- Living organisms can detect and respond to a stimulus, i.e. a change in the environment, such as light, temperature, etc.
- Receptors are stimulated by the stimulus and produce a rapid, involuntary (automatic) response. In other words, the organism responds without thinking. This is called a simple reflex
- The simplest animals rely on reflex actions for much of their behaviour. All their movements and reactions are simple reflex responses. The reflex actions ensure that the animal will respond in a way that is most likely to result in its survival.
- For example, a simple reflex response to chemicals can lead to an organism finding food quickly. A change in light level could indicate the presence of a predator, so the organism moves away.
- Newborn babies cannot think for themselves so they exhibit a range of simple reflexes for a short time after birth, which ensures that they can survive. The absence of the reflexes or their failure to disappear over time may indicate that the nervous system of the baby is not developing properly.
Some simple reflexes exhibited by newborn babies include the following:
- Stepping reflex - when held under its arms in an upright position with its feet on a firm surface, a baby makes walking movements with its legs.
- Grasping reflex - a baby tightly grasps a finger that is put in its hand.
- Sucking reflex - a baby sucks on a finger (or a mother's ******) when it is put into its mouth.
- Startle reflex - a baby shoots out its arms and legs when startled, e.g. by a sudden loud noise.
- Rooting reflex - a baby turns its head and opens its mouth, ready to feed, when its cheek is stroked.
Adults also exhibit a range of simple reflexes. They are the most efficient way of quickly responding to potentially dangerous events:
- Pupil reflex - bright light causes muscles in the iris in the eye to contract so that the retina is not damaged.
- Knee-jerk reflex - when the knee is struck just below the knee cap, the leg will kick out.
- Dropping hot object reflex - when picking up a very hot object, the response is to throw it away in order to prevent heat damage to the hand.
- There are two ways of sending signals in the body. The first is via electrical impulses through long, wire-like cells called neurons (nerve cells). This method is very quick and short lived.
- Sensory neurons carry nervous impulses (electrical signals) from receptors to the central nervous system: impulse travels towards cell body.
- Motor neurons carry impulses from the central nervous system to effectors: impulse travels away from cell body.
- The other way signals are sent in the body is via chemicals called hormones, for example insulin (which controls blood sugar levels) and oestrogen (the female sex hormone), which are secreted into the blood. Chemical signals are slower than electrical impulses and they move to target organs, but their effect lasts a long time.
- The nervous and hormonal communication systems are seen in larger, more complex organisms. This is a result of the evolution of multicellular organisms.
Nervous coordination in an animal requires the presence of one or more different receptors to detect stimuli. For example:
- Light - detected by receptors in the eyes
- Sound - detected by receptors in the ears
- Changes of position - detected by receptors for balance in the inner ear
- Taste - detected by receptors on the tongue
- Smell - detected by receptors in the nose
- Pressure - detected by receptors for pressure in the skin
- Temperature - detected by receptors for temperature in the skin
Coordinating the Response
The receptors are connected to a processing centre by sensory neurons. With simple reflexes, the processing centre is the spinal cord; the brain is not involved. The processing centre coordinates a response by sending back a message electrically via motor neurons to the effector, which carries out the response. This is called a spinal reflex arc.
A Simple Reflex Arc
- A receptor is stimulated by the stimulus causing impulses to pass along a sensory neuron into the spinal cord.
- The sensory neuron synapses with a relay neuron, by-passing the brain.
- The relay neuron synapses with a motor neuron, sending impulses down it to the muscles (effectors) causing them to contract in response to the stiimulus.
The arrangement of neurons into a fixed pathway in a spinal reflex arc means that the responses are automatic and very rapid because no processing is required.
If the signal had to travel to the brain and be processed before action was taken, then, by the time the response arrived, it may be too late.
Receptors and Effectors
Receptors and effectors can form part of the complex organs.
Muscle Cells in Muscle Tissue
The specialised cells that make up muscle tissues are effectors. Impulses travel along motor neurons and terminate at the muscle cells. These impulses cause the muscles to contract.
Light Receptors in the Retina of the Eye
The eye is a complex sense organ. The lens focuses light onto receptor cells in the retina, which are sensitive to light. The receptor cells are then stimulated and send electrical impulses along the sensory neurons to the brain.
Hormone Secreting Cells in a Gland
The hormone secreting cells in glands are effectors. They are activated by an impulse, which travels along a motor neuron from the central nervous system and terminates at the gland. The impulse triggers the release of the hormone into the bloodstream, which transports it to the sites where it is required.
The Structure of Neurons
Neurons are specially adapted cells that can carry electrical signals (nerve impulses). They are elongated to make connections from one part of the body to another. They have branched endings, which allow a single neuron to act on many other neurons or effectors, e.g. muscle fibres.
In motor neurons, the cytoplasm forms a long fibre surrounded by a cell membrane called an axon.
Some axons are also surrounded by a fatty sheath, which insulates the neuron from neighbouring cells (a bit like the plastic coating on a copper electrical wire) and increases the speed at which the nerve impulse is transmitted.
The Central Nervous System
The information from neurons is coordinated overall by the central nervous system (CNS). In humans and other vertebrates the CNS is made up of the brain and spinal cord.
- Sensory Neurons
- Relay Neurons
- Spinal Cord -------------------
- Brain ------------------------------- The Central Nervous System
- Spinal Cord ------------------
- Motor Neurons
The CNS is connected to the body via sensory and motor neurons, which make up the peripheral nervous system (PNS). The PNS is the second major division of the nervous system. Its sensory and motor neurons transmit messages all over the body, e.g. to the limbs and organs. They also transmit messages to and from the CNS.
Synapses are the gaps between adjacent neurons. They allow the brain to form interconnected neural circuits. The human brain contains a huge number of synapses. These are approximately 1000 trillion in a young child. The number decreases with age, stabilising by adulthood. The estimated number of synapses for an adult human varies between 100 and 500 trillion.
When an impulse reaches the end of a sensory neuron, it triggers the release of chemicals, called transmitter substances, into the synapse. They diffuse across the synapse and then bind with specific receptor molecules on the membrane of a relay neuron.
The receptor molecules will only bind with specific chemicals to initiate a nerve impulse in the relay neuron, so the signal can continue on its way. Meanwhile, the transmitter substance is reabsorbed back into the sensory neuron, to be used again. The sequence is as follows:
- Electrical signal (nerve impulse) moves through sensory neuron.
- Transmitter substances are released into the synapse.
- Transmitter substances bind with receptors on the motor neuron.
- Electrical signal (nerve impulse) is now sent through motor neuron.
Drugs and the Nervous System
Many drugs, such as Ecstasy, beta blockers and Prozac, cause changes in the speed at which nerve impulses travel to the brain, speeding them up or slowing them down. Sometimes false signals are sent.
Drugs and toxins can prevent impulses from travelling across synapses or they can cause the nervous system to become overloaded with too many impulses. For example, Ecstasy (an illegal recreational drug) and beta blockers (a drug used to prevent heart attacks) both affect the transmission of nerve impulses.
The drug Ecstasy, scientifically known as MDMA, in the nervous system affects a transmitter substance called serotonin which can have mood-enhancing effects, i.e. it is associated with feeling happy.
Serotonin passes across the brains synapses, landing on receptor molecules. Serotonin that is not on a receptor is absorbed back into the transmitting neuron by the transporter molecules. Ecstasy blocks the sites in the brain's synapses where the chemical serotonin is removed.
As a result, serotonin concentrations in the brain increase and the user experiences feelings of elation. However, the neurons are harmed in the process and a long-term consequence of taking Ecstasy can be memory loss.
The Cerebral Cortex
The cerebal cortex is the part of the brain most concerned with intelligence, memory, language and consciousness (our being aware of our own thinking and existence). Scientists have used a variety of methods to map the different regions of the cerebral cortex:
Physiological Techniques - Damage to different parts of the brain can produce different problems, e.g. long- and short-term memory loss, paralysis in one or more parts of the body, speech loss, etc. Studying the effects of accidents or illnesses, as well as directly stimulating the brain with elecrical impulses, has led to an understanding of which parts of the brain control different functions.
Electronic Techniques - An electroencephalogram (EEG) is a visual record of the electrical activity generated by neurons in the brain. By placing electrodes on the scalp and amplifying the electrical signals picked up throigh the skull, a trace can be produced showing the rise and fall of electrical potentials called brain waves.
By stimulating the patient's receptors (e.g. by flashing lights or making sounds), the parts of the brain that respond can be mapped.
The Cerebral Cortex.
Magnetic Resonance Imaging (MRI) scanning is a technique that produces images of cross-sections of the brain, showing its structure. The computer generated picture uses colour to represent different levels of electrical activity. The activity in the brain changes depending on what the person is doing or thinking.
In 2010, patients in a persistent vegetative state after being involved in serious accidents were tested using MRI. It was shown that, although they were outwardly comatose, they could answer questions by imagining playing tennis to represent 'yes' and sitting down to represent 'no'. The parts of the brain that show increased electrical activity when playing or imagining playing tennis are different to those that show electrical activity when doing or thinking about something else.
Although they are not conscious actions, reflex responses to a new stimulus can be learnt. Through a process of conditioning, the body learns to produce a specific response when a certain stimulus is detected. Conditioning works by building an association between the new stimulus (the secondary stimulus) and the stimulus that naturally triggers the response (the primary stimulus). This resulting reflex is called a conditioned reflex action.
The effect was discovered at the beginning of the 20th century by a Russian scientist named Pavlov, who received a Nobel prize for his work.Pavlov observed that whenever a dog sees and smells a piece of meat, it starts to salivate (produce saliva). In his experiment, a bell was rung repeatedly whenever meat was shown and given to the dog. Eventually, simply ringing the bell, without any meat present, caused the dog to salivate.
Another example of a conditioned reflex is when, after being stung by a wasp, you associate the yellow and black stripes with the painful sting. The next time you see a wasp (or similar insect) you feel fear.
In a conditioned reflex, the final response has no direct connection to the stimulus.
Some conditioned reflexes can increase a species' chance of survival.
Development of the Brain
Mammals have a complex brain that contains billions of neurons. This enables them to learn from experience, including how to respond to different situations, e.g. social behaviour.
The evolution of one particular mammal, homo sapiens, led to the development of a larger brain than in other animals. Early humans could use tools, coordinate hunting and formulate plans about what might happen in the future. Having a larger brain meant that early humans were more likely to survive and reproduce, passing on the genes for producing a larger brain.
In mammals, neuron pathways are formed in the brain during development. The way in which the animal interacts with its environment determines what pathways are formed. It is the huge variety of pathways available that makes it possible for the animal to adapt to new situations.
During the first few years after birth, the brain grows very rapidly. As each neuron matures it sends out multiple branches, increasing the number of synapses.
At birth, in humans, the cerebral cortex has approximately 2500 synapses per neuron. By the time an infant is two or three years old, the number of synapses is approximately 15,000 synapses per neuron.
Development of the Brain.
Each time an individual has a new experience, a different pathway between neurons is stimulated. Every time the experience is repeated, the pathway is strengthened. Pathways that are not used regularly are eventually deleted. Only the pathways that are activated most often are preserved.
These modifications mean that certain pathways of the brain become more likely to transmit impulses than others and the individual will become better at a given task. This is why certain tasks may be learned through repetition, e.g. riding a bike, revising for an exam, learning to ski or learning to play a musical instrument.
If neural pathways are not used then they are deleted. There is evidence to suggest that because of this, if a new skill (e.g. learning a language) has not been learned by a particular stage in development, an animal or child might not be able to learn it in the same way as normal.
One example of evidence showing this comes from the study of so-called feral (wild) children. Feral children are children who have been isolated from society in some way, so they do not go through the normal development process. This can be deliberate (e.g. inhumanely keeping a child in a cellar or locked room) or it can be accidental (e.g. through being shipwrecked).
In the absence of any other humans, the children do not ever gain the ability to talk (or they lose any ability they had already gained) other than making rudimentary grunting noises. Learning a language later in life is a harder and slower process.
After children are born, there are a series of milestones that can be checked to see if development is following normal patterns. If these milestones are missing or are late, it could mean that there are neurological problems or the child is lacking stimulation. For example, at three months, babies should be able to lift their heads when held to someone's shoulder or grasp a rattle when it is given to them.
Memory is the ability to store and retrieve information. Scientists have produced models to try to explain how the brain facilitates this but, so far, none have been able to provide an adequate explanation. Current models are limited.
Verbal memory (words and labels) can be divided into short-term and long-term memory. Short-term memory is capable of storing a limited amount of information for a limited amount of time (roughly 15-30 seconds). Long-term memory can store a seemingly unlimited amount of information indefinitely.
When using short-term memory, it is currently thought that up to seven (+/- two) separate pieces of information can be stored (e.g. for a number with eight digits like 24042003, each number could fill a slot if eight were available).
This capacity can be increased by chunking the information, i.e. putting it into smaller chunks. For example, the number 201054738087 could be stored as 2010, 5473 and 8087, using only three units of the sevent units of storage.
Long-term memory is where information is stored in the brain through repetition, which strengthens and builds up neuron pathways.
Humans are more likely to remember information when:
- it is repeated (especially over an extended period of time), e.g. going over key points several times as a method of revising for exams.
- there is a strong, stimulus associated with it, such as colour, light, smell or sound (the more senses that are involved, the better).
- there is a pattern to it (or if a pattern can be artificially imposed upon it).
There are a variety of models used to explain how memory works. The magical numbe seven rule, +/- two model was developed in 1956. In the late 1960s, the multi-store model was proposed by Atkinson and Shiffrin. This linked short- and long-term memory with a very quick sensory memory, lasting only 1-2 seconds. This model is useful as it suggests a relationship between memory centres for storage and retrieval, together with an explanation as to how repetition helps people to remember things. It also explains why people forget.
Biologists are still creating models to explain memory. Models are used to try to explain experimental results. Those that best explain and fit the data are maintained;those that do not are discarded. However, models will always be limited as they are not the real brain and memory, only a representation of how we think it works.