Neuronal Communication (OCR A Level Biology): Flashcards

Carries impulses from receptors to the CNS (brain or spinal cord).

Found entirely within the CNS, connecting sensory and motor neurones.

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

Sensory neurone:

• Cell body branches off in the middle of the cell

• Has a single long dendron carrying impulses to the cell body and a single long axon carrying impulses away

Motor neurone:

• Large cell body at one end, lying within the spinal cord or brain

• Many highly-branched dendrites extending from the cell body, giving a large surface area for the axon terminals of other neurones

False — the impulse jumps from one node of Ranvier to the next.

True

A cell that responds to a stimulus.

A structure that converts energy from one form (such as light, heat or sound) into an electrical impulse within a sensory neurone.

They convert energy from one form (e.g. light, heat or sound) into an electrical impulse in a sensory neurone.

Each receptor will only respond to a specific stimulus.

A receptor cell acts as a transducer, converting energy from a stimulus into an electrical impulse.

A mechanoreceptor.

Changes in pressure.

• It consists of the end of a sensory neurone surrounded by many layers of membrane

• The layers are separated by a gel

• The gel contains positively charged sodium ions (Na⁺)

An electrical potential difference established across the receptor cell membrane when a stimulus causes ions (e.g. Na⁺) to flow into the neurone.

Stretch-mediated sodium ion channels, which open when sufficient pressure distorts the layers of membrane.

• Pressure on the skin distorts the layers of membrane surrounding the end of the sensory neurone

• This causes stretch-mediated sodium ion channels to open

• Na⁺ flow into the neurone

• This establishes an electrical potential difference across the cell membrane — a generator potential

In a Pacinian corpuscle, sufficient pressure opens stretch-mediated sodium ion channels, allowing Na⁺ to enter the neurone.

• Whole cells

• Specifically adapted parts of a cell (e.g. the Pacinian corpuscle)

• Proteins on the cell surface membrane

Deep in the skin — including the fingers and soles of the feet, and in joints, tendons and ligaments.

True

False — it detects changes in pressure.

The potential difference across the axon membrane of a neurone that is not transmitting an impulse, where the inside of the axon is negatively charged compared with the outside.

About -70 mV (the inside of the axon is about 70 mV lower than the outside).

The inside of the axon always has a negative electrical potential compared with the outside.

• The active transport of sodium ions and potassium ions (by the sodium-potassium pump)\n\n- Differential membrane permeability to sodium and potassium ions

They use ATP to actively transport 3 sodium ions out of the axon for every 2 potassium ions moved in.

It creates a larger concentration of positive ions outside the axon than inside, helping to establish an electrochemical gradient.

Because it moves sodium and potassium ions against their concentration gradients by active transport, which requires energy from respiration.

A gradient of both charge and concentration across a membrane, established here by the net movement of ions via the sodium-potassium pump.

The protein channels are less permeable to sodium ions than potassium ions, so potassium ions diffuse back out of the axon down their concentration gradient faster than sodium ions diffuse in, leaving the inside more negative.

Facilitated diffusion down their concentration gradients.

The sodium-potassium pump uses ATP to actively transport sodium and potassium ions across the axon membrane.

The membrane's protein channels are less permeable to sodium ions than to potassium ions.

True

False — at rest the inside of the axon is negatively charged compared with the outside.

A brief change in the distribution of electrical charge across the cell surface membrane of an axon, caused by the rapid movement of sodium ions and potassium ions across the membrane.

It is not a flow of electrons like a normal electric current.

The rapid movement of sodium ions and potassium ions across the membrane of the axon.

A channel protein in the axon membrane that allows sodium ions or potassium ions to pass through, and which opens and closes depending on the electrical potential (voltage) across the axon membrane.

They are closed when the membrane is at its resting potential.

• Stimulus

• Depolarisation

• Repolarisation

• Hyperpolarisation

• Return to resting potential

A stimulus triggers sodium ion channels in the membrane to open, allowing sodium ions to diffuse into the neurone down an electrochemical gradient.

The stimulus can be an electrical impulse from another neurone or a chemical change to the membrane. If large enough, it raises the potential difference to the threshold.

The potential difference across the membrane must reach a threshold of around -55 mV to trigger depolarisation.

• Voltage-gated sodium ion channels in the axon membrane open

• Sodium ions move into the axon down the electrochemical gradient

• The inside of the axon becomes less negative, reducing the potential difference (depolarisation)

• This triggers more channels to open, causing more depolarisation (positive feedback), reaching around +30 mV

The entry of sodium ions causes depolarisation, which triggers more voltage-gated sodium ion channels to open, allowing more sodium ions to enter and causing more depolarisation.

• The voltage-gated sodium ion channels close (about 1 ms after reaching the peak), stopping sodium ions entering

• Voltage-gated potassium ion channels open, allowing potassium ions to diffuse out of the axon down their concentration gradient

• This returns the potential difference towards normal (about -70 mV)

Repolarisation is an example of negative feedback.

Potassium ion channels are slow to close, so too many potassium ions diffuse out of the neurone.

Once the potassium ion voltage-gated channels are closed, the sodium-potassium pump restores the resting potential.

The sodium ion channel proteins in this section of membrane then become responsive to depolarisation again.

As a wave of depolarisation that moves along the length of the neurone.

It briefly makes the potential difference across the membrane more negative than the normal resting potential.

False — it is a brief change in the distribution of electrical charge caused by the movement of sodium ions and potassium ions, not a flow of electrons.

True

It travels in one direction only, like a wave of depolarisation.

• An action potential triggers depolarisation of that section of the axon

• The local current causes sodium ion channels to open a little further along the axon

• This causes an influx of sodium ions, generating an action potential in the next section (in one direction)

• The previous section is in the repolarisation stage and is unresponsive, so the impulse continues to move forwards

The previous section of the axon is in the repolarisation stage: its sodium channels are closed and potassium channels are open, making it unresponsive.

Because the region behind cannot be re-stimulated, the impulse can only move forwards, making action potentials discrete events.

That section is in the repolarisation stage:

• Sodium ion channels are closed

• Potassium ion channels are open

This makes it temporarily unresponsive to further stimulation.

The opening of sodium ion channels a little further along the axon causes an influx of these ions, generating an action potential in the next section.

The membrane potential that must be reached for an action potential to be triggered. A stimulus must raise the membrane potential above this value for an impulse to be transmitted.

The principle that an impulse is only transmitted if the initial stimulus is sufficient to increase the membrane potential above the threshold potential. Below threshold, no action potential is produced; at or above it, a full action potential fires.

The receptor cell is not sufficiently depolarised, so the receptor potential does not reach the threshold.

As a result, the sensory neurone is not activated and no impulse is sent.

The receptor is depolarised enough to raise the receptor potential above the threshold potential, which stimulates the sensory neurone to send impulses. This is an example of the all-or-nothing principle.

The frequency of impulses increases (more impulses per second).

The amplitude of each action potential stays the same; it does not increase.

Rather than staying constant, threshold levels often increase with continued stimulation, so a greater stimulus is required before impulses are sent along the sensory neurone.

Depolarisation only occurs at the nodes of Ranvier.

It travels along the entire length of the axon, as a wave of depolarisation.

The previous node is in the repolarisation stage and cannot be re-stimulated, so transmission occurs in one direction only.

True

False — the frequency of impulses increases, while the amplitude stays the same.

The period of time after an action potential when a section of the axon membrane is unresponsive and cannot be excited to produce a new action potential, because it is recovering.

Both sets of channels are closed:

• Sodium ion channels are closed (during repolarisation)

• Potassium ion channels are closed (during hyperpolarisation)

The membrane is in a period of recovery, so it is unresponsive and a new action potential cannot be generated there.

It begins when repolarisation starts and ends when the resting potential is re-established.

The refractory period ensures that action potentials are discrete events, stopping them from merging into one another.

The region behind the action potential is still refractory (recovering), so a new action potential can only be generated in the region ahead, further along the axon.

The membrane behind the action potential is refractory (recovering and unresponsive), so a new action potential cannot form there.

This means the impulse can only propagate forwards, ensuring unidirectional transmission along the neurone.

The membrane must fully recover and return to its resting state before another action potential can be generated at that point, imposing a minimum time gap between successive action potentials.

The length of the refractory period determines the maximum frequency of impulses (typically between 500 and 1000 per second).

• It ensures action potentials are discrete events that do not merge together

• It ensures impulses travel in one direction only (new action potentials are generated ahead, not behind)

False — the membrane is recovering and unresponsive, so it cannot produce a new action potential until it returns to its resting potential.

True

How quickly the impulse is transmitted along a neurone.

• Myelination (whether or not the axon is insulated by a myelin sheath)\n\n- The diameter of the axon\n\n- Temperature

Because depolarisation must occur along the whole membrane of the axon, rather than jumping between points.

Schwann cells.

The myelin sheath stops the diffusion of sodium ions and potassium ions across the axon membrane, so depolarisation cannot occur there.

The small uninsulated sections of a myelinated axon, between adjacent myelin sheaths, where action potentials can occur.

The process by which action potentials 'jump' from one node of Ranvier to the next along a myelinated axon, allowing the impulse to travel much faster than in an unmyelinated axon of the same diameter.

In a myelinated axon, action potentials can only occur at the nodes of Ranvier, the small uninsulated sections of the axon.

It allows the impulse to travel much faster (up to 50 times faster).

• A thicker axon has an axon membrane with a greater surface area for the diffusion of ions, increasing the rate of diffusion of sodium and potassium ions and so the rate of depolarisation and action potentials\n\n- A greater diameter also means a greater volume of cytoplasm, reducing electrical resistance so an action potential can push into the next section faster

Mammals maintain very stable body temperatures, so temperature does not usually affect the speed of nerve impulses in these animals.

Colder temperatures mean there is less kinetic energy available for the facilitated diffusion of sodium and potassium ions during an action potential, slowing conduction.

A greater volume of cytoplasm in a thicker axon reduces the axon's electrical resistance, allowing an action potential to push into the next section faster.

True

False — action potentials can only occur at the nodes of Ranvier, the uninsulated gaps between myelin sheaths.

The junction where two neurones meet.

A chemical messenger released from vesicles at the presynaptic membrane that diffuses across the synaptic cleft to transmit an impulse to the postsynaptic neurone.

A synapse that uses acetylcholine (ACh) as its neurotransmitter.

• Presynaptic knob

• Synaptic cleft

• Postsynaptic membrane

Electrical impulses cannot 'jump' across the synaptic cleft, so transmission across a synapse must instead occur chemically, using neurotransmitters.

• An electrical impulse arrives at the synaptic knob of the presynaptic neurone.

• Neurotransmitters are released from vesicles at the presynaptic membrane.

• The neurotransmitters diffuse across the synaptic cleft.

• They temporarily bind with receptor molecules on the postsynaptic membrane.

• This stimulates the postsynaptic neurone to generate an electrical impulse that travels down its axon.

They are released from vesicles at the presynaptic membrane of the presynaptic neurone.

They diffuse across the synaptic cleft from the presynaptic membrane to the postsynaptic membrane.

They temporarily bind with receptor molecules on the postsynaptic membrane, stimulating the postsynaptic neurone to generate an electrical impulse.

They are destroyed or recycled.

Synapses that use acetylcholine (ACh) as their neurotransmitter are described as cholinergic synapses.

Neurotransmitters diffuse across the synaptic cleft to reach the postsynaptic membrane.

It prevents continued stimulation of the postsynaptic neurone, which could otherwise cause repeated impulses to be sent.

False — impulses cannot jump across; transmission occurs chemically using neurotransmitters.

False — they are released from vesicles at the presynaptic membrane.

A synapse that uses the neurotransmitter acetylcholine (ACh) to transmit an impulse across the synaptic cleft.

A neurotransmitter released at cholinergic synapses to transmit the impulse to the postsynaptic neurone.

The action potential causes depolarisation of the presynaptic membrane, which stimulates voltage-gated calcium ion channels to open.

• Ca²⁺ diffuses down an electrochemical gradient into the synaptic knob

• This stimulates ACh-containing vesicles to fuse with the presynaptic membrane

• ACh is released into the synaptic cleft by exocytosis

• ACh diffuses across the synaptic cleft and binds to cholinergic receptors on the postsynaptic membrane

• This causes sodium ion channels to open

• Sodium ions diffuse into the postsynaptic neurone, causing depolarisation once the threshold is reached

By exocytosis — vesicles fuse with the presynaptic membrane and release ACh.

It prevents the sodium ion channels from staying permanently open, stopping permanent depolarisation of the postsynaptic membrane.

The enzyme acetylcholinesterase catalyses the hydrolysis of ACh into acetate and choline.

The enzyme acetylcholinesterase catalyses the hydrolysis of acetylcholine into acetate and choline.

Neurotransmitter is released only from the presynaptic knob and its receptors are only on the postsynaptic membrane.

Therefore chemical transmission cannot occur in the opposite direction, preventing impulses travelling back the wrong way.

The process by which the effects of multiple impulses are added together at synapses to generate an action potential when a single impulse would be insufficient.

Temporal summation: multiple impulses arrive in quick succession at a single synaptic knob, and their effects add together to trigger an action potential.

Spatial summation: multiple impulses arrive simultaneously at different synaptic knobs stimulating the same cell body, together releasing enough ACh to trigger an action potential.

• Sodium ions enter the cell body from the excitatory synapse

• Potassium ions diffuse out of the cell body from the inhibitory synapse

• These effects cancel out, so the threshold is not reached and no action potential is generated

Excitatory: open sodium ion channels, causing depolarisation that can generate an action potential if the threshold is reached.

Inhibitory: open potassium ion channels, causing hyperpolarisation, which prevents an action potential from being generated.

By diffusion across the synaptic cleft.

Choline is absorbed back into the presynaptic membrane and reacts with acetyl coenzyme A to re-form ACh, which is packaged into vesicles ready for the next action potential.

True

False — they open potassium ion channels, causing hyperpolarisation.