Topic 6.5 Nerves, Hormones and Homeostasis Topic 6: Human Health and Physiology.

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

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!

• 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!

• This means it cannot be stimulated above its threshold potential.

• The AP travels in one direction only.

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

• 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

• 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

• Shivering – muscle contraction produces heat

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

• Piloerection of hair or feathers

Heating up

• 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

• 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

• Response: Convert amino acid into glucose

•Effect: Blood glucose levels rise to normal

Blood glucose levels increase

• 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

• 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

Topic 6.5 Nerves, Hormones and Homeostasis Topic 6: Human Health and Physiology.

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