MASTER CLASSTM THE BRAIN AND NEUROLOGICAL CONDITIONS
Anatomy of the brain
The nervous system is composed of the brain, spinal cord and peripheral nerves. The brain coordinates all the body functions. All mammals have a similar brain structure.
The cerebral hemispheres (blue in diagram) are where conscious thought and memory lie.
The cerebellum (shown in red) is responsible for fine control and co-ordination of movement.
The thalamus and hypothalamus (colored yellow) control sleeping, hormone release, thirst, appetite, sex and, it is thought, our emotions.
The pons (pink area) and the medulla (light blue) control the vital functions of the body such as heart rate and breathing.
Different mammalian brains all have the same basic components but the size differs depending on the species. The dog for example has a bigger olfactory bulb than the human because they use scent to hunt for food. Humans have very large cerebral hemispheres which suggests we think a lot more than other species.
The brain represents 2% of total body weight but it uses 15% of the cardiac output, 20% of the total oxygen consumption and 25% of glucose consumption.
Source: University of Wisconsin and Michigan State Comparative Mammalian Brain Collections and the National Museum of Health and Medicine.
The brain has no blood vessels – it is separated from the rest of the body by the blood brain barrier. This protects it from common bacterial infections. Viruses can pass through the blood brain barrier but unfortunately antibodies do not which is why brain infections are so serious. The brain is also protected by the skull, the meninges and surrounded by cerebrospinal fluid.
Messages are transmitted along nerves in the form of electric impulses. The myelin sheath speeds up the nerve impulses ten times. The myelin sheath is produced by the Schwann cells a type of glial cell. Neurons are not normally replaced when damaged but in some cases they can re grow if severed but this takes many months.
There are three types of neurons: motor neurons that control muscles and glands, sensory neurons and interneurons that connect neurons to other neurons. There are about 100 billion neurons in the human brain.
Each neuron is connected to many other neurons (up to 7000) via the dendrites. A message is passed from one neuron to another via chemicals called neurotransmitters. The chemical is released from one axon terminal and crosses the gap (called a synapse) where it bonds to the receptor site and triggers an electric impulse in the second nerve. An enzyme then comes along and removes the transmitter from the nerve so it is ready for another impulse. Acetylcholine is a neurotransmitter, and the enzyme which reverses it is acetylcholinesterase. In the peripheral nervous system acetyl choline activates muscles; in the central nervous system it causes stimulation of nerves. Organophosphate insecticides inhibit acetylcholinesterase causing excess levels of acetylcholine at the nerve end paralyzing the muscle.
A seizure is the symptom of temporary excessive activity in the brain. Seizures can cause involuntary changes in body movement or function, sensation, awareness, or behavior. A seizure can last from a few seconds to status epilepticus, a continuous seizure that will not stop without intervention. Seizures are often associated with a sudden and involuntary contraction of a group of muscles and loss of consciousness. However, a seizure can also be as subtle as numbness of a part of the body, a brief or long term loss of memory, seeing sparkling or flashes, sensing an unpleasant odor or a sensation of fear and total state of confusion which in some cases leads to death during seizure. The type of seizure depends on the area of the brain being affected. Small localized seizures are known as petit mal, full blown seizures are called grand mal. After a heavy seizure attack, since the brain is recovering, there is a sudden loss of memory; usually the short term memory.
In dogs and cats seizures usually take the form of loss of consciousness followed by rhythmic muscle contractions – shaking/running movements. Urination and defecation are common during a grand mal seizure. There may be a period before the seizure of abnormal behavior. Seizures can be caused by:
- Head injury
- Drug overdose
- Meningitis and encephalitis e.g. distemper, rabies
- Brain tumors
- Hepatic encephalopathy/portocaval shunt
- Advanced kidney failure
Idiopathic epilepsy is a disease in which seizures occur and no underlying cause can be found. It is diagnosed by excluding all other causes of seizures. Idiopathic epilepsy usually starts between 1 to 2 years of age. Idiopathic epilepsy may be inherited in certain breeds, including Beagles, Keeshonds, Irish Setters, Siberian Huskies, Springer Spaniels, Labrador Retrievers, Golden Retrievers, and German Shepherds. A specific type of seizure known as temporal lobe epilepsy appears to be familial in Cavalier King Charles Spaniels and is characterized by behavioral manifestations such as fly biting.
If epilepsy is suspected the following diagnostic tests should be carried out:, a complete blood count, serum chemistry profile, urinalysis, bile tests, and thyroid function tests. Seizures can be controlled with diazepam (valium). Long term treatment is with the drug phenobarbital. Potassium bromide is used with phenobarbital if it does not control the seizures on its own. Phenobarbital is damaging to the liver in high doses. Liver function tests and phenobarbital assay should be carried out every 3 to 6 months.
Hypoglycemia is low blood sugar. The brain uses 25% of the total body glucose utilization even though it only weighs 2% of the total body weight. Puppy hypoglycemia is an idiopathic syndrome in toy breeds of dogs that is seen in the first 6 months of life. It seems to relate to a relative immaturity of the liver and can usually be managed by providing frequent meals of a commercial puppy diet. The problem usually resolves as the animal matures. Puppies with hypoglycemia will be quiet and listless, rapidly followed by seizures, coma and death if the condition is untreated.
Hypoglycemia can also be caused by insulin overdose. Insulin causes glucose on the blood to be moved into the cells. If a diabetic is given too much insulin or it is given insulin and does not eat then its blood sugar levels can fall. The brain uses 25% of the total blood sugar so it is the first organ to be affected by low glucose levels. The signs of hypoglycemia are weakness, sleepiness, altered behavior and blurred vision followed by shaking and seizures. Hypoglycemia is defined as blood glucose below 70md/dL. The more rapidly the glucose levels drop the more pronounced the symptoms.
Eclampsia is an acute, life-threatening condition usually seen at peak lactation, 2-3 wk after whelping. Small-breed bitches with large litters are most often affected. Hypocalcemia may also occur during parturition and may precipitate dystocia It is rare in cats and most common in dogs weighing <20 kg, exacerbated by improper perinatal nutrition (excessive calcium/phosphorus supplementation or an imbalanced prenatal diet). It is caused by low levels of calcium in the blood stream due to the high demand for calcium in the milk. Calcium supplementation during pregnancy suppresses the normal calcium control pathways, so the body does not respond properly to the demand for calcium during lactation. The symptoms are panting, tremors, twitching, shaking and seizures in a lactating or heavily pregnant bitch. A pretreatment serum calcium concentration <7 mg/dL (<6 mg/dL in cats) confirms the diagnosis. Treatment is with slow IV administration of 10% calcium gluconate is given to effect (0.5-1.5 mL/kg over 10-30 min; 5-20 mL is the usual dose). This usually results in rapid clinical improvement within 15 min. Muscle relaxation should be immediate. The puppies should be weaned immediately.
Owners should be warned that this condition is likely to recur with future pregnancies. Steps to prevent eclampsia in the bitch include feeding a high-quality, nutritionally balanced, and appropriate diet during pregnancy and lactation, providing food and water ad lib during lactation, and supplemental feeding of the puppies with milk replacer early in lactation and with solid food after 3-4 wk of age. Oral calcium supplementation during gestation is not indicated and may cause rather than prevent postpartum hypocalcemia. Calcium administration during peak milk production may be helpful in bitches with a history of puerperal hypocalcemia.
Tumors of the nervous system occur in 1 – 2 % of dogs. Tumors are more common in the brain than in the spinal cord or peripheral nerves. Adult dogs of several related brachycephalic breeds—Boxers, English Bulldogs, and Boston Terriers—are often cited as having the highest incidence of brain tumors among domestic animals. There are many different types of brain tumors, some act as space occupying lesions and other are functional such as pituitary tumors causing Cushing’s disease. Brain tumors can exhibit almost any symptom: changes in behavior, ataxia, weakness, circling, head pressing, blindness, hormone disorders or no signs at all. CT scan or MRI are used to diagnose brain tumors.
Hydrocephalus literally means water on the brain. Patients with hydrocephalus have abnormal accumulation of cerebrospinal fluid (CSF) in the ventricles, or cavities, of the brain. This may cause increased intracranial pressure inside the skull and progressive enlargement of the head, convulsion, and mental disability. Hydrocephalus is most common in dogs, particularly in toy and brachycephalic breeds. Treatment relies on either corticosteroids or surgery to shunt CSF into the peritoneum. Manitol can be used in the short term to lower intracranial pressure.