Topic 6.5Nerves, Hormones and Homeostasis Topic 6: Human Health and Physiology
Responding to the environment • Our nervous system helps us interact with our environment. • It provides responses to stimuli affecting our senses. • The nervous system can be split into: • the central nervous system (CNS) consisting of the brain and spinal cord. • The peripheral nervous system (PNS) consisting of all other nerves
CNS and PNS • Stimuli are converted into electrical signals called NERVE IMPULSES. • These impulses travel rapidly along peripheral nerves (more accurately called NEURONES) to the CENTRAL NERVOUS SYSTEM. • The CNS coordinates all the nerve impulses to form a RESPONSE.
What happens next? • The CNS sends out nerve impulses to the parts of the body that will carry out the response. • These parts are called the EFFECTORS. • Effectors can be a muscle (produces movement) or a gland (releases a chemical).
Stimulus Receptors in sense organ Stimulus changed into a nerve impulse Sensory neurone CNS (contains relay neurones coordinates the best response) Nerve impulse is sent to the effector to create a response Motor neurone Effector
What are nerves? • NEURONES are different to all other cells in the body. • They are stretched out into long, thin NERVE FIBRES that can be over a metre long • Nerve impulses travel along nerve fibres in one direction only. • Sensory neurones take impulses from receptor to the CNS. • Relay neurones are found in the CNS. • Motor neurones take impulses from the CNS to the effector.
Motor Neurone Motor end plate on effectors.
Myelin Sheath • The myelin sheath is an insulating layer surrounding the axon that speeds up the transmission of the electrical impulse. • It has gaps in it called Nodes of Ranvier • The impulse “jumps” from one node of Ranvier to another rather than travelling the whole distance along the axon.
Reflex actions • The most rapid responses in the body are controlled by reflex arcs. • They achieve this by travelling through a very simple arrangement of nerves: One Sensory nerve One Relay nerve One Motor nerve Stimulus Muscle
relay neurone Reflex arc
Nerve impulse transmission • Ready for some serious Biology??! • You are going to learn about how nerve impulses travel down neurones. We will concentrate on non-myelinated nerves as they are slightly easier to understand!
Use this website: • http://outreach.mcb.harvard.edu/animations/actionpotential.swf • And fill in the handouts.
Action Potential • Look at P 109 in your text book and copy the diagram of the action potential (figure 627). • Note: • The resting potential of a neurone is -70mV • During the action potential the membrane potential rises to +50mV
Refractory period • Repolarization is also called the refractory period. • During the first part of the refractory period (the absolute refractory state) it is not possible to have a new impulse. • During the second part (relative refractory state) a stronger stimulus is required to produce a new impulse.
Threshold Potential • The resting potential across a neuron membrane is -70mV • An action potential only occurs if the membrane potential changes (depolarises) to -40mV - -50mV. This is called the threshold potential • This means that impulses can occur without an action potential being stimulated.
All or nothing • This kind of response is called “all or nothing”. • Each neurone is either “on” or “off” – it does not have degrees of excitation.
Direction of the AP movement • The excitation of one part of a neuron triggers the excitation of both neighbouring sections: Oooo! Oooo!
Direction of the AP movement • However, the part of the neuron that has just been stimulated is in the absolute refractory period and the inside is actually more negative than resting potential. Oooo! Absolute refractory! Go away!
Direction of the AP movement • This means it cannot be stimulated above its threshold potential. • The AP travels in one direction only. Oooo!
Synapses • A synapse is where two neurons meet. • The nerve impulse is transmitted across a narrow gap (20nm) by chemicals called neurotransmitters • Watch this animation Synaptic knob Pre-synaptic membrane Post-synaptic membrane
How a synapse works • The AP in the pre-synaptic neuron causes Ca2+ ions to diffuse into the membrane. • Vesicles move to membrane and release neurotransmitters by exocytosis • The neurotransmitter crosses the synaptic cleft and binds to receptors on the post-synaptic membrane
Excitatory synapses • In an excitatory synapse, this causes sodium ion channels to open so sodium can enter the post-synaptic membrane. • The membrane potential therefore becomes more positive. • If this stimulation causes the membrane potential to rise above threshold then a new AP starts in the post-synaptic axon.
Excitatory synapses Sodium (Na+) enters Membrane becomes more positive inside
Inhibitory synapses • In an inhibitory synapse, the neurotransmitter attaching to the receptors causes potassium and chloride ion channels on the post-synaptic membrane to open. • Potassium moves outside the membrane and chlorine moves inside making the inside even more negative compared to the outside.
Inhibitory synapses • This causes the membrane potential to move further away from the threshold so makes an AP less likely to happen in the post-synaptic axon.
Inhibitory synapses Potassium (K+) leaves Chloride (Cl-) enters Membrane becomes more negative inside
Aftermath • After the neurotransmitter has produced its effect it is broken down by enzymes. • The products of this breakdown diffuse back to the synaptic knob and are recycled to make more neurotransmitter.
Neurotransmitters Acetylcholine – very common! Noradrenaline – sympathetic nervous system Dopamine – found in the brain – it is released as a “reward” so give pleasurable feelings Serotonin – found in the brain – controls anger, aggression, appetite, sleep, mood and vomiting…
Synapses and drugs Option E The following section is from Option E – Neurobiology and Behaviour. You will be tested on this section with questions on Paper 3 (options paper).
More about synapses Option E To understand how drugs affect synapses you need to understand more about synapses: More than one pre-synaptic neuron can form a synapse with the same post-synaptic neuron. Some of these pre-synaptic neurons will be excitatory and some will be inhibitory.
EPSP Option E Excitatory = sodium channels open Membrane becomes depolarised – more positive inside. Membrane potential closer to zero AP more likely Excitatory Post Synaptic Potential (EPSP)
IPSP Option E Inhibitory = potassium and chloride channels open Membrane becomes hyperpolarised – more negative inside. Membrane potential further from zero AP less likely Inhibitory Post Synaptic Potential (IPSP)
Summation Option E Whether or not an action potential occurs in the post-synaptic neuron depends on the cumulative action of all the EPSP and IPSP. To make an AP may need more than one EPSP See this animation Choose “Information interpretation at the Synapse”
Summation Option E If the inputs are both EPSP and IPSP then the total stimulation needs to be added together. Try this activity Whether or not the summation produces an action potential depends on the depolarisation of all the EPSPs matched against the hyperpolarasation of any IPSPs.
Effect of psychoactive drugs Option E Psychoactive drugs change the way the the brain works by affecting the neurotransmitters. They can: Release the neurotransmitter Mimic the effect of the NT Reduce or delay re-uptake of NT Affect NT receptors on post synaptic membrane.
Effect of drugs on a synapse Option E Load up this webpage and click on “Repeat intro” at the bottom right hand corner (if you don’t see the intro to begin with) http://outreach.mcb.harvard.edu/animations/synapse.swf Fill in the handouts
Psychoactive drugs Option E Make sure you have detailed notes on how the following drugs affect the brain at the synapse: Excitatory drugs – Nicotine, Cocaine, Amphetamines Inhibitory drugs – Tranquilizers, Alcohol, Cannabis
Homework • Now you have covered this work you need to complete Option E section E.4. • This is homework to be completed by….???
Back to Topic 6!! NB: the notes to copy from the PowerPoint are getting fewer… You will need to listen to what I am saying to add information to your notes (justlike you’ll have to do in University).
Endocrine System A group of glands that make and release hormones Secrete directly into the blood Ductless (unlike exocrine glands e.g. sweat glands) Hormone bonds to receptors on target cells
Homeostasis Maintenance of an internal environment inside the blood and tissue fluid within certain limits Blood pH Carbon Dioxide concentration Water balance Blood glucose concentration Body Temperature
Blood pH pH 7.4 Carbon dioxide dissolves in blood to form carbonic acid – pH falls. Buffers in blood accept or give out H+ ions to keep pH in acceptable limits
Carbon dioxide concentration Special receptors (a type of chemoreceptor) in blood vessels measure the pH of the blood. Too much CO2 = increase ventilation rate
Water balance Water balance in the body is regulated by the kidney. Not enough water in body = concentrated urine (lots of urea – not much water) Too much water in body = dilute urine (lots of water – not much urea) This is closely linked to water lost in sweating.
Negative feedback The action results in a change that will cancel the action. For example – an air conditioning unit…
Negative feedback • Control of process by the outcome. • Requires: • Sensors to detect current situation • Control centre that knows the “normal” value • CC sets a mechanism going to correct any changes from normal • Feedback to CC to prove that factor is now normal • CC switches off correction mechanism.
Thermoregulation • Thermoreceptors in the skin • Heat centre in hypothalamus of brain • Together monitor temperature of surroundings and temperature of blood