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The Central Nervous System and the Peripheral Nervous System

In human anatomy, the nervous system is a whole network of specialised cells which coordinate actions by sending signals from one part of the body to another. The signals can either be in form of electrochemical waves or chemical releases. This system is divided into two parts: the Central Nervous System (CNS) and the Peripheral Nervous System (PNS).

Central Nervous System

The CNS is comprised of the brain and the spinal cord that make up a large part of the entire nervous system. The spinal cord is connected to the brain at the brainstem. The CNS contains millions of neurons (nerve cells).White matter within the CNS consists of axons which are insulated by a sheath of fatty material called myelin. Myelin insulates an axon from its neighbours so nerve cells can conduct electrical messages without interfering with one another. The grey matter of the CNS consists of cell bodies and the branched dendrites of nerve cells which effectively connect them together.

Functions of the CNS include: coordinating activities between the various parts of the human body (based on sensory information gathered), working in collaboration with the peripheral nervous system and serving important functions on the physical as well as mental aspects of our life. The brain receives sensory input from the spinal cord aswell as from its own nerves (for example olfactory and optic nerves) and sends coordinated motor output to the PNS.

The brain plays a major role in controlling various body functions such as: movement, sensation, thinking, memory, speech, etc. It is divided into two halves (right and left brain) each with specialised functions. The right brain functions include controlling the left side of the body, visual and spatial skills, memory storage, feelings and intuitions, holistic interpretations, gesturing and synthesising and creativity. The left brain functions include controlling the right side of the body, sequential analysis, memory storage in particular order, logical interpretations, reading and understanding language, objective, rational, analysing etc.

Damage to either side of brain can lead to adverse effects on the individual's body and behaviour and any damage caused to a specific side is known to show its adverse effects on the opposite side of the body. For example, damage to the right hemisphere of the brain leads to cognitive-communication problems, such as impaired memory, attention problems and poor reasoning.

The spinal cord and CNS neurons primary function is the transmission of messages back and forth between the brain and the peripheral nerves (both somatic and autonomic). The spinal cord connects a large part of the PNS to the brain and conducts sensory information from the PNS to the brain and conducts motor information from the brain to various effectors such as: skeletal muscle, cardiac muscle, glands etc. Although the brain and spinal cord work together to control various functions of the body, the spinal cord also serves as a minor reflex centre and reflex movements can occur through spinal cord pathways, without any of the structure of brain getting involved in the process (for example the withdrawal reflex).

Autonomic nervous system

The Peripheral Nervous System includes all the nerves outside the CNS and is further subdivided into two system; the Somatic Nervous system (somatic nerves carry sensory information from the skin and muscle, and motor commands to the skeletal muscles controlling movement) and the Autonomic Nervous System (autonomic nerves carry signals between the CNS and smooth muscles, glands, cardiac muscles and internal organs). The Autonomic Nervous System is separated into two branches, the Sympathetic and the Parasympathetic which control the body’s internal environment. These regulate the involuntary processes of the body, the viscera, the muscles, the sense organs, glands and blood vessels and plays an important role in the process of homeostasis.

The sympathetic system prepares the body for emergencies and is activated by any stimulus that is over an individual’s threshold (which can vary enormously) including: feelings, noise, light, drugs, chemicals etc and in response to the stimulus an immediate anticipatory state is generated by the release of adrenaline. This causes the heart to beat faster, increases blood supply to the muscles, raises blood pressure, dilates the bronchi and increases the breathing rate, raises the blood sugar level for increased energy, speeds up mental activity, increases tension in the muscles, dilates pupils and increases sweating. Non-emergency functions, such as digestion are lessened or suspended. Walter Cannon coined the phrase ‘fight or flight’ to describe the function of the rapid mobilisation of resources from the sympathetic system.

The parasympathetic governs relaxation, recuperation and digestion and comes into operation after the stimulus has been responded to and action take. It has the opposite effect to sympathetic activity, allowing the body to wind down and re-balance. The activation of the parasympathetic nervous system encourages relaxation of muscles, slowing the heart rate and restoring normal blood pressure. It assists the breathing to return to its normal rate, digestive juices flow, bladder and bowels are ready to function, the pupils constrict and immune functions, such as the production of white blood cells are re-commenced. The parasympathetic mode supports rest and sleep.

The standard physiological model of the ANS is of reciprocal tension. The two parts keep each other in check – when the sympathetic goes up, the parasympathetic goes down and vice versa. The controls are done automatically and outside of the conscious level; in most situations, we are unaware of the workings of the ANS. For example, we do not normally notice when blood vessels change size or when our heart beats faster. However, some people can be trained to control some functions of the ANS such as heart rate or blood pressure.

Endocrine System

The endocrine system is comprised of the glands within the body that release more than 20 major hormones directly into the bloodstream where they are transported to cells in other parts of the body. These physiological processes are carried out unconsciously by the endocrine system. Hormones act as chemical messengers that transmit information and instructions from one set of cells to another and they control and influence almost every cell, organ, and function within the body; cells are genetically programmed to receive and respond to specific hormones. The endocrine system is instrumental in regulating mood, growth and development, tissue function, and metabolism, as well as sexual function and reproductive processes.

Endocrine glands are the hypothalamus, pituitary, thyroid, parathyroid, adrenal and gonads (the reproductive glands which include the ovaries and testes). A number of glands that signal each other in a sequence are usually referred to as an axis, for example, the hypothalamic-pituitary-adrenal axis. In addition to the specialised endocrine glands, many other organs (such as the liver, kidney and heart) have secondary endocrine functions.

The Hypothalamus

The hypothalamus is located in the lower central part of the brain and is the primary link between the endocrine and nervous systems. In general, the hypothalamus acts as a integrator to regulate and coordinate basic functions necessary for life including fluid and electrolyte balance, feeding and energy metabolism, wake-sleep cycles, temperature regulation, stress responses, sexual behaviour and reproduction/reproduction cycles. The hypothalamus is important for survival because it maintains the body's homeostasis and accomplishes this by having neurons that are directly sensitive to variables or by having neurons that receive inputs from other sensory systems that monitor variables. The hypothalamus neurons attempt to regulate parameters against what amounts to a set point, comparable to a thermostat on a home central heating system. The hypothalamus contains sensors for the body’s internal environment such as blood temperature, blood sugar and mineral/hormone levels.

In contrast to the homeostatic system of the hypothalamus it also receives direct sensory inputs from external environments such as smell, taste and visual messengers aswell as information relayed from the limbic lobe of the brain (which receives highly processed sensory information from throughout the cerebral cortex, and determines it personal importance for the individual). All these inputs drive a wide range of emotional responses (known as allstatic). The responses can include increase in body temperature and blood pressure, as well as endocrine adjustments (such as adrenaline release when under threat) and many emotional expressions (changes in heart rate, blushing, hair standing on end, etc.) are mediated and controlled by the hypothalamus.

In addition to maintaining homeostasis, and responding to urgent external events, the hypothalamus also assists with anticipating behavioural daily events that are triggered by the external day-night cycle including predictable times for feeding, drinking, sleeping. All of these are regulated by the circadian timing system in the brain which enables the body to anticipate its various demands and needs.

System Interaction Example

In each 24-hour period, it is normal for people to be continuously awake for about 16 hours and then continuously asleep for approximately 8 hours. The sleep/awake cycle depends largely on the CNS/PNS and endocrine systems with the hypothalamus acting as a link between the three.

Several areas in the brainstem and hypothalamus promote wakefulness by sending neurotransmitters (arousal signals) to the cerebral cortex and basal forebrain regions of the brain. When neurons in the arousal areas are active, the cortex remains activated and we stay awake; at the same time this inhibits activity in other areas of the brain responsible for promoting sleep.

One area of the brain that promotes arousal/wakefulness is the tuberomammillary nucleus (TMN). Here, neurons release histamine and orexin (also known as hypocretin) which directly stimulate the arousal centres as well as the cerebral cortex itself. Many ‘anti-histamine’ medicines block this arousing transmission signal and cause sleepiness.

Neurons in a part of the hypothalamus called the ventrolateral preoptic nucleus (VLPO) transmit and receive signals to and from the cerebral cortex promoting activity (wake) or inhibiting activity (sleep). The areas of the brain that maintain wakefulness inhibit VLPO neurons. When VLPO neurons fire rapidly and induce sleep, they also inhibit activity in the arousal centres such as the TMN. The ability to remain in a stable period of sleep or wakefulness is a result of this switch between the wake-promoting neurons and the sleep-promoting neurons.

We normally change from one stable state to the other due to additional internal factors (such as the increasing drive to sleep that builds up during wakefulness with the accumulation of adenosine) and changing influences from our internal biological clock located in the suprachiasmatic nucleus (SCN). This tiny structure is made up from approximately 50,000 brain cells. It receives light signals directly from the eye, through the optic nerve which sets the clock to correspond to the day-night cycle. In turn, the clock regulates the timing of dozens of different internal functions, including temperature, hormone release, hunger, thirst aswell as sleep and wakefulness. The SCN promotes wakefulness by producing a powerful alerting signal and promotes sleep by turning off the alerting signal. In addition, the SCN actively maintains sleep throughout the night even after sleep drive has dissipated in the second half of the night through the circadian rhythm which is roughly a 24-hour cycle in the biochemical, physiological, or behavioural processes. Although endogenous these rhythms are adjusted to the external environment by external cues (such as daylight). Other external factors, such as noise, can also influence the likelihood if falling asleep or waking up. Caffeine inhibits the actions of adenosine and therefore helps maintain wakefulness.

People generally require several minutes to calm down and relax enough to fall asleep, and the deepest stages of sleep typically occur 20 or more minutes after the onset on sleep. Similarly, waking up from sleep can occur very quickly, for example in response to an alarm clock, although it typically takes people much longer to become fully alert after awakening.

TAQ2

The human brain is the complex organ that allows us to see, move, think, feel, see, hear, taste, and smell. It controls actions within the body by receiving, analyzing, processing and storing information 24 hours a day. Information, in the form of nerve impulses, travel to and from the brain via the central nervous system. This allows the brain to monitor and regulate all unconscious and voluntary movements and processes of the body. It is also the site of consciousness which give humans the ability to think, learn, create and to exhibit behaviour.

The brain is comprised of many parts, each of which has a specific function. It can be divided into four main areas: the cerebrum, the cerebellum, the diencephalons and the brain stem which are further divided into regions that control specific functions. While different parts of the brain specialise in one function they never work alone; the various areas of the brain collaborate together to generate a behaviour.

Cerebrum

The cerebrum is the largest part of the brain. The front section of the cerebrum is the frontal lobe which is involved in speech, thought, emotion, and skilled movements. Behind this is the parietal lobe which perceives and interprets sensations like touch, temperature and pain. At the centre back of the cerebrum, is a region called the occipital lobe which detects and interprets visual images. Either side of the cerebrum are the temporal lobes which are involved in hearing and memory storage. The cerebrum is split down the middle into two halves called hemispheres that communicate with each other.

Cerebellum

The cerebellum sits underneath the back of the cerebrum. It is involved in coordinating muscles to allow precise movements and control of balance and posture.

Diencephalon

The diencephalon is situated beneath the middle of the cerebrum on top of the brain stem. It includes two important structures: the thalamus and the hypothalamus. The thalamus acts as a relay station; it receives incoming sensory nerve impulses and sends them on to appropriate regions for processing. The hypothalamus plays a vital role in maintain homeostasis. It also controls the release of hormones from the nearby pituitary gland.

Brain stem

The brain stem is responsible for regulating many life support mechanisms such as: heart rate, blood pressure, digestion and breathing.

Synaptic transmission

Neurons carry nerve impulses from one part of the body to another within the nervous system; glial cells supply nutrients and protect the neurons. The biochemical mechanism that enables this is the synaptic neurotransmission. When neurons are stimulated they transmit an electrical impulse and upon reaching the axon terminals, release chemicals known as neurotransmitters. These diffuse across the synapse. Receptor molecules combine with the neurotransmitters causing a further burst of electro chemical impulses and if numerous enough the impulse begins to travel along the next nerve and so on. The neurons and glial cells are the building blocks of the brain and neuron to neuron communication with dynamic patterns of synaptic activation being the basic unit of brain function.

Behaviour

Human behaviour is the actions of an individual in response to various stimuli or inputs, whether internal or external, conscious or subconscious, overt or covert, and voluntary or involuntary. In terms of behaviour, normal refers to a lack of significant deviation from the average. Human behaviour can be acceptable, unusual or unacceptable and is usually judged based on social norms for that society. When looking at normal human behaviour then the environment where the individual lives needs to be taken into account aswell. For example, the behaviour patterns of an isolated African tribe will be very different to a Western, industrialised community.

Behaviour is primarily controlled by the brain and is believed for a large part to be innate or learned. We also know that bio-psychological factors are also involved in the normal/abnormal behaviour patters and that neural activity in the brain can also affect behaviour. Behaviour can be altered and affected from changes under the influence of the endocrine system and the nervous system. Research into brain and behaviour patterns is concerned with determining the neural and chemical correlation of motivation, development, and cognition. For example, some of the many topics studied include: maternal behaviour, biological rhythms, drugs, psychiatric disorders, physiology and chemistry of brain change associated with learning, aging, retardation, and epilepsy; and cognitive changes in brain-injured human patients. I have looked at one area to see how psychological factors are involved and how neural activity of the brain affects behaviour.

Stress Hormones

The sympathetic nervous system (SNS) turns on the fight or flight response and in contrast, the parasympathetic nervous system (PNS) promotes the relaxation response. The SNS and PNS carefully maintain metabolic equilibrium by making adjustments whenever something disturbs this balance. Hormones are the chemical messengers produced by endocrine glands that travel through the bloodstream to accelerate or suppress metabolic functions and to return the body to homeostasis.

The primary area of the brain that deals with stress is its limbic system. Whenever an individual perceives a threat, imminent or imagined then this activates the hypothalamic-pituitary-adrenal axis through the release of corticotrophin releasing hormone (CRH), leading to production of glucocorticoids that down-regulate immune responses. The adrenal glands release adrenaline (also known as epinephrine) and other hormones that increase breathing, heart rate, and blood pressure. This moves more oxygen-rich blood faster to the brain and to the muscles needed for the flight or fight response. Adrenaline causes a rapid release of glucose and fatty acids into your bloodstream. The senses become keener, memory sharper, and sensitivity to pain is reduced. Other hormones shut down functions unnecessary during the emergency so growth, reproduction and the immune system all go on hold. Blood flow to the skin is reduced. If the threat is severe or persistent the adrenal glands then release glucocorticoids (a class of steroid hormones more commonly know as corticosteroids or cortisol). Once in the brain cortisol remains much longer than adrenalin.

Once the perceived danger has passed, the body should return to normal. Studies by Robert M Sapolsky show that chronic over-secretion of glucocorticoids adversely affects brain function, and can damage the hippocampus, the part of the limbic brain which is central to learning and memory. Cortisol interferes with the function of neurotransmitters and excessive cortisol levels can make it difficult to think or retrieve long-term memories. The hippocampus is also a key activator in instructing the hypothalamus to turn off the cortisol-producing mechanism and if damaged so it cannot provide proper feedback to the hypothalamus and cortisol continues to be secreted. This, in turn, causes more damage to the hippocampus, and even more cortisol production. Thus, a Catch-22 cycle begins, which can be very difficult to stop.

Physically, chronic stress can lead to sexual dysfunction, a susceptible immune system (which often manifests as skin ailments). Stress can result in illnesses such as ulcers, depression, diabetes, digestive problems and even mental illnesses.

Stress hormones divert blood glucose to the muscles which diminishes the amount of glucose that reaches the hippocampus. This may explain why some people can't remember a very traumatic event, and why short-term memory is usually the first casualty of age-related memory loss resulting from a lifetime of stress.

Chronic stress can have lasting behavioural impacts. These will all impact on an individuals behaviour patterns. Chronic stress can alter how a person perceives and reacts to future stressful situations creating anxiety. Behaviourally, stress can make an individual feel under pressure, worried, tense, moody, upset, sad and angry. Due to ongoing stress the brain will begin to register irrational fears. The brain may create streams of thoughts about arguments and difficult situations from the past and you may dwell on these or create irrational fears about future events following the same patterns. All these behavioural manifestations can become exaggerated in the presence of adrenaline, and it can be hard to tune them out. The inability to concentrate or make decisions can result in aggressive behaviour in some individuals due to sheer frustration.

Stress can dramatically increase the ability of chemicals to pass through the blood-brain barrier (BBB). This is a physiological mechanism that is both a physical barrier and a system of cellular transport mechanisms. It restricting the entrances of potentially harmful chemicals from the blood while at the same time allowing the entrance of essential nutrients; not all areas of the brain have a blood- brain barrier.

During the Gulf War, Israeli soldiers took a drug to protect themselves from chemical and biological weapons. Normally, it should not have crossed the BBB, but scientists learned that the stress of war had somehow increased the permeability of the BBB. Nearly one-quarter of the soldiers complained of headaches, nausea, and dizziness – symptoms which occur only if the drug reaches the brain. (Sharabi, Y. et al. 1991). 

Stress can also be measured at a community or population level. At any point in time a population will consist of citizens at range of stress levels and studies have shown that as general pressures increase on a population (economic, environmental or political) then a higher proportion of individuals are likely to cross the threshold from anxiety to panic or violence. Increased stress therefore may also contribute to increased crime and violence in communities. This occurred in deprived communities in the UK during the recession of the late eighties. If a population is already stressed every additional source of stress increases the probability and number of individuals that will become violent; as pressures increases then normal social and moral restraints become weaker. (Dai Williams, D. et al. 1999). 

Stress seems to be a subjective and highly personal experience. The innate and learned behaviours that we all have may cause an array of physical symptoms that activates the hypothalamic-pituitary-adrenal axis in one person while in another the same situation fails to create a reaction. For example, I am personally petrified of wasps and bees. I know that this is irrational. I can feel my fight or flight reactions kicking in as soon as a buzzing noise enters my brain. However, over time I have altered my behaviour so that I can reverse this reaction. While it has not completely disappeared I can control it to some extent and my previous external behaviour has been modified, i.e. arms screaming, running away and arms flying about.

Stress is a result of our brain interpreting life events in ways that feel bad, negative, sad, and angry, etc. Stress affects behaviour. Behaviour affects the way we see the world which in turn how we interpret life events. People, through their behaviour can actually become addicted to their emotional state and their stress levels would probably rise if they did not get anxious or stressed. As with any addiction, their body acclimates to the current levels of bodily chemicals released by their emotions which again encourages the Catch-22 cycle.

In contrast, some stress can be relatively healthy and is a necessary part of life. During a period of stress norepinephrine is released. This is an excitatory neurotransmitter and is needed to create new memories. It improves mood. It can make problems feel more like challenges, which encourages creative thinking that stimulates your brain to grow new connections within itself.

Phrases such as abnormal behaviour or deviant behaviour is often applied in a negative sense when it may not necessarily have a negative impact on the individual or others; abnormality varies greatly in how pleasant or unpleasant this is for the individual and for those around them. Normal behaviour is also categorized on individual perspective and perception as we tend to use our own standards and values to define what is normal.

Everything we do, everything we think and everything we feel is mediated by our brain. Our brain guides us through life. By sensing the world around us, storing fragments and memories of each unique moment, cataloguing, sorting, organizing and acting on our experiences, our brain defines us.

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