nervous control

NERVOUS SYSTEM HAS TWO MAJOR DIVISIONS:

central nervous system (CNS)- which is made up of the brain and spinal cord 

Peripheral nervous system (PNS)- is made up of pairs of nerves that originate from the brain or spinal cord

THE PERIPHERAL NERVOUS SYSTEM IS DIVIDED INTO:

The sensory nuerones- carry nerve impulses from receptors towards the central nervous system

The motor neurones- carry nervous impulses away from the central nervous system to the effectors

THE MOTOR NEVOUS SYSTEM IS DIVIDED INTO:

The voluntary nervous system- carries nerve impulses to body muscles and is under voluntary control

The autonomic nervous system- carries nerve impulses to glands, smooth muscle and cardiac muscle and is under involuntary control 

The repsonse is totally involuntary. By the time the brain has revieved the nerve impulses the response has already taken place. 

invloves three neurones. one is in the spinal cord so is known as the spinal reflex

Main Stages Of A Reflex Arc:

1-Stimulus eg. heat from a hot object

2-Receptor eg. temperature receptors in hand create a nerve impulse in a sensory neurone

3-Sensory Neurone passes nerve impulse to spinal cord

4-Intermediate Neurone links the sensory neurone to the motor neurone in the spinal cord

5-Motor Neurone carries the nerve impulse from the spinal cord to a muscle in the upper arm

6-Effector the muscle in the upper arm is stimulated to contract

7-Response pulling hand away from the hot object

Why are reflex arcs useful?

They are involuntary so the brain is not involved and can continue to carry out more complex responses.

so it is not overloaded with situations, as the rsponse is always the same 

some impulses are sent to the brain so it is informed of what is heppening and can over-ride if needed.

They protect the body from harmful stimuli. 

we are born with them so they do not have to be learned.

They are fast, because the neurone pathway is short with very few synapses.

(synapses are the slowest link in the neurone pathway).

Autonomic means self governing

The autonomic nervous system conrols the involuntary activities of internal muscles and glands

THE TWO DIVISIONS:

sympathetic nervous system- stimulates effectors and so speeds up activity.                                 It stimulates effector when we exercise strenuously or experience powerful emotions.                    It helps us cope with stressful situations by heightening awareness and preparing us for fight or flight 

parasympathetic nervous system- inhibits effectors, so slows down any activity.                               controls activities under noremal resting conditions.                                                                  It's job is to conserves energy and replenishing the bodys reserves.

The heart rate must be altered to to meet the demands for oxygen.

changes to the heart rate are controlled by a region of the brain called the medulla oblongata.

TWO CENTRES:

A centre that increases heart rate, ehich is linked to the sinoatrial node by the sympathetic nervous system 

A centre that decreases heart rate, which is linked to the sinoatrial node by the parasypathetic nervous system

which of these centres stimulated depends upon the information they recieve from two types of receptor, which respond to one of the following:

chemical changes in the blood

pressure changes in the blood

chemoreceptors are found in the wall of the carotid arteries.                                                   They are sensitive to changes in the PH of the blood due to CO2 concentration                         Stages of control:

1-  chemoreceptors detect lowered PH and increase the frequency of nervous impulses to the centre of the medulla oblongata that increases the heart rate 

2- this centre increases the frequency of impulses via the sypathetic nervous system to the sinoatrial node, which increaes heart rate 

3- The increased blood flow leads to more CO2 being removed by the lungs and so CO2 level of the blood returns to normal

4- PH of blood returns to normal and the chemoreceptors reduce the frequency of nerve impulses to the medula oblongata

5- The medulla oblongata reduces the frequency of impulses to the sinoatrial node, which decreases heart rate to normal.

The Pacinian Corpuscle fucntions as follows:

In resting state the stretch mediating sodium channels are too narrow for sodium ions to pass along them. The Resting Potential 

when pressure is applied, it changes shape and the membrane around its neurone becomes stretched 

stretching widens the sodium channels in the membrane and sodium diffuse into the neurone

The influx of sodium ions changes the potential of the membrane (it becomes depolarised) producing a generator potential 

The generator potrntial in turn creates an action potential (nerve impulses) that passes along the neurone and then via the other neurones to the central nervous system