B1.2 Nerves and Hormones

The nervous system is a complex network of nerves cells that carry messages to and from the brain and spinal cord to various parts of the body. The nervous system includes both the Central Nervous System and Peripheral Nervous System. It allows the body to respond to changes in the environment in a process usually coordinated by the brain. Reflex actions are extra-rapid responses to stimuli; this process also involves the nervous system but bypasses the brain. Sense organs detect stimuli - a stimulus is a change in the environment which you may need to react to. The five sense organs are eyes, ears, nose, tongue and skin.

Receptors are groups of specialised cells that detect changes in the environment (stimulus). Receptors are often located in the sense organs, such as the ear, eye and skin. Each organ has receptors sensitive to particular kinds of stimulus that they detect.

• eyes - light
• ears - sound and position of the head
• tongue - chemicals in food
• nose - chemicals in the air
• skin - touch, pressure, pain and temperature

Neurones are nerve cells that carry information as tiny electrical signals. They are adapted to carry electrical impulses. The three types of neurones here have a slightly different function:

• sensory neurones carry signals from receptors to the spinal cord and brain
• relay neurones carry messages from one part of the CNS to another
• motor neurones carry signals from the CNS to effectors.

The tiny gap where two neurones meet, connection junction, is called a synapse. Information crosses this gap using (chemicals). One neurone releases chemicals into the synapse which diffuse across the gap to make the next neurone transmit an electrical impulse.

When a receptor is stimulated it sends a signal to the central nervous system, where the brain coordinates the response, but sometimes a very quick response is needed, one that does not involve the brain: this is a reflex action. A reflex action is a way for the body to automatically and rapidly respond to a stimulus to minimise any further damage to the body. It follows this general sequence and does not involve the brain:

stimulus → receptor → sensory neurone → relay neurone → motor neurone → effector

An effector is part of the body, muscles and glands, that produces the response. Examples of effectors are a gland releasing hormone into the blood and muscle contracting to move an arm.

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. Reflex actions are rapid and happen without us thinking, you would pull your hand away from a hot flame without thinking about it.

Muscles work in antagonistic pairs. This ensures that when a part of the body is moved, it can move back to its original position. Examples are:

• the biceps and triceps in the arm
• the quadriceps and hamstrings in the leg

Hormones are chemical messengers which travel in the blood to activate target cells. They are released directly into the blood and are carried in the plasma to certain parts of the body, but only affect particular cells (target cells). The chemical is produced in (and secreted by) various glands. They tend to have relatively long-lasting effects. Like the nervous system, hormones can control the body. There can be nervous or hormonal responses. 

Nerves:

• very fast response
• short-lived
• acts on a very precise area

Hormones:

• slower speed of response
• duration is long
• acts in a general way

The internal environment of the body is controlled by the nervous system and hormones. The maintenance of a constant internal environment is called homeostasis. The conditions inside our body must be very carefully controlled if the body is to function effectively. One example of homeostasis is the concentration of carbon dioxide in the blood being carefully controlled. Here are some of the other internal conditions that are regulated:

• Body temperature - is controlled to maintain the temperature at which the body’s enzymes work best, which is usually 37°C.
• Blood sugar levels - are ajusted to provide cells with a constant supply of glucose for respiration, in turn controlled by insulin.
• Water content - is kept at the right amount to protect cells by stopping too much water from entering or leaving them.
• Ion (salt) content of body - is maintained by the loss of ions from the skin and urine.

Homeostatic control is achieved using negative feedback mechanisms: if the level of something rises, control systems reduce it again or if the level of something falls, control systems raise it again.

The human body is designed to function most efficiently at 37ºC. If you become too hot or too cold, there are ways in which your body temperature can be controlled.

When we get too hot:

• Sweat glands in the skin release more sweat. The sweat evaporates, removing heat energy from the skin.
• Blood vessels leading to the skin capillaries become wider - they dilate - allowing more blood to flow through the skin, and more heat to be lost.

When we get too cold:

• Muscles contract rapidly - we shiver. These contractions need energy from respiration, and some of this is released as heat.
• Blood vessels leading to the skin capillaries become narrower - they constrict - letting less blood flow through the skin and conserving heat in the body

Diabetes is a condition in which the blood glucose levels remain too high. It can be treated by injecting insulin. The extra insulin causes the liver to convert glucose into glycogen, which reduces the blood glucose level. There are two types of diabetes - Type 1 and Type 2.

Type 1 diabetes is caused by a lack of insulin. It can be controlled by:

• monitoring the diet
• injecting insulin 

Type 2 diabetes is caused by a person’s body becoming resistant to insulin. It can be controlled by diet and exercise. There is a link between rising levels of obesity and increasing levels of Type 2 diabetes.

Water content is controlled by water loss from:

• the lungs - when we exhale
• the skin - through sweating
• passing urine - produced by the kidneys.

The menstrual cycle in women is a recurring monthly process in which the lining of the uterus (the womb) is prepared for pregnancy. If pregnancy does not happen, the lining is shed at menstruation. Several hormones control this cycle, which includes controlling the release of an egg each month from an ovary, and changing the thickness of the uterus lining.

Follicle-stimulating hormone (FSH) is secreted by the pituitary gland.

1. causes an egg to mature in one of the ovaries
2. stimulates the ovaries to release oestrogen

Oestrogen is produced in the ovaries.

1. stops further release of FSH - so that only one egg matures in a cycle
2. stimulates the pituitary gland to produce the hormone LH

Luteinising hormone (LH) causes the mature egg to be released from the ovary at around the middle of the menstrual cycle.

Human fertility is controlled by hormones. This means that knowledge of hormones can be used to decide to increase, or reduce, the chances of fertilisation and pregnancy. The oral contraceptive, commonly known as the pill (or birth control pill), greatly reduces the chances of mature eggs being produced. It contains oestrogen or progesterone because these hormones inhibit the production of FSH, which in turn stops eggs maturing in the ovaries.

Oral contraceptives allow couples to choose the time they start a family, and choose the time they stop having children.

However, there are some risks, the first birth-control pills contained higher amounts of oestrogen than the pills taken today. This caused women to have significant side effects, such as changes in weight, mood and blood pressure. Modern birth-control pills contain much less oestrogen. Some only contain progesterone, which also leads to fewer side effects.

Some women have difficulty becoming pregnant because they don't produce enough FSH to allow their eggs to mature. Fertility drugs contain FSH and LH, which stimulate eggs to mature in the ovary.

Fertility treatments increase a woman's chance of becoming pregnant, although the treatment may not always work. Because the treatment boosts the production of mature eggs, multiple conceptions sometimes occur, with twins or triplets being expected. This increases the risk of complications in pregnancy and childbirth, and may lead to premature or underweight babies.

If a couple are having difficulty conceiving a child because the quantity or quality of the man’s sperm is poor then they might decide to use IVF - In vitro fertilisation. This is where the egg is fertilised outside the woman’s body and then implanted back into her uterus. As FSH can also be used to encourage the production of several mature eggs at once, it is used as part of IVF to increase the number of eggs available for fertilisation. Some people worry about the ethical implications of IVF. They are concerned that couples may want 'designer babies' with 'desirable' qualities, so may only want certain fertilised eggs. For example, they may want a girl if they have lots of boys in the family, or they may wish to avoid producing a baby with an inherited defect.

Plants produce hormones and respond to external stimuli, growing towards sources of water and light, which they need to survive.

A tropism is a biological phenomenon, indicating growth or turning movement of a biological organism, usually a plant, in response to an environmental stimulus. An auxin is a plant hormone produced in the stem tips and roots, which controls the direction of growth. Plant hormones are used in weed killers, rooting powder and to control fruit ripening.

Plants need light and water for photosynthesis. Plant responses - called tropisms - help make sure that any growth is towards sources of light and water. There are two main types of tropism:

• positive tropism – the plant grows towards the stimulus
• negative tropism – the plant grows away from the stimulus.

Sensitivity in plants is shown by phototropism - a tropism where light is the stimulus. Then gravitropism (also called a 'geotropism') is a tropism where gravity is the stimulus.

The roots and shoots of a plant respond differently to the same stimuli. The tropisms of shoots mean that the shoots are likely to grow into the air, where there is light for photosynthesis. The tropisms of roots mean that the roots are likely to grow into the soil, where there is moisture. These are posistive tropisms but can be reversed to be negative tropisms.

Auxins are a family of plant hormones. They are mostly made in the tips of the growing stems and roots, and can diffuse to other parts of the stems or roots. Auxins change the rate of elongation in plant cells, controlling how long they become. Shoots and roots respond differently to high concentrations of auxins:

• cells in stems grow more
• cells in roots grow less

In a shoot, the shaded side contains more auxin. This means that the shaded side grows longer, causing the shoot to bend towards the light. 

Auxins have the opposite effect on root cells. In a root, the shaded side contains more auxin and grows less - causing the root to bend away from the light.

Auxins are also involved in geotropisms. In a root placed horizontally, the bottom side contains more auxin than the top side. This makes the bottom side grow less than the top side, causing the root to bend in the direction of the force of gravity.

The opposite happens in a shoot placed horizontally, the bottom side contains more auxin than the top side. This makes the bottom side grow more than the top side, causing the shoot to bend and grow against the force of gravity.

Synthetic plant hormones are used to control plant growth. For example, rooting powder contains growth hormones that make stem cuttings develop roots quickly. Plant hormones can be used as weed killers and rooting hormones by farmers and plant growers.

Selective weed killers work on some plants but not others. This can be useful for getting rid of dandelions in a lawn without killing the grass, or getting rid of thistles in a field without killing the wheat. Selective weed killers contain growth hormone that cause the weeds to grow too quickly and then die. The weed killer is absorbed in larger quantities by the weeds than the beneficial plants, which stay healthy.

Rooting powder contains growth hormones to make stem cuttings quickly develop roots.

Controling fruit ripening is used in shops and supermarkets. Some hormones slow the ripening of fruits and others speed it up. These hormones and their inhibitors are useful for delaying ripening during transport or when fruit is displayed in shops.