6.5 – Nerves, Hormones and Homeostasis
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
The somatic nervous system includes motor neurons attached to the skeletal muscles, and the sensory neurons attached to the receptor sense organs.
The autonomic nervous system includes the nerves from internal receptors and the nerves attached to smooth muscle.
6.5.2 – Draw and label a diagram of the structure of a motor neuron
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 motors neurons
The body contains many different receptors which detect specific stimuli and convert them into a nerve impulse. The central nervous system conducts the nerve impulses from sensory nerves along sensory neurons. This impulse is then passed to relay neurons that pass it through the brain and spine, then to a motor neuron and an effector (such as the muscles), where the response is produced.
𝑆𝑡𝑖𝑚𝑢𝑙𝑖 → 𝑅𝑒𝑐𝑒𝑝𝑡𝑜𝑟 → 𝑆𝑒𝑛𝑠𝑜𝑟𝑦 𝑁𝑒𝑟𝑣𝑒 → 𝑅𝑒𝑙𝑎𝑦 𝑁𝑒𝑟𝑣𝑒 → 𝑀𝑜𝑡𝑜𝑟 𝑁𝑒𝑢𝑟𝑜𝑛 → 𝐸𝑓𝑓𝑒𝑐𝑡𝑜𝑟 → 𝑅𝑒𝑠𝑝𝑜𝑛𝑠𝑒
6.5.4 – Define resting potential and action potential (depolarisation and repolarisation)
Resting Potential – An electrical potential across a cell membrane when not conducting an impulse Action Potential – The localised reversal, or depolarisation, and then restoration, or
Action Potential – The localised reversal, or depolarisation, and then restoration, or repolarisation, of electrical potential between the inside and outside of a neuron as the impulse moves along it
6.5.5 – Explain how a nerve impulse passes along a non-myelinated neuron
Nerve impulses move along the axon using a domino effect, with an action potential causing one in the adjacent part. Sodium and potassium ions move across the plasma membrane via active transport to alter the concentration and cause an action potential and then return to resting potential.
At resting potential:
Na+ ions are concentrated outside the membrane of the axon, while the K+ ions are concentrated inside the membrane. The Na+ and K+ ion channels are closed. For an action potential to be created, it must rise above a certain threshold before the action potential is created.
At action potential:
Na+ rush inside the membrane and K+ ion rush outside to reverse concentrations. The Na+ ion channels open, then close as the K+ ion channels open. The inside now has a net positive charge, while the outside has a net negative charge.
The ion pumps used to create the action potential must use active transport as they are moving the ions against the concentration gradient. This requires ATP for energy. The polarisation of the membrane changes during this process from -70mV at resting potential to +30mV at action potential. It is restored back to -70mV when the K+ ion channels open.
After the neuron is repolarised, there is a brief period in which that section of the axon cannot have an action potential, called the refractory period.
The all-or-nothing law is that a nerve impulse will only be passed on if it reaches the threshold of -55mV. If it does not, then no action potential will be created. If it does, then an action potential will be sent that is the same voltage as any other action potential. On a graph, all action potentials look the same, each rising to +35mV.
6.5.6 – Explain the principles of synaptic transmission
The synapse is the junction between two neurons – the presynaptic neuron passing the signal to the postsynaptic neuron. The synaptic cleft is the fluid-filled space between an axon terminal and the end of a dendrite. They are the location of communication between neurons and glands or muscles. They pass electrical or chemical signals to their target cells. When an action potential reaches the end of the neuron, the Ca+ ion channels open to allow Ca+ to flow in. This cause exocytosis of a neurotransmitter at the synaptic cleft.
The neurotransmitter diffuses across the synaptic cleft and then binds to a post-synaptic receptor. The post-synaptic membrane becomes polarised, causing the Na+ ion channels to open. An action potential is created in the post-synaptic neuron. The post-synaptic membrane then becomes depolarised. The K+ ion channels immediately open to cause hyperpolarisation of the post-synaptic membrane.
An enzyme binds to the neurotransmitter to hydrolyse it and prevent future function. It is recycled in the body.
6.5.7 – State that the endocrine system consists of glands that release hormones that are transported in the blood
The endocrine system is also called the hormone system. This is because it consists of glands that release hormones to our cells to aid bodily function. They assist in regulating mood, growth and development, tissue function, metabolism, sexual function and the reproductive processes. Whilst the nervous system controls processes that happen quickly such as breathing and movement, the endocrine system controls the slower processes like cell growth.
Hormones, as the body’s chemical messengers, transfer information by circulating through the bloodstream. The glands secrete hormones to be transported to another part of the body. The hormones go straight from the glands to the bloodstream, and may travel to any part of the body. However, they will only transmit messages to the cells that they are intended for.
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
Homeostasis controls the variables in our bodies to maintain health. The result of disrupting this is called stress, which will lead to disease if it is not corrected. These variables include blood glucose concentration, blood pH, body temperature, CO2 concentration and water balance. These levels are maintained at constant levels within narrow limits.
6.5.9 – Explain that homeostasis involves monitoring levels of variables and correcting changes in levels by negative feedback mechanisms
For all the variables controlled in homeostasis, there is a set point around which the body fluctuates within a certain range. Negative feedback is used to return the variable to its set point.
The body has many sensors which detect and signal when the variable fluctuates from the set point. This information is passed onto the control centre to direct the action that should be taken to rectify this. The effectors are the mechanism for returning the variable to its set point, and they switch on or off under the direction of the control centre. The response is produced by the effector.
The variables maintained by homeostasis include blood pH, CO2 concentration, blood glucose concentration, body temperature and water balance. Negative feedback relies on the nervous or endocrine systems. It has the opposite effect to try and stabilise the variable back at the set point. However, negative feedback is only triggered when there is a significant deviation from the set point.
6.5.10 – Explain the control of body temperature, including transfer of heat in blood, and the roles of the hypothalamus, sweat glands, skin arterioles and shivering
The hypothalamus, located in the brain, and the cerebral cortex maintain homeostasis. The hypothalamus sends signals to the pituitary gland in the form of hormones. The pituitary gland then secretes stimulating hormone into the bloodstream to the target gland, which in turn secretes its hormones. The hypothalamus and the pituitary gland can regulate the levels of hormones in the blood. The process used for regulating temperature is called negative feedback.
In humans, the set point for body temperature is 37°C. The body detects if the body goes above or below this using sensors such as the hypothalamus, skin warmth receptors and skin cold receptors.
Response Below the Set Point:
If the temperature is lower than 37°C, the sympathetic nervous system has an involuntary response:
- vasoconstriction to lower blood flow to the skin to decrease heat loss
- increased metabolism to increase heat production
- shivering to increase heat production
- piloerection, or goosebumps, to decrease heat loss
In addition, the cerebral cortex directs the following voluntary responses:
- rest to decrease heat loss
- behavioural responses such as warmer clothing, muscular activity, warm drink, curling up and eating
The effectors will increase heat production and decrease heat loss until the temperature is at 37°C.
Response Above the Set Point:
If the temperature is higher than 37°C, the sympathetic nervous system has an involuntary response to try an increase heat loss:
- decreased metabolism to decrease heat production
- sweating to increase heat loss
- lethargy to decrease heat production
- skin arterioles increase in diameter to increase heat loss
- relaxed skeletal muscles to lower heat production
Our bodies also have some voluntary responses controlled by the cerebral cortex:
- rest to decrease heat production
- behavioural responses such as cool drinks, cooler clothing and fanning
The effectors will try to decrease heat production and increase heat loss until the temperature reaches 37°C
6.5.11 – Explain the control of blood glucose concentration, including the roles of glucagon, insulin and a and b cells in the pancreatic islets
Blood glucose concentration fluctuate throughout the day, usually from about 4 to 8 millimoles dm-3. Using negative feedback, the body can alter the rate at which glucose is taken up into the blood. The set point for blood glucose concentrations is about 90 mg/ 100 mL. The control centre for this is the pancreatic islets.
Response Above Set Point
The beta cells in the pancreatic islets produce insulin, the chemical which stimulates the uptake of glucose from the blood for conversion into glycogen or for respiration. This lowers the blood glucose concentration.
The insulin binds to receptors on the muscle and liver cells to allow glucose to move from the blood into the cells. The glucose is either metabolised or stored. This continues until the concentration is at the set point.
Response Below Set Point
The alpha cells in the pancreatic islets produce the chemical glucagon, stimulating liver cells to convert glycogen into glucose. The blood glucose concentration rises as a result.
The glucagon binds to the receptors of liver cells to activate enzymes that break down glycogen to glucose. The glucose moves in to the blood until the concentration is at the set point.
6.5.12 – Distinguish between type I and type II diabetes
People who have diabetes have blood glucose levels that are too high. The glucose is not able to get form the blood to the cells to provide energy.