Nervous communication IGCSE Biology: The Human Nervous System and the Eye

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. The nerve impulse up to one metre ---->

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 Signalling between neurones

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.

- automatic reflexes ●are very fast (short pathway, no decision-making) ●cannot be trained - conscious reactions can be trained and are flexible 2 kinds of responses

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 The reflex arc

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 Transmission of impulses

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 synapse

100 billion neurons Memory / processing in the cortex The brain The cortex has many specialized zones

Homeostasis = keeping internal conditions constant The hypothalamus detects blood temperature, water and glucose The medulla controls the heart rate and breathing rate How the brain controls body functions

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 The human eye