Stimulus and Response
A STIMULUS is a detectable change in the internal or external environment of an organism that produces a response in the organism.
The ability to respond to a stimulus increases an organism’s chances of survival. Those organisms that survive hae a greater change of raising offspring and of passing their alleles to the next generation.
There is always, therefore, a selection pressure favouring organisms with more appropriate responses.
Stimuli are detected by cells or organs known as receptors. Receptors transform the energy of a stimulus into some form of energy that can be processed by the organism and leads to a response. The response is carried out by effectors which can include cells, tissues, organs and systems.
Receptors and effectors are often some distance apart so comunication is needed between the two. In animals the nervous system is used. These are linked to a central coordinator.
Stimulus > Receptor > Coordinator > Effector > Response
A TAXIS is a simple response whose direction is determined by the direction of the stimulus.
As a result, a motile organism responds directly to environmental changes by moving its whole body either towards a favourable stimulus or away from an unfavourable one.
Taxes are classified accordin to whether the movement is towards the stimulus (positive) or away from the stimulus (negative) and also by te nature of the stimulus.
A KINESIS is a form of response in which the organism does not move towards or away from a stimulus. Instead, the more unpleasant the stimulus, the more rapidly it moves and the more rapidly it changes direction.
A kinesis therefore results in an increase in random movements.
This type of response is designed to bring the organism back into favourable conditions.
It is important when a stimulus is less directional.
Humidity and temperature, for example, do not always produce a clear gradient from one extremem to another.
E.g. Woodlice favour moist conditions and move slower and turn slower in these types of environment.
A TROPISM is a growth movement of part of a plant in response to a directional stimulus.
In almost all cases the plant part grows towards or away from the stimulus.
Plant shoots grow towards light - Positive Phototropism
Plant roots grow away from light - Negative Phototropism
Plant roots grow towards gravity - Positive Geotropism
Plant roots grow towards water - Positive Hydrotropism
The nervous system has two major divisions:
- the central nervous system, which is made up of the brain and spinal cord
- the peripheral nervous system, which is made up of pairs of nerves that originate from either the brain or the spinal cord
The peripheral nervous system is divided into:
- sensory neurones, which carry nerve impulses from receptors towards the CNS
- motor neurones, which carry nerve impulses away from the CNS to effectors
The motor nervous system can be further subdivided as follows:
- the voluntary nervous system, which carries never impulses to body muscles and is under voluntary control
- the autonomic nervous system, which carries nerve impulses to glands, smooth muscle and cardiac muscles and is not under voluntary control, that it, it is involuntary
The spinal cord is a column of nervous tissue that runs along the back and lies inside the vertebral column for protection.
Emerging at intervals along the spinal cord are pairs of nerves.
A reflex arc is the nerve pathway in the body taken by an action potential that leads to a rapid, involuntary response to a stimulus.
A reflex is an involuntary response to a sensory stimulus.
1. the stimulus - heat from the hot object
2. a receptor - temperature receptors in the skin
3. a sensory neurone - passes the nerve impulse to the spinal cord
4. an intermediate neurone - links the sensory neurone to the motor neuron in the spinal cord
5. a motor neurone - carries the nerve impulse from the spinal cord to a muscle in the upper arm
6. an effector - the muscle in the upper arm, which is stimulated to contract
7. the response - pulling the hand away from the hot object
Importance of Reflex Arcs
- Involuntary – does not require the decision making power of the brain
- Brain can override the response if necessary
- Protects the body from harmful stimuli
- Effective from birth – does not need to be learnt
- Short pathway – fewer synapses
Synapses – slow
Neurons – fast
Control of Heart Rate - The Autonomic Nervous Syst
Controls subconscious activities of muscles and glands.
Has two main divisions:
The sympathetic nervous system – speeds up activities and thus allows us to cope with stressful situations (fight or flight response)
The parasympathetic nervous system – inhibits effects and slows down activities. This allows energy to be conserved. It controls activities under normal resting conditions.
The two divisions are antagonistic meaning that their effects oppose one another.
Control of Heart Rate
The resting heart rate of a typical adult human is around 70 beats per minute. However, it is essential that this rate can be altered to meet varying demands for oxygen. During exercise, for example, the resting heart rate may need to more than double.
Changes to the heart rate are controlled by a region of the brain called the medulla oblongata. This has two centres:
- a centre that increases heart rate, which is linked to the sinoatrial node by the sympathetic nervous system
- a centre that decreases heart rate, which is linked to the sinoatrial node by the parasympathetic nervous system
Which of these centres is stimulated depends upon the information they receive from two types of receptor, which respond to one of the following:
- chemical changes in the blood
- pressure changes in the blood
Control by Chemoreceptors
Chemoreceptors are found in the wall of the carotid arteries. They are sensitive to changes in the pH of the blood that result from changes in carbon dioxide concentration. In solution carbon dioxide forms an acid and therefore lowers pH. The process of control works as follows:
When the blood has a higher than normal conc of CO2, its pH is lowered. The chemoreceptors in the wall of the carotid arteries & the aorta detect this and increase the frequency of nervous impulses to the centre in the medulla oblongata that increases heart rate.
This centre increases the frequency of impulses via the sympathetic nervous system to the SAN, which in turn, increases the heart rate.
The increased blood flow that this causes leads to more CO2 being removed by the lungs ad so the CO2 level of the blood returns to normal.
As a consequence the pH of the blood rises to normal and the chemoreceptors in the walls of the carotid arteries and aorta reduce the frequency of nerve impulses to the medulla oblongata.
The medulla oblongata reduces the frequency of impulses to the sinoatrial node, which therefore decreases the heart rate to normal.
Control by Pressure Receptors
Pressure receptors occur within the walls of the carotid arteries and the aorta. They operate as follows:
When blood pressure is higher than normal, they transmit a nervous impluse to the centre in the medulla oblongata that decreases heart rate. This centre sends impulses via the parasympathetic nervous system to the SAN of the heart, which decreases the rate at which the heart beats.
When blood pressure is lower than normal, they transmit a nervous impulse to the centre in the medulla oblongata that increases heart rate. This centre sends impulses via the sympathetic nervous system to the SAN, which increases the rate at which the heart beats.