Brief details about Lysosomes enzyme

Lysosomes are spherical vesicles enveloped by a single membrane. Lysosomes are regarded as the digestive tract of the cell, since they are actively involved in digestion of cellular substances—namely proteins, lipids, carbohydrates and nucleic acids. 

Lysosomal enzymes are categorized as hydrolases. These include the enzymes (with substrate in brackets)—α-glucosidase (glycogen), cathepsins (proteins), lipases (lipids), ribonucleases (RNA). The lysosomal enzymes are responsible for maintaining the cellular compounds in a dynamic state, by their degradation and recycling. 

The degraded products leave the lysosomes, usually by diffusion, for reutilization by the cell. Sometimes, however, certain residual products, rich in lipids and proteins, collectively known as lipofuscin accumulate in the cell. 

Lipofuscin is the age pigment or wear and tear pigment which has been implicated in ageing process. As the cell dies, the lysosomes rupture and release hydrolytic enzymes that results in post-morteum autolysis. 

The digestive enzymes of cellular compounds are confined to the lysosomes in the best interest of the cell. Escape of these enzymes into cytosol will destroy the functional macromolecules of the cell and result in many complications. The occurrence of several diseases (e.g. arthritis, muscle diseases, allergic disorders) has been partly attributed to the release of lysosomal enzymes. 

Inclusion cell (I-cell) desease is a rare condition due to the absence of certain hydrolases in lysosomes. However, these enzyme are syntherized and found in the circulation. I-cell disease is due to a defect in protein targetting, as the enzymes cannot reach lysosomes.

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EATING HEALTHY DIET DURING COVID-19 PANDEMIC

Eating a healthy diet is very important during the COVID-19 pandemic. What we eat and drink can affect our body’s ability to prevent, fight and recover from infections.

While no foods or dietary supplements can prevent or cure COVID-19 infection, healthy diets are important for supporting immune systems. Good nutrition can also reduce the likelihood of developing other health problems, including obesity, heart disease, diabetes and some types of cancer.

For babies, a healthy diet means exclusive breastfeeding in the first six months, with the introduction of nutritious and safe foods to complement breast milk from age 6 months to 2 years and beyond. For young children, a healthy and balanced diet is essential for growth and development. For older people, it can help to ensure healthier and more active lives.

EATING HEALTHY DIET DURING COVID-19 PANDEMIC

Tips for maintaining a healthy diet:

Eat a variety of food, including fruits and vegetables

• Every day, eat a mix of whole grains like wheat, maize and rice, legumes like lentils and beans, plenty of fresh fruit and vegetables , with some foods from animal sources (e.g. meat, fish, eggs and milk).

Eat a variety of food, including fruits and vegetables

• Choose wholegrain foods like unprocessed maize, millet, oats, wheat and brown rice when you can; they are rich in valuable fibre and can help you feel full for longer.

• For snacks, choose raw vegetables, fresh fruit, and unsalted nuts.

Cut back on salt

• Limit salt intake to 5 grams (equivalent to a teaspoon) a day.

• When cooking and preparing foods, use salt sparingly and reduce use of salty sauces and condiments (like soy sauce, stock or fish sauce).

• If using canned or dried food, choose varieties of vegetables, nuts and fruit, without added salt and sugars.

• Remove the salt shaker from the table, and experiment with fresh or dried herbs and spices for added flavor instead.

• Check the labels on food and choose products with lower sodium content.

Cut back on salt

Eat moderate amounts of fats and oils

• Replace butter, ghee and lard with healthier fats like olive, soy, sunflower or corn oil when cooking.

• Choose white meats like poultry and fish which are generally lower in fats than red meat; trim meat of visible fat and limit the consumption of processed meats.

• Select low-fat or reduced-fat versions of milk and dairy products.

Eat moderate amounts of fats and oils

• Avoid processed, baked and fried foods that contain industrially produced trans-fat.

• Try steaming or boiling instead of frying food when cooking.

Limit sugar intake

• Limit intake of sweets and sugary drinks such as fizzy drinks, fruit juices and juice drinks, liquid and powder concentrates, flavored water, energy and sports drinks, ready-to-drink tea and coffee and flavored milk drinks.

• Choose fresh fruits instead of sweet snacks such as cookies, cakes and chocolate. When other dessert options are chosen, ensure that they are low in sugar and consume small portions.

Limit sugar intake

• Avoid giving sugary foods to children. Salt and sugars should not be added to complementary foods given to children under 2 years of age, and should be limited beyond that age.

Stay hydrated: Drink enough water

Good hydration is crucial for optimal health. Whenever available and safe for consumption, tap water is the healthiest and cheapest drink. Drinking water instead of sugar-sweetened beverages is a simple way to limit your intake of sugar and excess calories.

Stay hydrated: Drink enough water

Avoid hazardous and harmful alcohol use

Alcohol is not a part of a healthy diet. Drinking alcohol does not protect against COVID-19 and can be dangerous. Frequent or excessive alcohol consumption increases your immediate risk of injury, as well as causing longer-term effects like liver damage, cancer, heart disease and mental illness. There is no safe level of alcohol consumption.

Avoid hazardous and harmful alcohol use

Breastfeed babies and young children

Breast milk is the ideal food for infants. It is safe, clean and contains antibodies which help protect against many common childhood illnesses. Babies should be breastfed exclusively during the first 6 months of life, as breast milk provides all the nutrients and fluids they need.

• From 6 months of age, breast milk should be complemented with a variety of adequate, safe and nutrient-dense foods. Breastfeeding should continue under babies at 2 years of age or beyond.

Women with COVID-19 can breastfeed if they wish to do so and should take infection prevention and control measures.

Breastfeed babies and young children

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How to Protect yourself and others from the spread COVID-19

You can reduce your chances of being infected or spreading COVID-19 by taking some simple precautions:

• Regularly and thoroughly clean your hands with an alcohol-based hand rub or wash them with soap and water. Why? Washing your hands with soap and water or using alcohol-based hand rub kills viruses that may be on your hands. 

Safe use of alcohol based hand sanitizers

• Maintain at least 1 metre (3 feet) distance between yourself and others. Why? When someone coughs, sneezes, or speaks they spray small liquid droplets from their nose or mouth which may contain virus. If you are too close, you can breathe in the droplets, including the COVID-19 virus if the person has the disease.

Maintain Distance

• Avoid going to crowded places. Why? Where people come together in crowds, you are more likely to come into close contact with someone that has COIVD-19 and it is more difficult to maintain physical distance of 1 metre (3 feet).

Avoid going to crowded places

• Avoid touching eyes, nose and mouth. Why? Hands touch many surfaces and can pick up viruses. Once contaminated, hands can transfer the virus to your eyes, nose or mouth. From there, the virus can enter your body and infect you.

Avoid touching eyes

• Make sure you, and the people around you, follow good respiratory hygiene. This means covering your mouth and nose with your bent elbow or tissue when you cough or sneeze. Then dispose of the used tissue immediately and wash your hands. Why? Droplets spread virus. By following good respiratory hygiene, you protect the people around you from viruses such as cold, flu and COVID-19.

good respiratory hygiene

Stay home and self-isolate even with minor symptoms such as cough, headache, mild fever, until you recover. Have someone bring you supplies. 

Stay home and self-isolate

If you need to leave your house, wear a mask to avoid infecting others. Why? Avoiding contact with others will protect them from possible COVID-19 and other viruses.

Mask

If you have a fever, cough and difficulty breathing, seek medical attention, but call by telephone in advance if possible and follow the directions of your local health authority. Why? National and local authorities will have the most up to date information on the situation in your area. Calling in advance will allow your health care provider to quickly direct you to the right health facility. This will also protect you and help prevent spread of viruses and other infections.

Call for Help

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Best and Safe use of alcohol based hand sanitizers

To protect yourself and others against COVID-19, clean your hands frequently and thoroughly. Use alcohol-based hand sanitizer or wash your hands with soap and water. If you use an alcohol-based hand sanitizer, make sure you use and store it carefully.

use of alcohol based hand sanitizers

Keep alcohol-based hand sanitizers out of children’s reach. Teach them how to apply the sanitizer and monitor its use.

Safe use of alcohol based hand sanitizers

• Apply a coin-sized amount on your hands. There is no need to use a large amount of the product.

• Avoid touching your eyes, mouth and nose immediately after using an alcohol-based hand sanitizer, as it can cause irritation.

• Hand sanitizers recommended to protect against COVID-19 are alcohol-based and therefore can be flammable. Do not use before handling fire or cooking.

• Under no circumstance, drink or let children swallow an alcohol-based hand sanitizer. It can be poisonous.

• Remember that washing your hands with soap and water is also effective against COVID-19.

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The genetic code

Genetic information is information for synthesizing proteins, and since proteins consist of 20 different amino acids bonded in particular sequences, the genetic information must tell how to position the amino acids correctly in a polypeptide chain.

It is believed that each of the 20 different amino acids is represented in a DNA molecule by a particular sequence of 3-nucleotide groups. That is, the sequence C, G, A in a DNA strand represents one kind of amino acid; the sequence G, C, A represents another kind, and T, T, A still another kind.

Genetic Code For Certain Amino Acids 

Thus, the sequence in which the nucleotide groups are arranged within a DNA molecule can denote the arrangement of amino acids within a protein molecule. This method of storing information used for the synthesis of particular protein molecules is termed the genetic code.

Although DNA molecules are located in the chromatin within a cell's nucleus, protein synthesis occurs in the cytoplasm. Therefore the genetic information must somehow be transferred from the nucleus into the cytoplasm. This transfer of information is the function of certain RNA molecules.

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Thyroid Gland and Hormone Structure Function

The thyroid gland, is a very vascular structure that consists of two large lobes connected by a broad isthmus. It is located just below the larynx on either side and in front of the trachea. It has a special ability to remove iodine from the blood.

Structure of the Gland

The thyroid gland is covered by a capsule of connective tissue and is made up of many secretory parts called follicles. The cavities of the follicles are lined with a single layer of Cuboidal epithelial cells and are filled with a clear, viscous Glycoprotein called colloid. The follicle cells produce and secrete hormones that may be stored in the colloid or released into the blood of nearby capillaries.

Structure of the Gland

Thyroid Hormones and Their Functions

The thyroid gland produces several hormones that have marked effects on the metabolic rates of most body cells and one hormone that influences the level of blood calcium. Of the hormones that affect metabolic rates, the most important are thyroxine and triiodothyronine. 

Thyroxine and Triodothyronine

They act to increase the rate of energy release from carbohydrates and the rate of protein synthesis. They also accelerate growth in young persons and stimulate activities of the nervous system. As was explained earlier, the release of these hormones is controlled by the hypothalamus and pituitary gland.

Thyroid Gland

Before follicle cells can produce thyroxine and triiodothyronine, they must be supplied with iodine salts (iodides). Such salts are normally obtained from foods, and after they have been absorbed from the intestine, they are carried by the blood to the thyroid gland. 

An efficient active transport mechanism called the iodine pump moves the iodides into the follicle cells, where they are used together with an amino acid (tyrosine) in the synthesis of the hormones Follicle cells also secrete the substance called thyroglobulin, which is the main ingredient of thyroid colloid. 

Thvroglobulin is used to store thyroid hormones whenever they are produced in excess. The stored hormones are bonded to the thyroglobulin until the hormone concentration of the body fluids drops below a certain level; then enzymes cause the hormones to be released from the colloid, and they diffuse into the blood. 

Once they are in the blood, thyroid hormones combine with blood proteins (alpha globulins) and are transported to body cells in this form.

Although triiodothyronine is nearly five times more potent, thyroxine accounts for at least 95% of the circulating thyroid hormone.

The thyroid hormone that influences blood calcium levels is a polypeptide called calcitonin.

Calcitonin

This substance helps regulate the calcium level by inhibiting the rate at which calcium leaves the bones and enters the extracellular fluids.

This is accomplished by decreasing the bone resorbing activity of osteoclasts. At the same time, calcitonin causes an increase in the rate of calcium deposit in bone matrix by stimulating the activity of osteoblasts.

Thus, calcitonin acts to lower the concentration of blood calcium—an effect exactly opposite that promoted by parathyroid hormone. 

The secretion of calcitonin is thought to be controlled directly by the blood calcium level. As this level increases, so does the secretion of calcitonin.

Following chart reviews the actions and controls of the thyroid hormones -

Actions and controls of the thyroid hormones

Growth Hormone

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Basic knowledge about Skin with their functions Notes Summary

The skin is the largest and probably the most versatile organ of the body. 

Functions of skin

• Protective covering.
• Aids in the regulation of body temperature.
• Houses sensory receptors..
• Contains sensory (somatic) nerve endings of pain, temperature and touch.
• Synthesizes various chemical substances.
• Persons are recognized and their ages are often judged by their skin characteristics. 
• Protects the underlying structures from injury and from invasion by microbes.

Structure of skin

The skin has a surface area of about 1.5 to 2 m2 in adults and it contains glands, hair and nails.  

The skin is composed of two distinct layers of tissues. The outer layer, called the epidermis, contains stratified squamous epithelial cells. The inner layer, or dermis, is thicker than the epidermis, and it includes a variety of tissues, such as epithelial tissue, fibrous connective tissue, smooth muscle tissue, nerve tissue, and blood. Between the skin and underlying structures there is a layer of subcutaneous fat. 

Beneath the dermis are masses of loose connective and adipose tissues that bind the skin to underlying organs. This is called the subcutaneous layer.

Structure of skin

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Different types of Body Membrane Summary Notes

Two or more kinds of tissues grouped together and performing specialized functions constitute an organ. Thus, the sheetlike structures called membranes that cover body surfaces and line body cavities are organs.

Types of Membranes

Different types of membranes

There are four major types of membranes:

1. Serous membrane
2. Mucous membrane
3. Cutaneous membrane
4. Synovial membrane

Usually these structures are relatively thin. Serous, mucous, and cutaneous membranes are composed of epithelial tissue and some underlying connective tissue; synovial membranes are composed entirely of connective tissue.

Serous membranes

Serous membranes line the body cavities that lack openings to the outside. They form the inner linings of the thorax and abdomen, and they cover the organs within these cavities. Such a membrane consists of a layer of simple squamous epithelium (mesothelium) covering a thin layer of loose connective tissue. It secretes a watery serous fluid, which helps to lubricate the surfaces of the membrane.

Mucous membranes

Mucous membranes line the cavities and tubes that open to the outside of the body. These include the oral and nasal cavities and the tubes of the digestive, respiratory, urinary, and reproductive systems. Mucous membrane consists of epithelium overlying a layer of loose connective tissue; however, the type of epithelium varies with the location of the membrane. For example, stratified squamous epithelium lines the oral cavity, pseudostratified columnar epithelium lines part of the nasal cavity, and simple columnar epithelium lines the small intestine. Mucous membrane secretes the thick fluid called mucus.

Cutaneous membrane

The cutaneous membrane is an organ of the integumentary system and is more commonly called skin.

"Subcutaneous injections are administered into the layer beneath the skin. Intradermal injections, on the other hand, are injected between layers of tissues within the skin. Subcutaneous injections and intramuscular injections, which are administered into muscles, are sometimes called hypodermic injections".

Synovial membranes

Synovial membranes form the inner linings of joint cavities between the ends of bones at freely movable joints. These membranes usually include fibrous connective tissue overlying loose connective tissue and adipose tissue. They secrete a thick, colorless synovial fluid into the joint cavity. This fluid lubricates the ends of the bones within the joint.

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Clinical Uses of Reflexes

Since normal reflexes depend on normal neuron functions, reflexes are commonly used to obtain information concerning the condition of the nervous system.

An anesthesiologist, for instance, may try to initiate a reflex in a patient who is being anesthetized in order to determine how the anesthetic drug is affecting nerve functions. Also, in the case of injury to some part of the nervous system, various reflexes may be tested to discover the location and extent of the damage.

If any portion of a reflex arc is injured, the normal characteristics of that arc are likely to be altered. For example, a plantar reflex is normally initiated by stroking the sole of the foot, and the usual response includes a flexion of the foot and toes.

However, in persons who have suffered damage to certain nerve pathways (corticospinal tract) there may be an abnormal response called the Babinski reflex.

In this case the reflex response is plantar extension, in which the great toe extends upward and the smaller toes fan apart. If the injury is minor, the response may consist of plantar flexion with failure of the great toe to flex, or plantar flexion followed by plantar extension.

The Babinski reflex is, however, present normally in infants up to the age of 12 months and is thought to reflect a degree of immaturity in their corticospinal tracts.

Other reflexes that may be tested during a neurological examination include the following:

Biceps-jerk reflex

This reflex can be elicited by bending a person's arm at the elbow. The examiner's finger is placed on the inside of the bent elbow over the tendon of" the biceps muscle, and the finger is tapped. The biceps contracts in response, and the forearm is flexed at the elbow.

Triceps-jerk reflex

This reflex can be caused by flexing a person's arm at the elbow and tapping the short tendon of the triceps muscle close to its insertion near the tip of the elbow. The muscle contracts in response, and the forearm is extended slightly.

Abdominal reflexes

These reflexes occur when the examiner strokes the skin of the abdomen. For example, a dull pin may be drawn from the sides of the abdomen upward toward the midline and above the umbilicus. Normally, the abdominal muscles underlying the skin contract in response, and the umbilicus is moved toward the region that was stimulated.

Ankle-jerk reflex

This reflex is elicited by tapping the Achilles tendon just above its insertion on the calcaneus. The response is plantar flexion, produced by contraction of the gastrocnemius and soleus muscles.

Cremasteric reflex

This reflex is obtained in males by stroking the upper inside of the thigh. As a result, the testis on the same side is elevated by contracting muscles.

Anal reflex

This reflex is elicited by stroking the skin surrounding the anus. The anal sphincter muscles contract in response.

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Appendicular skeleton

The appendicular skeleton consists of the shoulder girdle with the upper limbs and the pelvic girdle with the lower limbs.

Shoulder (Pectoral) girdle

The shoulder girdle consists of two scapulae and two clavicles, which attach the bones of the upper limbs to the axial skeleton.

Clavicle (collar bone)

Each clavicle is a long, slender S shaped bone with two curves, one convex and one concave. It articulates with the manubrium of the sternum at the sternoclavicular joint and forms the acrominoclavicular joint with the acromion process of the scapula. The clavicle provides the only bony link between the upper limb and the axial skeleton.
 

Scapula (shoulder blade)

Each scapula is a large, triangular, flat bone situated in the posterior part of the thorax between the level of the second and seventh ribs and separated from them by muscles. At the lateral angle is a shallow articular surface, the glenoid cavity, which, with the head of the humorous, forms the shoulder joint.

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Impulse Conduction Notes Summary

An unmyelinated nerve fiber conducts an impulse over its entire membrane surface.

A myelinated fiber functions differently, because myelin serves as an insulator that prevents almost all flow of ions through the membrane.

Considering this, it might seem that the myelin sheath would prevent the conduction of a nerve impulse altogether, and this would be true if the sheath were continuous.

It is, however, interrupted by constrictions called nodes of Ranvier, which occur between adjacent Schwann cells.

At these nodes the fiber membrane is especially permeable to sodium and potassium ions, and a nerve impulse traveling along a myelinated fiber appears to jump from node to node.

This type of impulse conduction, called saltatory conduction, is many times faster than conduction on an unmyelinated fiber.

The speed of nerve impulse conduction is also related to the diameter of the fiber—the greater the diameter, the faster the impulse.

For example an impulse on a thick fiber, such as a motor fiber associated with a skeletal muscle, might travel 130 meters per second, while an impulse on an extremely fine fiber, such as a sensory fiber associated with the skin, might move only 0.5 meter per second.

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Some Factors Affecting Nerve Impulse Conduction

A number of substances affect nerve fiber membranes by influencing their permeability to ions.

For example, calcium ions seem to be needed for the closure of the sodium channels in the nerve fiber membrane during an action potential.

Consequently, if calcium is deficient, the sodium channels ma\ remain open, and sodium ions may diffuse through the membrane again and again so that impulses are transmitted repeatedly.

Such spontaneous impulses may travel along the nerve fibers to skeletal muscle fibers.

When this happens, the muscles may undergo continuous spasms (tetany). Such muscular spasms or cramps can sometimes be relieved by raising the level of calcium ions in the body fluids.

Certain drugs, such as procaine and cocaine, produce special effects by decreasing membrane permeability to sodium ions. When one of these drugs is present in the tissue fluids surrounding a nerve fiber, impulses are prevented from passing through the affected region.

Consequently, the drugs are useful as local anesthetics because they help keep impulses from reaching the brain and thus prevent the sensations of touch and pain.

“A decrease in the level of blood calcium, accompanied by tetany, sometimes occurs in women during pregnancy because of the increasing demand for calcium made by the developing fetus Tetany may also occur in persons who have an inadequate supply of calcium or vitamin D in their diets, or who suffer an increased loss of calcium due to prolonged diarrhea”.

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Excitatory and Inhibitory Actions

In the brain and spinal cord, some synaptic knobs release neurotransmitters that cause an increase in membrane permeability to sodium ions, and thus trigger nerve impulses. This action is said to be excitatory.

Substances of this type include serotonin, dopamine, and norepinephrine.

Other synaptic knobs release substances that decrease membrane permeability to sodium ions, thus causing the threshold of stimulation to be raised. This action is called inhibitory, for it lessens the chance that a nerve impulse will be transferred to an adjoining neuron.

Inhibitory substances include gamma-aminobutyric acid (GABA), which is an amino acid that is synthesized primarily in the brain and spinal cord, but is not incorporated into protein molecules. Another amino acid, glycine, seems to function as an inhibitor in some of the spinal cord synapses.

The synaptic knobs of a thousand or more neurons may communicate with the dendrites and cell body of a particular neuron.

Some of these knobs probably have an excitatory action, while others are likely to be inhibitory.

The effect on the neuron will depend on which knobs are activated from moment to moment.

In other words, if more excitatory than inhibitory knobs are functioning, the neuron's threshold may be exceeded, and a nerve impulse will be triggered to pass over its surface.

Conversely, if more inhibitory knobs are active, no impulse will be conducted.

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Some Factors Affecting Synaptic Transmission

If nerve impulses reach synaptic knobs at rapid rates, their supplies of neurotransmitters may become exhausted.

Impulses cannot be transferred between the neurons involved until more neurotransmitter is synthesized.

Such a condition seems to occur during an epileptic seizure. In this situation, abnormal and excessive discharges of impulses originate from certain brain cells.

Some of these impulses reach skeletal muscle fibers and stimulate violent contractions.

In time, the synaptic knobs seem to run out of neurotransmitter substances, and the seizure subsides.

“A drug called Dilantin (diphenylhydantoin) is commonly used to treat epilepsy. It seems to have a stabilizing effect upon excitable neuron membranes, apparently acting to increase the effectiveness of the sodium active transport mechanism. Consequently, the sodium ions move from inside the neurons, and the thresholds of their membranes are stabilized against excessive stimulation”.

Other factors that affect synaptic transmission include various drugs and chemicals.

For example, botulin, a toxin produced by certain bacteria {Clostridium botulinum), is responsible for botulism, a severe form of food poisoning. This toxin prevents the release of acetylcholine from synaptic knobs, so that nerve impulses are not transmitted across synapses.

A victim of botulism may die from paralysis of the breathing muscles. Caffeine, which is found in coffee, tea, and cola drinks, stimulates activity in the nervous system. It does this by somehow lowering the thresholds for triggering nerve impulses at synapses.

Thus, when caffeine is present, certain neurons are more easily excited than usual.

A number of drugs apparently produce their special effects by interfering with the normal actions of neurotransmitters.

These include LSD, which seems to counteract the function of serotonin; cocaine, which enhances the effects of norepinephrine by preventing its normal inactivation; and amphetamine, which promotes the excessive release of dopamine.

Some of the antianxiety drugs, including diazepam (Valium), seem to produce their effects by increasing the effectiveness of the inhibitory transmitter GABA.

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Neuropeptides Notes Summary

Several substances called neuropeptides occur in the brain and spinal cord.

They appear to be synthesized in neuron cell bodies and are composed of chains of two to thirty-nine amino acids. Some of them seem to serve as neurotransmitters, while others may act as neuromodulators substances that alter a neuron's response to a neurotransmitter.

The neuropeptides include two groups of neurotransmitters, called enkephalins and endorphins, whose molecules contain five amino acids each.

They influence the nervous system in much the same way as morphine, in that they relieve pain sensations.

Another neuropeptide, which consists of eleven amino acids and is widely distributed throughout the nervous system, is called substance P.

It seems to function as a neurotransmitter (or perhaps as a neuromodulator) in the neurons that transmit pain impulses into the spinal cord and on to the brain.

Some investigators think the enkephalins and endorphins may relieve pain by inhibiting the release of substance P from pain-transmitting neurons.

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The Synapse Notes Summary

Within the nervous system, nerve impulses travel from neuron to neuron along complex nerve pathways. The junction between the parts of two such neurons is called a synapse. Actually, the neurons are not in direct contact at a synapse. There is a gap called a synaptic cleft between them. For an impulse to continue along a nerve pathway it must cross this gap.

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Synaptic Transmission Notes Summary

A nerve impulse travels in both directions away from the point of stimulation.

Within a neuron, however, an impulse will usually travel from a dendrite to its cell body and then move along the axon to the end.

There it crosses a synapse and continues to a dendrite or cell body of another neuron. The process of crossing the gap at a synapse is called synaptic transmission.

The typical one-way transmission from axon to dendrite or cell body is due to the fact that axons usually have rounded synaptic knobs at their ends, which dendrites lack.

These knobs contain numerous membranous sacs, called synaptic vesicles, and when a nerve impulse reaches a knob, some of the vesicles respond by releasing a substance called a neurotransmitter.

The neurotransmitter diffuses across the synaptic cleft and reacts with the neuron membrane on the other side.

If a sufficient amount of neurotransmitter is released, the membrane is stimulated, and a nerve impulse is triggered. Neurotransmitters are usually destroyed through rapid decomposition by enzymes present in synaptic clefts or are somehow removed.

Such destruction or removal of the neurotransmitter is important in preventing a continuous stimulation of a neuron on the distal side of a synapse.

Acetylcholine is the neurotransmitter released by most axons outside the brain and spinal cord, and by some axons within these organs.

This is the same substance that is released from the ends of motor neurons at the motor end plates of muscle fibers. In both cases, it is decomposed by the action of the enzyme cholinesterase.

“Parkinson's disease, which is characterized by slowness of movement, difficulty in initiating voluntary muscle actions, and tremors, is thought to be caused by degeneration of certain neurons in the brain that synthesize a neurotransmitter, called dopamine. 

The resulting deficiency of this neurotransmitter interferes with normal nerve impulses, causing the symptoms. This condition is often treated with a drug called L-dopa, which can be converted to dopamine by cellular enzymes”.

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