Nelson Bio Chapter 9

The brain operates on the amount of electrical power that would light a 10 watt bulb

Your brain weighs 1/50 of your adult weight and uses 25% of the oxygen you take in

A nerve cell can send 1000 impulses a second

Neurons are the largest cells in the body – some are up to 3 feet long.

There are more nerve cells in the human brain than there are stars in the Milky Way

Nervous system contains 2 types of cells: Glial cells – non conducting cells which

provide structural support Neurons – the nerve cell itself

1) sensory(afferent) – sense & relay info(stimulus) to the CNS

2) interneurons – link neurons within the body

3) motor neurons (efferent) – relay info to the effectors

Glands or muscles that cause things to happen.

So why are they called effectors?

1) cell body – contains the nucleus and cell organelles; normal cell processes happen here

2) dendrites – short, highly branched fibers specialized for receiving impulses (stimuli)

3) axon – a long thin fiber(extension of the cytoplasm) which carries impulses away from the cell body to the effectors

More than 100 axons could be placed inside the shaft of a single human hair

Nerves are made of many neurons held together

The synapse is the junction between the terminal branches of a neuron and another cell.

Impulses are transmitted across this gap

An axon may synapse with 1000 other neurons

Nerve gas, botulism, some insecticides

Drugs Stimulants –produce a feeling of well

being, alertness and excitement –amphetimines bind to certain recept0rs so mimic norepinephrine, caffeine

Depressants – slow down body activites – barbituates block the formation of norepinephrine

A nerve impulse is both an electrical and a chemical event

A nerve at rest has a relative negative charge inside and a positive charge outside K ions are concentrated inside the

neuron and Na ions are concentrated outside the neuron

This neg. charge is maintained by sodium-potassium pumps which control the movement of Na and K ions through the cell membrane. (ATP supplies the energy to fuel the pumps).

The membrane is now said to be polarized

An external stimulus changes the ability of the membrane to keep out the Na ions & they rush in

This rapid inflow of Na causes a charge reversal – depolarization

This change in charge stimulates adjacent points in the nerve cell & a wave of impulse passes along the nerve cell

Animation An impulse ends when K ions rush to

the outside and repolarize the membrane

A nerve impulse can travel at the rate of 100m/sec

The recovery period is called the refractory period.

Saltatory conduction is when the impulse jumps from one node of Ranvier to the next – so this is faster in myelinated neurons

The impulse causes chemicals called neurotransmitters to be release from the vesicles in the synaptic knobs

The chemicals travel across the synaptic gap and cause an impulse to be started along the postsynaptic membrane

Animation

A stimulus must be above a certain level to produce a response

The all or none response- neurons either fire or not at all ( like a trigger of a gun)

What causes a greater intensity of response?

it is not stronger stimuli, but more impulses.

On pg. 426 – do questions 4 ,5,7,8,9 and 13.