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Hearing is taught in the note for Coordination. Read it below and look for the section on this topic.

Coordination

14 min read

Transmission of a nerve impulse

A nerve impulse is an electrical signal that travels along the axon of a neurone. At rest the outside of the neurone is positive compared with the inside: the resting potential is about −70 mV, kept by the sodium–potassium pump. When the neurone is stimulated, sodium ions rush in and the inside becomes positive: an action potential. Potassium ions then leave and the resting state returns. The wave of action potentials moves along the axon. In myelinated neurones the impulse jumps between the nodes of Ranvier (saltatory conduction), which is much faster.

A motor neuroneA nerve cell with a cell body containing the nucleus and several short branches called dendrites that receive signals. A long axon, wrapped in a fatty myelin sheath with gaps called nodes of Ranvier, carries impulses away. At the end, axon terminals pass the signal on to a muscle or another cell.cell body (nucleus)dendritesmyelin sheathnode of Ranvieraxonaxon terminalsimpulse
The membrane potential shoots up to about +40 mV then returns below the resting level before recovering.

The synapse

At a synapse, vesicles release a neurotransmitter, such as acetylcholine, across the gap. It binds to receptors on the next neurone and starts a new impulse. Impulses go one way only.

A synapseThe gap between two neurones. When an impulse arrives at the end of the first neurone, vesicles release a chemical called a neurotransmitter into the tiny gap. It diffuses across and binds to receptors on the next neurone, which starts a new impulse. Messages therefore pass one way only.vesicles of neurotransmitterreceptorssynaptic cleft (gap)first neuronenext neurone

Seeing

Light passes through the cornea, aqueous humour, lens and vitreous humour to the retina. The iris controls the size of the pupil: in bright light the pupil constricts, and in dim light it dilates. Accommodation is the change in the shape of the lens: for near objects the ciliary muscles contract, the suspensory ligaments slacken and the lens becomes thicker.

The human eyeA cross-section of the eye with its main parts: the cornea at the front which bends light most, the iris with the pupil in the centre that controls how much light enters, the lens that changes shape to focus, the retina at the back where the image forms with rods and cones, and the optic nerve that carries signals to the brain.corneairislensaqueous and vitreous humourretina (rods and cones)optic nervepupil
Cells in the retinaWorks inGives
RodsDim lightBlack-and-white vision
ConesBright lightColour vision and detail

Hearing and balance

Sound makes the eardrum vibrate. The three ear bones pass and amplify the vibrations to the oval window. Fluid movements in the cochlea bend sensory hair cells that start impulses in the auditory nerve. The semicircular canals detect movement of the head, and the utricle and saccule detect its position, which keeps the body balanced.

The human earSound waves enter the outer ear and make the eardrum vibrate. Three small bones, the hammer, anvil and stirrup, pass and increase the vibrations to the oval window of the cochlea, which is filled with fluid and lined with hair cells that turn the vibrations into nerve impulses. The semicircular canals sense balance, and the Eustachian tube equalises pressure.outer ear (pinna)eardrumhammer, anvil, stirrupcochleasemicircular canals (balance)auditory nerveEustachian tube
Try itSpeed of a nerve impulse

Impulses travel faster in myelinated neurones. Choose the length of the path and the speed.

Check yourself What makes the action potential rise?
Check yourself Why is the impulse faster in myelinated neurones?
Check yourself Which cells give colour vision?

Key terms

Resting potential
The membrane potential of a neurone at rest, about −70 mV.
Action potential
A brief reversal of the membrane potential.
Accommodation
Change in lens shape to focus.
Neurotransmitter
A chemical that carries a signal across a synapse.

Summary

  • The nerve impulse is a wave of action potentials caused by movement of Na⁺ and K⁺.
  • The synapse passes the message one way with a neurotransmitter.
  • The eye focuses by changing the shape of the lens; rods and cones detect light.
  • The cochlea detects sound and the semicircular canals detect balance.
Seeing Body balance