The human nervous system

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The Nervous System: Structure and Function

The human nervous system consists of the central nervous system (CNS) – the brain and spinal cord – and the peripheral nervous system (PNS) – all of the nerves in the body.
Information is sent through the nervous system as electrical impulses – electrical signals that pass along nerve cells known as neurones.
A bundle of neurones is known as a nerve.
The nervous system enables humans to react to their surroundings and to coordinate their behaviour.
The pathway through the nervous system is: stimulus → receptor → coordinator → effector → response.
The CNS is the coordinator that coordinates the response of effectors, which may be muscles contracting or glands secreting hormones.

Neurones and Synapses

Neurones have a cell body (where the nucleus and main organelles are found) and cytoplasmic extensions called axons and dendrites.
The axon is insulated by a fatty myelin sheath with small uninsulated sections called nodes, which the impulse jumps along.
There are three main types of neurones: sensory neurones carry impulses from sense organs to the CNS; relay neurones are found inside the CNS and connect sensory and motor neurones; motor neurones carry impulses from the CNS to effectors (muscles or glands).
Neurones never touch each other; they are separated by junctions called synapses.
At a synapse, the electrical impulse triggers the release of chemical messengers called neurotransmitters from vesicles; these diffuse across the synaptic gap and bind with receptor molecules on the postsynaptic membrane.
The neurotransmitters are then destroyed or recycled to prevent continued stimulation of the second neurone.
Synapses ensure that impulses only travel in one direction, avoiding confusion within the nervous system.

The Reflex Arc

A reflex response (involuntary response) does not involve the conscious part of the brain as the coordinator; awareness of the response occurs after it has been carried out.
Reflex actions are automatic and rapid – this helps to minimise damage to the body and aids survival.
The reflex arc is the pathway taken by electrical impulses as they travel along neurones.
Example: pain-withdrawal reflex when stepping on a pin: stimulus detected by a receptor in the skin → sensory neurone sends impulses to the spinal cord (coordinator) → relay neurone in the spinal cord → motor neurone carries impulse to a muscle (effector) → muscle contracts and pulls the foot away (response).
There are three main types of neurone in a reflex arc: sensory, relay and motor.
Sensory neurones are long and have a cell body branching off the middle of the axon; relay neurones are short with a small cell body at one end; motor neurones are long with a large cell body at one end.

A reflex arc

Required Practical: Reaction Time

Reaction time is the time taken to respond to a stimulus; reactions are usually very quick, typically taking less than a second.
Reaction time can be affected by factors such as age, stress or use of drugs (such as caffeine).
The ruler drop test measures reaction time: the further down the ruler is caught, the slower the individual's reaction time.
The experiment can be repeated several times to calculate a mean average distance caught, which can be equated to a reaction time.
Control variables include: using the dominant hand each time, dropping the same ruler from the same height, and orientating the ruler the same way (0 cm facing down).
Reaction time can also be recorded in milliseconds which is more accurate than seconds.

The Brain

The brain is made of billions of interconnected neurones and is responsible for controlling complex behaviours.
The cerebral cortex is the outer layer, divided into two hemispheres; it is highly folded and responsible for higher-order processes such as intelligence, memory, consciousness and personality.
The cerebellum is underneath the cerebral cortex and is responsible for balance, muscle coordination and movement.
The medulla controls unconscious activities such as heart rate and breathing.
The brain is incredibly complex and delicate, making it extremely difficult for neuroscientists to study; different regions can't be studied in isolation.
Neuroscientists have mapped regions of the brain by studying patients with brain damage, electrically stimulating different parts of the brain, and using MRI scanning techniques.
Our limited understanding means that treating brain damage and disease is very difficult; any potential treatment carries risks of further damage.

The Eye: Structure and Function

The eye is a sense organ containing receptor cells sensitive to light intensity and colour; its purpose is to receive light and focus it onto the retina.
The retina contains light receptor cells that detect light intensity and colour; the optic nerve carries electrical impulses from the eye to the brain.
The cornea is the transparent covering of the front of the eye that refracts (bends) light; the iris controls how much light enters the pupil.
The ciliary muscles are a ring of muscles around the lens which relax and contract to change the shape of the lens; the suspensory ligaments work with the ciliary muscles.
Accommodation is the process of changing the shape of the lens to focus on near or distant objects.
Focusing on a near object: ciliary muscles contract, suspensory ligaments loosen, lens becomes thicker and refracts light more strongly.
Focusing on a distant object: ciliary muscles relax, suspensory ligaments are pulled tight, lens is pulled thin and refracts light less.

The Pupil Reflex and Eye Defects

The pupil reflex is a reflex action that protects the retina from damage.
In dim light, the pupil dilates (widens) to allow as much light into the eye as possible.
In bright light, the pupil constricts (narrows) to prevent too much light from entering and damaging the retina.
Myopia (short-sightedness) occurs when the lens is more curved than normal or the eyeball is too long, so light is refracted too much and the focal point falls in front of the retina; distant objects appear blurry.
Hyperopia (long-sightedness) occurs when the lens is less curved than normal or the eyeball is too short, so light is not refracted enough and the focal point falls behind the retina; close objects appear blurry.
Myopia can be corrected with a concave lens; hyperopia can be corrected with a convex lens.
Other treatments include contact lenses, laser surgery (changing the shape of the cornea) and lens replacement surgery.

Control of Body Temperature

The human body needs to maintain a temperature at which enzymes work best, around 37°C.
Body temperature is monitored and controlled by the thermoregulatory centre in the brain, which contains receptors sensitive to the temperature of the blood.
The skin contains temperature receptors and sends nervous impulses to the thermoregulatory centre.
If body temperature is too high: blood vessels dilate (vasodilation) and sweat is produced from sweat glands; both mechanisms cause transfer of energy from the skin to the environment, cooling the body down.
If body temperature is too low: blood vessels constrict (vasoconstriction), sweating stops and skeletal muscles contract (shiver); these mechanisms reduce heat loss and increase heat released in the body.
Temperature control is an example of negative feedback.

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1.Which type of neurone carries electrical impulses from the CNS to effectors?
Easy
- ASensory neurone
- BRelay neurone
- CMotor neurone
- DReflex neurone
2.Muscles are one type of effector. Name another type of effector.
Easy
- AGland
- BNeurone
- CReceptor
- DSynapse
3.Place the components of a nervous response in the correct order, starting with the stimulus.
Medium
- Stimulus
- Receptor
- Coordinator
- Effector
- Response
4.Which sense organ contains receptor cells that respond to changes in light?
Easy
- AEar
- BEye
- CSkin
- DNose
5.What is the junction between two neurones called?
Easy
- AReflex arc
- BSynapse
- CMotor neurone
- DRelay neurone
6.Which of the following statements about nerve impulses and synapses are true? (Select all that apply.)
Medium
- AA nerve impulse is carried across a synapse by an electrical signal.
- BMotor neurones carry chemical signals from the CNS to effectors.
- CReflexes involve the conscious part of the brain.
- DAt a synapse, neurotransmitters diffuse across the synaptic cleft.
- ESynapses ensure impulses travel in only one direction.
7.Which part of the brain is responsible for balance, muscle coordination, and movement?
Easy
- ACerebral cortex
- BCerebellum
- CMedulla
- DSpinal cord
8.The pupil dilates (widens) in bright light to protect the retina.
Easy
True or false?

Mehr Fragen

9.Which structure controls the amount of light entering the eye?
Medium
- ACornea
- BIris
- CLens
- DRetina
10.Match each region of the brain to the function it controls.
Medium
- Cerebral cortex
- Cerebellum
- Medulla
- Balance, muscle coordination and movement
- Unconscious activities such as heart rate and breathing
- Higher-order processes such as intelligence, memory, consciousness and personality
11.Put the events that occur at a synapse into the correct order, starting with the arrival of an electrical impulse.
Medium
- Neurotransmitters are released from vesicles in the presynaptic neurone
- The electrical impulse travels along the axon of the presynaptic neurone
- The second neurone generates an electrical impulse
- Neurotransmitters diffuse across the synaptic cleft and bind to receptors on the postsynaptic membrane
- Neurotransmitters are destroyed to prevent continued stimulation
12.Which of the following statements about the reflex arc are correct? (Select all that apply.)
Medium
- AA reflex response does not involve the conscious part of the brain as the coordinator.
- BA reflex response is automatic and rapid.
- CA reflex response involves the conscious part of the brain as the coordinator.
- DAwareness of a reflex response occurs before the response is carried out.
- EReflex responses help to minimise damage to the body.
13.Which of the following correctly describes the function of the iris?
Medium
- AIt controls how much light enters the pupil.
- BIt refracts light onto the retina.
- CIt changes the shape of the lens.
- DIt carries electrical impulses from the eye to the brain.
14.In dim light, the pupil constricts to allow as much light into the eye as possible.
Easy
True or false?
15.Which of the following statements about the human nervous system is correct?
Medium
- AThe central nervous system consists of the brain and spinal cord.
- BThe central nervous system consists of all the nerves in the body.
- CThe peripheral nervous system consists of the brain and spinal cord.
- DThe central nervous system consists of the brain only.
16.Which part of the eye controls the amount of light entering it?
Easy
- AIris
- BCornea
- CLens
- DRetina
17.Which of the following are structures of the eye that help to change the shape of the lens? (Select all that apply.)
Medium
- ACiliary muscles
- BSuspensory ligaments
- COptic nerve
- DSclera
18.Which of the following statements about the transmission of impulses across a synapse is correct?
Medium
- AThe electrical impulse is converted to a chemical signal that diffuses across the synaptic cleft.
- BThe electrical impulse jumps directly across the synaptic cleft.
- CNeurotransmitters travel across the synaptic cleft by active transport.
- DThe chemical signal is converted back into an electrical impulse in the presynaptic neurone.
19.Two students investigate reaction times using the ruler drop test. Student A's distances caught (cm) are: 9, 8, 11, 9, 8, 9. What is Student A's mean distance caught?
Easy
- A9.0 cm
- B8.5 cm
- C9.5 cm
- D10.0 cm
20.Student B's distances caught (cm) are: 10, 8, 7, 7, 9, 7. Which student had the slower mean reaction time, and why?
Medium
- AStudent A, because a larger mean distance caught means a slower reaction time
- BStudent B, because a smaller mean distance caught means a slower reaction time
- CStudent A, because a smaller mean distance caught means a slower reaction time
- DBoth students had the same mean reaction time
21.A student's reaction times recorded in a computer test were 255 ms, 260 ms, 573 ms and 250 ms. Why should the value 573 ms be excluded when calculating the mean?
Medium
- AIt is an anomalous result that is much higher than the others
- BIt is the lowest reaction time recorded
- CIt was measured in seconds rather than milliseconds
- DIt was recorded using the non-dominant hand
22.The average speed of a nerve impulse in the human body is 100 metres per second. A man, who is 2 metres tall, accidentally stubs his toe in the dark. Calculate the time it takes the nerve impulse to travel to his brain. Give your answer in milliseconds.
Medium
- A20 ms
- B200 ms
- C2 ms
- D0.02 ms
23.A nerve impulse travels at 100 m/s. A person is 1.8 m tall. How long does the impulse take to travel from the toe to the brain?
Medium
- A18 ms
- B180 ms
- C1.8 ms
- D0.18 ms
24.A survey of 200 adults found that 120 wore glasses, 40 wore contact lenses, 10 had laser surgery and the rest had normal vision. What percentage of adults had normal vision?
Medium
- A15%
- B20%
- C10%
- D30%
25.Anatoxin is a neurotoxin that is similar in shape to the neurotransmitter that usually binds to the nicotinic acetylcholine receptor. Suggest how anatoxin can lead to death.
Hard
- AIt binds to the receptors, blocking the neurotransmitter and preventing impulses reaching the diaphragm, so breathing stops
- BIt destroys the myelin sheath, slowing impulses along the motor neurone
- CIt increases the speed of diffusion of neurotransmitters across the synaptic cleft
- DIt causes the diaphragm muscle to contract continuously, preventing exhalation
26.A drug binds to neurotransmitter receptors on the postsynaptic membrane and triggers impulses in different regions of the brain. Which of the following statements about this process are correct? (Select all that apply.)
Medium
- ANeurotransmitters normally cross the synaptic cleft by diffusion
- BThe receptors are complementary in shape to the neurotransmitter
- CThe drug acts at the only part of the nervous system where messages are chemical rather than electrical
- DFrequent drug use can lead to overstimulation of neurones and loss of function
- EThe drug travels along the motor neurone as an electrical impulse
27.In a reflex arc, there are synapses between the sensory and relay neurones, and between the relay and motor neurones.
Medium
True or false?
28.Diffusion of neurotransmitters across the synaptic cleft is the fastest part of the reflex arc.
Medium
True or false?
29.Match each part of the eye to its function.
Medium
- Retina
- Optic nerve
- Cornea
- Iris
- Contains light receptor cells that detect light intensity and colour
- Sensory neurone that carries electrical impulses from the eye to the brain
- Transparent covering of the front of the eye that refracts light
- Controls how much light enters the pupil
30.Place the events that occur when focusing on a near object into the correct order.
Medium
- The ciliary muscles contract
- The suspensory ligaments loosen
- The lens becomes thicker and refracts light more strongly
31.Which of the following is the independent variable in this investigation?
Medium
- AWhether the person has consumed caffeine
- BThe distance the ruler falls before being caught
- CThe hand used to catch the ruler
- DThe height from which the ruler is dropped
32.Which of the following are control variables that should be kept the same in this investigation? (Select all that apply.)
Medium
- AThe person catching the ruler uses their dominant hand each time
- BThe same ruler is dropped from the same height each time
- CThe ruler is orientated in the same direction (0 cm facing down) each time
- DThe person drinks a caffeinated drink before the first run
- EThe room is silent for the first run if background noise is being altered
33.A student investigates the effect of caffeine on reaction time using the ruler drop test. She drops the same ruler from the same height with the 0 cm mark facing downwards each time, and the volunteer always catches it with their dominant hand. Why is it important that the ruler is dropped from the same height every time?
Medium
- ASo that the distance caught depends only on the volunteer's reaction time
- BSo that the ruler is easier to catch
- CSo that the volunteer cannot see the ruler being released
- DSo that the ruler does not bend during the drop
34.A student uses a computer program to measure reaction time instead of the ruler drop test. Which of the following are advantages of using the computer method? (select all that apply)
Hard
- AIt measures reaction time in milliseconds, which is more precise
- BIt removes human error from the timing of the measurement
- CIt measures the distance the ruler falls
- DIt removes the need for the volunteer to concentrate
- EIt gives a numerical value that can be averaged over repeats
35.Reaction time is the time taken to respond to a stimulus.
Medium
True or false?
36.A student plans an investigation into the effect of background noise on reaction time. Which of the following is the independent variable in this investigation?
Medium
- AWhether background noise is playing or not
- BThe distance the ruler falls
- CThe hand the volunteer uses to catch the ruler
- DThe reaction time recorded in milliseconds
37.A student investigates the effect of caffeine on reaction time using the ruler drop test. Which of the following are control variables that should be kept the same in this investigation? (select all that apply)
Medium
- AThe hand used to catch the ruler
- BThe ruler used and the height it is dropped from
- CThe amount of caffeine consumed before the first run
- DThe colour of the ruler
- EThe room temperature
38.A drug binds to neurotransmitter receptors on the postsynaptic membrane and triggers impulses in different regions of the brain. Which of the following statements about this drug are correct? (select all that apply)
Hard
- AIt acts at synapses, the only place where messages in the nervous system are chemical
- BIt mimics the neurotransmitter by binding to complementary receptors
- CIt travels along the axon as an electrical impulse
- DFrequent use can lead to overstimulation of neurones and loss of function
- EIt works by destroying the neurotransmitter in the synaptic cleft
39.Many general anaesthetics reduce chemical transmission across a synapse. What effect would this have on a nerve impulse?
Hard
- AThe impulse would not be passed from one neurone to the next
- BThe impulse would travel faster along the axon
- CThe impulse would travel in both directions along the neurone
- DThe impulse would jump across the synaptic cleft as an electrical signal
40.Place the events that occur at a synapse into the correct order, starting with the arrival of an electrical impulse.
Hard
- Electrical impulse arrives at the end of the presynaptic neurone
- Neurotransmitters are released from vesicles into the synaptic cleft
- Neurotransmitters diffuse across the synaptic cleft
- Neurotransmitters bind with receptor molecules on the postsynaptic membrane
- The second neurone generates an electrical impulse
41.Name the two organs that make up the central nervous system (CNS).
Easy
42.Complete the sentence about neurones.
Easy
Neurones carry electrical impulses in the form of signals called ____.
43.Complete the sentence about the nervous system.
Medium
The ____ nervous system consists of the brain and spinal cord, while the ____ nervous system consists of all the nerves in the body.
44.The average speed of a nerve impulse in the human body is 100 metres per second. A man, who is 2 metres tall, accidentally stubs his toe in the dark. Calculate the time it takes the nerve impulse to travel to his brain. Give your answer in milliseconds; 1 000 milliseconds = 1 second.
Medium
45.In a reaction time experiment, three students played a computer game that measured their reaction time to click when they saw a blue light flash. Student A recorded 255 ms, 260 ms and 250 ms. Calculate Student A's mean reaction time in milliseconds.
Medium
46.Name the region of the brain that controls unconscious activities such as heart rate and breathing.
Easy
47.Complete the sentence about the structure of the nervous system. Use the words from the box: brain, spinal cord, nerves, receptors.
Easy
The central nervous system (CNS) consists of the ____ and the ____.
48.Complete the sentence about neurones. Use the words from the box: electrical impulses, chemical signals, hormones, neurotransmitters.
Medium
Neurones carry information in the form of ____ called ____.
49.A nerve impulse travels at a speed of 100 m/s. A person is 1.5 m tall. Calculate the time it takes for the impulse to travel from the toe to the brain. Give your answer in milliseconds (1 second = 1000 milliseconds).
Medium
50.Name the part of the brain that controls unconscious activities such as heart rate and breathing.
Easy
51.Name the region of the brain responsible for balance, muscle coordination and movement.
Medium
52.Complete the sentences about the eye. Use the words from the box: retina, lens, cornea, optic nerve.
Medium
The ____ is the transparent covering at the front of the eye that refracts light. The ____ at the back of the eye contains receptor cells that detect light intensity and colour.
53.Explain why reflex actions are important for survival. Give one example to support your answer.
Medium
54.Describe the pathway of a reflex action from the moment a sharp pin touches the skin to the hand being pulled away.
Hard
55.Explain why the ruler drop test is prone to errors and suggest one way to improve the reliability of the results.
Hard
56.Describe how body temperature is monitored by the brain.
Hard
57.Describe the changes that occur in the skin if the body temperature is too high.
Hard
58.The average speed of a nerve impulse in the human body is 100 metres per second. A man, who is 2 metres tall, accidentally stubs his toe in the dark. Calculate the time it takes the nerve impulse to travel to his brain. Give your answer in milliseconds; 1 000 milliseconds = 1 second.
Medium
- A18 milliseconds
- B24 milliseconds
- C22 milliseconds
- D20 milliseconds
59.In a reaction time experiment, three students played a computer game that measured their reaction time to click when they saw a blue light flash. Student A recorded 255 ms, 260 ms and 250 ms. Calculate Student A's mean reaction time in milliseconds.
Medium
- A307 ms
- B281 ms
- C229 ms
- D255 ms
60.A nerve impulse travels at a speed of 100 m/s. A person is 1.5 m tall. Calculate the time it takes for the impulse to travel from the toe to the brain. Give your answer in milliseconds (1 second = 1000 milliseconds).
Medium
- A16 ms
- B17 ms
- C14 ms
- D15 ms

Karteikarten

What two organs make up the central nervous system (CNS)?

The brain and the spinal cord.

What is the peripheral nervous system (PNS)?

All of the nerves in the body, which spread out from the CNS to all other regions of the body, including the sense organs.

How is information sent through the nervous system?

As electrical impulses — electrical signals that pass along nerve cells called neurones.

What is a nerve?

A bundle of neurones.

What is the role of the CNS in the nervous system?

It acts as the central coordinating centre for impulses that come in from, or are sent out to, any part of the body.

What is the function of sensory neurones?

They carry impulses from sense organs (receptors) to the CNS (brain or spinal cord).

What is the function of relay neurones?

They are found inside the CNS and connect sensory neurones to motor neurones.

What is the function of motor neurones?

They carry impulses from the CNS to effectors (muscles or glands).

Give the order of the components in a nervous response pathway.

Stimulus → receptor → coordinator → effector → response.

What are the two types of effector?

Muscles (which contract) and glands (which secrete hormones).

What is a synapse?

The junction between two neurones, separated by a very small gap called the synaptic cleft.

Why can't an electrical impulse cross a synapse directly?

Because of the synaptic cleft — electricity cannot jump the gap, so the signal is briefly converted to a chemical signal.

What are neurotransmitters and how do they cross the synaptic cleft?

Chemical messengers released from vesicles that diffuse across the synaptic cleft and bind with receptor molecules on the postsynaptic membrane.

What happens after neurotransmitters bind to receptors on the postsynaptic neurone?

The second neurone is stimulated to generate an electrical impulse that travels down its axon.

Why are neurotransmitters destroyed or recycled after crossing a synapse?

To prevent continued stimulation of the second neurone, which would cause repeated impulses to be sent.

Why do synapses ensure impulses travel in only one direction?

Neurotransmitters are released only from the presynaptic neurone and receptors are only on the postsynaptic neurone, avoiding confusion in the nervous system.

What is a reflex arc?

The pathway taken by electrical impulses as they travel along neurones during a reflex response.

Describe the reflex arc when someone steps on a pin.

Stimulus detected by a pain receptor in the skin → sensory neurone to spinal cord (coordinator) → relay neurone → motor neurone → muscle (effector) contracts, pulling the foot away.

Why are reflex actions important?

They are automatic and rapid and do not involve the conscious part of the brain, helping to minimise damage to the body and aid survival.

Give three examples of reflex responses that help avoid serious injury.

Pain-withdrawal, blinking and coughing.

Which region of the brain is responsible for balance, muscle coordination and movement?

The cerebellum.

What is the function of the cerebral cortex (cerebrum)?

It is the highly folded outer layer divided into two hemispheres, responsible for higher-order processes such as intelligence, memory, consciousness and personality.

What does the medulla control?

Unconscious activities such as heart rate and breathing.

Why is it difficult for neuroscientists to study the brain?

The brain is incredibly complex and delicate, and different regions cannot be studied in isolation.

What is the function of the retina?

It contains the light receptor cells (rods and cones) that detect light intensity and colour.

What is the function of the optic nerve?

It is a sensory neurone that carries electrical impulses from the eye to the brain.

What is the function of the cornea?

It is the transparent covering of the front of the eye that refracts (bends) light.

What is the function of the iris?

It controls how much light enters the pupil.

What is the function of the ciliary muscles and suspensory ligaments?

They work together to change the shape of the lens.

What happens to the pupil in dim light and in bright light?

In dim light the pupil dilates (widens) to let in as much light as possible; in bright light it constricts (narrows) to prevent too much light damaging the retina.

What is accommodation in the eye?

The process of changing the shape of the lens to focus on near or distant objects.

Describe the eye when focusing on a near object.

The ciliary muscles contract, the suspensory ligaments loosen, and the lens becomes thicker and refracts light more strongly.

Describe the eye when focusing on a distant object.

The ciliary muscles relax, the suspensory ligaments are pulled tight, and the lens is pulled thin and refracts light only slightly.

What is myopia and how is it corrected?

Short-sightedness, where the lens is too curved or the eyeball too long so light focuses in front of the retina; corrected with a concave lens.

What is hyperopia and how is it corrected?

Long-sightedness, where the lens is too flat or the eyeball too short so light focuses behind the retina; corrected with a convex lens.

What causes cataracts and how are they treated?

A build-up of protein clouds the lens, dispersing or absorbing light and causing blurred vision; treated by replacing the lens with an artificial one.

What causes colour blindness and can it be cured?

The cones in the retina do not work properly or are absent; there is currently no cure.

What is homeostasis?

The regulation of the internal conditions of a cell or organism to maintain optimum conditions.

What is the normal core body temperature and why is it maintained?

Around 37°C, the temperature at which enzymes work best.

How is body temperature monitored and controlled?

By the thermoregulatory centre in the brain, which contains receptors sensitive to the temperature of the blood; the skin also contains temperature receptors that send impulses to it.

What changes occur in the body when temperature is too high?

Blood vessels dilate (vasodilation) and sweat is produced by sweat glands, transferring energy from the skin to the environment to cool the body.

What changes occur in the body when temperature is too low?

Blood vessels constrict (vasoconstriction), sweating stops and skeletal muscles contract (shiver), reducing heat loss and releasing heat energy.

How is reaction time measured in the ruler drop test?

A ruler is dropped and caught; the further down the ruler it is caught, the slower the reaction time.

Name three factors that can affect reaction time.

Age, stress and use of drugs such as caffeine.

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