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Have fun and enjoy academic life!! We accept Powered by. 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. As a result, cone cells are responsible for higher visual acuity since they allow you to better distinguish between two points Light is concentrated by a lens to the centre of the eye called the fovea. This region receives a high light intensity and therefore has more cone cells.
The peripheries of the eye receive a low light intensity and therefore consist mainly of rod cells. Principles of coordination In mammals, there are two main forms of coordination: The nervous system — Uses nerve cells that can pass electrical impulses along their length.
The result is the secretion of chemicals by the target cells called neurotransmitters. The response is quick, yet short lived and only acts on a localised region of the body. The hormonal system — Chemicals are transported in the blood plasma which then reach target certain cells, thus stimulating them to carry out a function.
The responses due to secretion of hormones often act over a longer period of time, yet are slower to act. Chemical mediators Nervous and hormonal forms of communication are only useful at coordinating the activities of the whole organism.
At the cellular level they are complimented by chemical mediators. Chemical mediators are secreted by individual cells and affect other cells in the immediate vicinity. A common example of this type of coordination is the inflammation of certain tissues when they are damaged or exposed to foreign agents.
Two examples of chemical mediators are: Histamine — Stored in white blood cells and is secreted due to the presence of antigens. Histamine causes dilation of blood vessels, increased permeability of capillaries and therefore swelling the infected area. Prostaglandins — Found in cell membranes and cause dilation of small arteries and arterioles.
They release due to injuries and increase the permeability of capillaries. They also affect blood pressure and neurotransmitters. In doing so they relieve pain. Hormonal system Nervous system Communication by chemicals Communication by nervous impulses. IAA is used to ensure that plant shoots grow towards a light source. Cells in the tip of the shoot produce IAA, which is then transported down the shoot.
The IAA is initial transported to all sides as it begins to move down the shoot 3. Light causes the movement of IAA from the light side to the shaded side of the shoot. A greater concentration of IAA builds up on the shaded side of the shoot 5. The cells on the shaded side elongate more due to the higher concentration of IAA 6. The shaded side of the root therefore grows faster, causing the shoot to bend towards the source of light IAA can also effect the bending of roots towards gravity.
However in this case it slows down growth rather than speeds it up. IAA decreases root growth and increases shoot growth Section Can remove cell debris and are associated with nerve regeneration. A nerve impulse is not an electrical current! It is a self-propagating wave of electrical disturbance that travels along the surface of an axon membrane.
Nerve impulse — temporary reversal of the electrical p. This is called the sodium potassium pump. Sodium being positively charged causes the axon to become more positive in charge. The myelin sheath — Prevents the action potential forming in myelinated areas of the axon. The action potential jumps from one node of Ranvier to another salutatory conduction — this increases the speed of the impulse as less action potentials need to occur 2.
The greater the diameter of the axon the greater the speed of conductance — due to less leakage of ions from the axon 3. Temperature — Higher temperature, faster nerve impulse.
Energy for active transport comes from respiration. Respiration like the sodium potassium pump is controlled by enzymes. Refractory period After an action potential, sodium voltage-gated channels are closed and sodium cannot move into the axon. It is therefore impossible during this time for a further action potential to be generated. This time period, called the refractory period serves two purposes: It ensures that an action potential can only be propagated in one direction — An action potential can only move from an active region to a resting region.
It produces discrete impulses — A new action potential cannot be generated directly after the first. It ensures action potentials are separated from one another. It limits the number of action potentials — action potentials are separated from one another, therefore there is a limited amount that can pass along a neuron in a given time.
All or nothing principle Nervous impulses are all or nothing responses. A stimulus must exceed a certain threshold value to trigger an action potential A stimulus that exceeds the threshold value by a significant amount, will produce the same strength of action potential as if it has only just overcome the threshold value A stimulus can therefore only produce one action potential An organism can perceive different types of stimulus in two ways: The number of impulses in a given time larger stimulus, more impulses per second Having neurons with different threshold values — depending on which neurons are sending impulses, and how frequently impulses are sent, the brain can interpret the strength of the stimulus Section This means that several responses can be combined to give on single response Neurotransmitters are made in the presynaptic cleft only When an action potential reaches the presynaptic knob, it causes vesicles containing the neurotransmitter to fuse with the presynaptic membrane The neurotransmitter will the diffuse across the synaptic cleft The neurotransmitter then bind with receptors on the postsynaptic membrane, in doing so generating a new action potential in the postsynaptic neuron Features of synapses Unidirectionality.
Eventually the neurotransmitter will accumulate so as to overcome the threshold value of the postsynaptic membrane. Therefore generating a new action potential Inhibition Some postsynaptic membranes have protein channels that can allow chloride ions to diffuse into the axon making it more negative than usual at resting potential.
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