Download Nervous communication

Nervous
communication
IGCSE Biology: The Human
Nervous System and the Eye
The nerve impulse
Nervous communication:
● electrical signals travel fast
● Nerve cells (= neurones) are up to a metre long
Below is a motor neurone. It makes an effector respond.
In this case, the effector is a muscle.
up to
one
metre
---->
Signalling between neurones
Nervous signalling is not just fast, but also flexible
● neurones make multiple connections
● this allows us to have many possible responses
A sensory neurone passes impulses from receptors to
the Central Nervous System
Relay neurones and the central
nervous system (CNS)
The third kind of
neurone is a relay
neurone.
The CNS consists
mostly of relay
neurones.
Relay neurones
transmit impulses
between other
neurones.
2 kinds of responses
- automatic reflexes
● are very fast (short pathway, no decision-making)
● cannot be trained
- conscious reactions can be trained and are flexible
The reflex arc
A reflex arc is the nerve pathway of a reflex:
A sensory neurone, a relay neurone and a motor neurone.
In a reflex (e.g. withdrawing a finger from a hot object)
1.An impulse starts in a receptor
2.then is transmitted to a sensory neurone
3.then to a relay neurone in the brain or spine,
4.then to a motor neurone
5.and finally to an effector for a rapid response
Transmission of impulses
In the neurone:
Electrical impulses travel along the fibres of a neurone
● dendrons: towards the cell body
● axons: onwards away from the cell body
From one neurone to the next:
The communication between neurones is by chemical
transmitter across very narrow gaps called synapses
The synapse
When an electrical signal arrives at the synaptic terminals of
the axon, chemical transmitter is released.
The transmitter diffuses across the synapse and attaches to
the membrane of the next neurone
A new electrical impulse is generated
in the next neurone
The brain
100 billion neurons
Memory / processing in
the cortex
The cortex has many
specialized zones
How the brain controls body
functions
Homeostasis = keeping
internal conditions
constant
The hypothalamus
detects blood temperature,
water and glucose
The medulla controls the heart rate and
breathing rate
The human eye
Iris: controls light entering, protects retina
from too much light
● Dilates the pupil to let more light in
● Constricts the pupil to let less light in
Lens: focuses light on
the retina
● Flat for distant
objects
● Fatter for near objects to refract light more