The Nervous System
- The overall function of the nervous system is to control and coordinate our bodies and behaviour and to detect stimuli in the environment (sensitivity).
- The main organ in the nervous system is the brain.
The nervous system has two parts:
- Central Nervous System (CNS)
- Peripheral Nervous System (PNS)
- Neurones, or nerve cells, make up the majority of the nervous system.
→ These carry electrical impulses very fast.
- Neurones are specialised its 3 adaptations are:
1. Long
2. Have an insulating layer - Myelin sheath
3. Have branched endings
- There are three main types of neurones.
Types of Neurones
Motor Neurone:
- The effector connects or comes into contact with the nerve endings of a muscle or gland through the CNS (central nervous system).
Sensory Neurone:
- Receptors are cells that detect any changes in the environment.
- They're found in all sense organs.
| Sense organ | Stimulus detected | |
|---|---|---|
| Red cone cells in retina of eye | → | Light |
| Taste buds on tongue | → | Chemical |
| Olfactory cells in nose | → | Chemical |
| Receptors in skin | → | Movement and pressure |
| Hair cells in cochlea | → | Vibrations |
- Sensory neurones send the information from receptors to the central nervous system (CNS), which then coordinates the body's response. A motor neurone then carries the response from the CNS to an effector.
Relay Neurone:
- Relay neurones are an important part of reflexes.
- These are short neurones exclusively present in the central nervous system.
- They connect sensory neurones to motor neurones.
Myelin Sheath:
- Made of myelin, insulating, fatty material.
- It's not continuous; the impulse jumps between the gaps.
- This increases the transmission speed of electrical impulses.
- If the myelin sheath breaks down, it causes multiple sclerosis (MS); symptoms include problems with vision, balance, and muscle weakness.
Catch the Ruler
Experiment
Method:
1. One person holds a 30 cm ruler at the 30 cm mark and lets it hang vertically.
2. The other person places their thumb and index finger on either side of the 0 cm mark, ready to catch the ruler when it falls. Their fingers shouldn't touch the ruler and should be a set distance apart, e.g., 2 cm.
3. Without warning, the person holding the ruler lets go, and the subject tries to catch him as quickly as possible.
4. Record the level in millimetres (mm) just above the subject's index finger or the thumb where the ruler catches.
5. Test the same person three to five times and calculate the mean of the results to judge the repeatability.
6. Swap and test other people, then record the results in a table.
- Independent variable: Person catching ruler.
- We captured the dependent variable distance ruler at a distance of (mm).
- Control variables:
→ Use the same arm each time (left/right).
→ Thumb/Finger should be the same distance apart each time.
→ Each time, measure the level the ruler has travelled (cm) from the top of the index finger.
→ The tester holds exactly at the 30 cm mark each time.
→ The subject consistently positions their thumb and finger at a distance of 0 cm.
Repeats:
- Repeat 3-5x for each person and calculate a mean so we can judge repeatability.
- Look for anomalous results and exclude them from the mean.
- Look at how consistent the repeat readings are; the closer, the more repeatable.
Scientific Explanation:
The mean distance indicates the speed at which a person caught the ruler. During this period, the person must:
- Detect the stimulus (light receptors in the eye).
- Send an impulse along a sensory neurone to the brain.
- Process the information in the brain.
- Send out an impulse along (a) motor neurone(s) to muscles in the arm.
- Catch the ruler by contracting muscles in the arm.
- Limitation: During preparation for the ruler drop, both the tester and the subject will move slightly. (Drop from a clamp stand instead.)
- Errors: Reading the wrong distance on a ruler.
Reaction times differ because of:
→ age
→ using dominant/non-dominant hand
→ tired/alert/jogged by mood, caffeine, and alcohol
Reaction time in seconds can be calculated using:
d: distance in metres
a: acceleration as a result of gravity: 9.81 m/s²
Reflex Actions
- The reaction time experiment is an example of a voluntary response.
- The brain exerts conscious control over the reaction of catching the ruler.
- Reflexes are reactions that aren't under conscious control and happen without us thinking about them first.
- A reflex action is a fast, automatic, and protective response to a stimulus not consciously controlled by the brain.
- Examples include the knee-jerk reflex, the pupil reflex, and the removal of your hand from a hot plate before you experience any pain.
Example: Remove your hand from the hot plate.
1. Hot plate (stimulus)
2. Receptor
3. Sensory neurone
4. Relay neurone
5. Motor neurone
6. Effector: A hand moved away.
7. Response: A hand moved away.
8. Neurone sends message to brain
- Reflexes reduce the time between detecting a stimulus and responding.
- Because the brain does not process the information from the stimulus before activating the effector, it responds more quickly.
- Your brain still receives the information, but it does so with a slight delay.
- We also have cranial reflex arcs. The relay neurone in the brain, not the spinal cord, controls these reflex arcs, which include the pupil reflex.
Synapses
- Neurones must pass impulses to each other to send messages. But they don't actually touch.
- Synapse: The place where neurones meet. There is a tiny gap between the end of the first neurone and the start of the second neurone.
- An electrical impulse at a synapse releases a chemical into the gap. This chemical diffuses to the next neurone, causing an electrical impulse.
→ Electrical → Chemical → Electrical
- In the nerve ending of the 1st neurone there are hundreds of tiny vesicles that contain a chemical called transmitter substance/neurotransmitter.
- When an impulse reaches the nerve ending, vesicles fuse with the cell membrane and empty the neurotransmitter into the gap.
- The neurotransmitter diffuses across the gap and acts on a receptor on the dendrite of the next neurone.
- This triggers an electrical impulse in the next neurone, e.g., acetylcholine (ACh).
* A neurotransmitter is a chemical that diffuses between gaps and stores in vesicles.
