Toward the stimulus positive taxis 2. The central nervous system CNS — brain and spinal cord 2. The peripheral nervous system PNS — Made up of pairs of nerves that originate either from the brain or the spinal cord The peripheral nervous system This is divided into: Reflex arcs contain just 3 neurons: A sensory neuron 2.
An intermediate neuron 3. A motor neuron There are several stages of a reflex arc: Coordinator intermediate neuron 6. 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.
Controls under normal resting conditions The two divisions are antagonistic meaning that their effects oppose one another Control of heart rate Changes of the heart rate are controlled by a region of the brain called the medulla oblongata which has two main divisions One division is connected to the sinoatrial node through the sympathetic nervous system The other is connected to the sinoatrial node via the parasympathetic nervous system Control by chemoreceptors Chemoreceptors are found in the wall of the carotid arteries and detect hanges in pH as a result of CO2 concentration When CO2 concentration in the blood is too low, chemoreceptors detect the drop in pH and send impulses to the section of the medulla oblongata responsible for increasing heart rate This section then increases the number of impulses sent to the S.
A node via the sympathetic nervous system This results in an increase in heart rate which then causes blood pH to return to normal. Control by pressure receptors Pressure receptors occur in the wall of the carotid arteries and the aorta When blood pressure is too high — impulses are sent to the medulla oblongata which then sends impulses to the S.
A node via the parasympathetic nervous system decreasing the heart rate When blood pressure is too low — impulses are sent to the medulla oblongata which then sends impulses to the S. A node via the sympathetic nervous system, increasing the heart rate Section 9.
This means that it can convert the information to a form that the human body can interpret. This is achieved by using the energy of a stimulus into a nerve impulse called a generator potential. Both receptors convert light energy into a nervous impulse and are therefore acting as transducers Rod cells Cannot distinguish between different wavelengths Many rod cells are connected to the same neuron and so can function at low light intensities.
A threshold must be reached in the bipolar cells to which they are attached to and so since they can all contribute to reaching this threshold, they will function at lower light intensities Rod cells breakdown the pigment rhodopsin to generate an action potential.
Rhodopsin is easily broken down in low light intensity Since more that one rod cell is connected to the same neuron, only one impulse will be generated. It is impossible for the brain to determine which rod cells were stimulate to begin with and so it is not possible to determine exactly the source of light This results in rod cells having a relatively poor visual acuity and so are not very effective in distinguishing between two points close together Cone cells There are three types of cone cells, each of which respond to a different avelength The colour interpreted depends of the proportion of each type of cone cell stimulated Cone cells are connected only to one bipolar cells, this means that they cannot combine to reach a threshold.
As a result of this a high light intensity is required to create a generator potential Cone cells breakdown the pigment iodopsin to create a generator potential Iodopsin can only be broken down by a high light intensity Since cone cells are connected to a single bipolar cell, when two adjacent cells are stimulated, two separate nervous impulses will be sent to the brain.
This means that it is easier to determine the source of the light.GCE AS and A Level Biology AS exams onwards A2 exams onwards Unit 5 Specimen mark scheme Version 0.
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