The Central Nervous System
The central nervous system consists of the brain, spinal cord and peripheral nerves. Nerve cells respond to changes within the internal and external environment. They work alongside the endocrine system to maintain homeostasis. Their functions are; to detect changes, process sensory information, and to provide a response.
There are two types of nerve cells; Neurones which transmit electrical signals and Glial cells which provide nutrients and other materials. The synapse is the point in which one nerve impulses passes from one axon to another. Neurotransmitters facilitate this process. Schwann cells, produce myelin sheaths around the neurones. They are mainly required for insulation and also supplying nutrients to individual nerve fibres (axons). The gap between the Schwann cells is known as the Node of Ranvier, which facilitates the rapid conduction of nerve impulses.
There are three types of nerves
- Autonomic nerves. These nerves control the involuntary or partially voluntary activities of the body, including heart rate, blood pressure, digestion (Enteric Nervous System), and temperature regulation
- Motor nerves. These nerves control movement and actions by passing information from the brain and spinal cord to the muscles.
- Sensory nerves. These nerves relay information from the skin and muscles back to the spinal cord and brain. The information is then processed as pain and other sensations.
Action Potential. Prior to the body responding the cells are at rest. The charge outside of the membrane of the nerve cell is positive and inside negative, The inside of the cell is 70 millivolts more negative than the outside. Sodium NA+ is the main extracellular cation, Potassium K+ the main intercellular ion, but the cytoplasm also contains many negatively charged proteins. The cell membrane is impermeable to these charged ions, maintaining the difference between the outside and inside of the cell. When stimulated the permeability of membrane changes. Initially sodium floods into the neurone changing the electrical potential, initially depolarising, that is, when the electrical stimuli moves towards Zero. When it reaches -55 millivolts it is at threshold which results in the action potential being stimulated. It takes longer for the potassium channels to open and when they do potassium rushes out reversing the process. The sodium channels closes and sodium/potassium pumps helps restore the cell to its original resting state.
The body is constantly monitoring the levels of Sodium, Potassium, and Calcium within the blood. Excess or deficiency will impair the proper functioning of the muscles and other systems of the body. Other important minerals are; Chloride, which is a major contributor to the osmotic pressure gradient between the Intercellular fluid ICF and ECF extracellular fluid, and plays an important role in maintaining proper hydration of the cell. Magnesium contributes to the production of energy and cardiovascular function.
For an action potential to be initiated a signal is required. This is achieved through the involvement of Neurotransmitters.
The Neurotransmitter initiates a response. A particular neurotransmitter can have the opposite effects dependent upon the receptor it stimulates. For example, acetylcholine causes opposite effects in skeletal and heart muscle because these cell types produce different kinds of acetylcholine receptors that trigger different pathways. Once the neurotransmitter has been used it needs to be removed from the synaptic cleft, either through diffusion out of the cleft, degradation through the use of an enzyme, or by re-uptake.
Acetylcholine is generally an excitatory neurotransmitter causing muscle contraction, but also plays an important role in arousal, memory, and attention. It is removed by degradation.
The Amino acids glutamate and aspartate are excitatory neurotransmitters, Glutamate is released by nerve cells in the brain and plays an important role in learning and memory. It is the most common neurotransmitter in the brain. It is removed by re-uptake.
GABA (gamma-aminobutyric acid) and glycine act as inhibitory neurotransmitters. They are located in CNS (1/3 in brain). They are removed by re-uptake. Valium enhances GABA activity.
Catecholamines also function as neurotransmitters and hormones. Epinephrine is excitatory and causes an increase in heart rate, muscle strength, blood pressure, and sugar metabolism. Norepinephrine increases heart rate and blood pressure, triggers the release of glucose from energy stores, increases blood flow to skeletal muscle, reduces blood flow to the gastrointestinal system, and inhibits voiding of the bladder and gastrointestinal motility. It increases alertness and arousal, speeds up reaction time, and has been shown to play a role in a person's mood and ability to concentrate.
In response to low blood pressure and stress Norepinephrine promotes vasoconstriction, which is a narrowing of the blood vessels thereby increasing blood pressure. It is removed by re-uptake or degradation.
Monoamine oxidase enzyme MOA deactivates neurotransmitter activity, while Catechol -O- Methyltransferase enzyme COMT degrades.
Dopamine is excitatory. It is primarily located in the Substantia nigra of the brain. It is involved in movement and coordination, emotional responses, addictive behaviours, pleasurable experiences, and muscle tone. It is removed by re-uptake or the enzymes MAO, and COMT.
Serotonin is an excitatory neurotransmitter. It plays a role in regulating mood, sensory perception, temperature regulation, appetite, and sleep induction. It is removed by re-uptake
Nitric oxide is formed from arginine on demand and acts immediately. Plays a role in memory and learning. It is also a potent vasodilator.
Neuropeptides are small proteins produced by neurones which are responsible for slow onset and longer lasting effects. Enkephalins, endorphins, and dynorphins help with analgesia, mood stabilisation, pleasure, memory and learning.
Substance P is a peptide that is triggered by intense afferent painful stimuli. It modulates the neural response to pain and mood.
The Nervous system has unique energy requirements. High demand neurotransmitters require continual recycling and also the production of the enzymes required to synthesise the neurotransmitter. Its high energy requirements needs a constant supply of oxygen and glucose. One aspect of the workings of the nervous system is communication through the involvement of neurotransmitters. These have to be continually recycled and maintained. For nerve impulses to be effectively transmitted there is a need for the neurones to be insulated within a myelin sheath which is made up of a high concentration of lipids. Due to its high energy requirements and its high concentration of lipids the system is susceptible to cellular damage through the activity of free radicals . Some areas of the brain have high concentrations of iron which also makes it more susceptible to the risk of lipid peroxidation. The nervous system has a limited ability to replace damaged cells therefore it is important that the body has the essential nutrients for protecting against free radical damage; these include: zinc, copper, manganese, selenium, and Iron for the production of the enzymes involved in protecting from free radical damage. Also required are antioxidants, such as; Vitamin C and E, coenzyme Q10, carnitine and lipoic acid.
Neurotransmitters play an important role in nervous system health.
Serotonin is synthesised in the Central nervous system and also in enterochromaffin cells of the gastrointestinal tract, which produces around 90% of the body's serotonin levels. it is derived from tryptophan. Serotonin has various functions within the body, including that of the regulation of mood, sleep and appetite. Stress may have an effect of lowering serotonin levels. Low levels of serotonin have been associated with depression, greater impulsivity, insomnia, and eating disorders. While excess has found to lead to restlessness, agitation, muscle cramps and confusion. Activities that can naturally raise serotonin levels are; positive thinking, exposure to light, and exercise.
Gamma amino butyric acid (GABA) is a major inhibitory neurotransmitter, it is dependent upon vitamin B6 for its synthesis. GABA deficiency has been associated with anxiety, insomnia, panic attacks, palpitations, trembling, seizures and cold clammy hands, while in excess it has ben implicated in schizophrenia, alzheimer's, migraine, multiple sclerosis, epilepsy and bipolar disorders. Supplementation of GABA is difficult as it does not pass the blood brain barrier, so a more effective approach may be in boosting levels through foods that aid it its synthesis.
Catecholamines are hormones produced by the adrenal glands, which are found on top of the kidneys. They are released into the blood during times of physical or emotional stress. The major catecholamines are dopamine, norepinephrine, and epinephrine. Dopamine also acts as a hormone and is involved in the control of motor function. It has a role in cognition and also pleasure and motivation. Its deficiency results in rigidity, tremors, and brandykinesia.
Acetylcholine is the major neurotransmitter in the parasympathetic nervous system. It plays a role in learning, memory, cognitive functions, fine motor control and autonomic functioning. Low levels can lead to uncontrollable tremors and memory loss.
As is evident from the above brief descriptions, the neurotransmitters work very much in harmony alongside hormones (dopamine acts as both). That is one reason why mental and cognitive conditions are harder to treat. Cause and effect is another complicating factor, as seen with serotonin, low levels lead to depressive symptoms, while having a positive frame of mind causes serotonin levels to rise. For most higher level cognitive functioning there is an interaction between a number of neurotransmitters. Disruption within the endocrine system can also exhibit similar manifestations, for example hyperthyroidism increasing feelings of anxiety.
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