The Nervous System & The Endocrine System: Year 8 Biology Final Exam Summary

1.1 The Neuron

Functions of the Nervous System
The nervous system collects information about the body's internal and external environment, process the information, and respond to it.

> Peripheral Nervous System: consists of nerves and supporting cells, collects information about the body's external and internal environments.
> Central Nervous System: consists of the brain and spinal cord, process info and creates response (delivered to appropriate part of the body through the peripheral nervous system).

Neurons
Nervous system impulses are transmitted by cells called neurons. The types of neurons:

> Sensory neurons: carry impulses from sense organs (example: eyes, ears) to the spinal cord and brain.
> Motor neurons: carry impulses from the brain and spinal cord to the muscles and glands.
> Interneurons: process information from sensory neurons and sends commands to other interneurons or motor neurons.

Structure of Neurons

> Cell bodies: nucleus and cytoplasm, largest part of the neuron.
> Dendrites: receive impulses from other neurons and carry it to the cell body.
> Axon: long fiber that carries impulse away from the cell body.
> Myelin sheath: in some neurons, insulating (preventing any electricity to go through) membrane that surrounds the axon. Has many gaps– nodes. Impulse move through axon by jumping from node to node, resulting faster travel.

The Nerve Impulse
1. The Resting Neuron
> Active transport proteins pump sodium ions (Na+) out of the cells and potassium ions (K+) into the cell.
> Since there are more sodium ions outside than there are potassium ions inside, the membrane's interior has a negative charge. The membrane's exterior has a positive charge.
>  This produces the resting potential. Which mean the neuron is polarized.

2. The Moving Impulse
> An impulse begins when a neuron is stimulated by another neuron or by the environment.
> The neuron cell membrane contains thousands of "gated" ion channels.
> At the leading edge of each impulse, gated sodium channels open, allowing positively charged Na+ ions to flow into the cell. This causes the inside of the membrane to become more positive than the outside. (Called the nerve impulse or the action potential).
> This is depolarization.

> Once the impulse passes, sodium gates close and gated potassium channels open, allowing K+ ions to flow out.
> The sodium-potassium pump keeps working, ensuring that the axon will be ready for more action potentials. This pump allows the ions to return to their normal place, K+ inside, Na+ outside.

Threshold
> The minimum level of a stimulus that is required to cause an impulse to a neuron.
> Any stimulus weaker results to no impulse. It's either an impulse or none at all.
> All action potentials have the same strength. So how do we sense if a stimulus is weak or strong?
> A weak stimulus might produce only 3 or 4 action potential per second, while a strong one might result in as many as 100 per second.

The Synapse

> The end of the neuron is the axon terminal that passes the impulse to another cell. (Example: a motor neuron passing the impulse to a muscle cell, causing it to contract.)
> The neuron transfer point is called a synapse.
The synaptic cleft separates the axon terminal from the adjacent cell.
> Axon terminal at a synapse contain tiny vesicles called neurotransmitters.
> Neurotransmitters are chemicals that transmit an impulse across a synapse to another cell.
> When impulse arrives at synapse, neurotransmitters are released from the axon, diffuse across the synaptic cleft, and bind to receptors on the membrane of the receiving cell.
> If the stimulation is bigger than the cell's threshold, a new impulse begins.
> Once they're done with their work, the neurotransmitters are released from their receptors on the cell surface.
> They're then broken down by enzymes of synaptic cleft or taken & recycled by axon terminal.