6.5 Nerves, hormones & homeostasis 11/8/10 11:58AM

Topic 6: Human Health & Physiology6.5 Nerves, hormones & homeostasis

Orange book: pg. 240-252Green book: pg.

6.5.1 State that the nervous system consists of the central nervous system (CNS) and peripheral nerves, and is composed of cells called neurons that can carry rapid electrical impulses. (pg. 240, 107)

6.5.2 Draw and label a diagram of the structure of a motor neurone (pg. 240, 107)

6.5.3 State that nerve impulses are conducted from receptors to the CNS by sensory neurons, within the CNS by relay neurons and from the CNS to effectors by motor neurons. (pg. 240, 107)

6.5.4 Define resting potential and action potential (depolarization and repolarization) (pg. 242, 107)

6.5.5 Explain how a nerve impulse passes along a non-myelinated neuron (pg. 243, 108-109)

6.5.6 Explain the principles of synaptic transmission. (pg. 245)

6.5.7 State that the endocrine system consists of glands that release hormones that are transported in the blood.

6.5.8 State that homeostasis involves maintaining the internal environment between limits, including blood pH, carbon dioxide concentration, blood glucose concentration, body temperature and water balance. (pg. 247-249)

6.5.9 Explain that homeostasis involves monitoring levels of variables and correcting changes in levels by negative feedback mechanisms. (pg. 247)

6.5.10 Explain the control of body temperature, including the transfer of heat in blood, and the roles of the hypothalamus, sweat glands, skin arterioles and shivering (pg. 247-248)

6.5.11 Explain the control of blood glucose concentration, including the roles of glucagons, insulin and and cells in the pancreatic islets. (pg. 249)

6.5.12 Distinguish between type I and type II diabetes. (pg. 249)

6.5.1 The Nervous System 11/8/10 11:58 AM

6.5.1 State that the nervous system consists of the central nervous system (CNS) and peripheral nerves, and is composed of cells called neurons that can carry rapid electrical impulses.

Orange book pg. 240Green book pg. 107

To do: Read and highlight the notes below, read the relevant sections in the green book. Summary in Green book to include:the components of the CNS and its rolethe role of peripheral nervesneurons and electrical impulses.

The nervous system carries out a complex array of tasks, such as sensing various smells, producing speech, remembering, providing signals that control the body movements and regulating the operation of internal organs.

Our brain and spinal cord are our central nervous system (CNS). These two structures receive sensory information from various receptors and then interpret and process that sensory information. If a response is needed, some portion of the brain or spinal cord initiates a response which is called a motor response.

The cells that carry this information are called neurones. Sensory neurones bring information to the CNS and motor neurones carry response information to muscles.

Together, sensory neurones and motor neurones make up the peripheral nerves. A neurone is an individual cell which carries electrical impulses from one point in the body to another and does so very quickly. When many individual neurones group together into a single structure, that structure is called a nerve. Think of a nerve as being like a telephone cable, a protective sheath surrounding many individual wires. Each wire within that cable is like a neurone.

There are two categories of peripheral nerves:Spinal nerves – 31 pairs (left and right) of these emerge directly from the spinal cord. They are mixed nerves, as some of the neurones within them are sensory and some are motor.

Cranial nerves – 12 pairs of these emerge from an area of the brain known as the brainstem. One well known example is the optic nerve pair which carry visual information from the retina of the eyes to the brain.

6.5.2 Motor Neurone Structure 11/8/10 11:58 AM

6.5.2 Draw and label a diagram of the structure of a motor neurone

Orange book pg. 240Green book pg. 107

To do: Read and highlight the notes below, read the relevant sections in the green book. Summary in Green book to include:the components of the CNS and its rolethe role of peripheral nervesneurons and electrical impulses.

Use the books above and the diagram below to draw and label a diagram of the structure of a motor neurone in your book.

Structure of a Neurone

A neurone has: a cell body containing a nucleus surrounded by cytoplasm that

includes typical organelles such as mitochondria and Golgi complex.

numerous extensions which emerge from the cell body. These dendrites provide a large surface area for connecting with other neurones, and carry nerve impulses towards the cell body. The more dendrites the cell body has the more information it can receive from other cells.

a single long axon which carries the nerve impulse away from the cell body. The axon joins the cell body at the axon hillock. The axon is specialised for rapid conduction of nerve signals along its length. The end of the axon has numerous branches each of which end in a synaptic knob.

most neurones have many companion cells called Schwann cells, which wrap their cell membrane around the axon many times in a spiral to form a thick insulating lipid layer called the myelin sheath.

Class – Whiteboard ActivityPage 1. IntroductionThe structure of neurons reflects their function. One part of the cell receives incoming signals. Another part generates outgoing signals.

Page 2. Objective:Draw and label a diagram of the structure of a motor neurone

Page 3. Neurons Can CommunicateBecause of their unique anatomical design, and because they are excitable, neurons can communicate.What other structures do neurones communicate with?

Page 4. Neurons Have Three Characteristic Structural FeaturesNeurons come in many different shapes and sizes.

List the three characteristic structural features which all neurones have:

Multiple similar processes, called ____________________, extend from the ______________, forming a structure resembling the branches of a tree. A thin single process, called the ___________________, also extends from the cell body.

Page 5. Neuron Structure is Related to FunctionLabel the parts of this neurone:

Dendrites – Receptive and integrative region of the neurone.What is the function of the dendrites?

Cell Body – Receptive and integrative region of the neurone.What is the function of the cell body?

Axon – The transmitting or conductive region of the neurone.What is the function of the axon?

Page 6. Information Flow in Neurones is DirectionalWith arrows, label the input and output on this neurone:

When will an action potential be generated?

In which direction is an action potential conducted?

Page 7. Signals Are Received At SynapsesUsing arrows label one or two synapses in the diagram below:

The ________________ and cell body provide a large _______________________ for communication with other neurons. Signals from other neurons are received at synapses, the junctions between neurons.

Page 8. Axons Vary In LengthAxons vary in length. They can be short, just 1 or 2 millimeters, communicating only with cells in their immediate vicinity.

Axons can also be very long, more than a meter, and communicate over long distances. Give an example of a long axon.

In general, the longest axons are associated with the largest cell bodies.

Page 9. Signals Are Sent Out Along The AxonFrom which region of the cell body does the axon arise?

At their terminal ends, axons can branch profusely, forming thousands of endings called ______________.

In which direction is the action potential generated?

Label the diagram below:

** Now is a good time to go to quiz questions 1-2:

Page 10. Some Axons are Myelinated

Name the insulating material that surrounds some axons of the nervous system.

Which cells produce the insulating layer in the peripheral nervous system?

How does the Schwann cell insulate the axon?

Page 11. Many Schwann Cells Insulate an AxonLabel the diagram below:

Because Schwann cells are small compared to the length of an axon, it takes many of them to insulate a single axon.

Name the gaps between neighbouring Schwann cells.

Page 12. Summary Neurons have receptive and integrative regions, the dendrites and cell

body, which receive and integrate incoming signals. Neurons also have a conductive region, the axon, which generates and

transmits an outgoing signal.

Axons vary in length from 1 or 2 millimeters to more than 1 meter. Some axons are insulated with myelin sheath.

** Now is a good time to go to quiz questions 3-4:

6.5.3 Neurones 11/8/10 11:58 AM

6.5.3 State that nerve impulses are conducted from receptors to the CNS by sensory neurons, within the CNS by relay neurons and from the CNS to effectors by motor neurons.

Orange book pg. 240Green book pg. 107

To do: Read and highlight the notes below, read the relevant sections in the green book.Check the knee jerk reflect on class members Summary in Green book describing a reflex arc using the example of the knee jerk

The Reflex ArcThe three types of neurones are arranged in circuits and networks, the simplest of which is the reflex arc.

In a simple reflex arc, such as the knee jerk, a stimulus is detected by a receptor cell, which synapses with a sensory neurone. The sensory neurone carries the impulse from site of the stimulus to the central nervous system (the brain or spinal cord), where it synapses with a relay. The relay synapses with a motor neurone, which carries the nerve impulse out to an effector, such as a muscle, which responds by contracting.

Reflex arc can also be represented by a simple flow diagram:

The diagram below is a diagrammatic cross-section of the spinal cord to illustrate a reflex arc. The arrows indicate the direction in which impulses are transmitted through the nervous system.

Suppose you stand on a pin, you respond by quickly pulling your leg away. The reflex arc shown above shows the route taken by the impulse which causes you to move your leg away. The neurones are located in one of the spinal nerves which serve the leg. This nerve (and all spinal nerves), is attached to the spinal cord by two connections, a dorsal root and a ventral root.

The receptors in this reflex are nerve endings in the skin of the foot. The main effector is a muscle in the leg. The fibres of the sensory neurone enters the spinal cord via the dorsal root. The cell body of this neurone is located in the dorsal root ganglion, a swelling of the dorsal root. The ganglion contains the cell bodies of many other sensory neurones besides this one, which is why it is swollen. In the grey matter of the spinal cord the sensory neurone makes synaptic connection with the relay neurone. This in turn makes a synaptic connection with the effector neurone, which passes out of the spinal cord in the ventral root and supplies the flexor muscle.

*A ganglion is a localized part of the nervous system which contains a concentrated collection of nerve cells. The dorsal root ganglion in this case contains nerve cell bodies, but many other ganglia contain synapses too. A ganglion may be a swelling associated with a nerve or it may be a collection of nerve cells within the central nervous system.

Sensory Receptor

Dendrites or a sensory structure (e.g. pain receptor) respond to a specific stimulus. If the stimulus is strong enough to reach threshold level of depolarisation, it will trigger one or more nerve impulses (action potentials) in the sensory neurones.

Sensory NeuroneThe nerve impulses (action potentials) propagate from the sensory receptor along the axon of the sensory neurone to the axon terminals, which are located in the grey matter of the CNS.

Relay Neurone In the simplest type of reflex, the integrating of the message is carried out by one relay neurone between the incoming sensory neurone and the outgoing motor neurone.

Motor NeuroneImpulses triggered by the relay neurone propagate out of the CNS along a motor neuron to the part of the body that will respond.

EffectorThe part of the body that responds to the motor nerve impulse, such as a muscle or a gland, is the effector. Its action is called a reflex.

The Knee Jerk Reflex

6.5.4 Resting & Action Potentials 11/8/10 11:58 AM

6.5.4 Define resting potential and action potential (depolarization and repolarization)

Orange book pg. 242-243Green book pg. 107

To do: Read the relevant sections in the green book. Give a concise defintion of “Resting Potential” and “Action Potential”.

6.5.5 Nerve Impulses 11/8/10 11:58 AM

6.5.5 Explain how a nerve impulse passes along a non-myelinated neuron.

Orange book pg. 243Green book pg. 108-109

The Resting Membrane PotentialAll animal cell membranes contain a protein pump called Na+K+ATPase. This uses the energy from ATP splitting to simultaneously pump 3 sodium ions out of the cell and 2 potassium ions in. If this was to continue unchecked there would be no sodium or potassium ions left to pump.But there are also sodium and potassium ion channels in the membrane. These channels are normally closed, but even when closed, they “leak” (leakage channels), allowing sodium ions to leak in and potassium ions to leak out, down their respective concentration gradients.

The combination of the Na+K+ATPase pump and the leakage channels cause a stable imbalance of Na+ and K+ ions across the membrane. This imbalance causes a potential difference across all animal cell membranes, called the resting membrane potential.The resting membrane potential is always negative inside the cell because of a small build-up of negative ions in the cytoplasm and a build-up of positive ions in the extracellular fluid along the outside surface of the membrane. A typical value for the membrane potential is -70mV. A cell that exhibits a membrane potential is said to be polarised

The resting membrane potential is due to a small build-up of anions, mainly organic phosphates (PO4

3-) and proteins, in the cytoplasm just inside the membrane and an equal build-up of cations, mainly sodium ions (Na+), in the extracellular fluid just outside the membrane.

The Action PotentialIn nerve and muscle cells the membranes are electrically excitable, which means that they can change their membrane potential, and this is the basis of the nerve impulse.The sodium and potassium channels in these cells are voltage-gated, which means that they can open and close depending on the voltage across the membrane.

The normal resting membrane potential of nerve cells is –70mV (inside the axon). When a stimulating pulse is applied a brief reversal of the membrane potential occurs called the action potential:

An action potential is a sequence of rapidly occurring events that take place in two phases. During the depolarising phase, the negative membrane potential decreases towards zero and eventually becomes positive. Then the repolarising phase restores the membrane potential to the resting state of -70mV.

During an action potential two types of voltage-gated channels open and then close.These channels are present mainly in the plasma membrane of the axon and axon terminals.

DepolarisationThe first channels that open, the Na+ channels, allow Na+ to rush into the cell, which causes the depolarisation. The inflow of Na+ changes the membrane potential from -70mV to +30mV at the peak of the action potential. This phase is referred to as a depolarization since the normal voltage polarity (negative inside) is reversed (becomes positive inside).

RepolarisationWhen the membrane potential reaches 0V, the potassium channels open for 0.5ms, causing potassium ions to rush out, making the inside more negative again. Since this restores the original polarity, it is called repolarisation. The voltage-gated K+ channels open more slowly than the Na+ voltage-gated channels, so their opening occurs about the same time the voltage-gated Na+ channels are closing.

The pump runs continuously, restoring the resting concentrations of sodium and potassium ions.Action potentials arise according to the all-or-nothing principle. When depolarisation reaches a certain level termed the threshold (about -55mV), the voltage-gated channels open, and an action potential that is always the

6.5.6 Synapses 11/8/10 11:58 AM

6.5.6 Explain the principles of synaptic transmission.

Orange book pg. 245Green book pg. 109

SynapsesThe junction between two neurones is called a synapse. An action potential cannot cross the synaptic cleft between neurones, and instead the nerve impulse is carried by chemicals called neurotransmitters. These chemicals are made by the cell that is sending the impulse (the presynaptic neurone) and stored in synaptic vesicles at the end of the axon. The cell that is receiving the nerve impulse (the post-synaptic neurone) has neuroreceptors which have specific binding sites for the neurotransmitters.

Neurotransmitters are quite diverse in their action. Some are excitatory and some are inhibitory. The only neurotransmitter you need to know about is acetylcholine (ACh) which opens gates in the postsynaptic cell and depolarises (excites) it. A synapse where transmission is mediated by ACh is called a cholinergic synapse.

Structure and Function

1. At the end of the pre-synaptic neurone there are voltage-gated calcium channels. When an action potential reaches the synapse these channels open, causing calcium ions to flow into the cell.

2. These calcium ions cause the synaptic vesicles to fuse with the cell membrane, releasing their contents (the neurotransmitter chemicals) by exocytosis.

3. The neurotransmitters diffuse across the synaptic cleft.

4. The neurotransmitter binds to the neuroreceptors in the post-synaptic membrane, causing the channels to open. In the example shown these are sodium channels, so sodium ions flow in whilst K+ leaves the cell.

5. As Na+ enters the cell, it spreads out along the inside of the plasma membrane. This causes a depolarisation of the post-synaptic cell membrane (postsynaptic potential). If this is strong enough and persistent enough it may initiate an action potential.

As complex as the foregoing process may seem, it all happens in 0.5msec – an interval called synaptic delay. This is the time from the arrival of a signal at the axon terminal of a presynaptic cell to the beginning of an action potential in the postsynaptic cell.

Cessation of the signal6. The neurotransmitter is broken down by a specific enzyme in the synaptic cleft; for example the enzyme acetylcholinesterase breaks down the neurotransmitter acetylcholine. The breakdown products are absorbed by the pre-synaptic neurone by endocytosis and used to re-synthesise more neurotransmitter, using energy from the mitochondria. This stops the synapse being permanently on.

6.5.7 Endocrine System 11/8/10 11:58 AM

6.5.7 State that the endocrine system consists of glands that release hormones that are transported in the blood.

Orange book not coveredGreen book pg. 110

Although the nervous and endocrine systems act together to co-ordinate functions of all body systems, their means of control are different. The nervous system acts through nerve impulses conducted along axons of neurones. At synapses, nerve impulses trigger the release of messenger molecules called neurotransmitters. The endocrine system also controls body activities by releasing messengers, which are called hormones.

HormonesA hormone is a messenger molecule, which is released in one part of the body but regulates activity of cells in other parts of the body. Hormones are released by glands and travel in the blood stream to their target cells. The circulating blood delivers the hormones to cells throughout the body. Both neurotransmitters and hormones exert their effects by binding to receptors on or in their “target” cells.

Responses of the endocrine system often are slower than responses of the nervous system. Although some hormones act within seconds, most take several minutes to more to cause a response. Moreover, the effects attained by activating the nervous system are generally briefer than the effects produced by the endocrine system. The nervous system causes muscles to contract or relax and glands to secrete either more or less of their product. The influence of the endocrine system is much broader. It helps to regulate virtually all types of body cells.

The Hormone SystemHormones are secreted by glands into the blood stream.

Endocrine glands do not have ducts, they secrete chemicals directly into the blood stream. E.g. thyroid gland, pituitary gland, adrenal gland. The hormone-secreting glands are all endocrine glands.

Once a hormone has diffused into the blood stream it is carried all round the body to all organs. However, it only affects certain target organs, which can respond to it. These target organs have specific receptor molecules in their cells to which the hormone binds.

Complete the table below – it contains only the hormones you need to know about.

Gland Hormone Target ResponseFSHLHADHAdrenalineOestrogenProgesteroneTestosteroneInsulinGlucagon

6.5.8 Homeostasis 11/8/10 11:58 AM

6.5.8 State that homeostasis involves maintaining the internal environment between limits, including blood pH, carbon dioxide concentration, blood glucose concentration, body temperature and water balance.

Orange book pg. 247Green book pg. 110

IntroductionHomeostasis is the maintenance of a constant internal environment.

Homeostasis in the human body is continually being disturbed. Some disruptions come from the external environment in the form of physical insults such as intense heat or lack of oxygen. Other disruptions originate in the internal environment, such as blood glucose level that is too low.

Fortunately, the body has many regulating systems that usually bring the internal environment back into balance. Most often, the nervous system and the endocrine system, working together or independently, provide the needed corrective measures. The nervous system regulates homeostasis by sending messages in the form of nerve impulses to organs that can counteract deviations from the balanced state. The endocrine system includes many glands that secrete messenger molecules called hormones into the blood. Whereas nerve impulses typically cause rapid changes, hormones usually work more slowly. Both means of regulation, however, work toward the same end, namely maintaining homeostasis. As you will see both systems carry out their mission through negative feedback systems.

6.5.9 Negative Feedbck 11/8/10 11:58 AM

Explain that homeostasis involves monitoring levels of variables and correcting changes in levels by negative feedback mechanisms.

Ref:Orange book pg. 247Green book pg. 210-211

Feedback SystemsThe body can regulate its internal environment through feedback systems. A feedback system is a cycle of events in which the status of a body condition is continually monitored, evaluated, changed, re-monitored, re-evaluated and so on. Each monitored variable such as body temperature, blood pressure, or blood glucose level, is termed a controlled condition. Any disruption that changes a controlled condition is called a stimulus. Three basic components make up a feedback system – a receptor, a control centre and an effector.

A receptor detects a change.

A control centre receives information, processes it and co-ordinates an appropriate response.

An effector tries to bring the change back to a set point

The three basic components of a feedback system are receptors, a control centre and effectors.

Negative Feedback SystemsA negative feedback system reverses a change in a controlled condition. First, a stimulus disrupts homeostasis by altering the controlled condition. The receptors that are part of the feedback system detect the change and send input to a control centre. The control centre evaluates the input and, if necessary, issues output commands to an effector. The effector produces a physiological response that is able to return the controlled condition to it normal state.

All homeostatic mechanisms use negative feedback to maintain a constant value (called the set point). Negative feedback means that whenever a change occurs in a system, the change automatically causes a corrective mechanism to start, which reverses the original change and brings the system back to normal. It also means that the bigger then change the bigger the corrective mechanism.

So in a system controlled by negative feedback the level is never maintained perfectly, but constantly oscillates about the set point. An efficient homeostatic system minimizes the size of the oscillations.

6.5.10 Temperature Regulation 11/8/10 11:58 AM

6.5.10 Explain the control of body temperature, including the transfer of heat in blood, and the roles of the hypothalamus, sweat glands, skin arterioles and shivering.

Orange book pg. 247Green book pg. 111-112

HomeostasisThe ability of the body to maintain a constant internal environment is called homeostasis.

Internal conditions are not absolutely constant but fluctuate within a limited range, such as the range of body temperature. The internal state of the body is best described as a dynamic equilibrium, in which there is a certain set point or average value given for a variable (such as 37ºC for body temperature) and conditions fluctuate slightly around this point.

The fundamental mechanisms that keeps a variable close to its set point is negative feedback – a process in which the body senses a change and activates mechanisms that reverse it.

If a steady temperature is to be maintained in the human body, heat loss must equal heat gain. Most heat is generated internally from metabolic activities, though there may be some gain from the surroundings. The production of body heat is proportional to metabolic rate.

Heat is produced at a fairly constant rate from metabolic activities in various organs in the body, such as the liver and heart. Heat may also be gained from the environment in situations where air temperature is higher than the temperature of the skin. The body also gains heat by direct radiation from the sun or from a fire/ artificial heater. Some heat is gained directly from the consumption of hot food and drink. We also produce heat when we exercise as out metabolism increases.

When the body is hotter than the surrounding environment, heat may be lost by direct radiation from the body, by conduction through areas of the body touching cooler objects or by convection to the surrounding air.

Hypothalamic ThermostatThe control centre that functions as the body’s thermostat is a group of neurons in the anterior part of the hypothalamus. This area receives impulses from thermoreceptors in the skin and also monitors the temperature of blood flowing through it.

Nerve impulses are sent to two other parts of the hypothalamus known as the heat-losing centre and the heat-promoting centre, which, when stimulated, set into operation a series of responses that lower body temperature and raise body temperature, respectively.

ThermoregulationIf core temperature declines, mechanisms that help conserve heat and increase heat production act via several negative feedback loops to raise the body temperature to normal. Thermoreceptors in the skin send nerve impulses hypothalamus which sends messages to the heat-promoting centre in the hypothalamus. The nerve impulses from the hypothalamus then activate several effectors.

Each effector responds in a way that helps increase core temperature to the normal value.

Nerve impulses from the heat-promoting centre stimulate the blood vessels of the skin to constrict. Vasoconstriction decreases the flow of warm blood, and thus the transfer of heat, from the internal organs to the skin. Slowing the rate of heat loss allows the internal body temperature to increase as metabolic reactions continue to produce heat.

The heat-promoting centre stimulates parts of the brain that increase muscle tone and hence heat production. The resulting contraction and relaxation of muscles in a repetitive cycle is called shivering, which greatly increases the rate of heat production. During maximal shivering, body heat production can rise to about four times the resting rate in just a few minute.

In cold conditions, smooth muscles attached to hair follicles in the skin contract, raising the hairs. This is important in many mammals as it helps to trap a layer of air in the fur. Air is a poor conductor of heat so this helps to insulate the body against excessive heat loss. The insulating effect of hair is minimal in humans, except on the head, and lack of hair in bald adults and babies can lead to significant heat losses.

If core body temperature rises above normal, a negative feedback loop goes into action. The higher temperature of the blood stimulates thermoreceptors that send nerve impulses to stimulate the heat-losing centre. Nerve impulses from the heat-losing centre cause dilation of blood vessels in the skin. The skin becomes warm, and the excess heat is lost to the environment via radiation and conduction as an increased volume of blood flows from the warmer core of the body into the cooler skin. At the same time, metabolic rate decreases, and shivering does not occur. The high temperature of the blood stimulates sweat glands of the skin. As the water in perspiration evaporates from the surface of the skin, the skin is cooled. All these responses counteract heat-promoting effects and help return the body temperature to normal.

6.5.11 Glucose Regulation 11/8/10 11:58 AM

6.5.11 Explain the control of blood glucose concentration, including the roles of glucagons, insulin and and cells in the pancreatic islets.

Orange book pg. 249Green book pg. 111-112

Blood Glucose HomeostasisGlucose is the transport carbohydrate in animals, and its concentration in the blood affects every cell in the body. Its concentration is therefore strictly controlled within the range and very low levels (hypoglycaemia) or very high levels (hyperglycaemia) are both serious and can lead to death.

Blood glucose concentration is controlled by the pancreas. The pancreas has glucose receptor cells, which monitor the concentration of glucose in the blood, and it also has endocrine cells (called the islets of Langerhans), which secrete hormones. The α-cells secrete the hormone glucagon, while the β-cells secrete the hormone insulin. These two hormones are antagonistic, and have opposite effects on blood glucose:

insulin stimulates the uptake of glucose by cells for respiration, and in the liver it stimulates the conversion of glucose to glycogen. It therefore decreases blood glucose.

glucagon stimulates the breakdown of glycogen to glucose in the liver, and in extreme cases it can also stimulate the synthesis of glucose from pyruvate. It therefore increases blood glucose.

After a meal, glucose is absorbed from the gut into the hepatic portal vein, increasing the blood glucose concentration. This is detected by the pancreas, which secretes insulin from its β-cells in response. Insulin causes glucose to be taken up by the liver and converted to glycogen. This reduces blood glucose, which causes the pancreas to stop secreting insulin.

If the glucose level falls too far, the pancreas detects this and releases glucagon from its α-cells. Glucagon causes the liver to break down some of its glycogen store to glucose, which diffuses into the blood. This increases blood glucose, which causes the pancreas to stop producing glucagon.

These negative feedback loops continue all day, as shown in this graph:

The Negative Feedback Loop Controlling Blood Glucose Levels

Remember – the principal action of glucagon is to increase blood glucose level when it falls below normal. Insulin, on the other hand, helps lower blood glucose level when it is too high. The level of blood glucose controls secretion of glucagon and insulin via a negative feedback system.

6.5.12 Diabetes 11/8/10 11:58 AM

6.5.12 Distinguish between type I and type II diabetes.

Orange book pg. 251Green book pg. 113

Read the relevant sections in your book.Make notes on the differences between type I and type II diabetes.