6.5 Nerves, Hormones and Homeostasis

Assessment Statements 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 Draw and label a diagram of the structure of a motor neuron 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 Define resting potential and action potential (depolarization and repolarization) Explain how a nerve impulse passes along a non- myelinated neuron Explain the principles of synaptic transmission.

Why do we need control systems? Response to stimuli is essential for survival Multicellular organisms need coordination Nervous versus endocrine systems Both work together e.g. rabbit running from fox – what systems involved?

Sense organs detect change (receptors) Effectors respond Distance between two Every sensor and effector has at least 1 link to CNS Spinal cord links brain to rest of body

6.5.1 State that the nervous system consists of the central nervous system (CNS) and peripheral nerves

cells called neurons that can carry rapid electrical impulses.

6.5.2 Draw and label a diagram of the structure of a motor neuron Cell body with nucleus Dendrites transmit impulses to cell body Impulses leave via axon Myelin sheath composed of Schwann cells Axon terminal passes signal to effector Nodes of Ranvier direction of impulse

Neurones packed and wrapped to form nerves May contain sensory, effector or mixture

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.

Sensory neurone Relay neurone Motor neurone

Sensory neurons carry messages from receptors in sense organs to CNS. Relay neurones connect sensory neurons to motor neurones in the CNS. Motor neurons connect the CNS to the effector.

A Receptor in skin B Sensory neurone C Relay neurone D Motor neurone E Effector

6.5.4 Define resting potential and action potential (depolarization and repolarization). Neurons have an electrical potential (voltage) across the cell membrane, i.e. membrane is polarised. The inside of the cell is more negative than the outside This is called the Resting Membrane Potential = 70mV

Definitions Resting potential - the electrical potential across the cell membrane of a cell that is not conducting an impulse Action potential - is the reversal (polarisation) and restoration (depolarisation) of the electrical potential across the plasma membrane as an impulse passes along a neurone

Concentration of K+ is high inside the neuron Concentration of Na+ is high outside the neuron M embrane is more permeable to K+ than Na+ C oncentration of K+ inside 20x greater so K+ ions rapidly diffuse out until equilibrium reached This results in the inside being more negative than outside Difference in concentration of ions maintained by active transport against concentration gradient Sodium/potassiu m cation pumps transport Na+ out and K+ in This requires ATP

6.5.5 Explain how a nerve impulse passes along a non-myelinated neuron. Stimulation can reverse the charge on a neuron (-70 to +40 mV) Membrane becomes depolarised If stimulus exceeds certain threshold value an action potential results Action potential - rapid reversal of the resting membrane potential that travels down the axon

The nervous impulse

6.5.6 Explain the principles of synaptic transmission.

Synapses occur where neurons meet

Outline the use of four method of membrane transport in nerves and synapses (8)

Assessment Statements: State that the endocrine system consists of glands that release hormones that are transported in the blood State that homeostasis involves maintaining the internal environment between limits, including blood pH, carbon dioxide concentration, blood glucose concentration, body temperature and water balance Explain that homeostasis involves monitoring levels of variables and correcting changes in levels by negative feedback mechanisms 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 Explain the control of blood glucose concentration, including the roles of glucagon, insulin and α and β cells in the pancreatic islets Distinguish between type I and type II diabetes.

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

The endocrine system consists of glands. Glands secrete chemicals called hormones directly into the blood. Hormones travel in the blood to a target organ (effector) and bring about a response. The response becomes a feedback stimuli.

6.5.8 State that homeostasis involves maintaining the internal environment between limits Homeostasis literally means “same state” - refers to the process of keeping the internal body environment in a steady state. Very important - a great deal of the endocrine system and autonomic nervous system is dedicated to homeostasis.

What needs to be controlled? ●Blood pH ●Blood carbon dioxide levels ●Blood glucose concentration ●Body temperature ●Water balance All of these factors are maintained between limits within the blood and tissue fluid to o C mg/dL

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

Negative feedback loops 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.

The bigger the change the bigger the corrective mechanism. Applies to electronic circuits and central heating systems as well as to biological systems. In a system controlled by negative feedback the level is never maintained perfectly, but constantly oscillates about the set point. An efficient homeostatic system minimises the size of the oscillations.

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. Homeotherms - animals that maintain a fairly constant body temperature (birds and mammals) Animals with variable body temperature (all others) are called poikilotherms Homeotherms maintain body temperature at around 37°C (warm- blooded) Poikilothermic animals can also have very warm blood during the day by basking in the sun.

Receptors in the hypothalamus monitor the temperature of the blood as it passes through the brain (core body temperature) Receptors in the skin monitor the external temperature. In humans temperature is controlled by the thermoregulatory centre in the hypothalamus. Receives input from two sets of thermoreceptors The thermoregulatory centre sends impulses to several different effectors to adjust body temperature.

The skin and temperature control

Temperature homeostasis

Behavioural control of temperature

Explain the control of blood glucose concentration, including the roles of glucagon, insulin and α and β cells in the pancreatic islets. Glucose is the transport carbohydrate in animals, and its concentration in the blood affects every cell in the body. Concentration is therefore strictly controlled within a range of mg 100cm -3 Very low levels (hypoglycaemia) or very high levels (hyperglycaemia) are both serious and can lead to death.

Controlled by the pancreas. Glucose receptor cells monitor the concentration of glucose in the blood. Endocrine cells (called the islets of Langerhans), which secrete hormones. α cells secrete glucagon β cells secrete insulin. The two hormones are antagonistic, and have opposite effects on blood glucose.

glycogen glucose Increase in blood sugar High blood sugar promotes insulin release promotes glucagon release Low blood sugar Glucagon Insulin stimulates breakdown of glycogen stimulates formation of glycogen Decrease in blood sugar Stimulates uptake of glucose by cells α cells β cells

Control of blood glucose concentration

Distinguish between type I and type II diabetes. Diabetes is a disease caused by a failure of glucose homeostasis. Insulin-dependent diabetes (type 1 or early onset diabetes) - a severe insulin deficiency due to autoimmune killing of β cells (possibly due to a virus). Non insulin-dependent diabetes (type 2 or late-onset diabetes) - insulin is produced, but the insulin receptors in the target cells don’t work, so insulin has no effect.

In both cases: - high blood glucose concentration after a meal, - not reabsorbed by kidneys - much of the glucose is excreted in urine - osmosis causes water to follow producing large quantities of dilute urine - less glucose for cells means that proteins are metabolised in respiration - organ damage follows Diabetes mellitus means “sweet fountain”) - doctors used to test for diabetes by tasting urine!

Diabetes can be treated by injections with insulin or by careful diet. It can be monitored using clinistix or blood/urine analysis Until the discovery of insulin in 1922 by Banting and Best, diabetes was an untreatable, fatal disease.

What are causes of type I and type II diabetes? Type IType II A. autoimmune disease leading to reduced insulin secretion decreased responsiveness of the body to insulin B. decreased responsiveness of the body to insulin autoimmune disease leading to reduced insulin secretion C. increased responsiveness of the body to insulin autoimmune disease leading to increased insulin secretion D. autoimmune disease leading to increased insulin secretion increased responsiveness of the body to insulin

What is a role of the hypothalamus in homeostasis? A. Monitoring body temperature B. Monitoring blood glucose concentration C. Secretion of glucagon D. Secretion of sweat

Outline how the human body prevents blood glucose concentration from rising excessively (5 marks) blood glucose concentration monitored by pancreas/islets/beta cells; (more) insulin secreted in response to high blood glucose / glucose above threshold level; insulin stimulates cells to absorb glucose; glucose used in cell respiration (rather than lipids); glucose converted to glycogen; by liver/muscle cells; glucose converted to fatty acids / triglycerides / fat; negative feedback process; Accept these points if clearly made in an annotated diagram.

Explain the principle of homeostasis with reference to the control of body temperature (9 marks) homeostasis involves maintaining a constant internal environment; involves the concept of negative feedback; a deviation from the norm is the stimulus to trigger the mechanisms to restore the norm / OWTTE; body temperature in mammals must be maintained at a constant level for enzymes; controlled by the hypothalamus / hypothalamus as a thermostat; too hot causes vasodilation so more heat is lost from skin; too hot causes sweating as evaporation of sweat leads to cooling; too cold causes shivering/muscle contraction as (increased metabolic rate) generates heat; too cold causes vasoconstriction so less heat lost from skin; liver/muscles can generate heat which is distributed around the body by blood; hair can trap air which insulates against heat loss (goose bumps); behavioural example of heat retention; (e.g. adding layers of clothes, jumping up and down,huddling in groups)