The spleen's jobs

The spleen is an odd-shaped organ that lies hidden away behind our ribs, posteriorly, on our left sides. It's functions are threefold:

1. Red blood cell 'quality control' - the arterioles of the spleen mostly empty into a series of splenic cords. These lack an endothelial lining, and consist chiefly of a loose array of reticulin (a type of collagen) lined by macrophages and fibroblasts. To get back into the circulation, hopeful blood cells must squeeze themselves into the splenic sinuses that all ultimately empty into the splenic vein (and hence the general circulation). Old and damaged red blood cells aren't able to do this, and so they are doomed to remain in the cords where they are phagocytosed by the macrophages.
2. Immunity - the spleen is in many ways analogous to a a lymph node, in that blood-borne pathogens are amply exposed to macrophages and dendritic cells, which in turn present them to T and B cell aggregations within the spleen. The spleen seems particularly good at preventing infections by encapsulated organisms such as Streptococcus pneumoniae, Haemophilus influenzae and Neiserria meningitidis.
3. Extramedullary haemopoiesis - the spleen usually stops making our blood cells after about 7 months of gestation, but it has the ability to be reactivated if our bone marrows can't produce enough blood cells for whatever reason. (e.g. myelofibrosis, where the marrow is obliterated.)

These three functions are helpful to keep in mind when it comes to remembering the causes for splenomegaly. We'll cover that in the next post.

How many brain cells are there in the adult brain?

Get ready to put your little finger to the corner of your mouth; the answer is:

One hundred billion (100 000 000 000)

That's a lot of zeros, and the number is so big that it requires some sort of analogy to help place it in context. I like this one, from Daniel Levitin's book, "This is your Brain on Music":

Suppose each neuron was one dollar, and you stood on a street corner trying to give dollars away to people as they passed by, as fast as you could hand them out - let's say one dollar per second. If you did this twenty-four hours a day, 365 days a year, without stopping, and if you had started on the day that Jesus was born, you would by the present day only have gone through about two thirds of your money.

The American spelling of the scientific word for brain cells is "neuron", whereas the British (and most of the rest of the English speaking world) spell the word with an extra 'e': "neurone". Oh, and Levitin's book? Buy it. Now.

What is a "paradoxical embolus"?

An embolus is a detached intravascular mass of some kind that is carried by the blood to some distal point, where it lodges. Over 99% of emboli come from thrombi, although rare causes include fat emboli (usually from broken bones), amniotic fluid emboli and air emboli.

When a deep vein thrombosis (DVT) forms, the major risk is that a piece of it breaks off and lodges somewhere in the pulmonary circulation. The pulmonary capillaries are so small that even red blood cells can't make it through with out outrageous contortions, and so for all intents and purposes, dangerous clots can go no further. Any emboli from a DVT will cause pulmonary problems, but they can't, for instance, lodge in the systemic arterial system.

Except that, very rarely, they do. But how? Such "paradoxical emboli" arise when a great coincidence occurs: the patient has a DVT and the patient has some cardiac lesion whereby the right side communicates with the left side. Usually, this is an atrial septal defect. In such a case, it is possible for a DVT's embolus to bypass the lung's capillary filters altogether and skip straight to the left heart. From there, it can go on its merry way and block off a part of the systemic circulation. How rude!

Which cranial nerves are bilaterally innervated?

As I'm sure you're aware, spinal (as opposed to cranial) nerves are contralaterally innervated. This means that a lesion affecting the right side of the motor cortex will affect the left side of the body.

With cranial nerves things are different; those of the cranial nerves that have a motor function tend to be bilaterally innervated. In other words, the right accessory nerve (XI) receives input from both the right and the left motor cortex. The same goes for the left accessory nerve.

This is a handy advantage in strokes, since upper motor neurone lesions must therefore be bilateral in order to cause a deficit.

The one major exception to this rule is the facial nerve (VII). Only the forehead muscles are bilaterally innervated, so even a unilateral upper motor neurone lesion can cause mouth drooping, etc. However, this can still often be differentiated from a lower motor neurone lesion, which will not spare the forehead. This confusing bit of neuroanatomy is nicely illustrated below.

(Illustration by Patrick J. Lynch, medical illustrator)

Why do we see better at night with our peripheral vision?

It all comes down to anatomy.

Daylight is a period in which there is an abundance of photons flying around. In this time of plenty, our eyes make use of a high-demand, high-resolution colour system to provide us with images. Basically, at the "centre" of our eyes (actually, not the centre, but where the photons from something in the centre of our visual fields fall) there is an area called the fovea centralis. At this location, the retina is packed with cones - colour sensitive cells - that are so densely packaged as to provide outstanding resolution to an image. The downside is that they require large amounts of photons (of the particular wavelength) hitting them to fire.

Further out to the periphery, our retinas are manned by rods. These cells aim to scavenge any photons (regardless of their wavelengths); as such, they can collectively form an image from much less light than the cones need to fire. Unfortunately, they are not densely packed in the retina, and so give a relatively low quality image. Furthermore, they are not nearly so wave-length specific, and so only provide black-and-white images.

At night, photons are scarce. Therefore, our vision has to switch from the high-demand, high-resolution colour system of the cones to the low-demand, low-resolution black-and-white system of the rods. Since there are virtually no rods in the fovea (at the centre of our visual fields), objects are best appreciated at night by our peripheral vision, which has the rods. Astronomers are keenly aware of this fact, and usually advise you to look slightly to the left or right of a star cluster in order to get the best image of it.

Incidentally, this bit of anatomy also explains why objects at night appear black-and-white. (Amazingly, many people are unaware of this occurrence!) Furthermore, rods are so sensitive to any light that they are temporarily "blown" out of action by daylight conditions. This is why it takes time for our eyes to adjust to the dark conditions of a cinema if we walk in from the daylight outside.

Fascinating, don't you think?

In which order to the various layers of the heart go?

From outside to inside, the layers are:

• Parietal pericardium
• Pericardial cavity
• Visceral pericardium (also known as the epicardium)
• Myocardium
• Endocardium

Actually, this question can almost be worked out in its entirety by knowing a (very!) little of the Classical Greek and Latin from which the terms are derived.

• Cardium is latinised form of the Greek 'kardion', meaning heart.


• Peri- refers to 'around'. Thus, the layer around (i.e. outside) the heart would be the pericardium.


• Myo- denotes muscle, also from the Greek root. Thus, the heart muscle is the myocardium.


• Endo- comes from the Greek word for 'within'. In other words, the layer on the inside of the heart is the endocardium.


• Lastly, there's the issue of 'parietal' vs 'visceral'. This is a little more obscure, but 'visceral' refers to the inner organs of the chest and abdomen, whereas 'parietal' refers to a wall. Thus, the visceral pericardium hugs the heart (one of the viscera), whereas the parietal pericardium is nearer to the walls of the chest cavity, and is even fused to one of them - the diaphragm.

This may all seem a little silly, but knowing a few choice prefixes and suffixes (as opposed to studying the original languages, which would be crazy) really makes learning biology and medicine a lot easier.

Do men have fewer ribs than women?

NO!!

Both men and women have twelve ribs on each side, making 24 in total.

The ribs, and the musculature in between them, are an ingenious compromise between making the 'shield' for the heart and lungs as strong as possible, and still allowing the chest to expand when we breathe.

You can see in the diagram that the first 10 ribs are connected via a cartilage strip to the sternum* (breast bone), whilst the last 2 aren't (they go by the evocative term 'floating ribs'). Your 12th (last) rib actually ends quite a lot more laterally than you'd imagine. You can feel it by running your hand across the bottom of the ribs, starting from the centre. Suddenly, at your flanks, you feel an 'extra' lump = the 12th rib.

* Confusingly, the 10th rib's cartilage isn't properly connected in this picture. This can occur, but it's the exception, not the rule. Unfortunately, it was still the best picture of the anatomy I could find...

What is the structure of a neuron(e)?

Before tackling the question of a neurone's anatomy, we should ask ourselves just what it is that neurones need to do. Neurones are the basic structural and functional unit of the amazingly complicated system of circuits that is our brain. As such, like a computer's transistors, it must receive information, decide whether or not to fire (integrate that information) and then, if certain conditions have been met, it must be able to fire. It really is helpful to think of them as, fundamentally, biological transistors, because then their structure suddenly seems to make sense.

In this part, we'll tackle the anatomy of a typical neurone. Bearing the above in mind, it should come as no surprise that neurones have an input side. This is basically a tree of fine wires that connect to all the other neurones capable of sending our neurone a message. These wires are called dendrites, and there can be thousands of them. They usually receive information from other neurones, but sensory neurones receive information from the environment (e.g. did a photon just hit this bit of my retina?).

Then there's the cell body, which is boring from out present perspective. It's got the nucleus and all the basic stuff that any cell needs to survive for any decent length of time. (Remember, unlike transistors, neurones do still have to be alive to carry out their function. Their method of working is more complicated than a transistor, and requires continual production of, for example, neurotransmitters, to function.)

Sticking out in the opposite direction to the dendrites is cell's output side - the axon, which the neurone uses to send messages to other neurones. Each neurone usually has one axon, but near its end, the axon often divides into several branches so that it can communicate with numerous nerves. Each communication area is called an axon terminal, or terminal bouton. Basically, it all looks something like this:



So, one neurone's axon would communicate with another neurone's dendrites, and so on, making complicated circuits. If this were the whole story, though, then all neurones would be capable of it acting like wires - passively passing a signal on to the next bit. This is obviously not the case; wires don't make intelligence possible on their own. What is needed is for the neurone to make decisions, however trivial they may seem by our standards. In other words, the neurone must be able to take all the various inputs to its dendrites, and decide whether or not to fire.

More on this later, though. In the following post, we'll go into how nerves integrate their signals and fire, and how the firing is capable of communicating with other neurones.

What does a lesion of the oculomotor nerve look like?

The oculomotor nerve is the third cranial nerve (III).

As a MOTOR nerve, it supplies:

• Most of the extraocular muscles: inferior oblique, and the medial, inferior and superior rectus muscles.
• Around 2/3 of the muscle that lifts your upper eyelid: levator palpebrae.

It is also joined by PARASYMPATHETIC fibres with run along it for a while. The parasympathetic fibres, via the ciliary ganglion, control the smooth muscle within the eye. They constrict the pupil (pupillary reflex, due to bright light) and increase the convexity of the lens (accomodation reflex, when focusing on a nearby object).

The following is typical of people with a III nerve lesion: on the affected side, the eye deviates laterally, the pupil is dilated, and there is ptosis (drooping eyelid).

• The lateral deviation is due to unopposed action of the lateral rectus, which is supplied by a different cranial nerve
• The pupil dilatation: due to unopposed sympathetic supply to the pupil
• The ptosis is due to loss of the III nerve's share of the motor fibres to the eyelid levator muscle.

Note that the parasympathetic fibres run along the outside of the oculomotor nerve. Therefore, they will definitely be affected in the case of, say, a tumour that compresses the nerve as a whole. However, the middle of the nerve is farthest from the blood supply to the nerve (which also runs on the outside), and so in diseases affecting the blood vessels, like diabetes or hypertension, it is possible to affect just the oculomotor nerve, and not the parasympathetic supply that runs with it. The latter is right beside the blood supply, so it tends to get spared. If this occurs, the pupils will be normal but the rest still applies (and you will be given an important clue as to the diagnosis!).

Obviously, any testing of the ocular movments (e.g. getting them to follow an object with their eyes) will also be markedly off, as all extraocular muscles apart from lateral rectus and superior oblique will be affected by a III nerve lesion.

(The above image is from http://sprojects.mmi.mcgill.ca/braintumor/section1/subsection7/default.htm)

Are the extremities part of the appendicular skeleton?

The skeleton can be subdivided into the "axial" and "appendicular" skeletons.

The axial skeleton is composed of the bones that make up the central axis of the body - including the skull bones, the vertebrae and ribs.

The appendicular skeleton is composed of the bones making up the 'appendages' (hence the name). In humans, this basically entails the bones of the arms and legs.

The picture on the right comes from this website: http://www.besthealth.com/besthealth/bod

And for a full list of the bones in the appendicular skeleton, you can go here: http://www.botany.uwc.ac.za/sci_ed/grade10/manphys/appendicular.htmHope that helps!