2015 Exam Flashcards
The list below describes the events that constitute excitation-contraction coupling in a skeletal muscle. Place the following events in the correct order. [3 marks]
A) ATP binds myosin head
B) Ca2+ binds troponin causing a conformational change in actin
C) Myosin head binds actin
D) Thin filament returns to the relaxed state
E) Power stroke, ADP and P dissociate from myosin
F) Electrical signal causes release of Ca2+ from the sarcoplasmic reticulum
Event Sequence - Event
- Electrical signal causes release of Ca2+ from the sarcoplasmic reticulum
- Ca2+ binds troponin causing a conformational change in actin
- ATP binds myosin head
- Myosin head binds actin
- Power stroke, ADP and Pi dissociate from myosin
- The thin filament returns to the relaxed state
What is an isometric contraction and give an example? [3 marks]
- An isometric contraction is when a muscle contracts against a load which prevents shortening
- Maximum force is exerted, but the load is moved
- Example: carrying an object in front of you
Define the term haematocrit. Give two examples of physiological situations that would result in evelated and two examples of physiological situations that would result in reduced haematocrit values. [6 marks]
Definition:
1 + 2. The ratio of volume of red blood cells, i.e. the proportion of the total volume of blood occupied by red blood cells
Elevated haematocrit:
- Polycythemia (overstimulation of red blood cell production)
- Physiological adaptation to life at high altitude
Reduced haematocrit:
- Anaemia (add more)
- Haemorrhage (add more)
If a blood vessel is damaged, name the THREE processes that act to reduce blood loss? [3 marks]
- Initially loss of blood leads to reduced venous return and a reduced stimulation of the low-pressure receptors
- This results in vasoconstriction of the cutaneous, muscle, and splanchnic vessels, which occurs before there is any significant change in either mean blood pressure or pulse pressure
- Cardiac output will also fall; venous tone increases and heart rate increases
How are proteins moved across the plasma membrane of cells? [3 marks]
- Proteins are moved across the plasma membrane using vesicles via endocytosis and exocytosis
- Different types of cytosis (finish this)
3.
What features determine the conduction velocity of axons? [5 marks]
The conduction velocity of axons is determined by three factors:
- the axon’s internal electrical resistance;
- the electrical resistance of the axon membrane; and
- the electrical capacitance of the axon membrane
In unmyelinated axons, the electrical resistance and capacitance of each unit area of the membrane are the same regardless of diameter, but the internal electrical resistance of the axon decreases as axon diameter increases, causing an increase in conduction velocity.
Myelination electrically insulates the axon membrane causing in increase in the electrical resistance but a decrease in the electrical capacitance of the membrane.
These two factors combine to allow the depolarising influence of an action potential to spread much farther from the active region; thus action potential jumps from node to node, so conduction velocity is increased from 2m/sec to 120m/sec
List the main divisions of the nervous system. [4 marks]
The CNS - Brain and Spinal Cord
The PNS -
What is a motor unit? [2 marks]
A motor unit is made up of a motor neuron and the skeletal muscle fibres innervated by its axon terminals.
In what respects does the contractile response of cardiac muscle differ from that of skeletal muscle? [5 marks]
- Cardiac muscle has its own excitability where skeletal muscle is excited by a motor nerve
- In cardiac muscle the AP transmits to the contractile muscle via a gap junction;
- In cardiac muscle the AP activates dihydropyridine receptors on T tubules and cause an influx of extracellular Ca2+ into the muscle cell
- In skeletal ACh binds to muscle membrane to produce the AP
- In skeletal muscle AP stimulate dihydropyridine receptors on T tubules and does not cause an influx of extracellular Ca2+ into the muscle cell
How are hydrophobic hormones transported in the blood and give an example? [2 marks]
- They are transported in the blood bound to specific carrier proteins
- Thyroid hormones
Complete the following table with the principal target of each hormone using one of the following tissue descriptions [3 marks]
Hormone - Principal target
ACTH - adrenal gland Follicle stimulating hormone - gonad Prolactin - mammary gland Growth hormone - liver, bone, muscle Oxytocin - mammary gland Vasopressin - kidney
What is Starling’s Law of the heart and what is its main role in the normal circulation? [3 marks]
- “The energy of contraction of the ventricle is a function of the initial length of the muscle fibres comprising its walls”
- This means that during SYSTOLE the ventricle will eject the volume of blood that entered it during diastole
- Consequently, the heart automatically adjusts cardiac output to match venous return
Explain the processes by which glucose is absorbed by the small intestine when glucose levels are high and low? [5 marks]
The absorption of glucose in the small intestine depends on its concentration in the gut lumen.
- When glucose concentration is low, glucose is absorbed into the enterocytes against its concentration gradient by SGLT1
- 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
- After a meal, the glucose concentration in the gut lumen rises substantially so SGLT1 carriers become saturated
- 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
- As before, it can diffuse through the basolateral membrane using GLUT2 and then diffuse into the bloodstream
Outline the physiological factors that control the rate and composition of pancreatic exocrine secretion [5 marks]
- Control of exocrine pancreatic secretion is chiefly hormonal
- Although, the initial cephalic phase of secretion is under the control of parasympathetic nerves
- Gastrin contributes to the gastric phase of secretion
- However, about 70% of secretion occurs during the intestinal phase in response to secretin and CCK
- These hormones are released by the upper intestinal mucosa in response to H+ ions and the products of fat and protein digestion
Where does the cardiac action potential originate? Outline the pathway by which the cardiac action potential spreads from the pacemaker to the ventricles [3 marks]
3.
