CELLULAR PHYSIOLOGY

Membranes

What is the main function of the cell membrane?

To control the entry and exit of molecules from the cell and so regulate the intracellular environment.

Describe the basic structure of a cell membrane.

The cell membrane consists of a continuous lipid bilayer studded with protein molecules.

How does this structure allow control of the movement of molecules into and out of the cell?

The lipid bilayer has hydrophilic groups facing outwards while hydrophobic groups face each other across the middle. Most large water-soluble molecules, charged molecules and ions cannot cross the lipid barrier. Size, charge and water-solubility all decrease the ability of a molecule crossing the fatty membrane. These substances depend on the membrane proteins for their entry and exit from the cell. These proteins can act as channels sensitive to voltage or ligandbinding or as energy-dependent pumps. Fat-soluble substances like oxygen and carbon dioxide can cross easily as can water.

What is the overall charge of the outer surface of the cell membrane?

Negative.

What part of the membrane structure is responsible for this negative charge?

The cell has a “glycocalyx” formed by membrane carbohydrates, which are negatively charged. These carbohydrates also act as receptor substrates and can bind to carbohydrates on other cells.

Ion Channels

What is the basic structure of an ion channel?

They are proteins, which form tubular structures with a central pore which traverses the cell membrane and can allow communication between the extracellular fluid and the intracellular compartment.

GROWTH AND DIFFERENTIATION

Hyperplasia and Hypertrophy

Define hypertrophy.

The growth of a tissue or organ because cells increase in size without cell replication.

Define hyperplasia.

The growth of a tissue or organ because there is an increase in the number of cells due to cell division.

Give a physiological and pathological cause of hypertrophy.

Muscles undergo hypertrophy with exercise while cardiomyopathies involve heart muscle hypertrophy.

Give a physiological and pathological cause of hyperplasia.

Hyperplasia takes place in the breast during puberty and pregnancy while adrenal hyperplasia characterises Cushing's disease.

Can you name any conditions in which hypertrophy and hyperplasia occur in the same organ?

• Benign prostatic enlargement.

• Grave's disease of the thyroid.

What is autonomous hyperplasia?

Proliferation in the absence of a demonstrable stimulus as in psoriasis and Paget's disease of bone. This falls very close to the definition of neoplasia.

Metaplasia

Define metaplasia.

The reversible change of one fully differentiated cell type to another.

Can you give an example?

In Barrett's oesophagus the distal stratified squamous epithelium is replaced by columnar epithelium like that present in the proximal stomach.

What mechanism is thought to be responsible?

Prolonged gastro-oesophageal reflux leads to inflammation and eventually ulceration. Healing occurs by re-epithelialisation, which in the acidic environment created by gastro-oesophageal reflux disease (GORD) differentiate into gastric or intestinal type epithelium. These are more resistant to injury from gastric contents.

HEAD AND NECK

Salivary Glands

What important structures lie within the parotid gland?

• Facial nerve.

• Retro-mandibular vein.

• External carotid artery – bifurcating into its two terminal branches, the maxillary artery and the superficial temporal artery.

Can you outline the surface markings of the parotid gland for?

Using a marker begin a line from in front of the tragus of the ear and draw it into the middle of the cheek below the zygomatic arch. From here proceed downwards and backwards to a point 1 cm in front of the angle of the mandible. Then continue upwards and backwards 1–2 cm into the neck including the mastoid process. Finally draw your line around the ear to join up to the point from where we started. You will now have drawn a shape that approximates the position of the parotid.

What are the potential complications of parotid surgery?

• Facial nerve damage.

• Haematoma.

• Frey's syndrome (gustatory sweating).

• Numbness of the pinna (due to sacrifice of the greater auricular nerve).

• Salivary fistula (in superficial parotidectomy only, as the cut gland left behind continues to secrete saliva).

• Wound dimple.

During surgery how would you locate the facial nerve?

This can be done in three different ways:

1. by identifying the tip of the tragal cartilage with during the dissection – the facial nerve lies 1 cm inferior and deep to this cartilage;

2. […]

PERI-OPERATIVE CARE

The Diabetic Patient

How do you assess a diabetic patient in the clinic?

Take a history of the type of diabetic control used, the dosage schedule and the adequacy of control. Particular attention is paid to the propensity to develop hyperglycaemia, ketosis and hypoglycaemia. Ask specifically about the complications of diabetes: nephropathy, sensory and autonomic neuropathy, hypertension, peripheral and coronary arterial disease and retinopathy. The patient looking for these complications is then examined. Look for ongoing infection.

How do you manage the diabetic patient once they are on the ward?

Patients with diabetes often have gastroparesis, and they should fast at least 12 hours before elective surgery. Always try to put the patient first on the list. Patients with diet-controlled diabetes usually just require glucose monitoring. Patients on oral hypoglycaemic agents should have those agents discontinued on the day of surgery. Sulphonyl urea drugs should be withheld at least 1 day before surgery, because of their long half-life. For those on insulin prescribe 5% dextrose with potassium and start sliding scale insulin infusion. Continue the insulin and dextrose infusion until the patient has had a second meal with their normal dose of subcutaneous insulin post-operatively.

What are the potential operative complications in the diabetic patient?

• Infections: diabetics are prone to infection at the surgical site and elsewhere.

• Wound healing: this is impaired in diabetics due in part to microvascular disease.

• Cardiovascular complications: due to macrovascular disease.

AIRWAY ISSUES

Airway Obstruction

How would you define airway obstruction?

Partial or complete occlusion of the upper or lower respiratory tract, upper airway obstruction being more common than obstruction below the larynx.

In what situations does it occur?

Upper airway obstruction commonly occurs in the unconscious patient who is unable to maintain there airway due to the tongue falling backward. Other causes of upper airway obstruction include: laryngospasm, tumours, soft tissue swellings, oedema, infection (epiglottitis and diphtheria) and foreign objects as well as blood and vomit. In anaesthesia; lower airway obstruction may occur due to pulmonary secretions or mucus plugging, pulmonary oedema, pneumothorax or haemothorax.

What are the clinical features of airway obstruction?

• Hypoventilation.

• Increased work of breathing: accessory muscles of breathing are often employed, tracheal tug may be seen, see-saw paradoxical movement of the abdomen and the chest may also be noticeable.

• Change in noise of breathing: complete obstruction is silent; partial obstruction is noisy (e.g. stridor).

• Tachypnioea.

• Tachycardia.

• Lower respiratory signs will be present if there is lower airway obstruction, but this will depend on the cause of the lower airway obstruction.

How would you clinically assess an airway?

• Look: for accessory muscle movements, see-saw movements of abdomen and chest, foreign bodies in airway; and in late stages central cyanosis.

• Listen: for breath sounds, stridor, grunting and gurgling.

• Feel: for airflow at the nose and mouth; chest movement.

HEAD AND NECK

Fascial Compartments of the Neck

What is the significance of the fascial compartments of the neck?

They compartmentalise structures in the neck and form natural planes of cleavage through which tissues can be separated in surgery. They also form planes along which infections can spread.

What are the different fascial layers?

They are made up of superficial and deep cervical fascia. Superficial fascia lies between the skin and investing layer of deep fascia. As well as containing nerves, blood vessels and lymphatics, it encloses the platysma muscle anteriorly. The deep cervical fascia consists of four parts: investing; pretracheal; prevertebral; and the carotid sheath.

Can you tell me the margins of the investing layer?

The investing layer of fascia surrounds the neck deep to the superficial fascial layer. It splits to enclose the trapezius and sternocleidomastoid muscles on either side. The superior attachment of the investing layer extends from the superior nuchal line to the tip of the mastoid process. It extends to the zygomatic arch and the lower border of the mandible. Anteriorly, it attaches to the hyoid bone and posteriorly, it attaches to the ligamentum nuchae. Between the angle of the mandible and mastoid process, it splits to enclose the parotid and submandibular glands. Inferiorly, it attaches to the manubrium, clavicles and the spines and acromion of the scapulae. In attaching to the manubrium, the investing layer attaches to the anterior and posterior border forming a suprasternal space.