4.1.2 Nerves
a. Outline the roles of sensory receptors in mammals in converting different forms of energy into nerve impulses.
- Specialised cells that can detect changes in the surroundings.
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Energy transducers
- Convert one type of energy to another.
- Each one is adapted to detect changes in a particular form of energy.
- They convert the energy into a form of electrical energy called nerve impulses.
- A nerve impulse is created by altering the permeability of the nerve cell membrane to sodium ions.
- When sodium ion channels open, the membrane permeability is increased.
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The movement of the sodium ions causes a change in the potential difference so the inside of the cell is less negative. This is called depolarisation.
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A generator potential is caused when one or two sodium ion channels open and if more open and enough sodium ions enter the cell and action potential will be initiated.
b. Describe the structure and function of sensory and motor neurones
- Carry the action potential from a sensory receptor to the Central Nervous System.
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Surrounded by the myelin sheath which is made up of Schwann cells. The gaps where the cells meet are known as the nodes of Ranvier.
- They have a long dendron which is positioned just outside the central nervous system and a short axon which carries the impulse into the central nervous system.
- They have many dendrites connected to other neurones.
- Carry an action potential from the Central Nervous System to an effector e.g. a muscle or gland.
- Surrounded by the myelin sheath which is made up of Schwann cells with nodes of Ranvier.
- They have their cell body in the Central Nervous System and have a long axon that carries the action potential to the effector.
- They have many dendrites connected to other neurones.
c. Describe and explain how the resting potential is established and maintained
- When the neurone is not transmitting an action potential it is said to be at rest.
- The neurone is not actively transporting ions across its cell surface membrane.
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It is about -60mV inside the cell compared to the outside.
- ATP is used to pump three sodium ions out of the cell for every two potassium ions pumped in.
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The cell membrane is polarised because there is a potential difference across it.
d. Describe and explain how an action potential is generated
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When some of the sodium ion channels open causing some of the sodium ions to diffuse down their concentration gradient into the cell causing a depolarisation of the membrane.
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If the depolarisation is large enough to reach a threshold potential some nearby voltage-gated channels will open.
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When the depolarisation reaches +40mV the neurone will transmit an action potential which is self-perpetuating which means that once it has started it will continue to the end of the neurone.
e. Describe and explain how an action potential is transmitted in a myelinated neurone with reference to the roles of voltage-gated sodium ion and potassium ion channels
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The opening of sodium ion channels upset the balance of sodium and potassium ions so local currents are created in the cytoplasm of the neurone, which causes sodium ion channels to open further along the membrane.
- The concentration of sodium ions rises where the sodium ion channels are open.
- So sodium ions diffuse sideways away from the increased concentration.
- When the potential difference across the membrane has decreased, gates open to allow sodium ions to enter the neuron further down the membrane moving the action potential along the neurone.
- Sodium and potassium ions cannot diffuse through the myelin sheath, so the ionic movements of the action potential cannot occur over much of the neurone.
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Ionic exchanges can only happen at the nodes of Ranvier.
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The action potential appears to jump from one node to another which is called salutatory conduction. Action potentials move more quickly this way, conducting an action potential at up to 120ms-1.
f. Interpret graphs of the voltage changes taking place during the generation and transmission of an action potential
- The membrane is at its resting potential and is polarised.
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Sodium ion channels open and some sodium ions diffuse into the cell. The membrane depolarises as it becomes less negative the outside and reaches the threshold of -50mV. Voltage-gated sodium ion channels open and sodium ions flood in making the inside of the cell positively charged compared to the outside. The potential difference reaches +40mV.
- Sodium ion channels close and potassium channels open. Potassium ions diffuse out bringing the potential difference back to negative which is called repolarisation.
- The cell becomes hyperpolarised.
- The original potential difference is restored and the cell is back to its resting potential.
g. Outline the significance of impulse transmission
- A signal from a light receptor will only inform that there is light but not the intensity.
- When a stimulus is at a higher intensity it will produce more generator potentials therefore there will be more frequent actions potentials so more vesicles are released at synapses.
- The brain can determine the intensity of the stimulus due to the frequency of signal arriving.
- More signals=a more intense stimulus.
h. Compare and contrast the structure and function of myelinated and non-myelinated neurones
i. Describe the structure of a cholinergic synapse
Cholinergic synapse=A synapse that uses acetylcholine as the neurotransmitter.
j. Outline the role of neurotransmitters in the transmission of action potentials
- An action potential arrives at the synaptic knob.
- Voltage-gated calcium ion channels open.
- Calcium ions diffuse into the synaptic knob.
- Calcium ions cause the vesicles containing neurotransmitters to fuse with the presynaptic membrane.
- Acetylcholine is release by exocytosis.
- Acetylcholine molecules diffuse across the synaptic cleft.
- Acetylcholine binds to the receptor sites on the sodium ion channels on the postsynaptic membrane.
- Sodium ion channels open.
- Sodium ions diffuse into the postsynaptic neurone.
- A generator potential is created.
- If there are enough generator potentials then it will reach the threshold potential creating a new action potential in the postsynaptic neurone.
k. Outline the roles of synapses in the nervous system
- Connect two neurones together so a signal can be passed from one to another.
- Allow several presynaptic neurones to converge into one postsynaptic neurone to allow signals from different parts of the nervous system to create the same response.
- Allow one presynaptic membrane to diverge into several postsynaptic neurones to allow one signal to be passed to several parts of the body.
- Ensure that signals are transmitted in the right direction.
- To filter out unwanted low level signals.
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To amplify low level signals in a process known as summation which is when a low level signal is persistent, generating several action potentials which will combine together to produce an action potential.
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Acclimatisation-When after repeated stimulation a synapse runs out of vesicles containing the neurotransmitter and therefore no longer responds, helping to avoid overstimulation of an effector which could damage it.
- Allows the creation of specific pathways in the nervous system which is thought to be the cause of conscious thought and memory.
4.1.3 Hormones
a. Define the terms endocrine gland, exocrine gland, hormone and target tissue
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Endocrine gland-It is a ductless gland that secretes hormones straight into the blood.
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Exocrine gland-It is a gland that secretes molecules into a duct where they are carried to where they are needed.
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Hormone-These are secreted from the endocrine gland and are messengers carrying a signal from the endocrine gland to a specific target tissue.
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Target tissue-It is made up of target cell which have specific receptors on their plasma membrane to which the hormone complementary binds.
b. Explain the meaning of the terms first messenger and second messenger with reference to adrenaline and Cyclic AMP (cAMP)
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Adrenaline is the first messenger.
- When the adrenaline binds it makes the Adenyl Cyclase active.
- The Adenyl Cyclase converts ATP to Cyclic AMP.
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Cyclic AMP is the second messenger and it activates enzyme action in the cell.
c. Describe the function of the adrenal glands
- They lie just above the kidneys, one on each side of the body.
- Each gland has a medulla and a cortex region.
- Adrenal medulla
- In the centre of the gland.
- They manufacture and produce adrenaline in response to stress.
- Adrenaline prepares the body for activity and has effects including: increasing heart rate, relaxing the smooth muscle in the bronchioles, dilating the pupils and stimulating the conversion of glycogen to glucose.
- Uses cholesterol to produce certain steroid humans:
- Mineralcorticoids control levels of sodium and potassium.
- Glucocorticoids control the metabolism of carbohydrates and proteins in the liver.
d. Describe the histology of the pancreas and outline its role as an endocrine and exocrine gland
- Secretes enzymes.
- Manufactures and secretes digestive enzymes.
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The tubules join up to make the pancreatic duct.
- Enzymes are released in a fluid. The enzymes in the fluid are: Amylase, Trypsinogen and Lipase.
- Also contained in the fluid is sodium hydrogencarbonate.
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This is at the Islets of Langerhans which is split up into α and β cells.
- α cells secrete glucagon.
- β cells secrete insulin.
- It is well supplied with blood capillaries so the hormones can be released into the blood.
e. Explain how blood glucose concentration is regulated with reference to insulin, glucagon and the liver
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The islets of Langerhans monitor the concentration of blood glucose in the blood. The α and β cells detect the change and responds by releasing a hormone.
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The normal blood glucose levels are between 4 and 6 mmoldm-3.
- Blood glucose too high
- Change is detected by the β cells.
- Insulin is secreted into the blood.
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Insulin binds to specific receptors on the target cells which are hepatocytes (muscle cells).
- The binding activates the adenyl cyclase which converts ATP to cAMP.
- More glucose channels are in the cell surface membrane so more glucose enters the cell.
- Glucose in the cell is converted to glycogen for storage, more glucose is converted to fat.
- More glucose is used in respiration.
- Glucose concentration falls back to normal.
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Change detected by the α cells.
- Glucagon is secreted into the blood.
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Glucagon binds to specific receptors on hepatocytes.
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Glycogen is converted to glucose- glycogenolysis.
- More fatty acids used in respiration.
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Glucose produced by the conversion of amino acids and fats- gluconeogenesis.
- Increase in glucose concentration so it is back to normal.
f. Outline how insulin secretion is controlled with reference to potassium channels and calcium channels in beta cells
- Cell membranes of the β cells have calcium ion channels and potassium ion channels.
- Normally, the potassium ion channels are open and calcium ion channels close. Potassium ions diffuse out of the cell, so in the cell it is more negative.
- When glucose levels are high, glucose diffuses into the cell.
- Glucose is used quickly in metabolism to produce ATP.
- The extra ATP is used to close the potassium ion channels.
- Because the potassium cannot diffuse out it becomes less negative inside the cell.
- This causes calcium ion channels to open.
- When calcium enters the cell it causes the secretion of insulin that is in vesicles via exocytosis.
g. Compare and contrast the causes of Type 1 and Type 2 diabetes mellitus
h. Discuss the use of insulin produced by genetically modified bacteria and the potential use of stem cells to treat diabetes mellitus
- Advantages of using genetically modified bacteria
- Exact copy of human insulin so faster acting and more efficient than using animals.
- Less chance of developing intolerance.
- Less chance of rejection.
- Lower risk of infection.
- Cheaper to manufacture.
- Less likely to cause medical objection.
- They are not differentiated so can be any cell.
- Most common sources are bone marrow and the placenta.
- Similar precursor cells have been found in the pancreas of adult mice. It is thought that if similar ones are found in the human pancreas then it could be used to produce new β cells to treat type 1 diabetes.
i. Outline the hormonal and nervous mechanisms involved in the control of heart rate in humans
- Movement of the limbs are detected by stretch receptors which send impulses to the cardiovascular centre informing it that more oxygen may be needed increasing the heart rate.
- During exercise more carbon dioxide tends to be produced some of which will react with water in the blood plasma reducing the pH. This change is detected by chemoreceptors in the cartoid arteries, the aorta and the brain. These send impulses to the cardiovascular centre increasing the heart rate.
- When we stop exercise it leads to a decrease in carbon dioxide levels, decreasing the activity in the accelerating pathway leading to the heart rate decreasing.
- When adrenaline is released in response to stress, shock, anticipation and excitement, the heart rate increases to prepare the body for activity.
- Blood pressure is monitored by stretch receptors in the walls of the cartoid sinus. If it gets too high it send signals to the cardiovascular centre leading to the reduction in heart rate.