A hormone is a chemical messenger, secreted by an endocrine gland directly into the blood , that acts on specific target cells .

Define endocrine gland .

An endocrine gland is a ductless gland that secretes hormones directly into the bloodstream .

What are the key features of the endocrine system ?

Ductless glands secrete hormones directly into the blood . Hormones travel in the blood plasma to their target cells. They act only on target cells with complementary receptors . Responses are slower but longer-lasting and often widespread.

Why does a hormone only affect certain target cells ?

Only target cells have the specific complementary receptors for that hormone, so only they can respond to it.

Hormones are transported around the body dissolved in the .

Hormones are transported around the body dissolved in the blood plasma .

True or False? Hormones are transported to their target cells along nerve fibres.

False. Hormones are transported in the blood , not along nerve fibres.

Compare the speed of the nervous and endocrine systems.

The nervous system is fast (transmission in milliseconds). The endocrine system is slow (seconds, minutes or longer).

How is information transmitted in the nervous system compared with the endocrine system?

Nervous system: electrical impulses carried along neurones (with neurotransmitters at synapses). Endocrine system: chemical hormones carried in the blood .

Compare how long the response lasts in the nervous and endocrine systems.

Nervous responses are usually short-lived . Endocrine (hormonal) responses are usually long-lasting .

Compare how widespread the effects of the two systems are.

Nervous responses are localised — they act on a specific effector. Endocrine responses can be widespread , affecting many cells or organs.

Responses brought about by the nervous system are typically -lived, whereas hormonal responses last longer.

Responses brought about by the nervous system are typically short -lived, whereas hormonal responses last longer.

True or False? Nervous responses are generally slower than hormonal responses.

False. Nervous responses are generally faster than hormonal responses.

A neurone is a specialised cell that transmits electrical impulses through the nervous system.

Describe the structure and function of a sensory neurone .

Carries impulses from receptors to the CNS . Has one long dendron (bringing the impulse to the cell body) and an axon (carrying it to the CNS). The cell body is in a side-branch, part-way along.

Describe the structure and function of a motor neurone .

Carries impulses from the CNS to an effector (muscle or gland). Has a cell body at one end (within the CNS) with many dendrites , and a long axon .

What is the role of intermediate (relay) neurones ?

They connect sensory neurones to motor neurones within the central nervous system.

What is an effector ?

An effector is a muscle or gland that carries out a response when stimulated by a motor neurone.

A motor neurone carries impulses away from the CNS to an .

A motor neurone carries impulses away from the CNS to an effector .

True or False? A motor neurone's cell body is found within the central nervous system.

True. The cell body of a motor neurone lies within the CNS , with its long axon extending to the effector.

Define sensory receptor .

A sensory receptor is a cell (or nerve ending) that detects a specific stimulus and acts as a transducer .

A transducer converts one form of energy into another. A receptor converts the energy of a stimulus into an electrical nerve impulse .

What is the role of a sensory receptor cell ?

To detect a specific stimulus and convert its energy into an electrical impulse , stimulating the transmission of impulses in a sensory neurone .

Why are receptors described as energy transducers ?

Because they convert the energy of a stimulus into the electrical energy of a nerve impulse.

Define generator potential .

A generator potential is the small depolarisation produced when a receptor detects a stimulus. If it reaches threshold , it triggers an action potential in the sensory neurone.

Receptors act as , converting the energy of a stimulus into electrical impulses.

Receptors act as transducers , converting the energy of a stimulus into electrical impulses.

True or False? Each type of sensory receptor responds to many different stimuli.

False. Each type of receptor is specific , responding only to a particular stimulus .

How does a chemoreceptor in a taste bud detect a stimulus?

Chemicals from food bind to or enter the receptor cell membrane , changing its permeability and causing depolarisation .

Outline the sequence of events leading to an action potential in a taste bud chemoreceptor cell.

Chemicals from food bind to the receptor cell membrane . Sodium ion channels open, Na + enters and the membrane depolarises (a generator potential). If threshold is reached, voltage-gated calcium ion channels open and Ca 2+ enters. Ca 2+ triggers neurotransmitter release, which stimulates an action potential in the sensory neurone.

What triggers neurotransmitter release from a taste bud chemoreceptor cell?

Depolarisation opens voltage-gated calcium ion channels . The influx of Ca^2+^ causes vesicles to fuse with the membrane and release neurotransmitter.

Define generator potential .

A generator potential is the small depolarisation of a receptor cell produced by a stimulus. If large enough to reach threshold , it leads to an action potential.

In a taste bud, entry of ions depolarises the chemoreceptor cell membrane.

In a taste bud, entry of sodium ions depolarises the chemoreceptor cell membrane.

True or False? The taste bud receptor cell generates the action potential that travels to the brain.

False. The receptor cell depolarises and releases neurotransmitter ; the action potential is generated in the sensory neurone .

Define resting potential .

The resting potential is the potential difference across a neurone's membrane when it is not transmitting an impulse, about –70 mV (inside negative).

How is the resting potential maintained ?

The sodium–potassium pump actively pumps 3 Na^+^ out and 2 K^+^ in . The membrane is more permeable to K^+^ , which leaks back out. This leaves the inside negative relative to the outside (about –70 mV).

Define action potential .

An action potential is a rapid, temporary reversal of the membrane potential, from about –70 mV to +30 mV .

Describe the events during an action potential .

Depolarisation: voltage-gated Na^+^ channels open, Na + floods in, the membrane reverses to about +30 mV . Repolarisation: Na + channels close and voltage-gated K^+^ channels open, so K + leaves and the potential falls. Restoration: a brief hyperpolarisation occurs, then the Na^+^/K^+^ pump re-establishes the resting potential.

Define depolarisation .

Depolarisation is when the membrane potential becomes less negative (reverses) as Na^+^ ions enter the neurone.

Define repolarisation .

Repolarisation is the return of the membrane potential to a negative value as K^+^ ions leave the neurone.

What is meant by the all-or-nothing principle?

An action potential only fires if depolarisation reaches the threshold (about –55 mV). If it does, the action potential is always the same size , whatever the strength of the stimulus.

During depolarisation, voltage-gated ion channels open and Na + rushes into the neurone.

During depolarisation, voltage-gated sodium ion channels open and Na + rushes into the neurone.

True or False? A stronger stimulus produces a larger action potential.

False. Action potentials are all-or-nothing — all the same size. A stronger stimulus increases the frequency of impulses, not their size.

Define myelin sheath .

The myelin sheath is an insulating layer of membrane, formed by Schwann cells , wrapped around the axon of a neurone.

Define nodes of Ranvier .

The nodes of Ranvier are the gaps between segments of the myelin sheath , where the axon membrane is exposed.

Define saltatory conduction .

Saltatory conduction is the way an impulse 'jumps' from one node of Ranvier to the next in a myelinated neurone, speeding up transmission.

Explain how myelination speeds up the transmission of an impulse.

Myelin is an electrical insulator , so depolarisation (action potentials) can only occur at the nodes of Ranvier . The impulse therefore jumps from node to node ( saltatory conduction ), which is much faster than continuous conduction along the whole membrane.

Besides myelination, name two factors that increase conduction speed.

A larger axon diameter . A higher temperature .

In a myelinated neurone, action potentials can only occur at the of Ranvier.

In a myelinated neurone, action potentials can only occur at the nodes of Ranvier.

True or False? In a myelinated axon, the action potential forms at every point along the membrane.

False. Action potentials only form at the nodes of Ranvier ; the impulse jumps between them ( saltatory conduction ).

Define refractory period .

The refractory period is the short time after an action potential during which a neurone cannot be restimulated, because the voltage-gated Na^+^ channels are closed/inactivated.

Why can no new action potential occur during the refractory period?

The voltage-gated sodium ion channels are closed/inactivated, so the membrane cannot depolarise again while the resting potential is being restored.

A LevelBiologyCambridge (CIE)Flashcards15. Control & CoordinationControl & Coordination in Mammals

Control & Coordination in Mammals (Cambridge (CIE) A Level Biology): Flashcards

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FrontThe Endocrine System
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Cards in this collection (87)

Define hormone.
A hormone is a chemical messenger, secreted by an endocrine gland directly into the blood, that acts on specific target cells.
Define endocrine gland.
An endocrine gland is a ductless gland that secretes hormones directly into the bloodstream.
What are the key features of the endocrine system?
Ductless glands secrete hormones directly into the blood.
Hormones travel in the blood plasma to their target cells.
They act only on target cells with complementary receptors.
Responses are slower but longer-lasting and often widespread.
Why does a hormone only affect certain target cells?
Only target cells have the specific complementary receptors for that hormone, so only they can respond to it.
Name three hormones used as examples of the endocrine system.
ADH, glucagon and insulin.
Hormones are transported around the body dissolved in the .
Hormones are transported around the body dissolved in the blood plasma.
True or False?
Hormones are transported to their target cells along nerve fibres.
False.
Hormones are transported in the blood, not along nerve fibres.
Compare the speed of the nervous and endocrine systems.
The nervous system is fast (transmission in milliseconds).
The endocrine system is slow (seconds, minutes or longer).
How is information transmitted in the nervous system compared with the endocrine system?
Nervous system: electrical impulses carried along neurones (with neurotransmitters at synapses).
Endocrine system: chemical hormones carried in the blood.
Compare how long the response lasts in the nervous and endocrine systems.
Nervous responses are usually short-lived.
Endocrine (hormonal) responses are usually long-lasting.
Compare how widespread the effects of the two systems are.
Nervous responses are localised — they act on a specific effector.
Endocrine responses can be widespread, affecting many cells or organs.
Responses brought about by the nervous system are typically -lived, whereas hormonal responses last longer.
Responses brought about by the nervous system are typically short-lived, whereas hormonal responses last longer.
True or False?
Nervous responses are generally slower than hormonal responses.
False.
Nervous responses are generally faster than hormonal responses.
Define neurone.
A neurone is a specialised cell that transmits electrical impulses through the nervous system.
Describe the structure and function of a sensory neurone.
Carries impulses from receptors to the CNS.
Has one long dendron (bringing the impulse to the cell body) and an axon (carrying it to the CNS).
The cell body is in a side-branch, part-way along.
Describe the structure and function of a motor neurone.
Carries impulses from the CNS to an effector (muscle or gland).
Has a cell body at one end (within the CNS) with many dendrites, and a long axon.
What is the role of intermediate (relay) neurones?
They connect sensory neurones to motor neurones within the central nervous system.
What is an effector?
An effector is a muscle or gland that carries out a response when stimulated by a motor neurone.
A motor neurone carries impulses away from the CNS to an .
A motor neurone carries impulses away from the CNS to an effector.
True or False?
A motor neurone's cell body is found within the central nervous system.
True.
The cell body of a motor neurone lies within the CNS, with its long axon extending to the effector.
Define sensory receptor.
A sensory receptor is a cell (or nerve ending) that detects a specific stimulus and acts as a transducer.
Define transducer.
A transducer converts one form of energy into another. A receptor converts the energy of a stimulus into an electrical nerve impulse.
What is the role of a sensory receptor cell?
To detect a specific stimulus and convert its energy into an electrical impulse, stimulating the transmission of impulses in a sensory neurone.
Why are receptors described as energy transducers?
Because they convert the energy of a stimulus into the electrical energy of a nerve impulse.
Define generator potential.
A generator potential is the small depolarisation produced when a receptor detects a stimulus. If it reaches threshold, it triggers an action potential in the sensory neurone.
Receptors act as , converting the energy of a stimulus into electrical impulses.
Receptors act as transducers, converting the energy of a stimulus into electrical impulses.
True or False?
Each type of sensory receptor responds to many different stimuli.
False.
Each type of receptor is specific, responding only to a particular stimulus.
How does a chemoreceptor in a taste bud detect a stimulus?
Chemicals from food bind to or enter the receptor cell membrane, changing its permeability and causing depolarisation.
Outline the sequence of events leading to an action potential in a taste bud chemoreceptor cell.
Chemicals from food bind to the receptor cell membrane.
Sodium ion channels open, Na⁺ enters and the membrane depolarises (a generator potential).
If threshold is reached, voltage-gated calcium ion channels open and Ca²⁺ enters.
Ca²⁺ triggers neurotransmitter release, which stimulates an action potential in the sensory neurone.
What triggers neurotransmitter release from a taste bud chemoreceptor cell?
Depolarisation opens voltage-gated calcium ion channels. The influx of Ca^2+^ causes vesicles to fuse with the membrane and release neurotransmitter.
Define generator potential.
A generator potential is the small depolarisation of a receptor cell produced by a stimulus. If large enough to reach threshold, it leads to an action potential.
In a taste bud, entry of ions depolarises the chemoreceptor cell membrane.
In a taste bud, entry of sodium ions depolarises the chemoreceptor cell membrane.
True or False?
The taste bud receptor cell generates the action potential that travels to the brain.
False.
The receptor cell depolarises and releases neurotransmitter; the action potential is generated in the sensory neurone.
Define resting potential.
The resting potential is the potential difference across a neurone's membrane when it is not transmitting an impulse, about –70 mV (inside negative).
How is the resting potential maintained?
The sodium–potassium pump actively pumps 3 Na^+^ out and 2 K^+^ in.
The membrane is more permeable to K^+^, which leaks back out.
This leaves the inside negative relative to the outside (about –70 mV).
Define action potential.
An action potential is a rapid, temporary reversal of the membrane potential, from about –70 mV to +30 mV.
Describe the events during an action potential.
Depolarisation: voltage-gated Na^+^ channels open, Na⁺ floods in, the membrane reverses to about +30 mV.
Repolarisation: Na⁺ channels close and voltage-gated K^+^ channels open, so K⁺ leaves and the potential falls.
Restoration: a brief hyperpolarisation occurs, then the Na^+^/K^+^ pump re-establishes the resting potential.
Define depolarisation.
Depolarisation is when the membrane potential becomes less negative (reverses) as Na^+^ ions enter the neurone.
Define repolarisation.
Repolarisation is the return of the membrane potential to a negative value as K^+^ ions leave the neurone.
What is meant by the all-or-nothing principle?
An action potential only fires if depolarisation reaches the threshold (about –55 mV).
If it does, the action potential is always the same size, whatever the strength of the stimulus.
During depolarisation, voltage-gated ion channels open and Na⁺ rushes into the neurone.
During depolarisation, voltage-gated sodium ion channels open and Na⁺ rushes into the neurone.
True or False?
A stronger stimulus produces a larger action potential.
False.
Action potentials are all-or-nothing — all the same size. A stronger stimulus increases the frequency of impulses, not their size.
Define myelin sheath.
The myelin sheath is an insulating layer of membrane, formed by Schwann cells, wrapped around the axon of a neurone.
Define nodes of Ranvier.
The nodes of Ranvier are the gaps between segments of the myelin sheath, where the axon membrane is exposed.
Define saltatory conduction.
Saltatory conduction is the way an impulse 'jumps' from one node of Ranvier to the next in a myelinated neurone, speeding up transmission.
Explain how myelination speeds up the transmission of an impulse.
Myelin is an electrical insulator, so depolarisation (action potentials) can only occur at the nodes of Ranvier.
The impulse therefore jumps from node to node (saltatory conduction), which is much faster than continuous conduction along the whole membrane.
Besides myelination, name two factors that increase conduction speed.
A larger axon diameter.
A higher temperature.
In a myelinated neurone, action potentials can only occur at the of Ranvier.
In a myelinated neurone, action potentials can only occur at the nodes of Ranvier.
True or False?
In a myelinated axon, the action potential forms at every point along the membrane.
False.
Action potentials only form at the nodes of Ranvier; the impulse jumps between them (saltatory conduction).
Define refractory period.
The refractory period is the short time after an action potential during which a neurone cannot be restimulated, because the voltage-gated Na^+^ channels are closed/inactivated.
Why can no new action potential occur during the refractory period?
The voltage-gated sodium ion channels are closed/inactivated, so the membrane cannot depolarise again while the resting potential is being restored.
Explain how the refractory period limits the frequency of impulses.
A new action potential cannot start until the refractory period ends.
This sets a minimum time between impulses, placing an upper limit on the frequency of impulses.
Give two further roles of the refractory period.
It ensures action potentials travel in one direction only.
It ensures action potentials are discrete (separate) events.
During the period, a neurone cannot generate another action potential.
During the refractory period, a neurone cannot generate another action potential.
True or False?
The refractory period allows action potentials to travel in both directions along an axon.
False.
The refractory period ensures action potentials travel in one direction only.
Define synapse.
A synapse is a junction between two neurones (or a neurone and an effector) where they do not touch, separated by the synaptic cleft.
Define cholinergic synapse.
A cholinergic synapse is a synapse that uses acetylcholine as its neurotransmitter.
Describe the structure of a cholinergic synapse.
A presynaptic neurone containing vesicles of acetylcholine and many mitochondria.
A synaptic cleft between the neurones.
A postsynaptic membrane with acetylcholine receptors and sodium ion channels.
Explain how an impulse is transmitted across a cholinergic synapse.
An action potential reaches the presynaptic membrane and opens voltage-gated calcium ion channels, so Ca²⁺ enters.
Ca²⁺ causes vesicles to fuse with the membrane and release acetylcholine by exocytosis.
Acetylcholine diffuses across the cleft and binds receptors on the postsynaptic membrane, opening Na⁺ channels.
If threshold is reached, a new action potential is generated in the postsynaptic neurone.
What is the role of calcium ions at a cholinergic synapse?
The influx of Ca^2+^ triggers the vesicles of acetylcholine to fuse with the presynaptic membrane and release their neurotransmitter.
What happens to acetylcholine after it has acted?
It is broken down by acetylcholinesterase into choline and ethanoic acid.
These are reabsorbed into the presynaptic neurone to resynthesise acetylcholine, preventing continuous stimulation.
The neurotransmitter released at a cholinergic synapse is .
The neurotransmitter released at a cholinergic synapse is acetylcholine.
True or False?
Transmission can occur in both directions across a cholinergic synapse.
False.
Only the presynaptic neurone contains vesicles of neurotransmitter, so transmission occurs in one direction only.
Define neuromuscular junction.
A neuromuscular junction is a synapse between a motor neurone and a striated muscle fibre.
How is a neuromuscular junction similar to a cholinergic synapse?
It uses acetylcholine.
ACh binds receptors on the muscle membrane (sarcolemma), causing depolarisation and an action potential.
Define T-tubules (transverse tubules).
T-tubules are infoldings of the muscle cell membrane (sarcolemma) that carry the action potential deep into the muscle fibre.
Define sarcoplasmic reticulum.
The sarcoplasmic reticulum is a specialised endoplasmic reticulum in muscle that stores and releases calcium ions (Ca²⁺).
Describe how a nerve impulse triggers muscle contraction.
Acetylcholine at the neuromuscular junction depolarises the sarcolemma, generating an action potential.
The action potential spreads along the sarcolemma and down the T-tubules into the fibre.
This stimulates the sarcoplasmic reticulum to release Ca^2+^ into the sarcoplasm, triggering contraction.
What is the role of the T-tubule system?
It carries the action potential (depolarisation) from the sarcolemma deep into the fibre, so all the myofibrils are stimulated at the same time.
The reticulum releases calcium ions into the sarcoplasm to trigger contraction.
The sarcoplasmic reticulum releases calcium ions into the sarcoplasm to trigger contraction.
True or False?
The sarcoplasmic reticulum stores sodium ions for muscle contraction.
False.
The sarcoplasmic reticulum stores and releases calcium ions (Ca²⁺).
Define sarcomere.
A sarcomere is the functional (contractile) unit of a myofibril, the region between two Z-lines.
Define myofibril.
A myofibril is a long cylindrical organelle within a muscle fibre, made of repeating sarcomeres of actin and myosin.
What are the two types of protein filament in a sarcomere?
Thin filaments made of actin.
Thick filaments made of myosin.
Describe the bands and lines of a sarcomere.
I band: light — actin only.
A band: dark — the full length of the myosin (including overlap with actin).
H zone: central region of myosin only.
Z-line bounds the sarcomere; the M-line runs through its centre.
Which band contains only thin (actin) filaments?
The I band (the light band).
A sarcomere is the region of a myofibril between two .
A sarcomere is the region of a myofibril between two Z-lines.
True or False?
The A band is the light band that contains only actin.
False.
The A band is the dark band (the length of the myosin). The I band is the light band containing only actin.
Define the sliding filament model.
The sliding filament model states that muscle contracts as actin filaments slide past myosin, shortening the sarcomere. The filaments themselves do not shorten.
What happens to the sarcomere bands during contraction?
The sarcomere shortens.
The I band and H zone get shorter.
The A band stays the same length.
What is the role of calcium ions in muscle contraction?
Ca^2+^ binds to troponin, changing its shape.
This moves tropomyosin away from the myosin-binding sites on the actin filament.
What are the roles of troponin and tropomyosin?
At rest, tropomyosin blocks the myosin-binding sites on actin, held in place by troponin.
When Ca^2+^ binds to troponin, it moves tropomyosin aside, exposing the binding sites.
Describe the cross-bridge cycle.
Myosin heads bind to the exposed sites on actin, forming cross-bridges.
The power stroke pulls the actin towards the centre of the sarcomere; ADP + P~i~ are released.
ATP binds to the myosin head, detaching it; its hydrolysis re-cocks the head, ready to bind again.
What are the roles of ATP in muscle contraction?
It detaches the myosin head from actin.
Its hydrolysis provides energy to re-cock the myosin head for the next power stroke.
It powers the pumping of Ca^2+^ back into the sarcoplasmic reticulum during relaxation.
Calcium ions bind to , moving tropomyosin away from the binding sites on actin.
Calcium ions bind to troponin, moving tropomyosin away from the binding sites on actin.
How does a muscle relax?
Nerve impulses stop, so Ca^2+^ is actively pumped back into the sarcoplasmic reticulum (using ATP).
Tropomyosin moves back to cover the binding sites, so no more cross-bridges form.
True or False?
During contraction the actin and myosin filaments themselves shorten.
False.
The filaments do not shorten — they slide past each other, so the sarcomere shortens.

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