinephrine, dopamine, serotonin)