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Cutting Off the Blood Supply

Introduction
Popular medical lecture involving a pressure cuff.
Useful information
Kit List: 

Pressure cuff.
Willing committee 'victim'.

Explanation
Explanation: 

Start - Inflate cuff
- Intoduce ourselves
- Explain what we’re doing: cutting of the circulation to the arm in order to see the effects upon the function of different nerves
- Tell the audience that we don’t want to talk at them during this session – we want them to ask lots of questions, and have fun
- Ask for 6 volunteers
1) timer (he/she is in charge of alerting us at 5 minute intervals and not allowing us to keep the cuff on for too long)
2) someone to watch blood pressure cuff – keep it pumped above the pressure
3) Secretary
4) 4 people to test the arm at the different intervals

1) Talk about important safety issues in this experiment:
-Avoidance of movement of the the arm to prevent the build up of metabolites which would be painful (this occurs because we are cutting off the blood supply, creating an anaerobic environment…anaerobic metabolism produces lactic acid).
-Drain the arm before starting the experiment by raising the arm above your head.
-Cuff inflated to above 160 mm Hg in order to fully occlude arteries and veins….
–systolic blood pressure is approximately 120 mm Hg (this is when the heart is contracting/in its pumping phase of the cardiac cycle
-diastolic blood pressue is approximately 80 mm Hg (during diastole the resting/refilling stage)

Only 20% of the 5l blood volume contributes to blood pressure, 80% is required to fill the 50,000 miles of blood vessels

What shall we test over the course of the talk every 5/10mins:
1. Warm (spoon in warm water bath)
2. Cold (spoon in ice bath)
3. Light touch (with cotton ball)
4. Pain (with push pin)
5. Propriception
6. Motor

What is the nervous system?

The nervous system is divided into the central nervous system, essentially the brain and the spinal cord, and the peripheral nervous system, essentially the rest of the nerves in the body. We’re looking into the effects of anoxia (deprivation of oxygen) in the peripheral nervous system.

What is its function?

The nervous sytem’s functon is to collect information about the body’s external condition in relation to the body’s internal condition (sensory part of peripheral nervous system), to analyse this information (central nervous system) and to initiate an appropriate response (motor part of the peripheral nervous system). In essence, to maintain homeostasis.

By cutting off the circulation, we are depriving the nerves of oxygen…why? What is the purpose of circulation?
-What is the purpose of blood? (carries oxygen, immune system components, waste)
-Where exactly is the heart located in the body? (heart is on the slight left side of the chest, left lung is smaller with only 2 lobes)
-How does blood flow through the heart and how does it circulate through the body? (draw diagram of the chambers of the heat, explain how blood gets oxygen from the lungs, and describe difference between arteries/veins)
-Why are veins blue and arteries red? Why don’t we bleed blue blood?
(In humans and other haemoglobin-using creatures, oxygenated blood is a bright red in colour. Deoxygenated blood is a darker shade of red, which can be seen during blood donation and when venous blood samples are taken. However, due to an optical effect caused by the way in which light penetrates through the skin, veins typically appear blue in color.[5] This has led to a common misconception that before venous blood is exposed to air it is blue.)
heart picture

What is it made from?

Nerves which are bundles of neurons.

Draw/Explain the different parts of the nerve:

The cell body contains most of the cytoplasm and much of the metabolic activity, the nucleus and protein synthesis.

The dendrites are short branch extensions spreading out from the cell body. Dendrites receive a stimulus and carry impulses from the environment or from other neurons and carry them toward the cell body. They may receive stimuli from a large number of other neurons, in the cerebellum (Purkinje cells) receive some 250, 000 inputs. In contrast, in the retina, the bipolar cells receive input from just one cone receptor.

The axon is a long fibre that carries the impulse away from the cell body. Each neuron has only one axon. The axon ends in a series of small swellings called axon terminals. At the terminals a signal may then be sent to a target tissue/organ or to another neuron. The impulse may only be conducted in one direction.

Let’s make some predicitions…What do we expect to see?

Motor and discriminative touch disappearing first, followed by temperature and pain

Why? There are different types of nerves that detect touch vs. temp. vs. pain…

Due to the differences in the types of nerve fibres supplying each modality. Large Aβ fibres require more energy to function – as the circulation has been cut off, so has the supply of energy. Motor and touch nerves are large! Therefore they stop functioning first. Temperatue by Aδ. Pain by C fibres.

Why the difference in fibre types?

Fibres that have a larger diameter have a greater condution velocity. This is principally due to decrease in resistance to conduction.

A fibres are also myelinated. Myelin consists of the plasma membranes of Schwann cells and wraps around the axon of the neuron. There are more than 100 layers of Schwann cell membrane in each myelin sheath. There are gaps every 1 to 2 mm called nodes of Ranvier.

These nodes of Ranvier allow saltatory (from the Latin – saltare – to leap) conduction, this basically means the impulse will passed along from node to node, thus it is much quicker than passing along the entire axon. The myelin sheath also acts as insulation against impulse decline.

So, how much faster do you think an nerve fibre insulated with myelin is than a unmyelinated fibre is?

An unmyelinated fibre has a conduction velocity of ~0.5m/sec
A myelinated fibre has a conduction velocity of ~ 50m/sec

Talk about giant squid:
Invertebrates haven’t evolved a myelin sheath, yet some large species do exist such as the giant squid. It still contains a nervous system, however for this to be effective it has large diameter fibres of ~500um compared to the ~10um in man.

Why do we have these different speeds of transmission…why not keep all of the nerves the same?!

It depends upon the modality that is transmitted by the fibre and the need for it to be transmitted rapidly. Motor needs to be fast as a response to movement needs to be almost instant. Also think of touching something hot or sharp, there is a need for there to be a quick response to avoid (further) damage. However there may be other modalities that don’t require such velocities such as certain types of pain, as the stimulus will persist for a sustained period of time.

Think about this: What happens when you touch a hot stove? (You hand jerks back before you feel the pain…this is because the reflex signal that tells your muscles to move is fast, and travels just to your spinal cord and back…on the other hand, the pain signal travels through slower nerves all the way to your brain.

What happens if it all goes wrong?

Sometimes the myelin sheath can deteriorate. In multiple scelerosis, demyelination occurs in the central nervous system. Some people with MS experience muscle weakness, poor balance, poor coordination, muscle tightness or spasticity, tremors, or paralysis. As the insulation is lost, sometimes the impulse may not arrive in the axon terminal and the signal will be lost.

If there is extra time – teach the kids how to take blood pressure?
*Ask for a few kids to volunteer (one will take the other’s blood pressure) – if more time, we can let other kids try…
1) Find brachial artery
2) Place cuff on upper arm, with arrow aligning with brachial artery
3) Inflate cuff well above systolic pressure
-At this point you can explain the cardiac cycle:
-atrial systole, ventricular systole, diastole
-So must inflate cuff above systolic pressure to occlude blood flow both during systole and diastole. (remember, this is why we inflated the cuff above systolic pressure during our cutting off circulation to the arm experiment)
4) Slowly start reducing pressure, as the pressure drops, you will start to hear beating (take note of the pressure that this starts at as this is the systolic pressure)
– why do you hear the beating? When the pressure is above diastole, but below systole, you will hear spurts of blood going through the artery only during systole.
5) As the pressure drops further, you will stop hearing the beating
– the pressure at which this occurs is the diastolic pressure
– and this low pressure, the blood continuously flows through the artery since it isn’t occluded at all

Risk Assessment
Date risk assesment last checked: 
Tue, 05/01/2016
Risk assesment checked by: 
Charlotte Attwood
Date risk assesment double checked: 
Thu, 14/01/2016
Risk assesment double-checked by: 
Jess Gorman
Risk Assessment: 
DESCRIPTION An interactive talk in which a demonstrator cuts off the blood supply to his/her arm, and observes changes in nerve function in the arm over a short period of time.
RISKS
  • Cutting off the blood supply to the arm for greater than 20 minutes, may cause damage to the demonstrators arm
  • Cutting off the blood supply to the arm may be uncomfortable for the demonstrator
  • Push-pins are used to test the ability to feel pain during the experiments - risk of causing demonstrator to bleed.
  • Minor risk of fainting in some individuals.
  • ACTION TO BE TAKEN TO MINIMISE RISKS
  • Do not cut off blood supply for longer than 20 minutes. Both demonstrators should keep track of time during the experiment.
  • To minimize discomfort, demonstrator should avoid movement of arm to prevent the build-up of metabolites which would be painful. Demonstrator should drain arm before starting the experiment by raising arm above head. Cuff must be inflated at all times above 160 mmHg to fully occlude the arteris and veins.
  • Only demonstrator should be allowed to test the pain by poking gently with the push pin, children from the audience are not allowed to use the push pins.
  • The subject should remain seated for the duration of the occlusion.
  • ACTION TO BE TAKEN IN THE EVENT OF AN ACCIDENT
  • Call first aider in case of injury
  • Should a demonstrator bleed from pricks with the pin, normal first aid precautions should be followed with any blood contaminated objects.
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