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

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

CNS(contains relay neurones

coordinates the best response)

Stimulus changed into a nerve

impulse

Sensory neurone

Effector

Motor neurone

Nerve impulse is sent to the

effector to create a response

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 end plate on effectors.

Motor Neurone

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:

StimulusStimulus OneOneSensorSensor

y y nervenerve

OneOneRelay Relay nervenerve

OneOneMotor Motor nervenerve

MuscleMuscle

Reflex arc

relay neurone

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.

Absolute refractory!

Oooo!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

Chloride (Cl-) enters

Membrane becomes more negative inside

Potassium (K+) leaves

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

• The following section is from Option E – Neurobiology and Behaviour.

• You will be tested on this section with questions on Paper 3 (options paper).

Option E

More about synapses

• 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.

Option E

EPSP

• Excitatory = sodium channels open

• Membrane becomes depolarised – more positive inside.

• Membrane potential closer to zero

• AP more likely

• Excitatory Post Synaptic Potential (EPSP)

Option E

IPSP

• 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)

Option E

Summation

• 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”

Option E

Summation

• 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.

Option E

Effect of psychoactive drugs

• 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.

Option E

Effect of drugs on a synapse

• 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

Option E

Psychoactive drugs

• 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

Option E

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 (just

like 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

The endocrine system

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

Cooling down

• Vasodilation

If the temperature rises, the blood vessel dilates (gets bigger).

This means more heat is lost from the surface of the skin

Cooling down

• Vasodilation

• Sweating

Sweating

The skin

Cooling down

• Vasodilation

• Sweating

• Decreased metabolism – less heat given off as a by product

Cooling down

• Vasodilation

• Sweating

• Decreased metabolism – less heat given off as a by product

• Behaviour adaptations

Heating up

• Vasoconstriction

If the temperature falls, the blood vessel constricts (gets shut off).

This means less heat is lost from the surface of the skin

Heating up

• Vasoconstriction

• Shivering – muscle contraction produces heat

• Increased metabolism – releases heat as by product (e.g. in liver)

• Piloerection of hair or feathers

Heating up

• Vasoconstriction

• Shivering – muscle contraction produces heat

• Increased metabolism – releases heat as by product (e.g. in liver)

• Piloerection of hair or feathers

• Fat or blubber layer

• Special hair e.g. polar bears

Task

• Construct a negative feedback flowchart for thermoregulation

Regulation of Blood Glucose

• Controlled by pancreas

• The pancreas is on the left of the body below the stomach and close to the duodenum

The Pancreas

Anatomy of Pancreas

• The Pancreas has two main jobs:– to secrete digestive

enzymes (exocrine)– To secrete hormones

e.g. insulin and glucagon (endocrine)

• Different cells do these different jobs

Exocrine cells

Endocrine cells

The Endocrine Pancreas

• Hormones are made in the islets (or islands) of Langerhans.

• Chemoreceptors in the islets detect levels of glucose in the blood

The Endocrine Pancreas

• Each islet is made up of different cells. Each one synthesises and secretes a different hormone– Alpha cells secrete

Glucagon – Beta cells secrete

Insulin – (Delta cells secrete

somatostatin, another hormone)

Destinations of Glucose

Glucose

Respiration Storage

Absorbed from the blood by cells as

GLUCOSE

Stored in the liver as GLYCOGEN

Blood Glucose levels decrease

• E.g. Due to exercise• α (Alpha) cells secrete Glucagon

• Target cells: in liver (Hepatocytes)

• Response: Turn glycogen into glucose

• Target cells: in liver (Hepatocytes)

• Response: Convert amino acid into glucose

•Effect: Blood glucose levels rise to normal

Blood glucose levels increase

• Target cells: in liver (Hepatocytes)

• Response: Turn glucose into glycogen and use more glucose for respiration

• Target cells: Muscles

• Response: More glucose is absorbed

•E.g. Due to eating•β (Beta) cells secrete Insulin (easy to remember as you are doing the Iβ course!!)

•Effect: levels decrease to normal

• Target cells: Adipose (fat) tissue

• Response: Glucose is turned into fat

Task

• Construct a negative feedback flowchart for blood glucose control

Diabetes

• Diabetes is a condition where either:

– Insulin is not produced by the β-cells in the islets of Langerhans (Type I)

– The body does not react to the presence of insulin. (Type II)

Diabetes

• This results in hyperglycaemia which can cause:– Nerve damage– Retina damage– Damage to blood vessels– Renal (kidney) failure– Death

• No cure but can be treated/controlled

Type I Diabetes

• Also known as “Early onset diabetesEarly onset diabetes” or “Insulin dependant diabetesInsulin dependant diabetes”

• Insulin is not produced by the β-cells in the islets of Langerhans

• Treated by injecting insulin

• FACT: Insulin is a protein

• QUESTION: Why don’t diabetics take insulin tablets?

Type II Diabetes

• Also known as “Late onset diabetesLate onset diabetes” or “Non-Insulin dependant diabetesNon-Insulin dependant diabetes”

• Caused by a reduction in the insulin produced and insensitivity of target cells to insulin.

• Factors that lead to development:– Obesity– Age– Family history (Genetic factor?)

Type II Diabetes

• Treated by:– Controlled diet– Regular exercise– Weight loss– Medication

Further reading

• Read the article from Biology Course Companion P 25.

• Discuss your answers to the questions in groups.

Hand outs

relay neurone

Motor end plate on effectors.

Motor Neurone

What is happening in this picture? What does it show?

Why are these ions important?

What do these channels do? Write notes on their importance.

What is the sodium/potassium pump doing? Explain how it works including what “drives” it and what is meant by the “resting potential” (wait until after the activity for this!)

What is an action potential? What is the first step in this? How does it happen (try the activity!)?

(NB There is a mistake on the animation. When you get the activity correct it should say “Correct! When the potassium channel…”)

What happens during repolarization? Try the activity. Which protein maintains the resting potential?

Write a summary of what happens at each stage of the action potential to create the graphs.

Resting potential:

Depolarization:

Repolarization:

Return to resting potential:

Absolute refractory!

Oooo!

An action potential only goes in one direction because…

AP in pre-synaptic axon causes _______

________________________________

Vesicles _________________________

________________________________

The neurotransmitter_______________

________________________________

In excitatory synapses this

causes ______________

_____________________

_____________________

_____________________

In inhibitory synapses this

causes ______________

_____________________

_____________________

_____________________

Summarise how a synapse works using this diagram.

1. An action potential in the pre synaptic membrane…

2. Some neurotransmitters…

3. Despite the effect of any inhibitors, eventually the ions accumulate in the post synaptic neurone and…

Summarise how a synapse works using this diagram.

1. An action potential in the pre synaptic membrane…

2. Some neurotransmitters…

3. Despite the effect of any inhibitors, eventually the ions accumulate in the post synaptic neurone and…

Excitatory drugs: NicotineNicotine stimulates receptors that would normally bind to acetylcholine and cause an increase in adrenaline (raising heart rate, blood pressure and sugar levels). What other effects does this website says happen? How does this correlate with addiction to smoking?

Nicotine stimulates receptors that would normally bind to acetylcholine and cause an increase in adrenaline (raising heart rate, blood pressure and sugar levels). What other effects does this website says happen? How does this correlate with addiction to smoking?

Excitatory drugs: CocaineHow does cocaine work at a synaptic level? Explain in detail. Use P 287 in your book to add any extra information.

How does cocaine work at a synaptic level? Explain in detail. Use P 287 in your book to add any extra information.

Excitatory drugs: Amphetamines

How do amphetamines (such as Speed) work at a synaptic level? Use P 287 in your book to explain in detail.How do amphetamines (such as Speed) work at a synaptic level? Use P 287 in your book to explain in detail.

Inhibitory drugs: TranquilizersHow do tranquilizer drugs (Benzodiazepines) work at a synaptic level? Use P 287 in your book to explain in detail.How do tranquilizer drugs (Benzodiazepines) work at a synaptic level? Use P 287 in your book to explain in detail.

Inhibitory drugs: AlcoholHow does alcohol work at a synaptic level? Explain in detail. Use P 287 in your book to add any extra information.

How does alcohol work at a synaptic level? Explain in detail. Use P 287 in your book to add any extra information.

Inhibitory drugs: CannabisHow does Tetrahydrocannabinol (the drug found in cannabis) work at a synaptic level? Use P 287 in your book to explain in detail.How does Tetrahydrocannabinol (the drug found in cannabis) work at a synaptic level? Use P 287 in your book to explain in detail.

The endocrine system