Sense Organs

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SENSE ORGANS AND COORDINATION
1. describe the structures and functions of the nervous system

The nervous system
- This is the main system of control in the body.

It has the following functions:
* Controls thinking, planning and learning(voluntary actions) * Collects and interprets information from the sense organs. * Controls automatic(reflex) actions
* Controls the activities of other systems. Eg heart beat, digestion

The nervous system is made up of two main parts:
1. Central nervous system; brain and spinal cord
(C. N. S.)
2. Peripheral nervous system;cranial and spinal nerves, nerve fibers and sense organs

Brain
Part| Description| Function|
Cerebrum| Largest part, has a twisted and bumpy surface| Responsible for conscious thought and emotion centers| Cerebellum| Small, ridged surface| Center for muscle coordination and balance| Medulla oblongata| Stalk shaped, run into spinal cord| Controls involuntary processes such as breathing| Pituitary| Pea size, underneath| Links nervous and endocrine systems| The spinal cord lies within the cavity of the backbone (vertebrae). These bones protect the spinal cord as the skull protects the brain.

The nervous system links receptors to effectors.

Receptors are in the sense organs. These special cells receive stimuli. A stimulus is any change that occurs that can be detected by the body. Eg drop in temperature or a touch. SENSE ORGAN| RECEPTOR(sense cell)| STIMULUS DETECTED|

Eye| Rod cells on retina| Light|
| Cone cells on retina| Colour|
Ear| Cochlea cells| Sound|
| Semicircular canals| Position of body|
Tongue| Taste bud cells| Chemicals in solution|
Nose| Lining of nasal cavity| Chemicals in vapors|
Skin| Nerve endings| Pain, touch, hot , cold, pressure|

Effectors are responsible for carrying out the response to the stimuli. Eg muscles or glands

Nerve cells (neurons)
There are three main types of nerve cells
1. Sensory neuron – these carry messages from the sense organs to the C. N. S.

2. Intermediate neuron-these are found in the brain and spinal cord (C. N. S.). They connect sensory and motor neurons.

3. Motor neuron-these carry messages from the C. N. S. to the effector that carries out the response action.

Neurons carry messages in the form of an electrical impulse from one end to the next. Where two neurons meet they do not actually touch each other. There is a small gap called the synapse.

Nerve transmission
When the electrical impulse reaches the end a chemical is released from the 1st neuron this chemical crosses the synapse and goes to the 2nd neuron. In this way a nerve impulse is transmitted from one neuron to the next so the message is carried on.

Voluntary and involuntary actions
A voluntary action is not automatic. The brain needs to think about this action then send a message to the necessary muscles to carry out the action.

Eg. Picking up a dropped ball

STIMULUS; ball dropping

RECEPTOR;rods and cone cells

SENSORY NEURONE

C. N. S.;brain

MOTOR NEURONE

EFFECTORmuscles of legs
and hands
RESPONSE;pick up ball

Voluntary actions are not carried out unless a person consciously makes a decision to do them. The brain receives information from the sense cells and makes a decision, the response action is then carried out

Involuntary (Reflex) actions
These actions are not controlled by the person. They are:
* Automatic
* Quick
* Serve as a form of protection

Eg’s. Pulling your hand away from a hot pot, blinking when an object approaches the eye, knee jerk

The brain does not make a decision on involuntary (reflex) actions they just happen.

Knee-jerk reflex

STIMULUS; hit below knee

RECEPTOR;pressure receptors
in skin
SENSORY NEURONE

C. N. S.;intermediate neuron
in spinal cord
MOTOR NEURONE

EFFECTORthigh muscle

RESPONSE;leg springs forward...
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