Organisms with many cells need to be able to send messages between cells in parts of their body that are far apart. As MULTICELLULAR organisms evolved, so did their communication systems - the NERVOUS SYSTEM and the hormonal communication system. These two systems carry different types of messages. The nervous system sends messages using cells called NEURONS and specialised organs called the brain and the spinal cord. Neurons are used for fast responses when a change needs to happen quickly, for example: dropping something hot but neuron messages do not last very long.
The hormonal communication system produces HORMONES for longer-lasting messages. Hormones are chemicals. Their effect is much slower than neurons, but the long-lasting response is useful, for example: when a teenager goes through puberty.vNeurons are special cells that can conduct electricity. Neurons are some of the longest cells in the body, reaching from the bottom of the spine to the soles of the feet. There are different types of neurons:
SENSORY neurons connect RECEPTORS that detect changes in the environment, such as heat or pressure, with the brain and spinal cord.
MOTOR NEURONS connect the brain and the spinal cord to EFFECTOR cells such as muscles, which contract and produce a RESPONSE such as movement.
Hormones and neurons
The whole nervous system can be thought of as two parts. The CENTRAL NERVOUS SYSTEM (CNS) is made up of the brain and the spinal cord and "makes decisions" for the rest of the body. The PERIPHERAL NERVOUS SYSTEM (PNS) is made up o sensory and motor neurons. The CNS is connected to all parts of the body via the PNS.
Hormones are chemical messengers produced in GLANDS all over the body. They travel through the blood to their target cells and have many different roles in the body.
Insulin is a hormone made in the pancreas. It targets the liver and muscles to store sugar as glycogen. If someone's body stops producing insulin, they develop type 1 diabetes. Other important examples of hormones include the sex hormones. Oestrogen is made in the ovaries and testosterone is made in the testes. These hormones cause the changes that occur at puberty and help regulate sperm and egg production.
THYROID GLAND makes a hormone called THYROXIN. This controls the rate of chemical reactions in the body.
ADRENAL GLAND makes ADRENALINE. This gets the body ready for action in the "fight or flight" response.
The structure of a neuron
Neurons are covered with a layer of fat, which acts like an insulator. It protects the surround cells from the electricity and makes sure the signal travels in the right direction.
Nurons are highly SPECIALISED cells and look very different from other types of cells. They have three main parts:
- the CELL BODY - this contains the cell's nucleus, which controls what is going on in the cell, and DENDRITES, which detect changes in the environment called STIMULI that start the electrical signal in the cell.
- the AXON - a long extension of the cell that carries the eletrical signal or IMPULSE. It is surrounded by a fatty insulating sheath (cover).
- MOTOR END PLATES - these connect to another cell, which could be another neuron or another cell that produces a response, called an EFFECTOR. Effectors can be muscle cells, which cause movement responses, or GLANDS, which release chemicals made by the body, for example saliva in the mouth/
The cell body needs so many dendrites in it because they detect changes in the environment.
Eve n though it looks as if it is made from different sections, the whole of a neuron is one cell with one cell membrane. The axon is made of an extension of the membrane, which forms a long thin tube filled with cytoplasm. The axon of some neurons has to be long enough to reach from the bottom of the spine to the soles of the feet, which makes neurons the longest cells in the body.
The role of the axon is to send electrical impulses to effectors or to the central nervous system. Surrounding the axon is a fatty sheath made of MYELIN. The myelin sheath has two roles: it makes the impulse travel much faster, and it acts as an insulator, protecting surrounding cells from the electrical impulse.
A bunch of neurons can be grouped into a NERVE. Nerves can be easily seen by eye and are white and stringy, whereas individual neurons can only be seen using a microscope. You can think of a nerve as being like a thick rope made of lots of lengths of string (neurons). Rope is much stronger than string, and the same is true for nerves and neurons.
Neurons have to transmit impulses rapidly so the body can respond to stimuli as quickly as possible. The speed of the impulse is affected by three factors. Temperature, Axon-diameter and Fatty myelin sheath.
Temperature - the higher the temperature, the faster the impulse. In general, warm-blooded animals can respond to stimulus more quickly than cold-blooded animals.
Axon diameter - the larger the axon diameter, the faster the impulse. Some animals that live in cold conditions, such as the sqid, have evolved very thick axons to speed up their responses.
Fatty myelin sheath - neurons with a fatty sheath can transmit impulses up to 100m/s whereas nerons without a fatty sheath can onl manage 1m/s.
Neurons send information around the body, passing on their messages to specific cells. Some neurons (motor neurons) send messages to effector cells in muscles or glands, which do not conduct electricity. Some neurons send messages to other neurons, but the electrical impulse cannoy jump across the gap between adjacent neurons. To transmit the message from one cell to the next, the electrical impulse of the neuron is changed briefly into a chemical signal.
SYNAPSES are the special junctions (joining points) between a neuron and another cell where electrical impulses are changed into chemical signals. Synapses are specialised structures and messages can only travel across them in one direction.
The function of synapses
Synapses are found at junctions between a neuron and the cell next to it. Sometimes one neuron will have many synapses connecting it to many other cells. This allows different neurons to share information, and means that neurons in the central nervous system (CNS) can collect information from different types of stimuli. For example, in deciding whether food is good to eat, neurons in the brain can get information from taste and smell receptors, as well as from the receptors in the eyes. Synapses are very important in the brain, where the connections between neurons help us to learn and remember.
The message carried by a neuron is passed across a synapse by chemicals called TRANSMITTER SUBSTANCES that are released when the impulse reaches the end of the neuron. The transmitter substances diffuse across the gap and bind to RECEPTOR molecules on the membrane of the next cell. These initiate an impulse in the next cell. If this cell is another neuron, the new impulse will also be electrical. If it is a muscle cell, the impulse will be chemical and may initiate contraction of the muscle.
The chemical transmitters are specific to where the synapse is, for example, seratonin is a chemical transmitter substance that is found in the brain.
Mitochondria - These make the energy needed to release and re-absorb the chemical transmitter.
Linking nerves together
Goose bumps are an evolutionary response to fear from when humans had much more hair. If the effector muscles surrounding the hair contract, the hair stands on end. This made us look bigger and scarier to whatever was threatening us.
The nervous system responds to changes in the environment called STIMULI. Many different kinds of change can be detected, including temperature, pressure, light and sound.
The stimuli are detected by special cells called RECEPTORS.
For each kind of stimulus, there is a specific type of receptor, such as light receptors and temperature receptors. Sometimes the receptors are part of complex organs whose major role is to detect stimuli, for example:
- eyes detect changes in light using light receptors in the retina.
- ears detect changes in sound using sound receptors in the eardrum.
When your eyes detect an absense of light, the pupils dilate to let in more light to enable you to see more.
Effectors and responses
Once the central nerous system has made a decision about what to do about a specific stimulus, a response must be made. Responses are co-ordinated by efectors. The most common effectors are muscles and glands.
Glands make essential chemicals such as enzymes and hormones. Responses often control the release of these chemicals, for example, the hormone insulin only needs to be released after we eat sugar. Muscles are used for movement and muscle contraction can help the body to move away from dangerous stimuli. They also make the heart beat and ensure food passes through the digestive system. Tiny muscles are needed for the eyes to work. Like receptors, effectors can also be part of complex organs, for example, muscle cells are part of complex organs like the heart.
RELAY NEURONS in the central nervous system connect sensory neurons to motor neurons and so co-ordinate the body's responses to stimuli. INVOLUNTARY responses are called REFLEXES. An example of a simple reflex might be a pupil contracting in response to light shining in the eye. The light is the stimulus. Light receptors in the retina detect the change in light and send an impulse to the central nervous system using sensory neurons. The relay neurons in the central nervous system make a decision to make the pupil smaller and send an impulse to the effector muscles around the iris. The response is for the iris to contract, which makes the pupil smaller so that it lets less light into the eye.
The reflex arc
Stimulus (bright light) ---> Receptor (light receptors in the retina) ---> Sensory neurons ---> Brain ---> Motor neurons ---> Effectors (circular and radial muscles in the iris) ---> Response (pupils contract)
We detect the movement of the fly on our skin using touch receptors, which sends a message to the central nervous system. The response is to contract the muscles in our skin so that they are near to the fly, which knocks the fly off our skin.
A reflex is a simple response to a stimulus. The journey of the impulse from receptor to effector is called a REFLEX ARC. A spinal reflex arc has a fixed pathway:
1. A change in the environment, called a stimulus, is detected by special cells called RECEPTORS. 2. Receptors start an electrical impulse in sensory neurons. 3. The impulse travels through the sensory neurons into the spinal cord. 4. Relay neurons in the spinal cord co-ordinate a response to the stimulus and send another electrical impulse to the motor neurons. 5. Motor neurons carry the message to effetors (muscles and glands). 6. The response to the original stimulus occurs. This is either from a muscle contraction or a release of chemicals from the glands.
Over-riding reflex arcs
An example o a reflex is putting your hand over a flame. The change in temperature is the stimulus, detected by temperature receptors in your skin. The response is to move your hand away. The effectors are the muscles in your arms.
Simple reflexes are automatic, require no learning and happen from birth. We use reflex arcs to respond quickly to stimuli that could harm us. Simple reflexes include choking when we have food stuck in our throat, blinking when something comes towards out eyes, and sneezing when we have something stuck in our nose.
Reflex arcs are made of fixed pathways. Responses are rapid and automatic because the central nervous system does not need to process the information. If the stimulus comes from below the neck, the impulse bypasses the brain completely and the spinal cord co-ordinates the response. This makes the response much faster. However, sometimes it is important for the brain to be able to override reflex arcs. One important reflex response is breathing when we need oxygen. Breathing has to happen when we are asleep, and so much be an unconcious reflex response. However, if we are underwater it is essential that we do not breathe in, as we would fill our lungs with water and drown. In this situation, the brain overrides the reflex response by modifying the response to the motor neuron so it stops our muscles from contracting and breathing in. Overriding reflex arcs can be just as important to survival as the origianl reflexes.
Reflexes and behaviour
All animals have certain BEHAVIOUR that helps them to survive changes in their environment. Behaviour can be LEARNED or INSTINCTIVE. Instinctive behaviour comes from reflex responses. Reflex responses don't need to be learned and help all animals to survive.
For example, if you lift up a rock, any woodlice hiding underneath it will move away from the light to another dark place. This is the woodlouse's reflex response to light. The behaviour of the woodlous helps it to survive. By moving to a dark place, woodlice stay hidden from predators and are less likely to dry out.
All animals have instinctive behaviours controlled by reflex responses, for example, all animals will move away from fire. However, simple animals have a simple nervous system, which means that they cannot learn behaviours. These animals need reflex responses to survive changes in their environments.
Mos reflex responses have evolved to aid survival, for example searching for food and escaping from predators. Simple behaviours such as hiding or running away from larger animals are simple reflexes, but they lead to more complex behaviours such as an animal being able to escape from predators.
To help animals find food, they are born liking sweet tastes and disliking bitter tastes. Human tongues can detect sugar at 1 in 200 molecules, and bitter tastes at 1 in 2,000,000 molecules, which means we are much more likely to identify bitter tastes than sweet tastes.
Even though we are complex animals, humans also have involuntary responses that lead to behaviour. Adult reflexes include dropping hot objects and the contraction of the pupil of the eye in bright light. Doctors can check reflexes by tapping the lower leg just beneath the knee cap which will make the knee jerk upwards. This shows that the nerves and muscles are working correctly. These responses, and others such as blinking when objects come close to our eyes and breathing, remain with us for life. Other responses only occur in newborn babies.
Newborn babies have a number of reflexes that help them to survive when they are very young, but stop working as they get older. Important newborn reflexes are grasping, stepping and suckling. Newborn babies will grasp anything that touches the palm of their hand. This has evolved to help them cling onto their parents when they are being carried. The suckling response is also a reflex. Babies will such anything that is placed near their mouth, which helps babies to feed. The suckling reflex stops working when babies are around 2 months old and this is replaced by a voluntary reflex. Another reflex, which is not fully understood, is the stepping refles. When babies are placed with the soles of the feed on a flat surface their legs move up and down in a stepping motion. This could be walking practise.
The scientist John B. Watson wanted to test whether humans could learn to asociate two previously unlinked stimuli. He designed an experiment where he showed an 8-month boy named Albert a white rat, which Albert liked and tried to touch. He then showed Albert the rat again, but at the same time crept up behind him and made a loud noise. The noise made Albert jump and start to cry.
Watson repeated this several times, always making the loud noice when Albert was looking at the rat. He then showed Albert the rat without the noise. Albert showed signs of distress, even though there was no loud noise - Albert had been CONDITIONED to associate two unliked stimuli together. Later, Albert began to show signs of fear when he was shown any white furry object, including a fur coat, and a Santa Claus white beard.
A conditioned response is a LEARNED response. It occurs when animals link a response to two or more stimuli that are not conditioned. In the case of Little Albert:
- the loud noise was the PRIMARY STIMULUS, there should be a reflex response to it, because it might be a sign of something harmful
- the white rat was the SECONDARY STIMULUS - it should not have a reflex response associated with it
Pavlov and his dogs
Ivan Pavlov was the first scientist to carry out eperiments into conditioning. He won a Nobel Prize for his research in 1904. He worked with dogs and measured the amount they salivated (produced saliva). He noted that dogs began to salivate before they were given food. He started to ring a bell (SECONDARY STIMULUS) everytime they were given food (PRIMARY STIMULUS).
After a while, the dogs salivated when he rang the bell, even if they were not given food. The stimulus of the bell sound, normally unconnected to feeding, ha become linked to the stimuli of the smell and tasete of food - a conditioned reflex had been formed.
Conditioned responses are like reflex responses, as they have evolved to help us survive. A conditioned response is often called a CONDITIONED REFLEX. Sometimes it is useful for two stimuli that are unrelated in time to be linked by the same response. An example of this is when we eat something and later we are sick. Even though the response occurs much later the original stimulus, our brains link the food we eat earlier to the sickness we feel later and we are less likely to eat the same thing again. Animals have also evolved conditioned reflexes that help survival. Many poisonous animals and plants are bright red or yellow (a secondary stimulus). If an animal eats a poisonous animal or plant (primary stimulus), it will later be sick (final response). Animals quickly learned the conditioned response that links brightly coloured animals and plants with feeling sick. This is an advantage for brightly coloured plants/animals as they won't be eaten.
The brain is the CO-ORDINATOR of the body. It is made of billions of neurons all working together to respond to stimuli and to remember our experiences. The CERERAL CORTEX is the outer layer of the brain. It is made up of a very large sheet of tissue folded to fit into the skull The folds give the characteristic groove shapes on the outside of the brain. The cerebral cortex is much bigger in humans than in other animals. This part of the brain is concerned with traits that make us human, such as: intelligence, memory and language.
Scientists who study the nervous system are called NEUROSCIENTISTS. Neuroscientists have discovered how the brain works in two main ways. The first way is to stimulate various parts of the brain using electrical impulses and look at the responses that are caused. This technique was first used in the 19th century but is now used by neurosurgeons whilst they are performing brain surgery. The technique requires the brain to be exposed by removing part of the skull. Electrodes are then used to stimulate different parts of the brain. When conscious and alert, patients have different parts of their brain stimulated they can report memories and sensations and help to map the functionf of different parts of the brain. A non-invasive way of looking at the brain is to make images with special scanners, such as magnetic resonance imaging (MRI) machines. Brain images are useful for two main reasons. We can compare healthy people's brains with brains from people who have diseases like Alzheimer's. We can look at the activity of different parts of the brain from activity (eg: hearing music, speaking a foreign language).
The evolution of the brain
As humans have evolved, our brains have become bigger in comparison to our body size. Homo erectus lived 1.7 million years ago. They were a similar size to humans now, but their brains were only half the size of ours.
An important adaptation is not just the size of the brain, but the number of folds in the cerebral cortex. More intelligent mammels, such as dolphins and chimpanzees, have many more folds in their cerebral cortex than the smoother brains of less intelligens mammeals, such as mice. As humans evolved our brains have become larger with an increased number of folds in our cerebral cortex. As our brains have developed we have become better at adapting to new situations, which means indiviuals are more likely to survive.
The brain is a complex organ made of billions of neurons. The brain is involved in both INSTINCTIVE and LEARNED responses. It can respond to stimuli using reflexes and it can learn from experiences, which allows us to have more compex responses. Learning happens when certain pathways in the brain become more likely to transmit impulses. Transmitting impulses between neurons forms links between neurons called a NEURON PATHWAY. Th e more the same pathway is used, the stronger that pathway becomes. This means that the more times we repeat a task, the better our brain becomes at knowing how to complete the task. The neuron pathway also becomes stronger when there is a strong stimulus associated with a sense, which is why many baby toys are brightly coloured or noisy.
Development and learning
To improve at a skill we need to have strong neuron pathways. The best way to do this is to repeat the action, because every repetition strengthens the neuron pathway. This is why we get better at certain skills when we practise. Research shows you need to spend 10,000 hours to completely master a skill like playing the piano or hockey, or becoming fluent in a new language. It gets harder to learn new things as we get older, as children and teenagers make new neuron pathways more easily.
With the billions of neurons in our brain, the potential number of neuron pathways is huge. This means we can adapt to new situations and learn how to respond to new stimuli. However, if a child is isolated during development and is not presented with new situations, then that child may not progress in their learning.
Children who have been isolated during development are called "feral". Over history, there have been many anecdotal instances that are well-documented from the last 50 years. "Genie" was a feral child discovered in California. She had been kept imprisoned by her parents for the first 13 years of her life. When she was rescued, she could not talk, and even with extensive therapy she mostly communicated non-verbally with occasional one word answers. Incidences like Genie have supported the theory that children can only learn some skills at certain ages, and if they miss learning them it is almost impossible to learn the skills at a later date.
MEMORY is the storage and retrieval (bringing back or remembering) of imformation. It is not enough to store memories - we also need access to them when we need them. There are two types of memory:
SHORT-TERM MEMORY: this is information from our most recent experiences, which is only stored for a brief period of time.
LONG-TERM MEMORY: information from our earliest memories onwards is stored for a long period of time.
Remembering information is essential if we want to learn things from our experiences. Some things help us to remember information more easily:
- Patterns, our brains remember information in patterns more easily than random information. For example, 123456 is easier to remember than 829261.
- Repetition, repetition of information makes the neuron pathways in our brain stronger making it easier to retrieve the information
- Strong stimulus, strong colours, bright light, strong smells or loud sounds
Understanding how memories are made is a complex area of research. Scientists use models to help explain how we remember and retrieve information. One of the most famous models is the MULTI-STORE MODEL, which was proposed in 1968. The model splits memory into three different types of storage, called STORES.
Sensory memory (holds information for 1-2 seconds), short term memory (has a limited capacity and can only store information for a few seconds), long term memory (has an unlimited capacity and information can be stored for a lifetime).
Information is transferred between the stores by CONTROL PROCESSES. Paying attention to sensations allows information to pass from the sensory memory into the short-term memory. Once in the short-term memory, repetition of the information means it can pass into the long-term memory. At each store, memories can be lost if not enough attention is paid to the sensation or if it is not rehearsed.
Unfortunately, the way our brain stores memories is more complicated than most models can explain. Other models have been proposed since the multi-store model, but none provides an exact explanation as to how memory works. The multi-store model has been criticised for being too linear and now allowing enough sub-division for the short term and long term memory. It doesn't differentiate between the different types of stimulus and how they are stored.
Drugs and the nervous system
Many drugs and toxins affect the way our nervous system works. They affect the transmission of impulses across synapses. Sometimes, they stop the transmission altogether. Sometimes, they change the speed of the transmission, or make the impulse stronger or weaker.
Prozac is a prescribed ANTIDEPRESSANT that makes synapses in the brain more sensitive to a particular chemical TRANSMITTER SUBSTANCE. Transmitters substances carry the chemical impulses across the synapse and Prozac helps make the impulse stronger. Over time, this makes people feel less depressed. However, when people stop taking Prozac the synapses often go back to the way they worked before, so other treatments for depression are given alongside Prozac. Many toxins (poisons) also block synapses and stop impulses from being transmitted. For example, if a toxic blocks a transmission that was intended to make a muscle contract, it can cause muscle paralysis and even stop the heart from beating.
BETA BLOCKERS are prescribd drugs. They work in the opposite way to Prozac, as they block adrenaline recentops in the synapses and stop the transmission of impulses. Beta blockers stop the hormon adrenaline from making the heart beat faster, as so stops people feeling anxious.
The illegal drug ECSTASY affects the working of the same chemical transmitter substance as Prozac, effectively increasing its impact in the brain. Like antidepressants, Ecstacy only works short term.
How ecstasy works
Ecstasy, or 3,4-methylenedioxymethamphetamine (MDMA), prevents re-absorption (removal) of a transmitter substance called SERATONIN at synapses in the brain. It blocks the receptors on the synpase where serotonin is re-absorbed. This causes more seratonin than usual to be released into the gap, which means the neuron response is much greater and gives users a feeling of wellbeing and happiness.
The increase in seratonin associated with Ecstasy use leads to a feeling of wellbeing and self-acceptance.
However, the brain only has a limited amount of seratonin, so when the drug wears off there is a "come-down" during which users feel tired and irritable. Users have also reported physics effects of attention loss, anxiety and paranoia, and physiological effects of insomnia, teeth grinding and dizziness.
Long term repeated users of Ecstasy have increased rates of depression and 70-80% have impaired memory and learning difficulties. In a study on looking at the effect of Ecstasy on moneky brains, researchers found that neuron axons became shorter and axon terminals grew abnormally.