NERVOUS SYSTEM IN HUMANS Flashcards
The Nervous System
The human nervous system consists of the:
central nervous system (CNS) – the brain and the spinal cord
peripheral nervous system (PNS) – all of the nerves in the body
It allows us to make sense of our surroundings and respond to them and to coordinate and regulate body functions
Information is sent through the nervous system as nerve impulses – electrical signals that pass along nerve cells known as neurones
A bundle of neurones is known as a nerve
Types of Neurone
There are three main types of neurone: sensory, relay and motor
Sensory neurones carry impulses from sense organs to the CNS (brain or spinal cord)
Relay neurones are found inside the CNS and connect sensory and motor neurones
Motor neurones carry impulses from the CNS to effectors (muscles or glands)
Neurones have a long fibre (axon)
This means that less time is wasted transferring the impulse from one cell to another
The axon is insulated by a fatty sheath with small uninsulated sections along it (called nodes)
This means that the electrical impulse does not travel down the whole axon, but jumps from one node to the next
Their cell body contains many extensions called dendrites
This means they can connect to many other neurones and receive impulses from them, forming a network for easy communication
draw a diagram of a typical neurone
Identifying the types of neurone:
Sensory neurones are long and have a cell body branching off the middle of the axon
Relay neurones are short and have a small cell body at one end with many dendrites branching off it
Motor neurones are long and have a large cell body at one end with long dendrites branching off it
draw three types of neurone
Voluntary & Involuntary Responses
A voluntary response is one where you make a conscious decision to carry out a particular action therefore it starts with your brain
An involuntary (or reflex) response does not involve the brain as the coordinator of the reaction and you are not aware you have completed it until after you have carried it out
Involuntary actions are usually ones which are essential to basic survival and are rapid, whereas voluntary responses often take longer as we consider what the response might be before doing it
The Reflex Arc
An involuntary (or reflex) response does not involve the brain as the coordinator of the reaction and you are not aware you have completed it until after you have carried it out
This is an automatic and rapid response to a stimulus such as touching something sharp or hot
As it does not involve the brain, a reflex response is quicker than any other type of nervous response
This helps to minimise the damage to the body
Reflex actions are:
Automatic
Fast
Protective
Reflex actions
Heat (the stimulus) is detected by a temperature receptor in the skin
Sensory neurone sends electrical impulses to the spinal cord (the coordinator)
Electrical impulse is passed on to relay neurone in the spinal cord
Relay neurone connects to motor neurone and passes the impulse on
Motor neurone carries impulse to the biceps muscle in the arm (the effector)
Biceps muscle will contract and pull the arm up and away from the hot object (the response)
The reflex pathway
The Synapse: Definition
The junction between two neurones is known as a synapse
How an Impulse is Passed Across a Synapse
Neurones never touch each other
The junctions (gaps) in between them are called synapses
The electrical impulse travels along the first axon
This triggers the nerve-ending of the presynaptic neurone to release chemical messengers called neurotransmitters from vesicles which fuse with the presynaptic membrane
The neurotransmitters diffuse across the synaptic gap and bind with receptor molecules on the membrane of the second neurone (known as the post synaptic membrane)
This stimulates the second neurone to generate an electrical impulse that travels down the second axon
The neurotransmitters are then destroyed to prevent continued stimulation of the second neurone which would cause repeated impulses to be sent
Synapses ensure that impulses only travel in one direction, avoiding confusion within the nervous system if impulses were travelling in both directions
As this is the only part of the nervous system where messages are chemical as opposed to electrical, it is the only place where drugs can act to affect the nervous system – eg this is where heroin works
synapse : for maximum marks you will need to be able to understand
the structure and functioning of a synapse and explain what happens at each step.
receptors
Receptors are groups of specialised cells
They detect a change in the environment and stimulate electrical impulses in response
Sense organs contain groups of receptors that respond to specific stimuli
Once the receptor cell in the sense organ has been stimulated, it generates an electrical impulse
This is passed on to a sensory neurone which carries the impulse to the central nervous system
Here a response will be decided on and the impulse will be passed to a motor neurone (via a relay neurone)
The motor neurone carries the impulse to the effector (muscle or gland)
The effector carries out the response
sense organs
Sense organs contain groups of receptors that respond to specific stimuli
Structure of the Eye
what’s the eye ?
The eye is a sense organ containing receptor cells that are sensitive to light (rod cells) and colour (cone cells)
functions of structures in the eye
what is a Pupil Reflex?
This is a reflex action carried out to protect the retina from damage in bright light and protect us from not seeing objects in dim light
In dim light the pupil dilates (widens) in order to allow as much light into the eye as possible
In bright light the pupil constricts (narrows) in order to prevent too much light entering the eye and damaging the retina
How does the Pupil Reflex Work?
dim light
How does the Pupil Reflex Work?
bright light
whqt is accomodation?
The way the lens brings about fine focusing is called accommodation
The lens is elastic and its shape can be changed when the suspensory ligaments attached to it become tight or loose
The changes are brought about by the contraction or relaxation of the ciliary muscles
Viewing Near
Distant Objects
There are two types of receptor cells in the retina
Rods which are sensitive to dim light
Cones which distinguish between different colours in bright light
Rods & Cones
There are two types of receptor cells in the retina:
Rods which are sensitive to dim light
Cones which distinguish between different colours in bright light
There are 3 types of cone cells which are sensitive to different colours of light (red, blue and green)
The fovea is an area on the retina where almost all of the cone cells are found
Rod cells are found all over the retina, other than the area where the optic nerve attaches to the retina – there are no light-sensitive cells at all in this area, and so it is known as the blind spot
What is a Hormone?
A hormone is a chemical substance produced by a gland and carried by the blood, which alters the activity of one or more specific target organs i.e. they are chemicals which transmit information from one part of the organism to another and bring about a change
The glands that produce hormones in animals are known collectively as the endocrine system
Endocrine glands have a good blood supply as when they make hormones they need to get them into the bloodstream (specifically the blood plasma) as soon as possible so they can travel around the body to the target organs to bring about the response
Once a hormone has been used, it is destroyed by the liver
More about Adrenaline
Adrenaline is known as the fight or flight hormone as it is produced in situations where the body may be in danger
It causes a range of different things to happen in the body, all designed to prepare it for movement (ie fight or flight).
These include:
Increasing blood glucose concentration for increased respiration in muscle cells
Increasing pulse rate and breathing rate so glucose and oxygen can be delivered to muscle cells and carbon dioxide taken away from muscles cells more quickly
Diverting blood flow towards muscles and away from non-essential parts of the body such as the alimentary canal
Dilating pupils to allow as much light as possible to reach the retina so more information can be sent to the brain
Comparison of Nervous and Hormonal Control
Learning the list of … as it is a fairly common exam question and can be worth several easy marks.
effects of adrenaline on the body
Homeostasis
“The maintenance of a constant internal environment” means that internal conditions within your body (such as temperature, blood pressure, water concentration, glucose concentration etc) need to be kept within set limits to ensure that reactions in body cells can function, and therefore the organism as a whole, can live
When one of these conditions deviates far away from the normal if not brought back within set limits the body will not function properly and the eventual consequence without medical intervention will be death
This is why diabetics need to control glucose intake (as their body cannot regulate it for them); why an extremely high and prolonged fever will kill you; or why drinking too little or too much water can damage cells throughout the body – especially the kidneys and brain – and lead to death within days
Most homeostatic mechanisms in the body are controlled by a process known as negative feedback
Negative Feedback
Negative feedback occurs when conditions change from the ideal or set point, and returns conditions to this set point
It works in the following way:
if the level of something rises, control systems are switched on to reduce it again
if the level of something falls, control systems are switched on to raise it again
Negative feedback mechanisms are usually a continuous cycle of bringing levels down and then bringing them back up so that overall, they stay within a narrow range of what is considered ‘normal’
Control of Blood Glucose Levels
Blood glucose levels are controlled by a negative feedback mechanism involving the production of two hormones – insulin and glucagon
Both hormones which control blood glucose concentration are made in the pancreas
Insulin is produced when blood glucose rises and stimulates liver and muscle cells to convert excess glucose into glycogen to be stored
Glucagon is produced when blood glucose falls and stimulates liver and muscle cells to convert stored glycogen into glucose to be released into the blood
Diabetes
Type 1 diabetes is a condition where the blood glucose levels are not able to be regulated as the insulin-secreting cells in the pancreas are not able to produce insulin
This means that blood glucose levels are often far too high
It can be treated by injecting insulin
The extra insulin causes the liver to convert glucose into glycogen, which reduces the blood glucose level
Symptoms of diabetes include extreme thirst, weakness or tiredness, blurred vision, weight loss and loss of consciousness in extreme cases
People with Type 1 diabetes have to monitor their blood glucose levels throughout the day as their levels of physical activity and their diet affect the amount of insulin needed
They can help to control their blood glucose level by being careful with their diet – eating foods that will not cause large increases in blood glucose level, and by exercising, which can lower blood glucose levels due to increased respiration in the muscles
Remember:
Glucagon is the ..
Glycogen is the …
Learn the differences between the spellings and what each one does so you do not get confused in the exam!
- hormone
- polysaccharide that glucose is stored as
The Skin & Homeostasis
Control of body temperature is a homeostatic mechanism
Homeostasis is the maintenance of a constant internal environment
This means that internal conditions within your body (such as temperature, blood pressure, water concentration, glucose concentration etc) need to be kept within set limits in order to ensure that reactions in body cells can function and therefore the organism as a whole can live
The human body maintains the temperature at which enzymes work best, around 37°C
If body temperature increases over this temperature, enzymes will denature and become less effective at catalysing reactions such a respiration
Structure of the Skin
Regulating Temperature: Basics
Regulation is controlled by the brain which contains receptors sensitive to the temperature of the blood
The skin also has temperature receptors and sends nervous impulses to the brain via sensory neurones
The brain responds to this information by sending nerve impulses to effectors in the skin to maintain the temperature within a narrow range of the optimum, 37°C
Fatty tissue under the dermis acts as a layer of insulation to prevent too much body heat being lost through the skin
Vasodilation & Vasoconstriction
When we are cold blood flow in capillaries slows down because arterioles leading to the skin capillaries get narrower – this is known as vasoconstriction
This reduces the amount of heat lost from blood by radiation as less blood flows through the surface of the skin
When we are hot blood flow in capillaries increases because blood vessels to the skin capillaries get wider – this is known as vasodilation
This cools the body as blood (which carries heat around the body) is flowing at a faster rate through the skin’s surface and so more heat is lost by radiation
Plant Tropisms
lants can respond to changes in environment (stimuli) for survival, e.g. light, water, gravity
Their responses are usually much slower than animals
They grow either towards a stimulus (known as a positive response) or away from a stimulus (known as a negative response)
The responses are known as tropisms
plant tropism
ti is very important to a plant that its roots and shoots grow in the right directions
Shoots must grow upwards, away from gravity and towards light, so that leaves are able to absorb sunlight
This means that shoots have a positive phototropic response and a negative gravitropic response
Roots need to grow downwards into the soil, away from light and towards gravity, in order to anchor the plant and absorb water and minerals from the soil particles.
This means that roots have a negative phototropic response and a positive gravitropic response
Investigating Tropisms
Phototropisms
Three identical plants are set up as shown below (A, B and C)
The seedlings in A grow towards the light source
In B the effect of the light only coming from one direction has been cancelled out by using a clinostat (it revolves slowly and repeatedly)
This means all sides of the seedlings get an equal amount of light so they do not curve towards the light source but grow straight up
In C the seedlings grow straight up looking for light and the plant becomes tall and slender with yellowing leaves due to the lack of light
Gravitropism investigations
Add some damp cotton wool to two petri dishes
Place 3 bean seedlings in the cotton wool in each petri dish
P – horizontally
Q – radicle (root grows from here) facing upwards
R – radicle facing downwards
Cover with a lid and place petri dish A with its edge on a support
Attach petri dish B to a clinostat (as shown in the diagrams below)
Place both in a light proof box, leave for two days and then observe growth of the seedlings
In petri dish A all radicles have grown downwards (positive gravitropic response) regardless of which way they were initially facing (horizontal, up or down) and all plumules (shoots) have grown upwards (negative gravitropic response)
In petri dish B, all radicles and all plumules have all grown neither up nor down but straight outwards in whichever direction they were placed as the effect of gravity has been cancelled out by the revolving of the clinostat – they have shown no gravitropic response at all
The experiment needs to be done in a lightproof box in order to cancel out the effect of light on the growth of the seedlings
Know what a clinostat is and what it does
(cancel out the effect of light or gravity).
The Role of Auxin
Plants respond to stimuli by producing a growth hormone called auxin which controls the direction of growth of roots or stems
Therefore we say plants control their growth chemically
Auxin is mostly made in the tips of the growing stems and roots and can diffuse to other parts of the stems or roots
Auxin makes the cells behind the tip get longer; the more auxin there is, the faster they will grow
If light shines all around the tip, auxin is distributed evenly and the cells all grow at the same rate – this is what normally happens with plants growing outside
When light shines on the shoot from one side though, the auxin in the tip concentrates on the shady side, making the cells on that side grow faster than the cells on the sunny side
This unequal growth on either side of the shoot causes the shoot to bend and grow in the direction of the light
The role of auxin can be tested using
seedlings placed in a box that has a slit on one side, only allowing light in from one direction:
Investigating the roles of auxin on phototropic responses
Plant Hormones & Weedkillers
Most weedkillers contain synthetic hormones like auxin – known as 2,4D
They are selective so they are sprayed onto an area such as a lawn or farm crops and the synthetic auxin affects the weeds but not the grass / crop plants
The weeds respond by growing very fast and then dying, leaving more space, nutrients and water for the grass or crop plants to grow
