B6.2 How is information passed through the nervous system?
Nervous systems are made up of neurons (nerve cells) linking receptor cells (e.g. in eyes, ears and skin) to effector cells (in muscles/glands)
Neurons carry impulses from one place to another, around the many parts of the nervous system.
When neurons are stimulated they transmit an electrical impulse
They are elongated (lengthened) 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.
An axon is a long extension of the cytoplasm in a neuron and is surrounded by cell membrane.
The axon carries the electrical impulse and is protected by a fatty sheath – the fatty sheath insulated the neuron from neighbouring cells and increases the speed of transmission of a nerve impulse.
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 the CNS is connected to the body via the peripheral nervous system (PNS). The peripheral nervous system consists of motor and sensory neurons that carry impulses from the receptors to the CNS as well as impulses from the CNS to the effectors.
CNS coordinates an animal’s response via:
- Sensory neurones carrying impulses from receptors to the CNS
- Motor neurons carrying impulses from CNS to effectors
Within the CNS, impulses are passed from sensory neurons to motor neurons through relay neurons
The arrangements of neurons into a fixed pathway in a spinal reflex arc means that the responses are automatic and, hence, 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 time the response arrived it may be too late.
Synapses are the gaps between adjacent neurons – they allow the brain to form interconnected neutral circuits and impulses are transmitted across them.
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 the next neuron.
The receptor molecules will only bind with specific chemicals to initiate a nerve impulse in the next neuron (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.
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 be overloaded with too many impulses. For example ecstasy and beta blockers both affect the transmission of nerve impulses.
The drug ecstasy in the nervous system affects a transmitter substance called serotonin. Serotonin is a chemical that can have mood-enhancing effects – i.e. it is associated with feeling happy.
Serotonin passes the brain’s 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 brains synapses where the chemical serotonin is removed.
As a result serotonin concentrations in the brain increase and the user experiences feelings of elation (joy) – however the neurons are harmed in the process and a long term consequence includes memory loss.
The cerebral cortex is the part of the brain most concerned with intelligence, memory, language and consciousness.
Scientists have used different methods to find out which parts of the cerebral cortex do different jobs:
- Physiological techniques – damage to different parts of the brain can produce different problems – studying the effects of accidents or illnesses as well as by directly stimulating the brain with electrical impulses has led to an understanding of which parts of the brain control different functions
- Electronic techniques – scientists have stimulated different parts of the brain with a weak electrical current and then asked patients to describe what they experienced. By stimulating the patients receptors (e.g. by flashing lights or making sounds) the parts of the brain that respond can be mapped
- MRI scans – scans can show detail of brain structure and function