1. Notes Ch. 10c Nervous System 1
Chemistry

2. The Synapse
Synapses are points where neurons communicate with one another or another cell.
When you get a text message the person texting is the sender and you are the receiving. The neuron conducting an impulse to the synapse is the sender or presynaptic neuron. The neuron receiving input at the synapse is the postsynaptic neuron.

3. Synaptic transmission is a one-way process carried out by neurotransmitters. Most axons have several rounded synaptic knobs that have membranous sacs, called synaptic vesicles that contain neurotransmitters. When an impulse reaches this area it releases calcium ions and this increase releases neurotransmitters.

4. When the neurotransmitter binds to receptors on the postsynaptic cell the action is either excitatory (turning a process on) or inhibitory (turning a process off).

5. Cell Membrane Potential
A cell membrane is usually electrically charged, or polarized, so the inside of the cell is negatively charged compared to the outside. This is because there is an unequal distribution of positive and negative ions across the membrane. This is important in conducting an impulse in muscle fibers and neurons.

6. Distribution of Ions
Potassium ions are the major intracellular (inside the cell)positive ion and sodium ions are the major extracellular (outside the cell) positive ion. So the sodium/potassium ion pump actively transports sodium ions out of the cell and potassium into the cell.
There are gated channels and chemical and electrical factors can affect the opening and closing of the gate.

7. Resting Potential
A resting nerve cell is not being stimulated to send an impulse. Under resting conditions, non-gated channels (always open) determine the number of sodium and potassium ions in the cell. These ions move according to diffusion.
The membrane potential is the potential difference (stored electrical energy that can be used to do work in the future) in a cell and is measured in millivolts.

8. The resting potential of a resting neuron has a value of -70 millivolts.
The action potential is a rapid change in the membrane potential, first in a positive direction, then in a negative direction, then returning to the resting potential.

9. Local Potential Changes
Neurons are excitable (they can respond to changes in their surroundings). Some neurons can detect changes in temperature, light, or pressure outside the body. Some respond to changes inside the body. Any change will affect the membrane potential of any neuron exposed to the stimulus.
When there is an environmental change it will open a gated ion channel.
If the result is a membrane potential that becomes more negative than the resting potential it is called hyperpolarized. If the membrane becomes less negative than the resting potential it is called depolarized.
If neurons are sufficiently depolarized then there is a threshold potential, which is about millivolts and this will cause an action potential.

10. Ion Movements During Action Potentials
When threshold is reached it will open a voltage-gated sodium channel. When a threshold is met will allow sodium into the cell and depolarizes the cell. After the sodium gated channels open they close and potassium channels open. This repolarizes the cell. This continues to happen down the cell, sending a signal.

11. All-or-none response
An action potential is an all-or- none response. A greater intensity of stimulation produces more impulses per second, not a stronger impulse.

12. REfractory Period
There are only a certain number of impulses per second that an axon can generate. During an impulse a part of the axon becomes unresponsive. This is called a refractory period. It has 2 parts - the absolute refractory period when voltage gated sodium channels are not responsive at all. A relative refractory period follows when the membrane goes back to the resting potential.

13. The refractory period limits how many action potentials a neuron can generate.

14. Impulse Conduction
A myelinated axon acts differently. Myelin prevents the flow of ions through the membrane that it encloses and it acts as an electrical insulator.