Sending messages & the nervous system
Multicellular organisms need communication systems, so that the body works as a whole and not as individual cells or organs. The two communication systems are the nervous system and the hormonal system.
The nervous system:
The nervous system sends messages using nerve cells or neurons, which produce a quick, short response. The nerve impulse is electrical. The nervous sytem also has specialised organs called the brain and spinal cord. In the mammalian nervous system, the central nervous system is connected to the peripheral nervous system. This is the neurons which connect the CNS to the whole body.
There are two types of neurons:
- Sensory neurons connect receptors (eyes/ears/skin), which detect changes in the environment (called stimuli), with the CNS.
- Motor neurons connect the CNS to effectors (muscles etc.) which produce a response
The hormonal system
The hormonal system produces chemical messages in the form of hormones. The system is slower than the nervous system but the response is longer-lasting. For example:
- Insulin is produced by the pancreas. It acts on the liver, muscles and body cells to take up glucose from the blood.
- Oestrogen is produced by the ovaries. It is a sex hormone that controls the development of the adult female body at puberty, and the menstrual cycle.
Hormones are chemicals that are produced by glands. They are transported in the blood. This means that all organs in the body are exposed to them but they only affect thier 'target cells'.
Neurons are cells specialised for carrying nerve impulses so they are often very long. They communicate with other neurons but they do not physically touch each other. Neurons consist of the cell body which contains the nucleus and a long axon. Branches on the cell body called dendrites receive inputs from other cells (receptors and nerves) and conduct impulses towards the cell body.
Axons carry impulses away from the cell body (to ther nerves and muscles). The axon is a long extension of the cytoplasm in a neuron that communicates with the CNS or effector. Some neurons are therefore the longest cells in the body. Some axons are covered with an insulating fatty sheath called the myelin sheath.
The speed of the nerve impulse if affected by:
- temperature (the speed is increased; its always faster in warm blooded animals than cold blooded animals)
- the diameter of the axon (the wider the axon, the quicker the response)
- the myelin sheath (as well as insulating the neuron from neighbouring cells, the presence of the myelin sheath speeds up the nerve impulse - it is able to 'jump' from gap to gap along the sheath, making it travel much more quickly)
Some neurons send messagesto other neurons. There is a small gap between one neuron and the next, through which the message has to be transmitted. As the nerve impulse reaches the end of the nerve, it is changed to a chemical signal, which crosses the synapse and sets up an electrical impulse in the next neuron. Sometimes, a neuron has many synapses so it can communicate information with all these neurons. There is no physical connection between neurons. The presence of the synapse means that a nerve is able to communicate better with several neurons that may go to different locations.
As the nerve impulse reaches the end of the first neuron, a chemical transmitter substance is released. The transmitter diffuses across the synapse and binds with the receptor molecules on the membrane of the next neuron. This initiates a nerve impulse in the next neuron. After an impulse has been transmitted across, the chemical tansmitter is removed from the synapse (is taken back up by the neuron or broken down by an enzyme). There are many different types of transmitter molecules. These work on different nervous pathways. For example, seratonin is a transmitter that is important in brain function. Some transmitters work by inhibiting the next nerve instead of exciting it. Others work on muscles instead of nerves. Different transmitters have different receptor molecules.
The nervous system responds to changes in the environment called stimuli. Stimuli are detected by special cells called receptors. Sometimes, receptors are grouped together to form part of organs such as the eye or ear. A response to a specific stimulus may be required. The CNS coordinates the response. The response is made by an effector. Effectors include glands and muscles. Glands make and release chemicals such as enzymes and hormones eg. the hormone insulin is relased after a meal when blood sugar rises.
Muscles are used for movement. Their contraction helps the body to move away from dangerous stimuli and towards pleasant ones. Muscles are also used for movement we're not concious of such as our heartbeat.
Reflexes (continues on next card):
A reflex is a simple response to a stimulus. The pathway of a reflex action through the nervous system is called the reflex arc. The pathway is:
Stimulus --> Receptor --> Sensory neuron --> Relay neuron in CNS --> Motor neuron -->Effector
Simple reflexes are automatic; they require no learning and happen from birth. Reflexes enable us to respond quickly to stimuli that could harm us. Simple reflexes in humans include: dropping a hot object, the knee-jerk reflex, the pupil reflex. When a baby is born, its reflexes include stepping (taking steps when held under the arms with head supported), grasping (when a finger is placed into the baby's palm), sucking (when a nippl e or finger is placed in a baby's mouth), the breathing reflex (where babies do not breathe when they go under water).
Responses are rapid becase they used fixed pathways that do not involve the brain making a concious decision (relay neurons are in the CNS, but the brain is not involved at all if the stimulus is below the neck). Relay neurons in the CNS connect with other neurons that run to the brain, so we:
- know what's happened after the reflex action has occured
- can override an action eg. keeping hold of a hot object if you don't want to drop it (the brain sends a message to motor neurons, which changes the response of the muscles in your hand, so you keep hold of the object).
Instinctive and learned behaviour
Animals have certain behaviours that help them to survive in their environment. Behaviour can be instinctive or learned. Instinctive behaviours are controlled by reflex responses. Simple animals have simple nervous systems so they can't learn behaviours.
A reflex response to a stimulus can be learned by introducing a new, unrelated stimulus in association with the first. This is called conditioning. Two examples of conditioning are:
- Ivan Pavlov's dogs: dogs produce saliva in response to the smell, sight and taste of food. Pavlov rang a bell (seondary stimulus) immediately before giving dogs food (primary stimulus). This process was repeated. Soon, the dogs would produce saliva at the sound of the bell, even when not given food. They had learned to associate the sound of the bell with food.
- John B. Watsons's study with 8 month old Albert: Albert liked, and showed no fear of a white lab rat. Albert was then shown the rat (secondary stimulus) while Watson made a loud noise (primary stimulus) which made Albert cry. Later, when Albert was shown the rat, he showed signs of distress even when there was no loud noise.
A large number of folds in the cerebal cortex (the thin, folded, outer layer of thebrain) increases our ability to process information. It is involved with:
- Intelligence-how we think and solve problems
- Memory-how we remember experiences
- Language-how we communicate verbally
- Consciousness-being aware of ourselves and our surroundings
Neuroscientists map the regions of the brain using invasive and non-invasive methods. Invasive methods include:
- studying how a person is affected when a certain part of the brain is damaged
- during brain surgery, using electrodes to stimulate parts of the brain electrically, and seeing how the patient is affected, including reporting memories and sensations
Non-invasive methods include producing images and mapping activity with scanning techniques eg. MRI. These are useful in:
- comparing non-diseased brains with the brains of people with brain disease eg. Alzheimer's
- looking at activity in the brain when its stimulated (by music, language or images)
Transmitting impulses in the brain leads to links forming between the neurons. This is called a neuron pathway. When an experience is repeated, the pathway is strenghtened as more impulses follow the same pathway. The pathways can also be strenghtened by strong stimuli such as strong smells, colours, light or sound. Learning occurs as neuron pathways develop in the brain. Since repetition strengthens the pathways, we get better at certain skills,the more we practice. Learning results from experience where:
- new neuron pathways form (and other pathways may be lost)
- certain pathways in the brain become more likely to trnamit impulses than others
Neuron pathways form more easily in children than adults. However, children not presented with new, appropriate stimuli, or those isolated during development, may not progress in their learning. Evidence suggests that children can only acquire certain skills at a particular age. Feral children (children who have lived away from human contact since a very early age) develop only limited language skills when returned to civilisation.
We are able to adapt to new situations and respond to new stimuli as a result of the huge number of potential neuron pathways in our brains.
Memory is the storage and retrieval of information. There are two types of memory:
- Short term memory: involves information from our most recent experiences, which is only stored for a brief period of time
- Long term memory: involves information from our earliest experiences onwards that can be stored for a long period of time
You are more likely to remember information if...
- there is a pattern to it
- you use repetition, especially over a long period of time
- there is a strong stimulus associated with it
Scientists use models to try and explain how we store and retrieve information. The multi-store model is an example of this. However, models are limited in explaining how memory works. This is because: memory is more complicated than shown in the model; no model has an exact explanation of how memory works; the multi-store model is too linear and doesn't provide sub-divisions of short and long term memory; the model doesn't differentiate between different types of stimulus and the difference in performance of individuals.
The sensory memory can last from 1-3 seconds.
The short-term memory can last from 15-30 seconds.
Long-term memory can last from 1 second-lifetime.
Forgetting can be caused by biological factors.
Drugs and the nervous system
Many drugs and toxins work by affecting the transmission of nerve impulses across synapses, stopping the transmission, changing the speed of the transmission, or making the impulse stronger or weaker. For example:
- The antidepressant Prozac increases levels of the transmitter substance called seratonin.
- Like Prozac, Ecstacy (MDMA) works on seratonin. Following the transmission of a nerve impulse, the transmitter molecules should be removed from the synapse. MDMA blocks the site on the neuron where seratonin is reabsorbed, increasing its concentration. MDMA therefore gives a feeling of well-being, because of the increased levels of seratonin. After taking MDMA, the brain's seratonin is depleted, so the person is irritable and tired.
- Curare, used by South American Indians as an arrow poison, blocks the action of another type of transmitter molecule.
Beta blockers are prescription drugs that block the transmitter molecule adrenalaine, so they reduce the heart rate. They are used to treat people with problems with their heart rhythm but some people use them to control anxiety during public perfromances.