2013 Exam

Question 1

Outline how ions can be pumped into or out of a cell against their electrochemical
gradient. Illustrate you answer by reference to specific mechanisms [6 marks]

Answer 1

1. Direction in which charged particles such as ions move across the plasma membrane is influenced by the:
   
   • membrane potential
   • charge of the ion, and
   • concentration gradient
2. Transporting ions against their electrochemical gradient is achieved using carrier proteins by the process of active transport
3. In many cases, the activity of the carrier protein is directly dependent upon the metabolic energy derived from the hydrolysis of ATP
4. In other cases, the transport of the ion against its electrochemical gradient can be achieved by coupling its uphill movement to the downhill movement of sodium into the cell (secondary active transport)
5. The sodium/potassium pump is an example of primary active transport - using ATP to indue a conformational change, it pumps 3 Na+ out of the cell for every 2 K+ in.
6. The sodium gradient is used in secondary active transport mechanisms, e.g. Na+ - Ca2+ exchange

Question 2

What are typical values for the membrane potential of mammalian cells? How does
the membrane potential arise? [4 marks]

Answer 2

1. The magnitude of the membrane potential varies from one cell to the next; it is greatest in nerve and muscle cells, where it ranges from 70-90mV. In non-excitable cells, it may be somewhat lower, e.g. 40mV in hepatocytes (liver cells)
2. The membrane potential arises mainly due to the activity of the sodium/potassium pump - 3Na+ in, 2K+ out
3. The plasma membrane is more permeable to potassium ions than sodium ions so potassium ions diffuse back down their concentration gradient much more readily leading to an accumulation of negative charge inside the cell
4. This negative charge gives rise to the potential difference across the cell membrane, i.e. the membrane potential

Question 3

How does action potential propagation differ between myelinated and unmyelinated nerve fibres? [3 marks]

Answer 3

1. Action potential propagation is substantially faster in myelinated nerve fibres than unmyelinated nerve fibres
2. UNMYELINATED - action potential propagated by a continuous wave of depolarization so that every part of the membrane must be polarized; speed is roughly 2m/s
3. MYELINATED - myelin sheath covers and thus prevents sections of the axon membrane from becoming depolarized, so depolarization occurs by jumping from Node of Ranvier to another by the process of saltatory conduction

Question 4

How do motor nerves activate skeletal muscles? [5 marks]

Answer 4

1, The process of transmitting a signal from a motor nerve to skeletal muscle is called neuromuscular transmission.

2. The individual axons that make up a motor nerve are myelinated and branch as they enter the muscle, so that each individual motor neuron controls the activity of a number of muscle fibres.
3. The motor neuron and its muscle fibres are collectively known as a motor unit and an action potential in the motor neuron will cause the contraction of the muscle fibres to which it is connected. The region of contact between neuron and muscle fibre is called the neuromuscular junction.
4. The terminals of motor axons contain large numbers of mitochondria, and synaptic vesicles which contain acetylcholine. To innervate muscle, first an action potential depolarises the nerve terminal causing voltage-gated calcium channels to open so that calcium ions flow into the nerve terminal. This triggers the fusion of Ach vesicles with the synaptic membrane releasing Ach into the synaptic cleft. Ach diffuses across and binds to nAChRs on the post synaptic membrane.
5. Binding triggers the opening of non-selective cation channels depolarising the membrane causing an endplate potential. When the end-plate potential (EPP) has reached threshold action potential is propagated along muscle fibre triggering contraction in a process called excitation-contraction coupling.

Question 5

List THREE ways in which the contraction of cardiac muscle differ from that of skeletal muscle [3 marks]

Answer 5

Skeletal muscle:

• is excited by motor nerves which transmit ACh that binds to receptors on the muscle synaptic membrane to produce the action potential
• action potential stimulates dihydropyridine receptors on T tubules and does not cause an influx of extracellular Ca2+ into the muscle cell

Cardiac muscle:

• the action potential transmits to the contractile muscle via a gap junction
• action potential activates dihydropyridine receptors on T tubules and causes an influx of extracellular Ca2+ into the muscle cell

Question 6

What neurotransmitters are mainly used in the sympathetic ganglia and at their post-ganglionic synapses? [3 marks]

Answer 6

1. Majority of POSTGANGLIONIC fibres of the SYMPATHETIC system releases NORADRENALINE
2. Exceptions are the fibres which innervate the sweat glands and pilomotor muscles - secrete ACETYLCHOLINE
3. PREGANGLIONIC fibres tend to secrete ACETYLCHOLINE

Question 7

Name FIVE of the chemical classes that signalling molecules can belong to and give an example of each [5 marks]

Answer 7

1. ESTERS; e.g. acetylcholine
2. MONOAMINES; e.g. noradrenaline
3. AMINO ACIDS; e.g. GABA
4. PURINES; e.g. ATP
5. PEPTIDES; e.g. substance P
6. INORGANIC GASES; e.g. nitric oxide (NO)

Question 8

Name the hormones released from the posterior pituitary gland. What are their principal actions? [4 marks]

Answer 8

VASOPRESSIN: targets the KIDNEY
1. increases the permeability of the collecting ducts to water

OXYTOCIN: targets the BREASTS + ADRENAL MEDULLA

2. stimulates the ejection of milk from the mammary glands in response to suckling
3. may also play a role in expelling the foetus and placenta during labour
4. in males, it is believed to play a role in erection, ejaculation, and sperm progression

Question 9

What do you understand by the term anaemia? Give two principle types of anaemia and their causes [4 marks]

Answer 9

1. ANAEMIA is a conduction in which there is a deficiency of haemoglobin or red blood cells in the blood, resulting in pallor and weariness
2. MICROCYTIC anaemia - caused by a reduction in the size of red blood cells
3. MACROCYTIC anaemia - caused by insufficient absorption of VITAMIN B12 due to lack of intrinsic factor in the gastric mucosa; B12 is important for the maturation of erythrocytes in the bone marrow; in this condition, RBCs produced much larger than usual, but in greatly reduced numbers
4. APLASTIC anaemia - arises spontaneously as a result of damage to bone marrow
   * 5. SICKLE CELL anaemia - caused by abnormalities in the haemoglobin structure, causes RBCs to become sickled which can result in capillary blockage

Question 10

What is Starling’s Law of the heart and what is its main role in the normal circulation? [3 marks]

Answer 10

1. “The energy of contraction of the ventricle is a function of the initial length of the muscle fibres comprising its walls”
2. This means that during systole, the ventricle will eject the volume of blood that entered it during diastole
3. Consequently, the heart automatically adjusts cardiac output to match venous return

Question 11

What determines the heart rate and how is it adjusted to meet the demands of the circulation? [4 marks]

Answer 11

1. Heart rate is determined by the frequency at which action potentials are generated at the sinoatrial (SA) node
2. The SA node is innervated by the autonomic nervous system
3. The PARASYMPATHETIC branch - VAGUS nerve - releases ACETYLCHOLINE which SLOWS heart rate
4. The SYMPATHETIC branch - CARDIAC nerves - releases NORADRENALINE which INCREASES heart rate

Question 12

What effect would a raised partial pressure of CO2 have on respiratory rate? Where are the receptors for this stimulus located in the body? [4 marks]

Answer 12

1. The central chemoreceptors respond to changes in the pH of the cerebrospinal fluid resulting from alterations in the PCO2
2. They are located on or close to the ventral surface of the medulla near the origins of the vagus (X) and glossopharyngeal (IX) nerves
3. A rise in plasma CO2 leads to increased CO2 uptake into the brain, where it is converted to bicarbonate and hydrogen ions via carbonic acid
4. The hydrogen ions stimulate the central chemoreceptors and this increases the rate and depth of respiration

Question 13

Define the term compliance as applied to the respiratory system. What factors influence this compliance?

Answer 13

1. COMPLIANCE is the change in the volume of the chest that results from a given change in intrapleural pressure
2. It is a measure of the ease with which the chest volume can be changed and is determined when there is no movement of air into or out of the lungs
3. It can be influenced by
   
   • LUNG ELASTIC RECOIL
   • LUNG VOLUME
   • DISEASE
4. Compliance is high in a lung with low elastic recoil, as seen in emphysema; it is highest at moderate lung volumes and much lower at volumes which are very high/low

Question 14

List THREE functions of saliva. How is its rate of secretion regulated? [3 marks]

Answer 14

1. Lubricates food to facilitate swallowing
2. Contains the enzyme salivary-alpha-amylase that begins the process of starch digestion
3. Dissolves certain substances in food, making them available to taste cells
4. Contains IgA and lysozyme, which act on the walls of certain bacteria causing cell lysis and death
   * The rate of salivary secretion is controlled primarily by reflexes mediated by the autonomic nervous system

Question 15

Explain the processes by which glucose is absorbed by the small intestine when glucose levels are high and low? [5 marks]

Answer 15

The absorption of glucose in the small intestine depends on its concentration in the gut lumen.

1. When glucose concentration is low, glucose is absorbed into the enterocytes against its concentration gradient by SGLT1
2. The glucose is transported across the basoleateral membrane via GLUT2; the sodium gradient that drives this form of glucose transport is maintained by the Na+/K+ ATPase found in the basolateral membrane
3. After a meal, the glucose concentration in the gut lumen rises substantially so SGLT1 carriers become saturated
4. However, the increased activity of SGLT1 initiates a signalling cascade that leads to the insertion of GLUT2 carriers into the apical membrane, allowing glucose to diffuse into the cell down its concentration gradient
5. As before, it can diffuse through the basolateral membrane using GLUT2 and then diffuse into the bloodstream