Neural Signalling (DP IB Biology)

An impulse in a neurone is a momentary reversal in the electrical potential difference across the neurone cell surface membrane, not an electrical current flowing along the neurone as if it were a wire.

The resting potential is the electrical potential difference across the membrane of a resting neurone, typically around -70 millivolts (mV).

The negative charge inside a resting neurone is due to the active transport of sodium and potassium ions, a difference in the diffusion rates of these ions, and negatively charged proteins inside the axon.

The sodium-potassium pump actively transports 3 sodium ions (Na⁺) out of the axon and 2 potassium ions (K⁺) into the axon, creating a concentration gradient that contributes to the resting potential.

The neurone membrane is much less permeable to sodium ions than potassium ions, allowing potassium ions to diffuse out faster than sodium ions can diffuse back in, leading to a negative charge inside the neurone.

The unequal diffusion rates of sodium and potassium ions result in a resting membrane potential of about -70 mV, with the inside of the neurone being more negative compared to the outside.

A neurone membrane is polarised when it has reached its resting potential, with a negative charge inside and a positive charge outside.

A nerve impulse is initiated by depolarisation of the neurone membrane, which involves a reversal of the electrical potential difference, generating an action potential.

An action potential is a rapid change in the electrical potential difference across the axon membrane, reversing the membrane potential from around -70 mV to around +40 mV.

Sodium ions (Na⁺) and potassium ions (K⁺) are involved in generating an action potential, with sodium ions rapidly entering the axon and potassium ions exiting, causing the depolarisation of the membrane.

The speed of nerve impulse transmission is affected by the myelination of the neurone and the diameter of the axon.

Myelinated neurones conduct impulses much faster due to the insulation provided by the myelin sheath, which allows for saltatory conduction, where impulses jump between nodes of Ranvier.

Wider axons conduct impulses faster because they offer less resistance to the action potential, allowing the electrical impulse to travel more quickly.

Despite being significantly wider, the unmyelinated axon of a squid conducts impulses more slowly than a narrower, myelinated mammalian axon due to the absence of saltatory conduction.

Schwann cells form the myelin sheath around the axon by wrapping themselves around it, providing electrical insulation and facilitating faster impulse conduction.

Saltatory conduction is the process by which nerve impulses jump from one node of Ranvier to the next along a myelinated axon, greatly increasing the speed of transmission.

The relationship can be analysed by collecting data on these variables, plotting them on a scatter graph, and calculating the correlation coefficient to determine the strength of the correlation.

The correlation coefficient (r) indicates the strength and direction of a linear relationship between two variables. Values close to 1 or -1 indicate a strong correlation, while a value of 0 indicates no correlation.

Correlation is an association or relationship between two variables, while causation indicates that one variable directly influences or causes changes in another.

The coefficient of determination (R²) is calculated by squaring the Pearson correlation coefficient (r). It represents the proportion of the variance in the dependent variable that is predictable from the independent variable, with values closer to 1 indicating a stronger correlation.

An R² value closer to 1 indicates a strong correlation, meaning the dependent variable can be accurately predicted from the independent variable.

A synapse is the junction between two neurones, separated by a small gap called the synaptic cleft, where they communicate via chemical signals.

The synaptic cleft is the small gap between the presynaptic and postsynaptic neurones at a synapse, across which neurotransmitters diffuse to transmit nerve impulses.

When an impulse arrives at the presynaptic neurone, the membrane becomes depolarised, causing calcium ions to enter the neurone, which triggers the release of neurotransmitters into the synaptic cleft.

Calcium ions trigger the movement and fusion of neurotransmitter-containing vesicles with the presynaptic membrane, leading to the release of neurotransmitters into the synaptic cleft.

Acetylcholine (ACh) is a common neurotransmitter that, when released into the synaptic cleft, binds to receptors on the postsynaptic membrane, causing sodium ion channels to open and potentially generating an action potential.

After release, acetylcholine binds to receptors on the postsynaptic membrane and is then broken down by the enzyme acetylcholinesterase to prevent continuous stimulation of the postsynaptic neurone.

Impulses can only travel in one direction because neurotransmitters are released from the presynaptic neurone and receptors are only located on the postsynaptic membrane, ensuring unidirectional transmission.

The enzyme acetylcholinesterase breaks down acetylcholine, preventing the sodium ion channels from remaining open and stopping permanent depolarisation of the postsynaptic membrane.

The breakdown products, acetate and choline, are reabsorbed by the presynaptic neurone, where they are used to reform acetylcholine for future use.

Cholinergic synapses are synapses that use acetylcholine (ACh) as their neurotransmitter to transmit nerve impulses across the synaptic cleft.

Depolarisation is the process during an action potential in which voltage-gated sodium channels open, allowing Na⁺ ions to enter the neurone.

Repolarisation occurs when voltage-gated potassium channels open, allowing K⁺ ions to leave the neurone, and restoring the negative internal charge.

True.

Sodium channels will only open if the threshold potential is reached.

Threshold potential is the minimum membrane potential needed to open voltage-gated sodium channels and trigger an action potential.

Propagation occurs when local currents cause Na⁺ ions to diffuse and reach the threshold potential in adjacent areas, triggering successive action potentials.

Voltage-gated sodium channels open at threshold potential, allowing Na⁺ to enter the cell, leading to depolarisation.

Potassium channels allow K⁺ ions to exit the neurone, restoring a negative membrane potential.

An action potential is a rapid change in membrane potential due to depolarisation and repolarisation along a neurone.

True.

Local Na⁺ currents spread and cause the threshold potential to be reached in adjacent parts of the axon.

Local currents help propagate the action potential by spreading Na⁺ ions that depolarise nearby regions of the membrane.

An oscilloscope trace shows changes in membrane potential, including resting and action potentials, as they occur over time.

True.

Oscilloscope traces can show the number of impulses per second.

A spike indicates an action potential, which is the rapid depolarisation and repolarisation of the neurone membrane. This is caused by the opening of voltage-gated sodium ion channels followed by the closing of these channels and the opening of the voltage-gated potassium ion channels.

An action potential is a rapid, temporary change in the membrane potential of a neurone, represented on an oscilloscope trace by a sharp upward and downward spike in membrane potential.

The time interval between action potentials on the trace can be used to calculate impulse frequency and neurone firing speed.

True.

Resting potential shows as a flat line, indicating no change in membrane potential.

The rising phase corresponds to depolarisation, where Na⁺ ions enter the neurone.

The downward phase represents repolarisation, where K⁺ ions exit the neurone to restore the resting potential.

The trace shows a period of hyperpolarisation at D. This is where the membrane is briefly more negative than the resting potential.

True.

The K+ channels are open as K⁺ moves out of the neurone during repolarisation and the membrane potential decreases.

The threshold potential is -30mV because this is where the membrane potential rises sharply as N⁺ channels open.

Saltatory conduction is the process of action potentials jumping from one node of Ranvier to the next in myelinated fibers. This happens because depolarisation is only possible at the nodes where there is a break in myelination.

False.

Saltatory conduction occurs in myelinated neurones, where action potentials jump between nodes of Ranvier.

Nodes of Ranvier allow action potentials to regenerate, enabling faster transmission along myelinated fibers.

Ion pumps and channels are located at the nodes of Ranvier in myelinated neurones.

Saltatory conduction increases impulse speed by allowing action potentials to jump between nodes instead of moving continuously.

True.

ATP is used to power ion pumps that maintain resting potentials at the nodes of Ranvier.

Nodes of Ranvier are gaps in the myelin sheath where ion channels and pumps are concentrated to allow saltatory conduction of nerve impulses.

Saltatory conduction is more energy-efficient because only the nodes of Ranvier require active ion exchange.

Action potentials are regenerated at each node of Ranvier to maintain signal strength along the myelinated axon.

True.

The myelin sheath is made of Schwann cells which provide insulation to prevent loss of electrical energy along the length of the axon.

Hyperpolarisation is when the postsynaptic membrane potential becomes more negative than resting potential, decreasing likelihood of an action potential.

Cocaine blocks the reuptake of neurotransmitters, leading to prolonged stimulation of the postsynaptic neurone.

An inhibitory postsynaptic potential is a hyperpolarisation event that reduces the likelihood of action potential in the postsynaptic neurone.

True.

Summation is the integration of signals from multiple presynaptic neurones affecting postsynaptic depolarisation.

Hyperpolarisation lowers the membrane potential below the resting potential, this reduces the chance of the postsynaptic neurone reaching the threshold for an action potential.

True.

Excitatory neurotransmitters cause depolarisation, increasing the likelihood of an action potential.

An all-or-nothing response occurs when the combined excitatory and inhibitory signals reach the threshold, triggering or preventing an action potential.

Summation is the combined influence of excitatory and inhibitory neurotransmitters on the postsynaptic neurone’s membrane potential.

False.

Neonicotinoids act as pesticides by binding to receptors, blocking synaptic transmission. They do not break down the neurotransmitters.

Neonicotinoids are pesticides that block synaptic transmission by binding to neurotransmitter receptors in insects.

False.

When free nerve endings in the skin detect pain stimuli, they have channels that open to allow positively charged ions to enter. This triggers an action potential if the threshold is reached.

Free nerve endings found in the skin are responsible for detecting pain such as that caused by high temperature, acid, or chemicals like capsaicin. These stimuli, trigger the ion channels in free nerve endings to open.

Capsaicin opens ion channels in free nerve endings, allowing positive ions to enter and triggering a pain response if a threshold is reached.

Free nerve endings detect high temperatures, acidic conditions, and chemicals such as capsaicin.

Entry of positively charged ions into free nerve endings may increase the membrane potential. If the threshold level is achieved, depolarisation occurs and an action potential travels to the brain where pain is perceived.

True.

Interactions between neurones in the brain leads to consciousness which incorporates qualitative perception of feelings that give an awareness of the environment.

Consciousness is considered an emergent property, arising from the collective interactions of many neurones and leading to a collection of different and complex behaviours.