Download the nervous system

The Nervous System
Is a system that collects information from the external and internal environment and coordinates a
response to the stimulus. It consists of two parts:
Peripheral Nervous System
1. receptors which transduce a form of energy into a nervous impulse e.g. eye
2. sensory neurones which carry sensory impulses to the Central Nervous System (CNS)
3. motor neurones which carry impulses from the CNS to effectors (muscles, glands) to cause a
response (muscular contraction, release of a hormone, etc).
Central Nervous System (CNS)
1. spinal cord which links the Peripheral Nervous System to the brain
2. brain which receives sensory impulses from the receptors converts them into a 'sense' (e.g. vision)
and then controls and coordinates a response by sending out motor impulses.
Sense organs are groups of receptor cells that respond to specific stimuli (a particular type of energy).
They are able to transduce the form of energy they detect into a nervous impulse
Stimulus
Type of
Receptor
Photo
Light
Mechano
Sound
Mechano
Touch
Temperature Thermo
Chemo
Chemicals
Example
Notes
Eye
Ear
Skin
Skin and arteries
Nose
Wavelength = colour
Sound wave vibration
Pain
Homeostasis
Chemical shape
Nerve cells (neurones) are cells able to conduct an electrical (nervous) impulse (see above).
Diagram of a sensory neuron
Diagram of a relay neuron
Diagram of a motor neuron including the nerve ending in muscle
Effectors are organs which respond to a nervous impulse e.g. muscle by contracting, endocrine gland
by secreting a hormone.
Coordination Of Stimulus And Response
S A Voluntary action is an action initiated in the brain, e.g. picking up a banknote. It is alterable and
the response is based upon learning and memory and is not inherited.
light (stimulus)  eye  visual cortex (brain)  cerebral hemispheres  relay neurones 
various muscles (effectors) to bend legs, arms, back and pick up note  spinal cord 
A Reflex action is an automatic response to a stimulus which does not vary (e.g. coughing, sneezing.) It
is initiated in a receptor e.g. pain receptor in the skin and the stimulus is coordinated with the response
in the Central Nervous System but not in the voluntary centres of the brain via a Reflex Arc. It is
inherited.
A Reflex arc is a quick automatic response to a stimulus. It is involuntary and cannot be altered.
Reflexes are advantageous in that they are very quick responses but disadvantageous in that they are
unalterable and so may not be the best response. The reflex arc follows a particular pathway and does not
involve the higher centres.
STIMULUS
RECEPTOR
RECEPTOR
SENSORY
NEURON
CENTRAL
NERVOUS
SYSTEM
MOTOR
NEURON
EFFECTOR
Diagram of a spinal reflex arc
If more pain receptors are stimulated (pain over a greater area of skin) with more energy (i.e. greater the
pain), more effector cells are stimulated to a greater degree giving a stronger response. The relay
neurones relay impulses to the brain which can then inhibit the reflex with a voluntary action (e.g.
retaining hold of a hot plate). If no relay neurone is involved, e.g. knee jerk reflex, the brain cannot
override the response.
A Cranial reflex arc is similar to spinal but involves 'involuntary centres' of brain (as stimulus is in the
head region) and not spinal cord (e.g. pupil reflex, blinking).
Vision.
The most important sense in humans – more than half the total of all human sensory receptors are located
in the eyes.
Structures outside the eyeball:
Eyelids – protect eyelid from excessive light, spread secretions over conjunctiva.
Lachrymal glands – produce lubricating tears containing lysozyme, an antibacterial enzyme.
Conjunctiva – thin transparent skin to protect the cornea.
Eye muscles – six antagonistic muscles to move eyeball up, down, left right and swivel.
Structures within the eyeball:
Cornea – curved transparent coat at the front of the eye. Refracts light to aid accommodation.
Sclera – tough outer coat of the eyeball, gives a rigid shape to the eye essential to accommodation.
Choroid – provides nutrients to the retina, black pigment absorbs light to prevent internal reflection of
light which would give multiple images.
Ciliary body/ciliary muscles/suspensory ligaments are for accommodation (focusing).
Iris – a pigmented layer that gives the eye its colour and contains smooth muscles that regulate the size of
the pupil by means of a cranial reflex action The pupil reflex adjusts the size of the pupil to allow the
correct amount of light to enter the eye and prevent damage to the retina
Dim light – Brain stimulates involuntary contraction of radial
muscles to increase the size of the pupil (dilation).
Circular muscles reflexively relax.
Bright light – Brain stimulates involuntary contraction of circular muscles
to decrease the size of the pupil (constriction).
Radial muscles reflexively relax
Lens – a clear, fluid, transparent body made up of layers of a protein, crystalin whose width is adjusted to
focus light to give a clear image on the fovea.
Anterior chamber – filled with aqueous humour, a fluid secreted from blood which travels through the
chamber and lens providing nutrients and then returns to blood.
Posterior chamber – filled with the vitreous body, a jelly like substance (vitreous humour) formed in
embryonic life which holds the retina against the choroid.
Both humours maintain the pressure and shape of the eyeball and prevent it from collapsing.
Retina – contains photoreceptors which convert light to a nervous impulse that is transmitted to the brain
which perceives the image.
Optic nerve – transmits impulses from the retina to the visual centres of the brain.
Labelled diagram of a cross section of the eye
Accommodation:
Involves the refraction (bending) of light rays from an object to create a sharp (focussed) image on the
retina.
The cornea achieves approximately 75% of the refraction of light. As the shape of the cornea is constant,
the amount of refraction cannot be altered therefore,
The lens shape is adjusted to focus the incoming light rays.
For far vision (6m or more), the light rays require less refraction. The ciliary muscle is relaxed and the
pressures of the humours on the sclera pull on the suspensory ligaments to draw the lens out to a thin
shape.
For near vision, 10cm to 6m, greater refraction is required so the ciliary muscles contract pulling the
ciliary process forward towards the lens, releasing the tension in the ligaments and the elasticity of the
lens causes it to become more convex increasing refraction. As the ciliary muscles are contracted, near
vision can fatigue the eye.
Diagram to show accommodation of a near and a far object
S There is two types of light receptors in the retina:
Rods are very sensitive to light and therefore only functional in dim light. There are @ 120 million rods
in the retina, (none in the yellow spot) and their density is greatest at the periphery of the retina. They can
only sense the brightness of light and not colour.
Cones are sensitive to one of three wavelengths of light, blue, green or yellow-orange so are responsible
for colour vision. There are @ 6 million cones concentrated in and around the fovea which has the
greatest density of cones (and no rods). It therefore has the higher visual acuity (detail) and the eyes move
to focus objects of interest on the fovea in bright light
Antagonistic muscles in the movement of the forearm
Muscles can only do work when contracting. Therefore, two antagonistic muscles are required in each
part of a limb:
An extensor muscle to straighten the limb
A flexor muscle to flex (bend) the limb.
In the forearm, the flexor is the biceps muscle which contracts to bend the limb and the extensor is the
triceps muscle which contracts to straighten the limb. When one of the pair is stimulated to contract, the
other relaxes and is stretched (contraction of the antagonist is prevented by a reflex action).
The biceps and triceps are attached at the top end to the humerus and scapula(the origin which does not
move) and at the bottom end to the ulna (the insertion on the bone which moves). When either contracts
it pulls on the ulna which acts as a lever to move an object held in the hand. The biceps and triceps are
antagonistic as they have opposite effects (flexion and extension).
Diagrams to show the action of the antagonistic muscles of the human forearm.
The Endocrine System And Hormones
A hormone is a chemical substance produced by an endocrine gland and released into the blood stream
to alter the activity of one or more target organs; e.g. insulin is released by the pancreas to cause the
liver to absorb sugar from the blood and convert it to glycogen.
When released by the gland, hormones travel in blood plasma all over the body but only affect the
working of the target organ which absorbs them.
The liver continually breaks hormones down so that their affects have a limited time span (unless, like
thyroxin, a growth hormone, it is continually produced to stimulate growth over several years).
Hormones are found in both animals and plants and are used to control long-term or cyclical activities
(e.g. growth, flowering, seed germination, menstrual cycle).
Adrenaline. The "fight or flight" hormone released from the adrenal gland which is stimulated by the
brain when organism is frightened/excited or about to take action, etc. It causes
1. increased heart rate
2. increased ventilation rate (1,2,3 increase O2 availability to muscle)
3. increased blood flow to muscles
4. decreased blood flow to skin and gut (so that muscle blood flow can increase)
5. release of glucose from the liver (for use in respiration)
6. increase in metabolic rate
It prepares the body for action by maximising glucose and O2 supply to tissues. It also causes
wakefulness and alertness. It is destroyed by the liver to limit the action time.
Comparison between nervous and hormonal response
NERVOUS
Electrical impulse in nerve
Fast response
Stimulates small area/localised
Short-lived response
HORMONAL
Chemical carried in blood
Slow response
Can be widespread
Long-lived response/cyclical
Hormones control growth and artificial growth hormones can be included in cattle and poultry feed to
increase growth rates.
Homeostasis
Homeostasis is the maintenance of the internal environment within close limits. This includes regulation
of blood water and solute concentrations, pressure and temperature. It is vital to the correct functioning of
the body.
S Insulin. is a homeostatic hormone that is one of the two antagonistic hormones that regulate blood
sugar level. In the human, the fasting blood sugar level is maintained at a constant 80mg/100cm3 blood
by the liver but it can rise after a meal or fall on starvation.
After a meal, blood travelling from gut in hepatic portal vein to liver has a high sugar concentration which
causes blood sugar level to rise. Insulin is then released from the pancreas which
1. causes liver to absorb glucose from blood and convert it to glycogen which is stored
2. causes muscles to absorb sugar from blood and convert it to glycogen which is stored
and these effects combine to cause blood sugar level to drop to normal. When this happens, insulin is no
longer released, negative feedback
Glucagon is the antagonistic hormone to adrenaline that is released from the pancreas when blood sugar
drops below 80mg/100ml. It causes
1. liver to release glucose into the bloodstream
2. breakdown of glucagon into glucose in the liver for release
both of which cause the blood glucose level to rise to normal and glucagons is no longer released,
negative feedback.
RISE
BLOOD
INSULIN
RELEASE
MEDULLA
GLUCOS
E
LEVEL
OF
BRAIN
RETURN TO
-VE FEEDBACK
NORM
FALL
GLUCAGON
RELEASE
A rise in blood sugar causes release of insulin – when the level drops, insulin ceases to be released. This
system allows antagonistic hormones working by negative feedback to maintain homeostatis.
Hormones can be used in food production e.g. cattle can be given hormones to increase growth rate.
Homeostatic control of body temperature
Temperature sensors in the skin and brain measure external and blood temperature and control body
responses to cause heat gain or loss to maintain the blood temperature at 37.4 C.
Blood transfers heat round the body from sites with a high rate of respiration e.g. active muscle, liver to
cooler areas or to skin for loss.
Heat loss mechanisms are activated when the body temperature rises:
Skin vasodilation – local arterioles dilate filling skin capillary beds with blood and causing loss by
radiation/conduction
Sweat glands – secrete sweat causing loss by evaporation
Heat gain mechanisms are activated when the body temperature falls:
Skin vasoconstriction – local arterioles constrict lowering loss by radiation/conduction
Shivering – increased muscle metabolism to produce heat
Diagram of cross section of the skin with annotation giving the function of each label.
Labels: hair, sweat gland, heat (temperature) receptors, blood vessels, fatty tissue.
Drugs
Drug – any substance taken into the body that affects or modifies chemical reactions in the body. it is by
modifying reactions that drugs have their effect.
Examples
Antibiotics are chemicals that affect the chemical reactions that in occur in bacteria but not in animal
cells e.g. penicillin prevents bacteria from manufacturing cell walls. Animal cells do not have cell walls
so are unaffected.
They do not affect viruses because a virus does not reproduce itself but takes over the host cell metabolic
processes to manufacture new virus particles (antiviral drugs slow host cell processes)
Sedatives depress brain e.g. alcohol, heroin
Stimulants quicken brain e.g. caffeine
(Narcotics - drugs with very powerful effect e.g. morphine)
(Side effects – the harmful effect that the drug has on the body e.g. aspirin can cause bleeding)
Addiction – the body becomes accommodated to the drug, which then becomes essential to body
function either or both:
1. physiologically on brain and body e.g. heroin, nicotine and therefore causes withdrawal
symptoms on removal of the drug
2. psychologically on behaviour when the individual needs the drug to feel as they wish e.g. cocaine,
heroin
Heroin abuse
Heroin is a highly addictive drug derived from morphine, which is obtained from the opium poppy. It is a
"downer" or depressant that affects the brain's pleasure systems and interferes with the brain's ability to
perceive pain.
What are its short-term effects?
The short-term effects of heroin abuse appear soon after a single dose and disappear in a few hours. After
an injection of heroin, the user reports feeling a surge of euphoria ("rush") accompanied by a warm
flushing of the skin, a dry mouth, and heavy extremities. Following this initial euphoria, the user goes "on
the nod," an alternately wakeful and drowsy state. Mental functioning becomes clouded due to the
depression of the central nervous system. Other effects included slowed and slurred speech, slow gait,
constricted pupils, droopy eyelids, impaired night vision, vomiting, and constipation.
What are its long-term effects?
Long-term effects of heroin appear after repeated use for some period of time. Chronic users may develop
collapsed veins, infection of the heart lining and valves, abscesses, cellulites, and liver disease.
Pulmonary complications, including various types of pneumonia, may result from the poor health
condition of the abuser, as well as from heroin's depressing effects on respiration. In addition to the
effects of the drug itself, street heroin may have additives that do not really dissolve and result in clogging
the blood vessels that lead to the lungs, liver, kidneys, or brain. This can cause infection or even death of
small patches of cells in vital organs. With regular heroin use, tolerance develops. This means the abuser
must use more heroin to achieve the same intensity or effect.
As higher doses are used over time, physical dependence and addiction develop. With physical
dependence, the body has adapted to the presence of the drug and withdrawal symptoms may occur if use
is reduced or stopped. Withdrawal, which in regular abusers may occur as early as a few hours after the
last administration, produces drug craving, restlessness, muscle and bone pain, insomnia, diarrhoea and
vomiting, cold flashes with goose bumps ("cold turkey"), kicking movements ("kicking the habit"), and
other symptoms. Major withdrawal symptoms peak between 48 and 72 hours after the last does and
subside after about a week. Sudden withdrawal by heavily dependent users who are in poor health can be
fatal.
http://www.drugfree.org/Portal/drug_guide/Heroin
accessed 12;40 03/10/2007
Alcohol abuse
(Alcohol is a depressant drug obtained from fermentation of sugar-containing natural products. It is
legally available above a certain age)
Sensible drinking
This factsheet is written for people who want to know more about the effects of alcohol and how to drink
sensibly.
Research has shown that, for men over 40 and women after the menopause, having one or two small
drinks a day can help prevent coronary heart disease. However, it is estimated that nearly one in three
adults in the UK are risking their health by drinking more than the recommended daily amount of alcohol.
How much is too much?
Current UK guidelines recommend that men don't drink more than three or four units of alcohol a day,
and that women limit their intake to two or three units a day. You shouldn't save up units through the
week and use them to binge at the weekend, and at least one day a week should be alcohol-free.
If you do have an episode of heavy drinking, as a short term measure, you shouldn't drink alcohol for 48
hours.
Strength and units
One unit is 8 grams, or about 10ml, of pure alcohol - regardless of how diluted it is. Below is a list of
some common drinks and how many units they have in them.





One pint of strong lager (alcohol 5% vol) = 3 units
One pint of standard strength lager (alcohol 3 - 3.5% vol) = 2 units
One 275ml bottle of an alcopop (alcohol 5.5% vol) = 1.5 units
One standard (175ml) glass of wine (alcohol 12% vol) = 2 units
One measure (25ml) of a spirit strength drink = 1 unit
Units for women
The recommended limits are lower for women than for men because the body composition of women has
less water than men. So, even if a man and woman weigh the same and are of a similar size, the woman
will tend to get drunk faster.
Some experts also think that women develop liver disease at lower levels of drinking than men, although
this appears to only be the case in higher levels of alcohol consumption.
Alcohol dependence
Some people who drink frequently or in large quantities can become addicted to alcohol. Doctors use a
number of techniques to diagnose patients with drink problems. They may ask you how much and how
often you drink and whether you have "blackouts". You may be asked if you have tried to cut down,
whether you feel guilty about your drinking, or whether you have a drink in the morning.
According to the Institute of Alcohol Studies, a person is considered to be dependent on alcohol when
they have experienced three or more of the following symptoms during a year.





A strong urge to drink, difficulty controlling how much they drink, or difficulty stopping.
Physical withdrawal symptoms, such as sweating, shaking, agitation and nausea when they try to
reduce drinking.
A growing tolerance to alcohol - needing larger quantities to get the same effect.
Gradual neglect of other activities.
Persistent drinking even though it is obviously causing harm.
Alcohol abuse, or problem drinking, happens when a person is not dependent on alcohol, but is drinking
enough to cause themselves actual physical or psychological harm.
Short-term effects
A small amount of alcohol will relax you and make you feel less anxious. But alcohol is a depressant of
the central nervous system. In increasing amounts it suppresses the part of your brain that controls
judgement, resulting in a loss of inhibitions. It also affects your physical co-ordination, causing blurred
vision, slurred speech and loss of balance. Drinking a very large amount at one time (binge drinking) can
lead to unconsciousness, coma, and even death. Vomiting while unconscious can lead to death by
asphyxiation (suffocation). Alcohol is involved in a large proportion of fatal road accidents, assaults and
incidents of domestic violence.
Long-term effects
Alcohol can be a dangerous drug. Drinking too much too often will cause physical damage, increase the
risk of getting some diseases, and make other diseases worse. Excessive drinking over time is associated
with:








hepatitis and cirrhosis of the liver
gastritis (inflammation of the stomach lining) or pancreatitis (inflammation of the pancreas)
high blood pressure (which can lead to stroke)
certain types of cancer, including mouth and throat
damage to the brain
heart failure
neurological problems such as epilepsy
certain types of vitamin deficiency
Excessive drinking has also been linked to:





obesity
sexual problems
infertility
muscle disease
skin problems
Alcohol and pregnancy
Women who drink heavily during pregnancy are at risk of having babies with a condition called foetal
alcohol syndrome. This can result in growth deficiencies, nervous system problems, lowered intelligence,
and facial abnormalities in the child. It is also called foetal alcohol spectrum disorder - all the symptoms
are not always present and can vary in how serious they are.
There is some evidence that pregnant women who drink 10 to 15 units a week are more likely to have
underweight babies. It is not known if there is an absolutely safe limit for drinking during pregnancy, but
research indicates that it may be wise to avoid alcohol altogether.
Psychological effects
Although alcohol initially makes people feel relaxed, long term excessive use can ultimately increase
anxiety and cause depression. It is also related to problems with sleeping, mood-swings, violence and
suicide (about two-thirds of suicide attempts are thought to involve alcohol).
http://hcd2.bupa.co.uk/fact_sheets/html/alcohol_abuse.html
Accessed 03/10/2007
Social effects of drug abuse in terms of alcohol and heroin.
Due to the addictive properties, the drug becomes the priority in the person’s lifestyle leading to
1.
2.
3.
4.
Family dysfunction – as family life is limited by dependence and physiological affects
Crime – to source an ever more expensive habit due to tolerance
Violence – direct affect of alcohol abuse due to depressive quality
Poverty/Economic difficulties – lowered earning capacity and increasingly expensive habit.
Alcohol and heroin abuse also can lead to other health problems such as HIV/Aids due to needle sharing
and unprotected intercourse.
Tobacco
Are dried leaves of the tobacco plant processed to allow smoking. It contains many toxic substances such
as:
Substance
Carbon
monoxide
Tar
Physiological
Binds
irreversibly
haemoglobin
Carcinogenic chemicals
Effect
On health
to Reduces oxygen carrying capacity of blood leading to
anoxia, shortness of breath.
Cause carcinomas (uncontrolled cell growth, cancers) of
lung, throat, mouth
Slow diffusion rates of oxygen leading to anoxia. Damage
High viscosity liquid binds to cilia leading to dust and microbe retention in trachea and
lung increasing severity of respiratory infections
epithelia of lung and trachea
Nicotine
Smoke
particles
Excitatory, addictive narcotic, Increased risk of cardiovascular disease, addictive leading
increases blood pressure
to increased demand
Bind to epithelia
Cause irritation, coughing