Homeostasis and Response

Homeostasis - regulating the conditions inside of the body to maintain a stable internal environment, in response to changes in internal and external conditions

A negative feedback loop is used in order to maintain a stable internal evnironment

Effectors continue to produce a response for as long as they are stimulated by the coordination centre. This may cause the opposite problem, in which case the receptor will detect the change and negative feedback will start again.

The nervous system is made up of the central nervous system, sensory neurones, motor neurones, and effectors.

Central Nervous System (CNS) - this consists of the brain and the spine. In mammals, the CNS is connected to the body by sensory and motor neurones

Sensory Neurones - carry information as electrical impulses from the receptors to the CNS

Motor Neurones - neurones that carry electrical impulses from the CNS to effectors

Effectors - all your muscles and glands, which respond to nervous impulses

Receptors are cells that detect stimuli.

1. A bird sees a cat walking towards it

2. Receptors in the bird's eye are stimulated

3. Sensory neurones carry the information from the receptor to the CNS

4. The CNS decides what to do

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

6. The muscles contract, and the bird flies away

Stimulus -> Receptor -> Sensory Neurone -> CNS -> Motor Neurone -> Effector -> Response

The connection between two neurones is called a synapse.

When an electrical impulse reaches the end of a neurone, it triggers the release of chemical neurotransmitters. These diffuse across the gap, and trigger a new electrical impulse in the next neurone.

Reflexes are rapid, automatic responses to stimuli that don't involve the conscious part of the brain. They can greatly reduce the chances of being injured.

For example, pupils automatically adjust to the light level, and saliva is automatically released when food enter the mouth.

The Reflex Arc:

1. A stimulus is detected by receptors, causing impulses to be sent along a sensory neurone to the CNS

2. Impulses go along the sensory neurone to the relay neurone, then from the relay neurone to a motor neurone

3. The impulse travels along the motor neurone to the effector, which carries out a response

As you don't have to think about the response, reflexes are quicker that normal responses.

The brain is part of the central nervous system, and is made up of billions of interconnected neurones. Different functions are carried out by different regions of the brain.

Cerebral Cortex - responsible for things such as consciousess, intelligence, memory and language

Cerebellum - responsible for muscle coordination

Medulla - controls unconscious activities such as breathing and heartbeat

Hypothalamus - regulates the body's temperature and water balance

Studying patients with brain damage: If a small part of the brain has been damaged, this can give a lot of information about the role of that part of the brain. Scientists can study patients with brain damage to find out more about the functions of different parts of the brain.

Electrically stimulating the brain: The brain can be electrically stimulated, by putting an electrode into the tissue, and electrically charging it. By observing what the stimulations do, you can work out what that part of the brain does.

MRI scans: Magnetic resonance imaging (MRI) scanners can produce very detailed images of the brain's structures. Scientists can use it to find what parts of the brain are active when patients do activities such as listening to music or trying to recall a memory.

However, investigating brain function and carrying out brain surgery can be very risky - the brain is incredibly complex and delicate, with lots of very intertwined, interconnecting parts. This means it is very easy to accidentally damage part of the brain while operating, or damage one part while trying to do something to another, which can have disastrous consequences.

The eye is a sense organ.

Parts of the eye:

• Sclera - tough, supporting wall of the eye
• Cornea - transparent outer layer found at the front of the eye. Refracts light into the eye
• Iris - contains muscles, allowing it to control the diameter of the pupil, and how much light enters the eye
• Lens - focuses light onto the retina (which contains receptor cells, sensitive to light intensity and colour)
• Ciliary muscles and suspensory ligaments - control the shape of the lens
• Optic nerve - carries impulses from the receptors on the retina to the brain

Very bright light can be damaging to the retina, but dim light can make it hard to see. Because of this, we have a reflex action controlling the amount of light that can enter the eye.

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

In dim light, radial muscles contract and the circular muscles relax, making the pupil wider.

Accomodation is when the lens changes shape in order to focus light on the retina, depending on the distance from the eye to an object.

Looking at near objects:

• The ciliary muscels contract, and the suspensory ligaments relax.
• The lens becomes fatter, and more curved
• This increases the amount by which light refracts.