Neuronal Communication (OCR A Level Biology)

The greater blue-ringed octopus, Hapalochlaena lunulata, is one of the most venomous of all animals.

Its bite contains tetrodotoxin (TTX), a neurotoxin that can cause paralysis and death within minutes. 

The following information has been discovered about the effects of TTX on nerve cells:

• • TTX binds to the external surface of the voltage-gated sodium ion channels in the axon membrane.
  • Binding of TTX changes the tertiary structure of the channel.
  • This means the channel cannot open.

Using the information provided, explain how TTX affects the activity of neurones.

[4] 

A common cause of death from TTX poisoning is suffocation (not getting enough oxygen) as a result of paralysis of the diaphragm.

Explain how paralysis of the diaphragm could lead to suffocation.

[2] 

TTX is also known to reduce the speed of conduction in the Purkyne fibres of the heart.

Suggest and explain what effect this would have on the heart rate.

Molluscs such as H. lunulata have unmyelinated neurones. Saltatory conduction cannot occur in these neurones.

Why is transmission of action potentials along the axon slower in the absence of saltatory conduction?

A student looked at slides of different tissues under a light microscope.

The four viewed images are labelled W, X, Y and Z in Fig. 23.1 below.

Fig. 23.1

Identify tissues W, X and Y.

The student wrote the following summary about the control of heart rate.

Identify and correct any biological errors in the student’s summary.

Reflex actions are rapid responses that protect the body from harm.

The Moro reflex is found in babies up to five months of age, and occurs when the baby feels its head is suddenly no longer supported. The Moro reflex is made up of the following responses:

• The baby spreads out its arms then brings them together rapidly.
• The baby cries.

Suggest how the Moro reflex helps to prevent harm to a newborn baby.

[2]

(ii)

The Moro reflex gradually disappears and usually stops completely after babies reach nine months. Other reflexes develop as children grow older.

Describe a reflex response a 3-year-old child would make to an object moving towards their eyes and explain the advantage of this response.

[3]

Fig. 1 below shows some structures inside the ear.

Within the cochlea of the ear, fluid surrounds thousands of mechanoreceptor cells that transmit nerve impulses to the brain via the cochlear nerve. These receptor cells are called sensory hairs.

Fig. 1

Identify the stimulus for these sensory hairs.

[1]

(ii)

Sensory hair cells are highly specialised cells shown to contain many voltage gated Ca²⁺ channels within the cell membrane and many clusters of vesicles tethered to the internal cell membrane.

Suggest why these cells are specialised in this way. 

[3]

Fig. 2 shows the changes in membrane potential across the membrane of the cochlear nerve over a period of time.

The membrane was stimulated after a quiet sound at A and after a louder sound at B.

Fig. 2

Explain why the quiet sound at A did not result in an action potential being produced.

[2]

(ii)

State which of the letters C, D and E on Fig. 2 correspond to each of the events below.

You may use each of the letters C, D or E once, more than once or not at all.

• The Na⁺/K⁺ pump is operating
• The voltage gated Na⁺ channels are open
• The voltage gated K⁺ channels are open

[3]

(iii)

The period between around 2.9 ms and 3.3 ms in Fig. 2 is known as the refractory period

Describe the importance of the refractory period in the transmission of action potentials.

[2]

Exposure to loud sounds can result in damage to the sensory hairs found in the cochlea of the ear due to breakage of the cell tips and disruption of the synapses, resulting in varying degrees of hearing loss.

Suggest how damage to hairs may result in hearing loss for individuals.

Stem cell therapy provides one potential treatment for sensory hair damage.

Describe how stem cells could provide a successful treatment

Table 1 shows the speed of impulse transmission in neurones of different type and diameter.

Table 1

Calculate the percentage increase in impulse transmission speed in myelinated neurones when the axon diameter is increased from 7 μm to 16 μm.

Describe the effect of myelination on the speed of impulse transmission in neurones shown in Table 1.

Explain the following from Table 1:

The effect of myelination on speed of nerve impulse transmission.

[3]

(ii)

The effect of axon diameter on speed of nerve impulse transmission.

[1]

Scientists investigated the relationship between myelin in brain tissue and the different types of dementia.

The scientists measured the mean myelin content in brain tissue samples from 3 different groups:

• A control group of 15 people without dementia
• 22 people with vascular dementia (VD)
• 18 people with Alzheimer's dementia (AD)

Their results are shown in Fig. 1 below. The smaller vertical bars represent standard deviation.

Fig. 1

A research assistant reviewing Fig. 1 concluded that there was a relationship between the concentration of myelin present in an individual’s brain and the likelihood of dementia.

Evaluate this conclusion.