Nervous System and the Eye
How impulses travel along neurones, how reflexes protect the body, and how the eye detects light. Part A of Chapter 14.
The Nervous System
Electrical impulses travel along neurones. The nervous system is made up of the central nervous system and the peripheral nervous system.
| Part | Made up of |
|---|---|
| Central nervous system (CNS) | The brain and the spinal cord |
| Peripheral nervous system (PNS) | The nerves outside the brain and spinal cord |
Role of the nervous system
The nervous system has the role of coordination and regulation of body functions. It receives information from receptors, and sends impulses to effectors, so that the body responds.
Worked example (naming the system): Name the system to which the spinal cord and the nerves in the arm belong. Step 1. The spinal cord is part of the central nervous system. Step 2. The nerves in the arm are outside the brain and spinal cord, so they are part of the peripheral nervous system. Answer: CNS; PNS.
Neurones
There are three kinds of neurone in a simple reflex pathway. Each has a different shape and position.
| Neurone | Carries impulses | Position |
|---|---|---|
| Sensory | From a receptor to the CNS | Mostly outside the CNS |
| Relay | Between sensory and motor neurones | Inside the CNS |
| Motor | From the CNS to an effector | Mostly outside the CNS |
Worked example (identifying a neurone): A neurone carries impulses from the spinal cord to a muscle. Name the neurone. Step 1. It carries impulses away from the CNS to an effector. Step 2. A neurone that does this is a motor neurone. Answer: Motor neurone.
Reflexes
A reflex action is a way of automatically and rapidly integrating and coordinating stimuli with the responses of effectors, which are muscles and glands.
| Part of the arc | Role |
|---|---|
| Receptor | Detects the stimulus |
| Sensory neurone | Carries impulses to the CNS |
| Relay neurone | Passes impulses on to the motor neurone |
| Motor neurone | Carries impulses to the effector |
| Effector | A muscle or gland that produces the response |
Worked example (describing a reflex): Describe what happens when a hand touches a hot object and is pulled away. Step 1. Receptors in the skin detect the heat and sensory neurones carry impulses to the spinal cord. Step 2. A relay neurone passes the impulses to motor neurones. Step 3. Motor neurones carry them to the arm muscles (effectors), which contract. Answer: Automatic and rapid response.
Synapses
A synapse is a junction between two neurones.
ExtendedStructure of a synapse
Neurones do not touch each other. A synapse has vesicles containing neurotransmitter molecules in the first neurone, a synaptic gap, and receptor proteins on the next neurone.
ExtendedEvents at a synapse
(a) An impulse stimulates the release of neurotransmitter molecules from vesicles into the synaptic gap.
(b) The neurotransmitter molecules diffuse across the gap.
(c) They bind with receptor proteins on the next neurone.
(d) An impulse is then stimulated in the next neurone.
Synapses ensure that impulses travel in one direction only.
Worked example (ordering the events): Put these in order: neurotransmitter binds to receptors; impulse arrives; neurotransmitter diffuses across the gap. Step 1. The impulse arrives and releases neurotransmitter from the vesicles. Step 2. The neurotransmitter diffuses across the gap. Step 3. It binds to receptors, which stimulates an impulse in the next neurone. Answer: Arrives, diffuses, binds.
Sense Organs and the Eye
Sense organs are groups of receptor cells that respond to specific stimuli: light, sound, touch, temperature and chemicals.
| Stimulus | Example of a sense organ |
|---|---|
| Light | Eye |
| Sound | Ear |
| Touch and temperature | Skin |
| Chemicals | Nose and tongue |
Structures of the eye
You need to identify the cornea, iris, pupil, lens, retina, optic nerve and blind spot in diagrams and images.
Worked example (naming the structure): Name the structure that forms the opening at the centre of the iris. Step 1. The iris surrounds a gap. Step 2. The gap is the pupil. Answer: Pupil.
Functions of the Eye and the Pupil Reflex
Each part of the eye has its own function.
| Part | Function |
|---|---|
| Cornea | Refracts light |
| Iris | Controls how much light enters the pupil |
| Lens | Focuses light on to the retina |
| Retina | Contains light receptors, some sensitive to light of different colours |
| Optic nerve | Carries impulses to the brain |
The pupil reflex
The pupil reflex is a reflex action that changes the diameter of the pupil when the light intensity changes. In bright light the pupil becomes smaller, so less light enters the eye. In dim light it becomes larger, so more light enters.
Worked example (predicting the response): A person walks from a dark room into bright sunlight. What happens to the pupils? Step 1. The light intensity increases. Step 2. The pupil reflex makes the pupils smaller, so less light enters the eye. Answer: Pupils become smaller.
The Iris and the Lens
ExtendedThe pupil reflex: circular and radial muscles
The iris contains circular and radial muscles, which act as an antagonistic pair.
In bright light the circular muscles contract and the radial muscles relax, so the pupil gets smaller. In dim light the radial muscles contract and the circular muscles relax, so the pupil gets larger.
ExtendedAccommodation
| Near object | Distant object | |
|---|---|---|
| Ciliary muscles | Contract | Relax |
| Suspensory ligaments | Slack | Tight |
| Lens shape | Fatter | Thinner |
| Refraction of light | More | Less |
Rods, Cones and the Fovea
ExtendedRods and cones in the retina
The retina contains two kinds of light receptor. Cones are packed most closely at the fovea and are fewer elsewhere; rods are spread across the rest of the retina, with none at the fovea. There are no receptors at the blind spot.
ExtendedPosition of the fovea
The fovea lies on the retina directly behind the lens and pupil, away from the optic nerve and blind spot.
ExtendedFunction of rods and cones
| Receptor | Function |
|---|---|
| Rods | Greater sensitivity to light, so used for night vision |
| Cones | Three different kinds, absorbing light of different colours, give colour vision |
| Fovea | Gives the sharpest vision; where the cones are packed most closely |
Exam advice
Common mistakes
Model answer
Recall checklist
- Define the CNS and the PNS.
- Identify sensory, relay and motor neurones.
- Describe a reflex arc and a reflex action.
- Label the parts of the eye and state their functions.
- State the stimuli sense organs detect; describe the pupil reflex.
- Describe a synapse and its events (Extended)
- Explain the iris muscles and accommodation (Extended)
- Outline rods, cones and the fovea (Extended)
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