Homeostasis

1) The conditions inside your body need to be kept steady, even when the external environment changes

2) This is really important because your cells need the right conditions in order to function properly, including the right conditions for enzyme action

3) Homeostasis is all about the regulation of the conditions inside your body (and cells) to maintain a stable internal environment, in response to changes in both internal and external conditions

4) You have loads of automatic control systems in you body that regulate your internal environment - these includ both nervous and hormonal communication systems. For example, there are control systems that maintain your body temperature, your blood glucose level, and your water content

5) All your automatic control systems are made up of three main components which work together to maintain a steady condition - cells called receptors, coordination centres (including the brain, spinal chord and pancreas) and effectors

If one factor goes too high or low, what does the brain do to get it back to a normal level?

Negative feedback

What hormone does the pancreas release if glucose in the blood gets too high?

Insulin

Where will sugar be stored?

Liver (and muscles)

What then happens to the blood sugar level?

Returns to normal

Temperature:

Body temperature increases

Becomes too high

Homeostasis via negative feedback brings it back to normal (sweating)

Temperature:

Body temperature decreases

Becomes too low

Homeostasis via negative feedback brings it back to normal (shivering)

Blood glucose:

Blood glucose increases

Becomes too high

Homeostasis via negative feedback brings it back to normal (insulin brings it down)

Blood Glucose:

Blood glucose decreases

Becomes too low

Homeostasis via negative feedback brings it back to normal (glucagon)

Negative Feedback Counteracts Changes

Your automatic contol systems keep your internal environment stable using a mechanism called negative feedback. When the level of something (e.g. water or temperature) gets too high or too low, your body uses negative feedback to bring it back to normal

Receptor detects a stimulus - level is too high/low

The coordination centre receives and processes the information, then organises a response

Effector produces a response, which counteracts the change and restores optimum level - the level decreases/increases

The effectors will just carry on producing the responses for as long as they're stimulated by the coordination centre. This might cauase the opposite problem - making the level change too much (away from the ideal). Luckily the receptor detects if the level becomes too different and negative feeback starts again

The process happens without you thinking about it - it's all automatic

The Nervous System Detects and Reacts to Stimuli

1) Organisms need to respond to stimuli (changes in the environment) in order to survive.

2) A single-celled organism can just respond to its environment, but the cells of multicellular organisms need to communicate with each other first.

3) So as mulitcellular organisms evolved, they developed nervous and hormonal communication systems.

1) Central Nervous System (CNS). In vertebrates, this consists of the brain and spinal cord only. In mammals, the CNS is connected to the body by sensory neurons and motor neurones.

2) Sensory Neurones. The neurones that carry information as electrical impulses from the receptors to the CNS.

3) Motor Neurones. The neurones that carry electrical impulses from the CNS to effectors.

4) Effectors. All your muslces and glads, which respond to nervous impulses.

Receptors and Effectors can form part of Complex Organs

1) Receptors are the cells that detect stimuli.

2) There are many different types of receptors, such as tase receptors on the tongue and sound receptors in the ears.

3) Receptors can form part of larger, complex organs, e.g. the retina of the eye is covered in light receptor cells.

4) Effectors respond to nervous impulses and bring about a change.

5) Muscles and glands are known as effectors - they respond in different ways. Muslces contract in reponse to a nervous impulse, whereas glands secrete hormones.

The CNS Coordinates the Response

The CNS is a coordination centre - it receives information from the receptors and then coordinates a response. The response is carreid out by effectors.

For example, a small bird is eating some seed...

1)...when it sees a cat (stimulus)

2) The receptors in the bird's eyes are stimulated.

3) Sensory neurones carry the information from the receptors to the CNS.

4) The CNNS decides what to do about it.

5) The CNS sends information to the muscles in the bird's wings (the effectors) along motor neurones.

6) The muscles contract and the bird flies away safely.

Synapses Connect Neurones

1) The connection between two neurones is called a synapse.

2) The nerve signal is transferred by chemicals which diffuse across the gap.

3) These chemicals then set off a new electrical signal in the next neurone.

Reflexes Help Prevent Injury

1) Reflexes are rapid, automatic responses to certain stimuli that don't involve the conscious part of teh brain - they can reduce the chances of being injured.

• For example, if someone shines a bright light in your eyes, your pupiles automatically get smaller so that less light gets into the eye - this stops it getting damaged.
• Or if you get a shock, your body releases the hormone adrenaline automatically - it doesn't want for you to decide that you're shocked.

2) The passage of information in a reflexe (from receptor to effector) is called a reflex arc.

1) The neurones in reflex arcs go through the spinal cord or through an unconscious part of the brain.

2) When a stimulus (e.g. a painful bee sting) is detected by receptors, impulses are sent along a sensory neurone to the CNS.

3) When the impulses reach a synapse between the sensory neurone and a relay neuone, they trigger chemicals to be released. These chemicals cause impulses to be sent along the relay neurone.

4) When the impulses reach a synapse between the relay neurone and a motor neurone, the same thing happens. Chemicals are released and cause impulses to be sent along the motor neurone.

5) The impulses then travel along the motor neurone to the effector (in this example it's a muscle).

6) The muscle then contract and move your hand away from the bee.

Reaction time is the time it takes to respond to a stimulus - affected by factors such as age, gender or drugs (caffeine). 

1) The person being tested should sit with their arms resting on the edge of a table (this should stop them moving their arm up or down during the test).

2) Hold a ruler vertically between their thumb and forefinger. Make sure that the zero end of the ruler is level with their thumb and finger. Then let go without giving any warning. The person being tested should try to catch the ruler as quickly as they can - as soon as they see it fall.

3) Reaction time is measured by the number on the ruler where it's caught - the futher down it's caught (i.e. the higher the number), the slower their reaction time. Repeat the test several times then calculate the mean distance that the ruler fell.

4) The person being tested should then have a caffeinated drink. After 10 minutes, repeat steps 1 to 3.

5) You need to control any variable to make sure that this is a fair test: For example, you should use the same person to catch the ruler each time, and that person should always use the same hand to catch the ruler. Also, the ruler should always be dropped from the same height, and you should make sure that the person being tested has not had any caffeine (or anything else that may affect their reaction time) before the start of the experiment.

6) Too much caffeine can cause unpleasant side-effects, so the person being tested should avoid drinking any more caffeine for the rest of the day after the experiment is completed.

Reaction Time Can Be Measured Using a Computer

1) Simple computer tests can also be used to measure reaction time.

For example, the person being tested has to click the mouse (or press a key) as soon as they see a stimulus on the screen, e.g. a box change colour.

2) Computers can give a more precise reaction time because they remove the possibility of human error from the measurement.

3) As the computer can record the reaction time in milliseconds, it can also give a more accurate measurement.

4) Using a computer can also remove the possibility that the person can predict when to to respond - using the ruler test, the catcher may learn to anticipate the drop by reading the tester's body language. 

The Brain is Responsble for Complex Behaviours

1) Along with the spinal cord, the brain is part of the central nervous system.

2) It's made up of billions of interconnected neurones (neurones that are all connected together).

3) The brain is in charge of all of our complex behaviours. It controls and coordinates everything you do.

4) We know that different regions of the brain carry out different functions:

Cerebral Cortex - This is the outer wrinkly bit. It's responsible for things like consciousness, intelligence, memory and language.

Medualla - Controls unconscious activites like breathing and your heartbeat.

Cerebellum - Responsible for muscle coordination. 

Spinal Cord - Main pathway for information connecting the brain and peripheral nervous system (i.e. sends messages from the brain to different parts of the body).

Scientists Use a Range of Methods to Study the Brain

Scientists (scientists that study the brain are called neuroscientists) use a few different methods to study the brain and map out which bits do what:

Studying patients with brain damage - If a small part of the brain has been damaged, the effect this has on the patient can tell you a lot about what the damaged part of the brain does. E.g. if an area at the back of the brain was damaged by a stroke and the patient went blind, you know that that area has something to do with vision.

Electrically stimulating the brain - The brain can be stimulated electrically by pushing a tiny electrode into the tissue and giving it a small zap of electricity. By observing what stimulating different parts of the brain does, it's possible to get an idea of what those parts do. E.g. when a certain part of the brain (known as the motor area) is stimulated, it causes muscle contraction and movement.

MRI Scans - A magnetic imaging (MRI) scanner is a big fancy tube-like machine that can produce a very detailed picture of the brain's sturcutres. Scientists use it to find out what areas of the brain are active when people are doing things like listening to music or trying to recall a memory.

Messing With the Brain Can Have Consequences

1) Knowledge of how the brain works has led to the development of treatments for disorders of the nervous system. For example, electrical stimulation of the brain can help reduce muscle tremors caused by nervous system disorders such as Parkinson's disease.

2) However, the brain is incredibly complex and delicate - the investigation of brain function and any treatment of brain damage is difficult. It also carries risks, such as physical damage to the brain or increased problems with brain function (e.g. difficulties with speech).

 Do not need to learn fovea or conjunctiva

Iris - Controls the size of the pupil

Lens - Focusses light onto the retina

Retina - Area containing light receptors

Pupil - Allows light into eye

Ciliary muscles - Controls shape of lens

Optic nerce - Carries information from eye to the brain

Cornea - Refracts light as it enters the eye

Suspensory ligaments - Helps muscle control shape of the lens

1) The sclera is the tough, supporting wall of the eye.

2) the cornea is the transparent outer layer found at the front of the eye. It refracts (bends) light into the eye.

3) The iris contains muscles that allow it to control the diameter of the pupil (the hole in the middle) and therefore how much light enters the eye.

4) The lens focuses the light onto the retina (which contains receptor cells sensitive to light intensity and colour).

5) The shape of the lens is controlled by the ciliary muscles and suspensory ligaments.

6) The optic nerve carries impulses from the receptors on the retina to the brain.

Very bright light can damage the retina - so you have a reflex to protect it.

1) When light receptors in the eye detect very bright light, a reflex is triggered that makes the pupil smaller. The circular muscles in the iris contract and the radial mucles relax. This reduces the amount of light that can enter the eye.

2) The opposite process happens in dim light. This time, the radial muscles contract and the circular mucles relax, which makes the pupil wider.

Focusing on Near and Distant Objects - Another Reflex

The eye focuses light on the retina by changing the shape of the lens - this is known as accomodation

To Look at Near Objects:

• The ciliary muscles contract, which slackens the suspensory ligaments
• The lens becomes fat (more curved)
• This increases the amount by which it refracts (bends) light

As you get older, your eye's lens loses flexibility so it can't easily spring back to a round shape. This means light can't be focused well for near viewing, so older people often have to use reading glasses

To Look at Distant Objects:

• The ciliary muscles relax, which allows the suspensory ligaments to pull tight
• This makes the lens go thin (less curved)
• So it refracts light by a smaller amount

If the lens cannot refract the light by the right amount (so that it focuses on the retina), the person will be short-or long-sighted

Some People are Long-Sighted

Long-sighted people are unable to focus on near objects:

• This occurs when the lens is the wrong shape and doesn't refract the light enough or the eyeball is too short
• The image of near objects are brought into focus behind the retina
• You can use glasses with a convex lens (a lens which curves outwards) to correct it
• The lens refracts the light rays so they focus on the retina
• The medical term for long-sightedness is hyperopia 

Some People are Short-Sighted

Short-sighted people are unable to focus on distant objects:

• This occurs when the lens is the wrong shape and refracts the light too much or the eyeball is too long
• The image of distant objects are brought into focus in front of the retina
• You can use glasses with a concave lens (a lens which curves inwards) to correct it, so that the light rays focus on the retina
• The medical term for short-sightedness is myopia 

There are Several Treatments for Vision Defects

Contact lenses:

• Contact lenses are thin lenses that sit on the surface of the eye and are shaped to compensate for the fault in focusing
• They're popular because they are lightweight and almost invisible. They're also more conveniant than glasses for activities like sports
• The two main types of contact lenses are hard lenses and soft lenses. Soft lenses are generally more comfortable, but carry a higher risk of eye infections than hard lenses

Replacement lens surgery

• Sometimes long-sightedness may be more effectively treated by replacing the lens of the eye (rather than altering the shape of the cornea with laser eye surgery)
• As it involves work inside the eye, replacing a lens carries higher risks than laser eye surgery, including possible damage to the retina (which could lead to loss of sight)

Laser eye surgery

• Bad eyesight can sometimes be corrected with laser eye surgery
• A laster can be used to vaporise tissue, changing the shape of the cornea (and so changing how strongly it refracts light in the eye)
• Slimming it down makes it less powerful and can improve short sight. Changing the shape so that it's more powerful will improve long sight
• The surgeon can precisely control how much tissue the laser takes off, completely correcting the vision
• However, like all surgical procedures, there is a risk of complications, such as infection or the eye reacting in a way that makes your vision worse than before 

Body Temperature Must be Kept Constant - body has to be around 37 °C - optimum temp for enzymes in the body

• The body has to balance the amount of energy gained (e.g. through respiration) and lost to keep the core body temperature constant
• There is a thermoregulatory centre in the brain, which contains receptors that are sensitive to the temperature of the blood flowing through the brain
• The thermoregulatory centre also receives impulses from temperature receptors in the skin, giivng information about skin temperature

Body Temperature is Controlled via Negative Feedback 

• Temperature receptors detect that core body temperature is too high/low
• The thermoregulatory centre acts as a coordination centre - it receives information from the temperature receptors and triggers the effectors automatically
• Effectors, e.g. sweat glands (when too hot) and muscles (when too cold) produce a response and counteract the change

Antagonistic Effectors Oppose Each Other's Actions: Some effectors work antagonistically, e.g. one effector heats and another cools - they'll work at the same time to achieve a very precise temperature. This mechanism allows a more sensitive response

Different responses are produced by effectors to counteract an increase / decrease in body temp

When You're Too Hot:

• Hairs lie flat
• Sweat is produced by sweat glands and evaporates from the skin. This transfers energy to the environment
• The blood vessles supplying the skin dilate so more blood flows close to the surface of the skin. This is called vasodilation. This helps transfer energy from the skin to the environment

When You're Too Cold:

• Hairs stand up to trap an insulating layer of air
• No sweat is produced
• Blood vessels supplying skin capillaries constrict to close of the skin's blood supply. This is called vasoconstriction
• When you're cold you shiver too (you're muscles contract) automatically. This needs respiration, which transfers some energy to warm the body