Homeostasis and The Nervous System

Homeostasis keeps conditions within an organism constant, to keep cells ticking over. It’s partly controlled by the nervous system, which sends messages between the brain and the rest of the body.

Homeostasis

It’s important that the conditions inside our body remain fairly stable. If temperature or blood pH was constantly changing, it would wreak havoc to the enzymes inside. Enzymes would denature and would be unable to catalyse metabolic reactions. Homeostasis is the maintenance of a constant internal environment so that cells can function effectively.

In the human body, the internal conditions which are kept constant are:

• Blood glucose concentration

• Body temperature

• Water levels

Homeostasis is carried out by our nervous system and chemical responses:

• Cells called receptors detect stimuli (changes in the environment).

• Coordination centres (such as the brain, the spinal cord and the pancreas) receive and process information from receptors.

• Effectors (such as muscles or glands) bring about responses which restore the body to optimum levels.

For example, if you go outside on a cold day, your internal body temperature might drop. Temperature receptors inside the body will detect the fall in temperature and send this information to the brain. The brain processes the information and sends impulses to various effector organs. Effectors carry out responses which will increase body temperature — things like muscles shivering, blood vessels constricting and the hairs on our skin standing on end. These responses will help to warm us up and restores our body temperature to its optimum level.

Structure and function of the nervous system

We’re responding to our environment all the time – running for the bus, waving goodbye or dropping a burning hot object all involve stimulation by our nervous system. The nervous system allows us to react to our surroundings and coordinate our behaviour appropriately.

The nervous system detects changes in our environment (stimuli) through cells called receptors. Receptors are sensitive to a number of different aspects of our environment, such as light, pressure (touch) and chemicals in the air (smell).

When receptors detect certain stimuli, they signal to the coordination centre through initiating an electrical impulse through a neuron (nerve cell). The neuron which sends an electrical impulse from the receptor within the sense organ to the coordination centre is called the sensory neuron.

The coordination centre receives impulses from various receptors around the body, processes the information and coordinates a response by signalling to other parts of the body. Coordination centres include the brain, spinal cord and pancreas.

These organs will signal to an effector (a muscle or gland) by releasing an electrical impulse along a motor neuron. Stimulation of an effector will produce a response such as muscle contraction or hormonal release.

This process occurs when we need to respond immediately to harmful stimuli in our environment, such as accidentally touching a hot object. These unconscious responses are called reflex actions and protect our body from harm through a coordinated response which bypasses the brain. The information is sent directly to the spinal cord, where the electrical impulse is passed from the sensory neuron to motor neuron via a relay neuron. Reflex actions are extremely quick and do not require any conscious input from the brain.

The brain (triple science only)

The brain controls complex behaviour, such as speaking and reasoning. It is made of billions of interconnected neurones and has different regions which carry out different functions.

The brain can be divided into specific regions which have specific functions. Most of our brain is made up of the cerebrum, which is found at the top of the brain. It is divided into two cerebral hemispheres joined together by a band of nerve fibres called the corpus callosum. The thin, outer layer of the cerebrum is called the cerebral cortex. It is highly folded, giving it a really large surface area. The cerebrum is involved in ‘higher-brain functions’, such as processing language, vision, thinking and emotions.

The cerebellum is a leaf-shaped structure found towards the back of the brain. It is positioned underneath the cerebrum and is highly folded. It plays an important role in movement and balance. Things like learning to ride a bike or the movement involved in writing will involve a large input from the cerebellum.

Right at the base of the brain and above the spinal cord is a structure called the medulla oblongata. This is involved in unconscious processes, such as the regulation of breathing rate and heart rate.

Neuroscientists have been able to research the functions of brain areas in various ways:

• Studying patients with brain damage – for example, people who have lost a particular function, such as the ability to speak, can undergo a brain scan. Any areas of brain damage on the scan are likely to be involved in the formation of language.

• Electrical stimulation – different brain regions can be stimulated with a weak electrical current and observe the effects. For example, if stimulation to a certain brain region results in an involuntary movement of their arm muscle, we can conclude that this brain region is involved in movement.

• MRI scanning – MRI scanners use magnetic fields and radio waves to produce highly detailed images of the brain. Patients are asked to perform a task within the MRI scanner and the scientists can look at the MRI scan to see which brain regions are activated. For example, if particular regions of the cerebrum light up when the person inside the scanner solves maths problems, we can determine that the cerebrum is involved in mathematical reasoning and calculations.

Brain damage and brain disease can be difficult to treat. One of the few treatment options is brain surgery, which carries a lot of risk. However, if the brain disorder is left untreated, it could get worse and reduce the quality of life of the patient. Medical staff will therefore weigh up the pros and cons of brain surgery before deciding whether to perform the procedure.

The eye (triple science only)

The eye is a sense organ containing receptors which detect colour and light intensity. Light is detected by the retina at the back of our eye. The retina contains receptors to detect light intensity and colour (rods and cones) and converts the light into an electrical impulse. The optic nerve sends the electrical impulse to the brain.

The muscles of the iris control how much light enters the eye, depending on whether we are in a bright or dim environment. This is an example of a reflex action - a rapid, involuntary response to our environment.

Other important structures of the eye include:

• Cornea – refracts light as it enters our eye

• Lens – further refracts light and allows us to focus on objects

• Sclera – a tough, protective layer covering the eye, protecting it from injury

 In bright light, the circular muscles of the iris contract while the radial muscles relax, making our pupils smaller and allowing less light into our eye. In dim light, the circular muscles relax (while radial muscles contact), making our pupils wider and allowing more light into our eye.

 The lens is a structure behind our pupil which refracts (bends) light to focus it on the retina.

• When we focus on an object which is near to us, the lens becomes thicker to refract light more strongly.

• When we focus on an object further away in our visual field, the lens becomes thinner to refract light less strongly.

The shape of the lens is controlled by a circular ring of muscle, called ciliary muscle, which are connected to suspensory ligaments.

• When the ciliary muscles contract, the suspensory ligaments slacken which increases the thickness of the lens, helping to focus on object close in our visual field.

• When the ciliary muscles relax, the suspensory ligaments tighten, making the lens thinner to focus on objects far-away.

 Two common defects of the eyes are short sightedness (myopia) and long sightedness (hyperopia). In these conditions, the lens does not focus the rays of light on the retina. This is treated with glasses (spectacle lenses) which refract the light rays so that they do focus on the retina. New technologies now include hard and soft contact lenses, laser surgery to change the shape of the cornea and a replacement lens in the eye.

Control of body temperature (triple science only)

Body temperature is monitored and controlled by the thermoregulatory centre in the brain. The thermoregulatory centre contains receptors called thermoreceptors which are sensitive to changes in blood temperature. The skin also contains thermoreceptors and can send nerve impulses to the thermoregulatory centre in the brain when the temperature of the skin changes.

If our body temperature gets too low, the thermoregulatory centre sends impulses to effector organs which results in vasoconstriction, reduced sweating and shivering. Vasoconstriction is the narrowing of blood vessels and results in less heat loss through radiation. Shivering occurs when skeletal muscles contract, which means they have to carry out more aerobic respiration which produces heat as a by-product. These responses reduce the transfer of energy from our skin to the environment and help to warm us up.