Homeostasis
• Homeostasis is the regulation of conditions inside the body to maintain a stable internal
environment, in response to both internal and external conditions. The body needs certain
conditions to function properly – for optimal enzyme action and cell function.
- We have automatic control systems with...
- Receptors that detect a change in the internal or external environment.
- Coordination centres consisting of the brain and spinal cord that interpret a change.
- Effectors that carry out a change – usually muscles (contract) or glands (release hormones).
• The nervous system sends very fast and precise electrical impulses through nerves.
• The endocrine system relies on hormones (chemicals released into the bloodstream). They
only affect certain cells despite travelling through the entire body. The endocrine system is
slower, longer-lasting and more generalised.
• Negative feedback – When the level gets too high, this acts to decrease it. When the level
gets too low, this acts to increase it. It does the opposite of a change. This creates a loop.
The Nervous System
• A neurone (or nerve cell) are adapted to carry electrical impulses from one point to another.
They’re long, thin and have lots of branched connections to pass messages on.
- Synapses are connections between nerve cells. When an electrical impulse hits the end of a
nerve, it causes the release of chemicals that diffuse across the gap to trigger another electrical
impulse in the next nerve cell.
• The central nervous system consists of the brain and spinal cord.
- It receives information via sensory neurons (receptors --> CNS), for example temperature,
CO2 in bloodstream.
- It sends information back out via motor neurons to effectors (usually muscles which contract
or glands to release hormones).
• The reflex arc involves unconscious reflexes. A stimuli is detected by receptor cells. Sensory
neurone carries an impulse to spinal cord. Impulse is transferred to a relay neurone which
passes the impulse to a motor neurone. This skips out the brain to make the response quicker.
- Stimulus --> Receptor --> Sensory Neurone --> Relay Neurone --> Motor Neurone --> Effector
--> Response
The Brain
• Made up of billions of interconnected nerve cells. Responsible for complex behaviours.
- Brain and spinal cord make up the central nervous system (CNS).
,
- Main section at the top is called the cerebral cortex. Responsible for consciousness,
intelligence, memory and language. Senses. Split into two halves called hemispheres. Left
controls muscles on opposite side of body (right).
- At the back bottom is the cerebellum. Responsible for balance and muscle coordination.
- In the middle is the hypothalamus. Responsible for regulating body temperature and sends
signals to the pituitary gland.
- The brain stem is found at the bottom. Within this is the medulla which controls unconscious
activities such as breathing and our heart beating.
• Scientists can study the brain by...
- Studying people with brain damage. For example, if someone has a stroke damaging a
specific part of the brain and this has an effect we can decipher what this region was
responsible for.
- Electrically stimulating different parts of the brain with an electrode. Certain areas have certain
effects such as muscle contraction.
- Scanning the brain is less invasive. CT scans (x-rays), PET scans (radioactive chemicals),
MRI scans (magnetic fields). CT scans for damage, PET and MRI for activity.
• Treating the brain is difficult because...
- Wide range of things that can go wrong with the brain – tumours, infection, trauma, mental
health problems.
- Encased within the skull so difficult to access. Surrounded by delicate tissue.
- Complex and we don’t fully understand it, so difficult to treat with drugs and chemicals.
The Eyes
• The eye is a spherical organ encased in a tough white layer called the sclera which protects it
from damage.
- At the front it is transparent to allow light in – known as cornea. Curved shape refracts light.
- The pupil is a ‘hole’ in the middle of the eye.
- The iris (coloured part) can adjust the size of the pupil depending on light levels.
- Behind the pupil is a transparent disk called the lens, held in place by suspensory ligaments
and ciliary muscles which work together to adjust the shape to focus on near and distant
objects.
- It falls on light-sensitive receptor cells in the retina (stretch around edge of eye). Convert light
signals into electrical impulses.
- Impulses sent via optic nerve to the brain.
Adaptation -
• The retina contains rod cells which are sensitive to DIM LIGHT and cone cells which allow us
to see in COLOUR. These are delicate and easily damaged by too much light. To prevent this a
, reflex controls the size of the pupil. It constricts (smaller) in bright light and dilates in low light.
- In bright light, the circular muscles contract (makes pupil smaller) and the radial muscles relax
so they can be stretched longer. This lets less light into the eye.
- In dark light, the circular muscles relax (allows pupil to expand) and the radial muscles
contract (makes shorter and pulls pupil open). This lets more light into the eye.
Accommodation -
• The lens automatically changes shape to allow the eye to focus on near or distant objects.
- The suspensory ligaments anchor the lens and the ciliary muscles attach these.
- Light reflected off distant objects arrives at the eye in almost parallel light rays, therefore they
don’t need to be refracted much to focus them on the retina. Therefore the CILIARY MUSCLES
RELAX making the circle wider. This STRETCHES the SUSPENSORY LIGAMENTS creating
high tension on the lens that thins out and flattens the lens leading to less curvature and less
refraction of light.
- In near objects, the light rays to the lens at an angle and need more refraction to focus. The
CILIARY MUSCLES CONTRACT making the circle narrower and thicker. The SUSPENSORY
LIGAMENTS SLACKEN creating less tension on the lens, allowing it to be fatter and more
curved creating greater refraction.
Correcting Sight Problems -
• Short-sightedness (myopia) is when people can’t focus on distant objects, either because the
lens is too curved or the eyeball shape is too long. Light is refracted too much and the focus
point falls short of the retina. CONCAVE LENSES can be worn (inwards facing) in glasses to
adjust the focal point.
• Long-sightedness (hyperopia) is when people can’t focus on close objects. The lens is too thin
or the eyeball shape is too long. Not refracting enough, focal point is beyond the retina.
CONVEX lenses can be used to adjust it.
• People can alternatively use contact lenses. Hard contact lenses have to be kept sterile. Soft,
disposable contact lenses can also be used. They can cause the eye to feel dry, but are good
for sports.
- Laser eye surgery can also be used. Surgically changes the lens shape so it has the correct
level of refraction. Only done on adults as in children, the suspensory ligaments and ciliary
muscles are adjusting constantly. It’s expensive and risky. The lens can be replaced completely.
- Remember with C. When pupil CONSTRICTS the CIRCULAR muscles CONTRACT. With
CLOSE objects the CILIARY muscles CONTRACT and the lens becomes more CURVED.
• Homeostasis is the regulation of conditions inside the body to maintain a stable internal
environment, in response to both internal and external conditions. The body needs certain
conditions to function properly – for optimal enzyme action and cell function.
- We have automatic control systems with...
- Receptors that detect a change in the internal or external environment.
- Coordination centres consisting of the brain and spinal cord that interpret a change.
- Effectors that carry out a change – usually muscles (contract) or glands (release hormones).
• The nervous system sends very fast and precise electrical impulses through nerves.
• The endocrine system relies on hormones (chemicals released into the bloodstream). They
only affect certain cells despite travelling through the entire body. The endocrine system is
slower, longer-lasting and more generalised.
• Negative feedback – When the level gets too high, this acts to decrease it. When the level
gets too low, this acts to increase it. It does the opposite of a change. This creates a loop.
The Nervous System
• A neurone (or nerve cell) are adapted to carry electrical impulses from one point to another.
They’re long, thin and have lots of branched connections to pass messages on.
- Synapses are connections between nerve cells. When an electrical impulse hits the end of a
nerve, it causes the release of chemicals that diffuse across the gap to trigger another electrical
impulse in the next nerve cell.
• The central nervous system consists of the brain and spinal cord.
- It receives information via sensory neurons (receptors --> CNS), for example temperature,
CO2 in bloodstream.
- It sends information back out via motor neurons to effectors (usually muscles which contract
or glands to release hormones).
• The reflex arc involves unconscious reflexes. A stimuli is detected by receptor cells. Sensory
neurone carries an impulse to spinal cord. Impulse is transferred to a relay neurone which
passes the impulse to a motor neurone. This skips out the brain to make the response quicker.
- Stimulus --> Receptor --> Sensory Neurone --> Relay Neurone --> Motor Neurone --> Effector
--> Response
The Brain
• Made up of billions of interconnected nerve cells. Responsible for complex behaviours.
- Brain and spinal cord make up the central nervous system (CNS).
,
- Main section at the top is called the cerebral cortex. Responsible for consciousness,
intelligence, memory and language. Senses. Split into two halves called hemispheres. Left
controls muscles on opposite side of body (right).
- At the back bottom is the cerebellum. Responsible for balance and muscle coordination.
- In the middle is the hypothalamus. Responsible for regulating body temperature and sends
signals to the pituitary gland.
- The brain stem is found at the bottom. Within this is the medulla which controls unconscious
activities such as breathing and our heart beating.
• Scientists can study the brain by...
- Studying people with brain damage. For example, if someone has a stroke damaging a
specific part of the brain and this has an effect we can decipher what this region was
responsible for.
- Electrically stimulating different parts of the brain with an electrode. Certain areas have certain
effects such as muscle contraction.
- Scanning the brain is less invasive. CT scans (x-rays), PET scans (radioactive chemicals),
MRI scans (magnetic fields). CT scans for damage, PET and MRI for activity.
• Treating the brain is difficult because...
- Wide range of things that can go wrong with the brain – tumours, infection, trauma, mental
health problems.
- Encased within the skull so difficult to access. Surrounded by delicate tissue.
- Complex and we don’t fully understand it, so difficult to treat with drugs and chemicals.
The Eyes
• The eye is a spherical organ encased in a tough white layer called the sclera which protects it
from damage.
- At the front it is transparent to allow light in – known as cornea. Curved shape refracts light.
- The pupil is a ‘hole’ in the middle of the eye.
- The iris (coloured part) can adjust the size of the pupil depending on light levels.
- Behind the pupil is a transparent disk called the lens, held in place by suspensory ligaments
and ciliary muscles which work together to adjust the shape to focus on near and distant
objects.
- It falls on light-sensitive receptor cells in the retina (stretch around edge of eye). Convert light
signals into electrical impulses.
- Impulses sent via optic nerve to the brain.
Adaptation -
• The retina contains rod cells which are sensitive to DIM LIGHT and cone cells which allow us
to see in COLOUR. These are delicate and easily damaged by too much light. To prevent this a
, reflex controls the size of the pupil. It constricts (smaller) in bright light and dilates in low light.
- In bright light, the circular muscles contract (makes pupil smaller) and the radial muscles relax
so they can be stretched longer. This lets less light into the eye.
- In dark light, the circular muscles relax (allows pupil to expand) and the radial muscles
contract (makes shorter and pulls pupil open). This lets more light into the eye.
Accommodation -
• The lens automatically changes shape to allow the eye to focus on near or distant objects.
- The suspensory ligaments anchor the lens and the ciliary muscles attach these.
- Light reflected off distant objects arrives at the eye in almost parallel light rays, therefore they
don’t need to be refracted much to focus them on the retina. Therefore the CILIARY MUSCLES
RELAX making the circle wider. This STRETCHES the SUSPENSORY LIGAMENTS creating
high tension on the lens that thins out and flattens the lens leading to less curvature and less
refraction of light.
- In near objects, the light rays to the lens at an angle and need more refraction to focus. The
CILIARY MUSCLES CONTRACT making the circle narrower and thicker. The SUSPENSORY
LIGAMENTS SLACKEN creating less tension on the lens, allowing it to be fatter and more
curved creating greater refraction.
Correcting Sight Problems -
• Short-sightedness (myopia) is when people can’t focus on distant objects, either because the
lens is too curved or the eyeball shape is too long. Light is refracted too much and the focus
point falls short of the retina. CONCAVE LENSES can be worn (inwards facing) in glasses to
adjust the focal point.
• Long-sightedness (hyperopia) is when people can’t focus on close objects. The lens is too thin
or the eyeball shape is too long. Not refracting enough, focal point is beyond the retina.
CONVEX lenses can be used to adjust it.
• People can alternatively use contact lenses. Hard contact lenses have to be kept sterile. Soft,
disposable contact lenses can also be used. They can cause the eye to feel dry, but are good
for sports.
- Laser eye surgery can also be used. Surgically changes the lens shape so it has the correct
level of refraction. Only done on adults as in children, the suspensory ligaments and ciliary
muscles are adjusting constantly. It’s expensive and risky. The lens can be replaced completely.
- Remember with C. When pupil CONSTRICTS the CIRCULAR muscles CONTRACT. With
CLOSE objects the CILIARY muscles CONTRACT and the lens becomes more CURVED.
