Interpreting nervous system data

Nervous system data can be presented as numbers in a table or as points, lines or bars on a graph. To interpret the data, first identify what was measured and the condition or measurement each value belongs to.

Reading and comparing nervous system data

Use the title, headings, axis labels and key to identify the quantities shown. Check the scale to see what each division represents, and check the units before reading or comparing values. A key is especially useful when a graph shows more than one set of data.

Compare values to look for patterns, such as whether reaction time increases or decreases between conditions. A shorter reaction time means a faster response. State conclusions using the values and conditions shown; the data alone may not explain why a difference occurred.

Translating reaction-time data

Suppose a table gives a reaction time of $$0.20,\text{s}$$ for condition A and $$0.30,\text{s}$$ for condition B. The response was faster in condition A, and the difference was $$0.10,\text{s}$$.

To represent these values on a bar chart, put the conditions on the horizontal axis and reaction time in seconds on the vertical axis. Choose an even scale that includes both values, such as $$0$$ to $$0.4,\text{s}$$ in divisions of $$0.1,\text{s}$$. Draw the bar for A up to $$0.20,\text{s}$$ and the bar for B up to $$0.30,\text{s}$$.

To read values from the chart, use the condition label to select the correct bar, then read its height against the vertical scale. Record each value with its unit and its condition: A is $$0.20,\text{s}$$ and B is $$0.30,\text{s}$$. When converting in either direction, keeping each value matched to its condition prevents the comparison from being reversed.

Central nervous system

The central nervous system (CNS) consists of the brain and spinal cord. It receives information, processes it and coordinates responses. This allows the body to respond to changes in its surroundings and helps coordinate behaviour.

A stimulus is a change that can be detected, such as a hot surface touching the skin. A receptor detects the stimulus and sends information along a sensory neurone to the CNS. The CNS processes the information and can send a signal along a motor neurone to an effector, which produces a response.

The pathway can be followed as:

Stimulus → receptor → sensory neurone → CNS → motor neurone → effector → response

Effectors include muscles, which respond by contracting, and glands, which respond by releasing substances such as hormones.

Central nervous system and reflex responses

Some responses are reflexes: rapid, automatic responses that occur without waiting for a conscious decision. For example, touching a hot surface can cause the arm to be pulled away. The receptor detects the heat, and signals pass through the nervous system to muscles that contract. Such reflex responses help protect the body from injury.

Neurones and synapses

Information travels through the nervous system along specialised cells called neurones. The signal moving along a neurone is an electrical impulse. A nervous pathway usually contains several neurones linked together, so an impulse can pass from one part of the body to another.

In a simple pathway, three types of neurone carry information in different directions:

• A sensory neurone carries impulses from a receptor towards the central nervous system. A receptor detects a change in the surroundings or inside the body.
• A relay neurone passes impulses from one neurone to another within the central nervous system, which consists of the brain and spinal cord.
• A motor neurone carries impulses from the central nervous system to an effector. An effector, such as a muscle or gland, produces a response.

For example, an impulse may travel from a receptor along a sensory neurone, pass to a relay neurone in the central nervous system, and then pass to a motor neurone leading to an effector.

Synapses between neurones

A synapse is a junction between two neurones. Neurones in a pathway are linked at synapses, allowing information to pass from one neurone to the next. The neurones are separate cells, rather than one continuous cell running through the whole pathway.

Recognising neurones in diagrams

The position of a neurone in a pathway and the direction in which it carries impulses are useful clues to its type. The shape of its cell body can also help:

• Sensory neurone: connects a receptor to the central nervous system. Its cell body is often shown along one side of its long fibre.
• Relay neurone: lies entirely within the central nervous system and connects other neurones. It is often shown as a relatively short neurone.
• Motor neurone: runs from the central nervous system to an effector. Its cell body is usually shown at one end, within the central nervous system, with a long fibre extending towards the effector.

Diagrams can vary, so use the neurone’s connections as well as its shape to identify it.

Reflex arcs and reflex actions

A reflex action is a rapid, automatic response to a stimulus, such as withdrawing a hand from a hot surface. It happens without a conscious decision, allowing the body to respond quickly.

A reflex connects a detected stimulus to an action by an effector. An effector may be a muscle, which contracts, or a gland, which releases a substance.

The simple reflex arc

A reflex arc is the pathway followed during a reflex action. Signals called nerve impulses pass along neurones in the following sequence:

1. A receptor detects a stimulus: a change in the internal or external environment.
2. A sensory neurone carries an impulse from the receptor towards the central nervous system (CNS), which consists of the brain and spinal cord.
3. A relay neurone passes the impulse within the CNS.
4. A motor neurone carries the impulse away from the CNS to an effector.
5. The effector produces the response.

For example, when a hand touches a hot surface, a receptor detects the stimulus and impulses pass through the reflex arc. The motor neurone carries an impulse to a muscle, which contracts and pulls the hand away. The reflex arc therefore coordinates the stimulus with a quick response.

Sensory, relay and motor neurones

Neurones are specialised cells that transmit electrical impulses through the nervous system. Different neurones carry impulses along different parts of a pathway: from a sensory receptor to the central nervous system, and then from the central nervous system to a part of the body that responds.

The central nervous system (CNS) consists of the brain and spinal cord. A sensory receptor detects a change, while an effector, such as a muscle or gland, produces a response. A muscle responds by contracting; a gland can respond by releasing a substance.

Neurone pathways and structure

The parts of a neurone are arranged to carry impulses in a particular direction. A dendron carries an impulse towards the cell body; an axon carries an impulse away from the cell body, towards another neurone or an effector.

• A sensory neurone carries impulses from a sensory receptor to the CNS. It typically has a long dendron leading towards a cell body positioned to one side, followed by a shorter axon leading into the CNS.
• A relay neurone is found within the CNS. It passes impulses between other neurones, linking incoming information to outgoing signals. Relay neurones typically have short fibres compared with sensory and motor neurones.
• A motor neurone carries impulses from the CNS to an effector. Its cell body is at one end, and it typically has a long axon leading to the effector.

In a pathway containing all three types, an impulse travels from a receptor along a sensory neurone to the CNS. A relay neurone passes the signal to a motor neurone, which carries it to an effector. The effector then produces a response.

Many neurones have a myelin sheath around a fibre. This sheath insulates the fibre and helps the impulse travel more quickly.

Synaptic transmission

A synapse is a junction between two neurones. The neurones do not touch: a small gap called the synaptic cleft separates the end of the first neurone from the next. An electrical impulse cannot cross this gap directly, so a chemical carries the signal across it.

How a signal crosses a synapse

1. An electrical impulse reaches the end of the first neurone and causes a neurotransmitter to be released.
2. The neurotransmitter diffuses across the synaptic cleft.
3. The neurotransmitter binds to receptors on the next neurone. This can trigger a new electrical impulse in that neurone.

Transmission across a synapse is one-way: the first neurone releases the neurotransmitter, and the next neurone has the receptors that receive it. Synapses therefore allow signals to pass between successive neurones in a reflex pathway.