why does impulse condition starts in receptors?

The impulses from the receptor pass along a
sensory neurone to the central nervous system. A
motor neurone carries the impulses to the muscles which
moves the hand away from the source of heat. This is the
RESPONSE

There are
two types of responses controlled by the nervous system.

1.
Learned responses. These responses are slower and need to
be learned (you are not born with them). Example imagine an athlete is waiting to start
a 100m race. The starter starts the race with a gun.

The
noise of the gun stimulates the sensitive cells in the ears, these cell then send
electrical impulses (messages) to the central nervous system, that is the spinal cord
and brain, via sensory neurone. Once the impulses get to the brain, it sorts out the
message and then co-ordinates a response. The impulses then travel from the brain to the
muscles via the motor neurone. When the muscles receive the impulses they contract and
move the athlete.

The stimulus
is the starter shooting the gun.

The
receptors are the cells in the athletes'
ears.

The co-ordinator is the
athlete's brain.

The effectors
are the muscles in the athletes' legs.

The
response is to start
running.

Stimulus - receptor - sensory neurone -
co-ordinator - motor neurone - effector - response

2.
Reflex actions. These types of reactions are very fast and
are automatic (you are born with them). These actions involve three neurones called
sensory, relay and
motor neurones. For example: if you touch something hot
with your hand.

Pain sensitive receptors in the skin detect
pain. The impulses from the receptor pass along a sensory
neurone to the central nervous system. At a junction
(synapse) between the sensory
neurone system and the relay neurone, in the
central nervous system, a chemical is released that causes
an impulse to be sent along the relay neurone. At a junction
(synapse) between the relay
and the motor neurone, a chemical is released that causes
impulses to be sent along the motor neurone. The
motor neurone carries the impulses to the muscles (the
EFFECTOR) which moves the hand away from the source of
pain. This is the RESPONSE.

A
reflex action always follows the
path:

Stimulus -
receptor - co-ordinator
- effector -
 response

The
major difference between learned responses and the reflex action is that the body
increases the speed of reflex actions by cutting out a part of the nervous system, that
is, the brain.

NOTE: The
effector can be either a muscle or a gland. Muscles respond to impulses by contracting,
glands respond by secreting.