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Nerves and Hormones
The nervous system
The human nervous system consists of:
 the central nervous system (CNS) – the brain and spinal cord
 the peripheral nervous system – nerve cells that carry
information to or from the CNS
Nerve cells; Nerve cells are also called neurones
...

Receptors; Receptors are groups of specialised cells
...
Sense organs contain groups of
receptors that respond to specific stimuli
...
For
example:

a muscle contracting to move an arm
muscle squeezing saliva from the salivary gland
a gland releasing a hormone into the blood

REFLEX ACTIONS
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
...
For example, a simple reflex arc happens if we
accidentally touch something hot
...

2
...

4
...

Sensory neurone sends impulses to relay neurone
...

Effector produces a response (muscle contracts to move hand away)
...
This ensures that when a part of the body is moved, it can move back to its
original position
...
It is moved downwards (extended) when the triceps is contracted and the biceps is relaxed
...
Information crosses this gap using
neurotransmitters, rather than using electrical impulses
...

These diffuse across the gap and make the other neurone transmit an electrical impulse
...
Homeostasis is
the maintenance of a constant internal environment
...

One example of homeostasis is the concentration of carbon dioxide in the blood being carefully controlled
...

Blood Sugar level; This is controlled to provide cells with a constant supply of glucose for respiration
...

Water content; This is controlled to protect cells by stopping too much water from entering or leaving them
...
The sweat evaporates, removing heat energy from the skin
...
(vasodilatation)

Too cold;



2

Muscles contract rapidly - we shiver
...

Blood vessels leading to the skin capillaries become narrower - they constrict - letting less blood flow
through the skin and conserving heat in the body
...
They lie
flat when we are warm, and rise when we are cold
...

The hypothalamus is the part of the brain which monitors the body's
temperature
...

The hypothalamus responds to this information by sending nerve
impulses to effectors to maintain body temperature
...
This raises the skin
hairs and traps a layer of air next to the skin
...
They
include the amount of:



shivering (rapid muscle contractions release heat)
sweating (evaporation of water in sweat causes cooling)



blood flowing in the skin capillaries

VASOCONSTRICTION AND VASODILATATION
The amount of blood flowing through the skin capillaries
is altered by vasoconstriction and vasodilation
...
It is important that the concentration of glucose in the blood is
maintained at a constant level
...

Effect on pancreas
Effect on liver
Effect on blood glucose level

LOW GLUCOSE
Insulin not secreted into the blood
Does not convert glucose into glycogen
Increases

HIGH GLUCOSE
Insulin secreted into the blood
Converts glucose into glycogen
Decreases

Plant and Animal Hormones
HORMONES AND NERVES
A hormone is a 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)
...
There are important differences between the two systems
as described in the table
...

SOURCE
Pituitary gland

ADH

TARGET ORGAN(S)
Kidneys

Adrenaline Adrenal glands

Several targets including
the respiratory and
circulatory systems

Insulin

Liver

Pancreas

ROLE
Controlling the water
content of the blood
Preparation for ‘fight
or flight’

Controlling blood
glucose levels

EFFECTS
Increases reabsorption of
water by the collecting ducts
Increases breathing rate,
heart rate, flow of blood to
muscles, conversion of
glycogen to glucose
Increases conversion of
glucose into glycogen

PLANT TROPISMS
Plants need light and water for photosynthesis
...

There are two main types of tropisms:



positive tropisms – the plant grows towards the stimulus
negative tropisms – the plant grows away from the stimulus

Phototropism is a response where the stimulus is light, whereas geotropism (also called gravitropism) is a response
where the stimulus is gravity
...
They are mostly made in the tips of the growing stems and roots, and can
diffuse to other parts of the stems or roots
...

Stems and roots respond differently to high concentrations of auxins:



cells in stems grow more
cells in roots grow less

In a stem, the shaded side contains more auxin and grows longer – causing the stem to bend towards the light
...
In a root, the shaded side contains more auxin and grows less - causing
the root to bend away from the light
...

The retina; Light passes through the eyeball to the retina
...
Rods are more sensitive to light than cones
...

The pupil reflex; The amount of light entering the eye is controlled by a reflex action
...
This is controlled by the muscles of the iris
...

the nearer the distance the more contracted the muscle is
STRUCTURE
Cornea
Iris
Lens
Retina
Optic nerve

5

FUNCTION
Refracts light - bends it as it enters the eye
Controls how much light enters the pupil
Focuses light onto the retina
Contains the light receptors
Carries impulses between the eye and the brain



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