▶️ Answer/Explanation
Neurons in embryonic tissue develop through several stages to enable communication within the nervous system. First, axons grow out from immature neurons, guided by chemical signals that direct their growth towards target areas. During development, some neurons or their axons migrate or extend to specific regions in the embryo. As the neurons reach their destinations, dendrites begin to form, increasing the surface area for potential connections.
Synapses, or communication junctions between neurons, form between these cells. With repeated activity and stimulation, frequently used synapses are strengthened, making neural pathways more efficient. Conversely, synapses that are not used undergo neural pruning, where they are eliminated. Throughout life, the nervous system retains neural plasticity, meaning it can form new synapses and connections in response to learning or environmental changes.
Markscheme:
a. Axon develops from (immature) neuron;
b. Chemical stimuli cause (direction of) growth;
c. Some neurons/axons migrate/extend to other areas;
d. Dendrites develop;
e. (Multiple) synapses form between neurons;
f. Synapses are strengthened with use;
g. Neural pruning causes unused synapses to be eliminated;
h. Neural plasticity allows new connections/synapses;
▶️ Answer/Explanation
(a)
The membrane potential at point X is approximately 0 millivolts (mV). This point represents the peak of depolarization during the action potential when the inside of the neuron is about as positive as the outside.
(b)
When the membrane potential reaches the threshold value (Y), it triggers the opening of voltage-gated sodium channels in the axon membrane. This causes sodium ions (Na⁺) to rush into the cell, starting the depolarization phase. If the threshold is reached, an action potential is initiated, allowing the electrical signal to propagate along the neuron.
(c)
During the time period labeled t, the following sequence of ion movements occurs:
- In the first half of t (depolarization phase), sodium ions (Na⁺) enter the neuron by diffusing through open voltage-gated sodium channels. This influx causes the membrane potential to become more positive.
- In the second half of t (repolarization phase), potassium ions (K⁺) exit the neuron through voltage-gated potassium channels, restoring the negative membrane potential as the inside of the axon becomes less positive.
These ion movements are crucial for generating and resetting the action potential.
(d)
A nerve impulse is passed from one neuron to another at a synapse, which is the small gap between the axon terminal of one neuron and the dendrite or cell body of the next:
- When the action potential reaches the axon terminal of the presynaptic neuron, it causes voltage-gated calcium channels to open.
- Calcium ions (Ca²⁺) enter the presynaptic terminal, triggering vesicles containing neurotransmitters (e.g. acetylcholine) to fuse with the membrane and release their contents into the synaptic cleft.
- The neurotransmitters diffuse across the synapse and bind to receptor proteins on the postsynaptic membrane.
- If enough neurotransmitter binds and the threshold potential is reached in the postsynaptic neuron, sodium channels open, and a new action potential is generated in the next neuron.
This process allows the electrical signal to continue across a network of neurons.
Markscheme:
(a) 0 mV; (accept answers in the range of -10 mV to +10 mV) (Units required)
(b)
• sodium channels (start to) open
OR
• depolarization/axon begins to depolarize
OR
• action potential occurs;
(c)
• Na+/sodium ions diffuse into the axon (in the first part/half of t);
• K+/potassium ions diffuse out of the axon (in the second half/part of t);
(d)
• impulses pass to another neuron at a synapse/across synaptic gap/cleft;
• (depolarization causes) Ca2+/calcium ions to diffuse into the (presynaptic) neuron/axon;
• depolarization (of presynaptic neuron) causes release of a neurotransmitter
OR
• neurotransmitters diffuse across the synapse;
• (neurotransmitters) bind to receptors on postsynaptic neuron/membrane;
• (if the threshold potential is reached) an action potential occurs/sodium gates open (in the postsynaptic neuron);
▶️ Answer/Explanation
a.
b.
The heart pumps blood through a coordinated sequence of muscular contractions and valve actions:
- Both atria fill with blood — the right atrium receives deoxygenated blood from the vena cava, and the left atrium receives oxygenated blood from the pulmonary veins.
- The sinoatrial (SA) node, located in the right atrium, acts as a pacemaker by sending electrical impulses that cause the atria to contract (atrial systole).
- As the atria contract, blood flows into the ventricles through the open AV valves.
- During this time, the semilunar valves in the pulmonary artery and aorta remain closed, allowing the ventricles to fill completely.
- Next, the ventricles contract (ventricular systole), forcing the AV valves to close and semilunar valves to open.
- Blood is pumped from the right ventricle into the pulmonary artery (to the lungs) and from the left ventricle into the aorta (to the body).
- After contraction, the ventricles relax (diastole), causing the semilunar valves to close and prevent backflow.
c.
The transmission of a nerve impulse across a synapse occurs as follows:
- When an action potential reaches the end of the presynaptic neuron, it triggers the opening of voltage-gated calcium channels.
- Calcium ions (Ca²⁺) enter the neuron and stimulate synaptic vesicles to move to and fuse with the presynaptic membrane.
- These vesicles then release neurotransmitters (e.g. acetylcholine) by exocytosis into the synaptic cleft.
- The neurotransmitter molecules diffuse across the synapse and bind to specific receptors on the postsynaptic membrane.
- This binding causes ion channels to open, allowing sodium ions (Na⁺) to enter the postsynaptic neuron.
- The influx of sodium causes depolarization of the postsynaptic membrane, and if the threshold is reached, a new action potential is generated.
- The neurotransmitter is then broken down by enzymes or reabsorbed by the presynaptic neuron to terminate the signal.
Markscheme:
a. Drawings must be correctly proportioned and clearly drawn showing connections between structures.
- Atria/right atrium/left atrium – shown above the ventricles and must not be bigger than ventricles;
- Ventricle/left ventricle/right ventricle – shown below the atria, must have thicker walls than atria;
- Vena cava/superior vena cava/inferior vena cava – connected to right atrium;
- Pulmonary artery – shown from right ventricle (to lungs);
- Pulmonary vein(s) – shown (from lungs) to left atrium;
- Aorta – shown as large artery from left ventricle out of heart;
- AV valves/atrioventricular valves/mitral/bicuspid and tricuspid – named correctly and shown between both atria and ventricles;
- Semilunar valves – shown in aorta/pulmonary artery;
Valves need to open in correct direction.
b.
- (Both) atria collect blood (from veins);
- Sinoatrial/SA node sends impulses to muscle/fibres initiating contraction;
- Blood is pushed to ventricles by contraction of atria/atrial systole;
- AV (atrioventricular) valves are open (as atria contract);
- Semilunar valves are closed so that ventricles fill with blood;
- Ventricles contract/ventricular systole;
- AV (atrioventricular) valves close (preventing backflow);
- Blood is pushed out through semilunar valves into pulmonary artery and aorta;
- When ventricles relax/diastole, semilunar valves close preventing backflow;
[4 max] if suggests left and right sides contract at different times.
c.
- Nerve impulse reaches end of presynaptic neuron;
- (Depolarization causes) calcium channels in membrane to open;
- Calcium diffuses into presynaptic neuron;
- Vesicles of neurotransmitter move to and fuse with presynaptic membrane;
- Neurotransmitter released (by exocytosis) into synaptic cleft;
- Neurotransmitter diffuses across synapse;
- Neurotransmitter attaches to receptors on postsynaptic neuron;
- Receptors cause ion channels to open and sodium diffuses in;
- Postsynaptic neuron membrane is depolarized;
- (Depolarization) causes new action potential;
- Neurotransmitter on postsynaptic membrane is broken down;
- Neurotransmitter is reabsorbed into presynaptic neuron;