Biology B1.2JaskiratK

Nerves & Hormones

The nervous system allows the body to respond to changes in the environment. This is a process usually co-ordinated by the brain. Reflex actions are extra-rapid responses to stimuli, and this process also involves the nervous system, but bypasses the brain.

Receptors:Receptors are groups of specialised cells. They can detect changes in the environment, which are called stimuli, and turn them into electrical impulses. 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.

Receptors & Effectors

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Sense Organs: Stimulus:

Skin Touch, Pressure, Pain, Temperature

Tongue Chemicals In Food

Nose Chemicals In The Air

Eyes Light

Ears Sound & Position Of The Head

The central nervous system – CNS- in humans consists of

the brain and spinal cord. When a receptor is

stimulated, it sends a signal along the nerve cells –

neurons – to the brain. The brain the co-ordinates the

response.

Effectors:An effector is any part of the body that produces the response. Here are some examples of effectors:- A Muscle Contracting To Move The Arm.- A Muscle Squeezing Saliva From The Salivary Gland.- A Gland Releasing A Hormone Into The Blood.

NeuronesNeurons are nerve cells. They carry information as tiny electrical signals. There are three different types of neurones, each with a slightly different function.1. Sensory Neurones - Carry Signals From Receptors To The Spinal Cord And Brain.2. Relay Neurones - Carry Messages From One Part Of The CNS To Another.3. Motor Neurones - Carry Signals From The CNS To Effectors.

The diagram below shows a typical neurone – in this case, a motor neurone. It has tiny branches at each end and long fibre carries the signals.

A Motor Neurone

Synapses:Where two neurones meet, there is a tiny gap called a Synapses, Signals cross this gap using chemicals. One neurone releases the chemical into the gap. The chemical diffuses across the gap and makes the next neurone transmit and electrical signal.

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Reflex ActionsWhen a receptor is stimulated, it sends a signal to the central nervous system, where the brain co-ordinates the response. But sometimes a very quick response is needed, One that does not need the involvement of the brain. This is a reflex action.Reflex actions are rapid and happen without us thinking. For example, you would pull your hand away from a hot flame without thinking about it. The animation below allows you to step through each stage of the reflex arc.

c c c

c c c

c

c

c c

cc

c

1. Stimulus (Heat)

2. Pain Receptor Stimulated

c c3. Signal Sent Along Sensory Neuron

5. Signal Sent Along Motor Neuron

c 6. Effector Muscle Contracts (Response)

cc

c

4. Signal Passed Along Relay Neuron (Message Sent To Brain)

c

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This is what happens:1. Receptor Detects A Stimulus – A Change In Environment.2. Sensory Neurone Sends Impulses To Relay Neurone.3. Motor Neurone Sends Impulses To Effector.4. Effector Produces A Response. – Muscle Contracts To Move Hand Away.

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.

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 Response

The EyeThe eye is a sense organ that responds to light.

Structure: Function:

Comea Refracts Light – Bend It As It Enters The Eye

Iris Controls How Much Light Enters The Pupil

Lens Focuses Light Onto The Retina

Retina Contains The Light Receptors

Optic Nerve Carries Impulses Between The Eye And The Brain

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The RetinaLight 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. There are three different types of cone cells which produce colour vision.

A photograph of the human retina seen through the eye.

Rods: Cones:

Number In The Retina 12 Million 7 Million

Where Concentrated Outer Edges Of The Retina The Fovea

Works Best In Dim Light Bright Light

Accommodation:The ability of the lens to change it shape to focus near and distant objects is called ‘Accommodation’.

Position Of Object

Ciliary Muscles Suspensory Ligaments

Muscle Tension On Lens

Lens Shape

Near Contract Slackened Low Fat

Distant Relax Stretched High Thin

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The Pupil Reflex

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.The way the iris in our eye adjusts the size of the pupil in response to bright or dim light is also a reflex action:

In Dim Light:- Radial Muscles Of The Iris Contract.- Circular Muscles Of The Iris Relax.- More Light Enters The Eye Through The

Dilated Pupil.

In Bright Light:- Radial Muscles Of The Iris Relax.- Circular Muscles Of The Iris Contract.- Less Light Enters The Eye Through The

Contracted Pupil.

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Antagonistic PairsMuscles work in antagonistic pairs. This ensure that when apart 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 Hamstring In The Leg.The slideshow explains how the lower arm is moved upwards (flexed) when the biceps muscle contracts and the triceps muscle is relaxed. It is moved downwards (extended) when the triceps is contracted and the biceps is relaxed.

The Synapse:Where two neurones meet there is a tiny gap called a ‘Synapse’. Information crosses this gap using neurotransmitters, rather than using electrical impulses. One neurone releases neurotransmitters into the synapse. These diffuse across the gap and make the other neurone transmit electrical impulses.

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Control In The Human Body And In Plants

A hormone is chemical substance, produced by a gland and carried by the blood, which alters the activity of more specific target organs (and is then destroyed by the liver). Like the nervous system, hormones can control the body.There are important differences between the two systems as described in the table.

What It Is

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Nervous: Hormonal:

Type Of Signal: Electrical (Chemical At Synapses)

Chemical

Transmission Of Signal: By Nerve Cells By Nerve Cells By The Bloodstream

Effectors: Muscles Or Glands Target Cells In Particular Tissue

Type Of Response: Muscle Contraction Or Secretion

Chemical Change

Speed Of Response: Very Rapid Slower

Duration Of Response: Short (Until Nerve Impulses Stop)

Long (Until Hormone Breaks Down)

Control In The Human BodyHormones are chemical substances that regulate processes in the body. Hormones are secreted by glands and travel to their target organs in the blood stream. Several hormones are involved in the female menstrual cycle. Hormones can be used to control human fertility and have advantages and disadvantages.

Control Of Internal Conditions:The internal environment of the body is controlled by the nervous system and hormones. The maintenance of a constant internal environment is called homeostasis. Below are some of the internal conditions that need to be controlled.

Water Content In The Body:Water in the body has to be controlled to protect cells from either too much water entering them or too much water leaving them. Water content is controlled by water loss from:- The Lungs = When We Exhale- The Skin = Through Sweating- Passing Urine = Produced By The Kidneys.

Ion (Salt) Content In The Body:

Ion levels are controlled to protect cells from too much water entering or leaving them. Ion content is controlled by the loss of ions from:- The Skin = Through

Sweating.- Passing Urine = Produces

By The Kidneys.

Temperature Of The Body;This is controlled to maintain the temperature at which enzymes work best. Body temperature is controlled by sweating, shivering and controlling blood flow to the skin.

Blood Sugar Levels:This is controlled to provide cells with a constant supply of energy. Blood sugar level is controlled by the release and storage of glucose controlled by insulin.

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Water Content In The Body:

Too Much Water Too Little Water

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Hormones In The Human BodyHormones are chemical secreted by glands in the body. Different hormones affect different target organs. The bloodstream usually transports hormones from the glands to their target organ. Hormones regulate the functions of many cells and organs.

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Hormones In The Menstrual CycleThe menstrual cycle in women is a recurring monthly process in which the lining of the uterus (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. These hormones are secreted by the ovaries and pituitary gland. Follicle Stimulating Hormone (FHS):The hormone FSH is secreted by the pituitary gland. FHS makes two things happen:- It causes an egg to mature in an ovary.- It stimulates the ovaries to release the hormone oestrogen.

Oestrogen:The hormone oestrogen is secreted by the ovaries. Oestrogen makes two thing happen:- It stop FHS being produced = So that only

one egg matures in a cycle.- It stimulates the pituitary gland to release

luteinizing hormone (LH), which triggers ovulation (the release of the mature egg from the ovary).

How the level of oestrogen changes during the menstrual cycle.

Luteinising Hormone (LH):This photo shows how the levels of FSH and

LH change during the menstrual.

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Oral ContraceptivesHuman 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.

Contraceptives:The oral contraceptive, commonly known as the pill, greatly reduces the chances of mature eggs being produced. It contains oestrogen and progesterone (another hormone). These hormones inhibit the production of FSH, which in turn stops eggs maturing in the ovaries.

Benefits And Risks:Oral contraceptives allow couples to chose the time they start a family, and choose the time they stop having children.The first birth-control pills contained higher amount 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 o fewer side effects.

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Fertility TreatmentsSome 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. On the other hand, 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.

In Vitro Fertilisation (IVF):If a couple are having difficulty conceiving a child because the quantity or quality of the mans sperm is poor then IVF can be used. This is where the egg Is fertilised outside the women’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 od egg 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,

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Control In PlantsPlants produce hormones and respond to external stimuli, growing towards sources of water and light, which they need to survive.A tropism is a growth in response to a stimulus and an auxin is a plant hormone produces 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.

Sensitivity In Plants: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.Phototropism is a tropism where light is the stimulus. A 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 stimulus.

Part Of Plant: Light Stimulus: Gravity Stimulus:

Shoot Positive Phototropism (Grows Towards Light)

Negative Gravitropism (Grows Against The Force Of Gravity)

Root Negative Phototropism (Grows Away From Light)

Positive Gravitropism (Grows In The

Direction Of The Force Of Gravity)

Different Types Of Tropism:

The tropism 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.

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