8. Grey matter
8.a Responding to the environment
Nervous and hormonal communication
The nervous system
1. A stimulus is detected by receptor cells
2. Electrical impulse sent along a sensory neurone
3. At the end of the neurone chemical neurotransmitters are released into the synapse
via exocytosis and diffuse across
4. When neurotransmitters bind to receptors on the postsynaptic neurone it regenerates
the impulse which is then carried throughout the central nervous system (CNS)
5. The CNS processes the information and sends impulses along motor neurones to
effector cells so a response can be produced
→ sends information as electrical impulses
The hormonal system
1. The hormonal system is made up of glands and hormones
● Glands are a group of cells that are specialised ro secrete a substance (e.g
hormone)
● Hormones are chemical messengers - many hormones are proteins, peptides
(e.g. insulin), some hormones are steroids (e.g. progesterone)
2. Hormones are secreted when a gland is stimulated
● Glands can be stimulated by a change in concentration of a specific
substance (e.g. hormone) or by electrical impulses
3. Hormones diffuse directly into the blood then taken around the body via the
circulatory system
4. They then diffuse out of the blood and bind to specific receptors target cell
membranes
5. This triggers a response in the target cells (the effectors)
→ sends information as chemical signals
Photoreceptors
● Photoreceptors are light receptors in your eye which converts light into an electrical
impulse
1. Light enters the eye, hits the photoreceptors and is absorbed by
light-sensitive pigments
2. Light bleaches the pigments causing a chemical change triggering a nerve
impulse along a bipolar neurone
3. Bipolar neurones connect photoreceptors to the optic nerve, which then takes
the impulse to the brain
● The human eye has two types of photoreceptor - rods and cones
1. Rods are mainly found in the peripheral parts of the retina, only give
information in black and white (monochromatic vision)
, 8. Grey matter
2. Cones are packed together in the fovea, they give information in colour
(trichromatic vision). There are three types of cones: red-sensitive,
green-sensitive and blue-sensitive, they’re stimulated in different proportions
so you can see different colours
Rod cells
● Rod cells contain a light-sensitive pigment called rhodopsin
● Rhodopsin is composed of two chemicals joined together - retinal and opsin
● Rod cells are only stimulated when its light:
1. Light energy causes rhodopsin to break apart into retinal and opsin in a
process called bleaching
2. Causes the sodium ion channels to close meaning sodium ions are actively
transported out of the cell but can’t diffuse back in causing it to build up on the
outside of the cell making the inside of the membrane much more negative
meaning it is hyperpolarised
3. When the rod cell is hyperpolarised it stops releasing neurotransmitters - this
means there is no inhibition of the bipolar neurone so it depolarises
4. If the change in potential difference reaches the threshold then an action
potential is transmitted to the brain via the optic nerve
● However, when there is no light …
1. Sodium ions are pumped out of the cell via active transport
2. BUT sodium ions diffuse back into the cell through the open sodium channels
3. This makes the inside only slightly negative compared to the outside, the cell
membrane is depolarised
4. Neurotransmitters are released and inhibit the bipolar neurone meaning no
action potential and no information to the brain
Neurones
● Neurones are specialised cells of the nervous system which carry electrical impulses
around the body
● There are some common features
- A long fibre known as an axon
- A cell body that contains the nucleus and other cellular structures
- The end of the axon, known as the axon terminal, has many nerve endings
● Some neurones have a myelin sheath made up of specialised cells called Schwann
cells which help increase the rate of impulse travel. There are also uninsulated gaps
called the nodes of Ranvier which allow impulses to jump between neurones
1. Motor neurones
● A large cell body at one end that lies within the spinal cord or brain
● Many highly-branched dendrites extending from the cell body,
providing many connections with the axon terminals of other neurones
2. Sensory neurones
● Short neurones with axons and highly branched dendrites
8.a Responding to the environment
Nervous and hormonal communication
The nervous system
1. A stimulus is detected by receptor cells
2. Electrical impulse sent along a sensory neurone
3. At the end of the neurone chemical neurotransmitters are released into the synapse
via exocytosis and diffuse across
4. When neurotransmitters bind to receptors on the postsynaptic neurone it regenerates
the impulse which is then carried throughout the central nervous system (CNS)
5. The CNS processes the information and sends impulses along motor neurones to
effector cells so a response can be produced
→ sends information as electrical impulses
The hormonal system
1. The hormonal system is made up of glands and hormones
● Glands are a group of cells that are specialised ro secrete a substance (e.g
hormone)
● Hormones are chemical messengers - many hormones are proteins, peptides
(e.g. insulin), some hormones are steroids (e.g. progesterone)
2. Hormones are secreted when a gland is stimulated
● Glands can be stimulated by a change in concentration of a specific
substance (e.g. hormone) or by electrical impulses
3. Hormones diffuse directly into the blood then taken around the body via the
circulatory system
4. They then diffuse out of the blood and bind to specific receptors target cell
membranes
5. This triggers a response in the target cells (the effectors)
→ sends information as chemical signals
Photoreceptors
● Photoreceptors are light receptors in your eye which converts light into an electrical
impulse
1. Light enters the eye, hits the photoreceptors and is absorbed by
light-sensitive pigments
2. Light bleaches the pigments causing a chemical change triggering a nerve
impulse along a bipolar neurone
3. Bipolar neurones connect photoreceptors to the optic nerve, which then takes
the impulse to the brain
● The human eye has two types of photoreceptor - rods and cones
1. Rods are mainly found in the peripheral parts of the retina, only give
information in black and white (monochromatic vision)
, 8. Grey matter
2. Cones are packed together in the fovea, they give information in colour
(trichromatic vision). There are three types of cones: red-sensitive,
green-sensitive and blue-sensitive, they’re stimulated in different proportions
so you can see different colours
Rod cells
● Rod cells contain a light-sensitive pigment called rhodopsin
● Rhodopsin is composed of two chemicals joined together - retinal and opsin
● Rod cells are only stimulated when its light:
1. Light energy causes rhodopsin to break apart into retinal and opsin in a
process called bleaching
2. Causes the sodium ion channels to close meaning sodium ions are actively
transported out of the cell but can’t diffuse back in causing it to build up on the
outside of the cell making the inside of the membrane much more negative
meaning it is hyperpolarised
3. When the rod cell is hyperpolarised it stops releasing neurotransmitters - this
means there is no inhibition of the bipolar neurone so it depolarises
4. If the change in potential difference reaches the threshold then an action
potential is transmitted to the brain via the optic nerve
● However, when there is no light …
1. Sodium ions are pumped out of the cell via active transport
2. BUT sodium ions diffuse back into the cell through the open sodium channels
3. This makes the inside only slightly negative compared to the outside, the cell
membrane is depolarised
4. Neurotransmitters are released and inhibit the bipolar neurone meaning no
action potential and no information to the brain
Neurones
● Neurones are specialised cells of the nervous system which carry electrical impulses
around the body
● There are some common features
- A long fibre known as an axon
- A cell body that contains the nucleus and other cellular structures
- The end of the axon, known as the axon terminal, has many nerve endings
● Some neurones have a myelin sheath made up of specialised cells called Schwann
cells which help increase the rate of impulse travel. There are also uninsulated gaps
called the nodes of Ranvier which allow impulses to jump between neurones
1. Motor neurones
● A large cell body at one end that lies within the spinal cord or brain
● Many highly-branched dendrites extending from the cell body,
providing many connections with the axon terminals of other neurones
2. Sensory neurones
● Short neurones with axons and highly branched dendrites
