Homeostasis and Response

Question 1

Coordination and control - The nervous system

Answer 1

The nervous system enables humans to react to their surroundings and to coordinate their behaviour. It comprises millions of neurones and uses electrical impulses to communicate very quickly.

Question 2

Homeostasis

Answer 2

Homeostasis maintains optimal conditions for enzyme action throughout the body, as well as all cell functions.

In the human body, these include the control of:
○ blood glucose concentration
○ body temperature
○ water levels

These automatic control systems may involve nervous responses (nervous system) or chemical responses (endocrine system).

Question 3

Nervous System

Answer 3

The human nervous system consists of:
○ the central nervous system - the brain and spinal cord
○ the peripheral nervous system - nerve cells that carry information to or from the CNS

Question 4

The structure and function of the nervous system

Answer 4

The conditions inside our body must be carefully controlled if the body is to function effectively. The conditions are controlled in two ways with chemical and nervous responses.

All control systems include:
○ Cells called receptors (organs which recognise and respond to stimuli).
○ The coordination centre, such as the brain, spinal cord or pancreas, which receives and processes information from receptors around the body.
○ Effectors bring about responses, which restore optimum levels, such as core body temperature and blood glucose levels. Effectors include muscles and glands, and so responses can include muscle contractions or hormone release.

Question 5

Nerve Cells

Answer 5

Nerve cells are called neurones. They are adapted to carry electrical impulses from one place to another.
A bundle of neurones is called a nerve.

There are three main types of neurone: sensory, motor and relay.
They have some features in common:
○ A long fibre (axon) which is insulated by a fatty (myelin) sheath. They are long so they can carry messages up and down the body.
○ Tiny branches (dendrons) which branch further as dendrites at each end. These receive incoming impulses from other neurones.

Question 6

Receptors to Effectors

Answer 6

Information from receptors passes along neurones, as electrical impulses to co-ordinators such as the central nervous system or CNS. The CNS is the brain and spinal cord. Muscles contracting or glands secreting hormones are the response of effectors coordinated by the CNS.

Stimulus –> receptor –> coordinator –> effector –> response

Question 7

Receptors

Answer 7

Receptors are groups of specialised cells. They detect a change in the environment (stimulus) and stimulate electrical impulses in response. Sense organs contain groups of receptors that respond to specific stimuli.

○ Skin - Touch, temperature and pain
○ Tongue - Chemicals (in food and drink, for example)
○ Nose - Chemicals (in the air, for example)
○ Eye - Light
○ Ear - Sound and position of head

Question 8

Effectors

Answer 8

Effectors include muscles and glands - that produce a specific response to a detected stimulus.

For example:
○ a muscle contracting to move an arm
○ muscle squeezing saliva from the salivary gland
○ a gland releasing a hormone into the blood

Question 9

Reflex actions

Answer 9

There are different types of neurones that work together in a reflex action.
This creates an automatic and rapid response to a stimulus, which minimises any damage to the body from potentially harmful conditions, such as touching something hot.
A reflex action follows this general sequence and does not involve the conscious part of the brain, which makes it much quicker.

Stimulus
↓
Receptor
↓
Sensory neurone
↓
Relay neurone
↓
Motor neurone
↓
Effector
↓
Response

Question 10

Reflex Arc

Answer 10

The nerve pathway followed by a reflex action is called a reflex arc. For example, a simple reflex arc happens if we accidentally touch something hot.

1. Receptor in the skin detects a stimulus (the change in temperature).
2. Sensory neurone sends electrical impulses to relay neurone, which are located in the spinal cord. They connect sensory neurones to motor neurones.
3. Motor neurone sends electrical impulses to an effector.
4. Effector produces a response (muscle contracts to move hand away).

Question 11

Synapse

Answer 11

Where two neurones meet there is a small gap, a synapse.

1. An electrical impulse travels along the first axon.
2. This triggers the nerve-ending of a neurone to release chemical messengers called neurotransmitters.
3. These chemicals diffuse across the synapse (gap) and bind with receptor molecules on the membrane of the second neurone.
4. The receptor molecules on the second neurone bind only to the specific neurotransmitters released from the first neurone. This stimulates the second neurone to transmit the electrical impulse.

Question 12

Required Practical Activity 7

Answer 12

Reaction time

Question 13

Plan and carry out an investigation into the effect of a factor on human reaction times.

Answer 13

You can carry out a number of investigations to determine the effect of a specific factor on human reaction times.
A suitable investigation could be the effect of caffeine or the amount of background noise in the room. A simple method to measure the effect is to use the rule drop test.

Question 14

Ruler Drop Test

Answer 14

1. Work with a partner
2. Person A holds out their hand with a gap between their thumb and first finger
3. Person B holds the ruler with the zero at the top of person A’s thumb
4. Person B drops the ruler without telling Person A and they must catch it
5. The number level with the top of person A’s thumb is recorded in a suitable table. Repeat this ten times.
6. Swap places, and record another ten attempts
7. You can use the conversion table to help convert your ruler measurements into reaction time or just record the catch distance in cm.

Question 15

The Brain

Answer 15

The brain control complex behaviour. It is made of billions of interconnected neurones and has different regions that carry out different functions.

Question 16

Main Areas of the Brain

Answer 16

There are four main areas in the brain:
○ The cerebrum (the outer layer is called the cerebral cortex), which is split into two hemispheres and is highly folded. It controls intelligence, personality, conscious thought and high-level functions, such as language and verbal memory.
○ The cerebellum, which controls balance, co-ordination of movement and muscular activity.
○ The medulla, which controls unconscious activities such as heart rate and breathing rate.
○ The hypothalamus, which is the regulation centre for temperature and water balance within the body.

Question 17

Investigating the brain - Higher

Answer 17

Modern science has allowed scientists to discover how different parts of the brain function. Neuroscientists have been able to map various regions of the brain to particular functions by studying patients with brain damage, electrically stimulating different parts of the brain and using MRI scanning techniques.

Question 18

Brain Damage

Answer 18

A well-documented example of brain damage is of Phineas Gage, who in 1848 had a serious accident whilst laying railway tracks and an iron rod went through his skull.

Phineas survived the accident, but it was documented that his personality changed following it. It was noted that he lost his inhibitions socially and emotionally.

Doctors realised the changes in Phineas were due to the damage in the particular parts of the brain that the iron rod had passed through. This important case allowed scientists to examine the effect of the injuries on his brain activity.

Question 19

Electrical Stimulation

Answer 19

Scientists have stimulated different parts of the brain with a weak electrical current and asked patients to describe what they experienced. If the motor area is stimulated, the patient makes an involuntary movement. If the visual area is stimulated, they may see a flash of colour. EEGs (Electroencephalograms) can be created and studied, to observe the electrical activity in the brain.

Question 20

MRI Brain Scans

Answer 20

Modern imaging methods such as MRI (Magnetic Resonance Imaging) scans, use strong magnetic fields and ratio waves to show details of brain structure and function. Patients are asked to perform various tasks and, by looking at the scan, scientists can see which parts of the brain are active when the task is carried out.

Question 21

Risks of MRI Scans

Answer 21

Brain surgery may be needed to remove a tumour or excess fluid, such as blood. All surgery carries a level of risk, but due to the complexity and delicacy of the brain, investigating and treating brain disorders can be very difficult. If surgery is undergone more damage or side-effects may be created, which could affect the patients’ quality of life. Serious considerations about the risks involved against the benefits need to be undertaken first.

Question 22

The Eye

Answer 22

The eye is a sense organ containing receptors sensitive to light intensity and colour.

Question 23

Structure and Function of the Eye

Answer 23

○ Cornea - Refracts light - bends it as it enters the eye
○ Iris - Controls how much light enters the pupil
○ Lens - Further refracts light to focus it onto the retina
○ Retina - Contains the light receptors
○ Optic Nerve - Carries impulses between the eye and the brain
○ Sclera - Tough white outer layer of the eye. It helps protect the eye from injury.

Question 24

The Retina

Answer 24

Light passes through the eyeball to the retina. There are two main types of light receptors - rods and cones. Rods are more sensitive to light than cones so they are useful for seeing in dim light. There are three different types of cone cells which produce colour vision.

Question 25

The Pupil Reflex

Answer 25

The amount of light entering the eye is controlled by a reflex action. The size of the pupil changes in response to bright or dim light. This is controlled by the muscles of the iris.

○ Dim light
- Radial muscles of iris contracted
- Circular muscles of iris relaxed
- Dilated pupil (more light enters eye)

○ Bright light
- Radial muscles of iris relaxed
- Circular muscles of iris contracted
- Contracted pupil (less light enters eye)

Question 26

How the eye works - Higher

Answer 26

Accommodation is the process of changing the shape of the lens to focus on near or distant objects.
To focus on a near object - the lens becomes thicker, this allows the light rays to refract (bend) more strongly.
To focus on a distant object - the lens is pulled thin, this allows the light rays to refract slightly.

Position - ciliary muscle - suspensory ligament - muscle tension - lens shape - refraction

○ Near - contract - slacken/loosen - low - fat/thicker - light is refracted strongly

○ Distant - relax - stretched/tighten - high - thin - light is only refracted slightly

Question 27

Common defects of the eye and how they can be overcome

Answer 27

This includes vision defects

Question 28

Correcting Vision Defects

Answer 28

Two common defects of the eyes are myopia (short-sightedness) and hyperopia (long-sightedness). In both cases rays of light do not focus on the retina so a clear image is not formed.
These two defects are treated with spectacle lenses, which refract (bend) the light rays so that they do focus on the retina.

Question 29

Short Sight (Myopia)

Answer 29

Someone with short-sight can see near objects clearly, but cannot focus properly on distant objects.

Short sight is caused by one of the following:
○ The eyeball being elongated - so that the distance between the lens and the retina is too great.
○ The lens being too thick and curved - so that light is focused in front of the retina.

Short-sightedness can be corrected by placing a concave lens in front of the eye.

Question 30

Long Sight (Hyperopia)

Answer 30

Someone who is long-sighted can see distant objects clearly, but they cannot focus properly on near objects.

Long-sightedness is caused by one of the following:
○ The eyeball being too short - so the distance between the lens and retina is too small.
○ A loss of elasticity in the lens - meaning it cannot become thick enough to focus (which is often age-related).

As a result, the lens focuses light behind the retina instead of onto it. Long-sightedness is corrected by putting a convex lens in front of the eye.

Question 31

Technology

Answer 31

New technologies have provided alternatives to wearing spectacle lenses: the hard and soft contact lenses, laser surgery to change the shape or the cornea and a replacement lens in the eye. Contact lenses - work by being in ‘contact’ with your eye. They float on the surface of the cornea. They work like spectacle lenses, by focusing and refracting the light.

○ Laser surgery - reshapes the cornea surgically. Common for myopia but can be used for some hyperopia conditions.
○ Replacement lens - implanting artificial lenses is a recent development, and can be placed in front of the original lens, through a small cut in the cornea, to correct an eye defect.

Question 32

Coordination and control - The human endocrine system

Answer 32

The endocrine system secretes hormones into the bloodstream from glands throughout the body. Hormones produce an effect on specific target organs in the body.

Question 33

Hormones and Nerves

Answer 33

A hormone is a chemical substance, produced by a gland and carried in the bloodstream, which alters the activity of specific target organs. An example of this is the release of the hormone adrenaline, which is released by the adrenal gland. One of its target organs is the heart, where it increases the heart rate.

Once a hormone has been used, it is destroyed by the liver.

Hormones can control the body, and the effects are much slower than the nervous system, but they last for longer.

Question 34

There are important differences between nervous and hormonal control

Answer 34

○ Nervous
- Type of signal - electrical (chemical at synapses)
- Transmission of signal - by nerve cells (neurones)
- Effectors - muscles or glands
- Type of response - muscle contraction or secretion
- Speed of response - very rapid
- Duration of response - short (until nerve impulses stop)

○ Hormonal
- Type of signal - chemical
- Transmission of signal - by the bloodstream
- Effectors - target cells in particular tissues
- Type of response - chemical change
- Speed of response - slower
- Duration of response - long (until hormone is broken down)

Question 35

Master Gland

Answer 35

The pituitary gland in the brain is known as a ‘master gland’. It secretes several hormones into the blood in response to the body’s condition, such as blood water levels. These hormones can also act on other glands to stimulate the release of different types of hormones and bring about effects.

Question 36

Different hormones

Answer 36

The body produces a range of different chemical hormones that travel in the bloodstream and affect a number of different organs or cells in the body.

○ Pituitary gland - the ‘master gland’, situated at the base of the brain
○ Thyroid gland - produces thyroxine
○ Adrenal glands - produces adrenaline
○ Pancreas - produces insulin
○ Testes - produce testosterone
○ Ovaries - Produce oestrogen

Question 37

Other hormones

Answer 37

Important hormones released into the bloodstream include ADH (anti-diuretic hormone), adrenaline and insulin.

○ ADH
- Source - Pituitary gland
- Organ(s) - Kidneys
- Role - Controlling the water content of the blood
- Effects - Increases reabsorption of water by the collecting ducts

○ Adrenaline
- Source - Adrenal glands
- Organ(s) - Several targets include the respiratory and circulatory systems
- Role - Preparation for ‘fight or flight’
- Effects - Increases breathing rate, heart rate, flow of blood to muscles, conversion of glycogen to glucose

○ Insulin
- Source - Pancreas
- Organ(s) - Liver
- Role - Controlling blood glucose levels
- Effects - Increases conversion of glucose into glycogen for storage

Question 38

Negative feedback systems in hormonal control - Higher

Answer 38

This includes homeostatic control.

Question 39

Homeostatic Control

Answer 39

In animals, conditions such as water concentration, temperature, and glucose concentration must be kept as constant as possible. Control systems that keep such conditions constant are examples of homeostasis; this is the maintenance of constant internal conditions in an organism.
A negative feedback mechanism is an important type of control that is found in homeostasis. A negative feedback control system responds when conditions change from the ideal or set point and returns conditions to this set point. There is a continuous cycle of events in negative feedback.

Question 40

General Stage sin negative feedback

Answer 40

In general this works by:
○ if the level of something rises, control systems reduce it again
○ if the level of something falls, control systems raise it again

↑→Conditions in the body change from set point
↑ ↓
↑ Change detected
↑ ↓
↑ Corrective mechanisms activated
↑ ↓
↑Conditions returned to set point
↑ ↓
↑Corrective mechanisms switched off
↑←

Question 41

An example of negative feedback

Answer 41

An example of negative feedback is the control of body temperature. Body temperature is controlled by the hypothalamus in your brain, and if your body gets too hot, your body begins to sweat to try and reduce it. Conversely if the body gets too cold, it begins to shiver to try and raise the temperature.

Question 42

Thyroxine and Adrenaline - Higher

Answer 42

Thyroxine is produced from the thyroid gland, which stimulates the basal metabolic rate. It controls the speed at which oxygen and food products react to release energy for the body to use. Thyroxine plays an important role in growth and development. Thyroxine levels are controlled by negative feedback.