Synapses

Figure 2 shows a drawing of a myelinated motor neurone.

Name the parts labelled A to D.

Osmotic demyelination syndrome (ODS) results in destruction of the myelin sheath of neurones in the brain.

ODS can increase reflex response times.

Explain why.

The parasympathetic nervous system (PNS) uses acetylcholine as its neurotransmitter. Atropine is an anticholinergic drug that attaches to postsynaptic receptors.

Atropine can be used to increase the heart rate in patients with bradycardia (a slower than normal heart rate).

Use the information provided to explain how atropine causes an increase in the heart rate.

One study has suggested that the atropine dosage for severely obese male patients should be 0.01 mg kg^–1 of lean body weight (LBW).

LBW is determined by using this equation.

where TBW = total body weight BMI = body mass index

Calculate the suggested dosage of atropine, in mg, for a severely obese male patient whose:

• TBW = 115.2 kg

• BMI = 41

Give your answer to 3 significant figures.

Show your working.

Figure 1 shows a drawing of a neuromuscular junction.

Name the parts labelled A to D.

Following the release of acetylcholine into a neuromuscular junction, a muscle contraction occurs.

Describe the sequence of events, following the release of acetylcholine, that leads to stimulation of this contraction.

Do not include in your answer the events following the release of calcium ions in the myofibril.

Inhibitory synapses cause hyperpolarisation in postsynaptic neurones.

Explain how this inhibits synaptic transmission.

Glutamate is a neurotransmitter involved in the transmission of nerve impulses from pain receptors to the brain. Ziconotide is a drug that can reduce severe, constant pain. Ziconotide blocks the calcium ion channels at some of the synapses which use glutamate.

The transmission of glutamate at synapses is similar to that of acetylcholine.

Explain how ziconotide reduces severe, constant pain.

Ziconotide is a polypeptide and acts on synapses in the spinal cord. Scientists investigated the effectiveness of ziconotide in reducing severe, constant pain.

Ziconotide was injected into each patient’s cerebrospinal fluid that bathes the brain and spinal cord. Patients recorded the intensity of their pain using a statistically valid scale.

Suggest two reasons why the patients had ziconotide injected into their cerebrospinal fluid rather than taking a pill containing the drug.

Ziconotide was injected at 3 ng kg^–1 h^–1 for 8 days into each patient.

Calculate the total mass in grams of ziconotide injected after 8 days into a patient with a body mass of 82 kg

Show your working.

When the patients recorded the intensity of pain, suggest two reasons why it was important to use a statistically valid scale.

Give two reasons why transmission across a cholinergic synapse is unidirectional.

Figure 4 shows the changes in membrane potential in a postsynaptic neurone after repeated stimulation from a single presynaptic neurone.

Name and explain the type of summation shown in Figure 4.

Type of summation

Myasthenia gravis (MG) is an autoimmune disease caused when antibodies bind to the sarcolemma (postsynaptic membrane) of neuromuscular junctions. This can weaken contraction of muscles.

Mestinon is a drug that inhibits the enzyme acetylcholinesterase. Mestinon can help in the treatment of MG.

Suggest and explain how MG can weaken contraction of muscles.

Do not include details of myofibril or muscle contraction in your answer.

Mestinon can help in the treatment of MG. Explain how.

Figure 4 shows a diagram of a Pacinian corpuscle.

Name the structures labelled P, Q and R shown in Figure 4.

Two students (A and B) investigated reaction time in response to touch.

• Student A sat with her eyes shut and her forearm resting on a worktop so that her hand was over the edge.

• Student B held a ruler vertically between student A’s thumb and first finger, with the ruler at 0 mm lightly touching student A ’s first finger.

• Student B released the ruler.

• As soon as student A felt the ruler fall, she closed her thumb and first finger to catch the ruler as shown in Figure 5.

• Student B measured the distance the ruler had fallen to the nearest mm

The test was repeated three more times using the same hand to catch the ruler. Table 3 shows student A ’s results.

The student was able to convert these distances into reaction times using Table 4.

Calculate the percentage uncertainty in the measurement of Trial 1 in Table 3.

Put a tick () in the correct box below.

In this investigation, it is not possible for a student to react in less than 45 ms

Suggest one explanation for the value recorded in Trial 3 in Table 3.

Student A estimated that the length of the nerve pathway involved was 175 cm

Use Table 3 and Table 4 to calculate the mean speed of nerve impulse transmission.

Do not use the value for Trial 3 in your calculation.

In response to touch, nerve impulses can be transmitted at speeds of 76.2 m s^–1

Suggest three reasons why, in this investigation, the estimated speed of student A ’s impulse transmission was less than 76.2 m s^–1

Describe the sequence of events involved in transmission across a cholinergic synapse.

Do not include details on the breakdown of acetylcholine in your answer.

Dopamine is a neurotransmitter released in some synapses in the brain. The transmission of dopamine is similar to that of acetylcholine.

Dopamine stimulates the production of nerve impulses in postsynaptic neurones.

Describe how.

Do not include in your answer the events leading to the release of dopamine and the events following production of nerve impulses at postsynaptic neurones.

Dopamine has a role in numerous processes in the brain including pain relief. The release of dopamine can be stimulated by chemicals called endorphins produced in the brain. Endorphins attach to opioid receptors on presynaptic neurones that release dopamine.

Morphine is a drug that has a similar structure to endorphins and can provide pain relief.

Explain how.

GABA is a neurotransmitter released in some inhibitory synapses in the brain. GABA causes negatively charged chloride ions to enter postsynaptic neurones.

Explain how this inhibits postsynaptic neurones.

Alzheimer’s disease (AD) is a non-reversible brain disorder that develops over a number of years. At the start of 2014 the number of Americans with AD was estimated to be 5.4 million. Every 30 seconds another person in America develops AD.

In the brain of a person with AD there is a lower concentration of acetylcholine. 5 This affects communication between nerve cells and initially results in memory loss and confusion. Some of the symptoms of AD that are associated with communication between nerve cells are reduced by taking the drug donepezil. Donepezil inhibits the enzyme acetylcholinesterase.

A gene mutation called E280A found on chromosome 14 causes early-onset AD 10 at a mean age of 49 years. The age at which the E280A mutation is expressed to cause AD varies.

Yaramul is a town in a historically isolated region of the Andes Mountains. The population of this town has the highest frequency of the E280A mutation in the world. The origin of the E280A mutation in this population has been traced back 15 to a common ancestor in the 17th century. Natural selection has not reduced the frequency of the E280A mutation in the population.

This autosomal dominant mutation involves a change in triplet 280 from GAA to GCA. Scientists analysed chromosome 14 from 102 individuals from Yaramul. They recorded a sample size of 204 and detected 75 E280A mutations but only 20 74 potential AD cases. The scientists identified individuals with the mutation by whole genome sequencing. They had decided that a DNA probe would not be a suitable method to detect the E280A mutation.

Assuming no one with AD died in 2014, calculate the annual percentage increase in AD cases in America for 2014 (lines 2–4).

Explain how donepezil could improve communication between nerve cells (lines 7–9).

Suggest and explain two reasons why there is a high frequency of the E280A mutation in Yaramul (lines 13–15).

Explain why natural selection has not reduced the frequency of the E280A mutation in the population (lines 16–17).

The age at which the E280A mutation is expressed to cause AD can vary (lines 11 – 12).

Suggest and explain one reason for this.

One scientific study which analysed chromosome 14 involved 102 individuals. The scientists recorded a sample size of 204. In this sample they detected 75 E280A mutations but only 74 potential AD cases (lines 19 – 21).

Suggest explanations for the figures the scientists recorded.

Suggest why a DNA probe for the mutated triplet was not considered a suitable method for detection of the E280A mutation (lines 22–23).

When a nerve impulse arrives at a synapse, it causes the release of neurotransmitter from vesicles in the presynaptic knob.

Describe how.

The presynaptic knob contains actin filaments and myosin molecules.

The myosin molecules can attach to mitochondria and move them towards the presynaptic membrane, as shown in Figure 1.

Use your knowledge of how myosin and actin interact to suggest how the myosin molecule moves the mitochondrion towards the presynaptic membrane.

Do not include the roles of calcium ions and tropomyosin in your answer.

This movement of mitochondria happens when nerve impulses arrive at the synapse.

Suggest and explain one advantage of the movement of mitochondria towards the presynaptic membrane when nerve impulses arrive at the synapse.