(a) Paranasal sinuses and their functions in ox (10 marks) (b) Use of electron microscopy in the study of tissue structure and…

(a) Paranasal sinuses of the ox and their functions (10 marks)

Paranasal sinuses are air-filled cavities in the skull bones, lined by respiratory mucosa, that open into the nasal cavity. In the ox they are large:

• Frontal sinus: the largest, divided by septa into compartments, extending into the neighbouring bones and, through the cornual diverticulum, into the horn core.
• Maxillary sinus and palatine sinus, which communicate with each other and open into the middle nasal meatus.
• Lacrimal sinus in the lacrimal bone, sphenoidal sinus in the sphenoid, and conchal sinuses in the dorsal and ventral nasal conchae.

Functions:

• Lighten the skull, especially the large frontal region that carries the horns.
• Protect the brain and eyes from shock and trauma, as an air cushion.
• Warm and humidify inspired air, and give thermal insulation to the brain.
• Give resonance to the voice.

Clinically, dehorning opens the frontal sinus, which can lead to sinusitis.

(b) Use of electron microscopy in the study of tissue structure and cytology (10 marks)

An electron microscope uses a beam of electrons, accelerated at 60 to 120 kV, whose very short wavelength (about 0.005 nm) is focused by electromagnetic lenses in a vacuum. It resolves details of about 0.1 to 0.2 nm, against about 200 nm for the light microscope.

• Transmission electron microscopy (TEM): ultrathin sections (50 to 90 nm), fixed in glutaraldehyde and osmium tetroxide, embedded in resin and stained with uranyl acetate and lead citrate, show the internal ultrastructure: membranes, mitochondria, endoplasmic reticulum, Golgi, ribosomes, cell junctions and viruses.
• Scanning electron microscopy (SEM): the surface is coated with gold, and gives a three-dimensional view of the surface of cells and tissues, such as microvilli, cilia, sperm and bacteria.
• Other techniques: negative staining for virus identification, immunogold labelling to locate proteins, and cryo-EM.

Uses include diagnosis (kidney biopsy, storage diseases, muscle and ciliary defects, viral diseases) and research. Limitations are cost, vacuum, and processing artefacts in dead material.

(c) Autacoids and their classification (10 marks)

Autacoids ("self-remedies") are biologically active substances made locally in tissues and cells, which act near their site of production and are short-lived, unlike hormones, which are made in glands and act at a distance. They take part in inflammation, allergy, pain, fever and smooth muscle tone.

Classification:

• Amine autacoids: histamine (from mast cells; H₁ and H₂ actions) and 5-hydroxytryptamine (serotonin).
• Lipid-derived autacoids (from membrane phospholipids): eicosanoids, that is prostaglandins, prostacyclin, thromboxanes and leukotrienes, and platelet-activating factor.
• Polypeptide autacoids: kinins (bradykinin, kallidin), angiotensin, endothelin and substance P.
• Others: nitric oxide and adenosine.

Drugs act on them: antihistamines, NSAIDs (which inhibit cyclooxygenase), 5-HT antagonists, leukotriene inhibitors, ACE inhibitors.

(d) Porcine stress syndrome (10 marks)

Porcine stress syndrome (PSS), or malignant hyperthermia of pigs, is an inherited disorder of heavy-muscled breeds (Pietrain, Landrace, Poland China) in which stress causes a sudden, often fatal, muscle and metabolic crisis.

• Genetic basis: an autosomal recessive mutation in the RYR1 gene (ryanodine receptor 1, the calcium-release channel of the sarcoplasmic reticulum; the Arg615Cys point mutation, the "halothane gene"). Affected pigs are homozygous (nn); carriers are Nn.
• Triggers: transport, mixing, mating, fighting, heat, handling, or halothane anaesthesia. Calcium leaks uncontrollably from the sarcoplasmic reticulum.
• Signs: muscle tremors and rigidity, hyperthermia (above 41 °C), open-mouth breathing, blotchy skin, tachycardia, lactic acidosis, and sudden death.
• Meat: pale, soft, exudative (PSE) pork, from rapid post-mortem glycolysis and a fall of pH while the carcass is warm.
• Diagnosis and control: halothane test and PCR test for the RYR1 mutation, and removal of carriers from breeding; reduce stress and rest pigs before slaughter; cool the affected pig, and give sedatives such as azaperone, or dantrolene.

(e) Key aspects of regional anaesthesia in dogs (10 marks)

Regional anaesthesia is the injection of a local anaesthetic near a nerve or the spinal cord to block sensation in an area, while the dog stays conscious or lightly sedated.

• Mechanism: local anaesthetics (lidocaine, bupivacaine, ropivacaine) block voltage-gated sodium channels and stop conduction.
• Techniques: infiltration and line or ring block, nerve blocks (brachial plexus, dental and infraorbital, mandibular, testicular), epidural (lumbosacral) and intravenous regional anaesthesia.
• Drugs and doses: lidocaine (2%) acts in a few minutes for about 60 to 90 minutes, maximum about 6 mg/kg; bupivacaine (0.5%) acts more slowly but lasts 4 to 6 hours, maximum about 2 mg/kg and never intravenous. Adrenaline prolongs lidocaine, except at end-arteries.
• Precautions: aseptic technique, correct landmarks, aspiration before injection, and avoiding overdose.
• Benefits: pre-emptive, multimodal analgesia, less general anaesthetic needed, and safer in high-risk patients.
• Complications: systemic toxicity (seizures, arrhythmia), nerve injury, haematoma, infection, and hypotension after epidural.