The Brain Part II

In the first part of my post on the brain, I discussed the anatomy and introduced some of the early ideas that gave the brain its special status in vertebrates, in particular in man. The question I want to address in this post is how does the brain interpret signals and generate sensations such as the sound of music, the sweet taste of sugar or the detailed, moving images on a sports field, for example. [Did you know it took over 60 years to finally produce a moving image after the first still photograph was taken (1826-1888]. I want to consider these phenomena in the context of evolution, since Biologists often use the device of investigating complex structures like the human brain in simpler organisms, in order to work out "the rules". For those interested I am currently challenging my new Biochemistry undergraduate students with a justification for the use of  simple, "model organisms" in Molecular Life Science research here. 

We know that the brain interprets inputs from our senses. But we also know intuitively that the smell of fresh coffee is something quite different than the feeling we get when we see a full moon, or interestingly how some people turn up the volume when they hear Hey Jude by the Beatles, while others turn it down (that's mostly what the family tell me to do when I am listening to Bob Dylan: there's no accounting for taste!). We can clearly appreciate that individuals react differently to smell, taste, touch etc., so the brain has three roles to play here. The first is to detect the input, the second is to interpret that input and finally we respond to the interpreted smell, sound etc. In Biology (and many areas of Science and Health) there is another important phenomenon to consider here: "threshold values": this is easy to understand. Simply put if something is too bright, or too loud, or too hot to touch; we try and avoid it; because we have "crossed a threshold". All of these subtle behaviours have their roots in physiology and chemistry. That's why I love Biochemistry so much!

 In order to understand these phenomena we need to know how the brain connects to the rest of our body, not just physically: yes it is on the top of our head (but is it always at the front end of an organism?). The most important connection are the blood flow (for nutrition, waste disposal and inputs) and nerve connections (inputs and outputs: chemicals, electrons and protons). Let's look at blood flow first. The human brain sits in our skull largely "fed" through blood vessels entering from underneath. The first thing that I should say here is the vocabulary of physiologists and anatomists is a bit of a nightmare! (I can already hear Biologists say what about Physics and Chemistry?). All scientific subjects have their own language historically derived from Latin and Greek (usually), often with the unhelpful interjection of a few modern "bits", to try and make the terms "systematic". You only have to look at today's new drug names to appreciate this:  Abagovomab or Ciprofloxacin. 

Back to blood vessels. The brain is one of the most highly "serviced" organs in the body. There are two pairs of arteries: the right and left internal carotid and the right and left vertebral arteries (these are the "eyes" and the "legs" respectively, of the alien-like figure that is shown above (LHS)! The carotids supply blood to the Cerebrum (above your eyes), while the Vertebral supply feeds the Cerebellum and the Brain Stem (the back of your neck). Interestingly the supplies are connected to form the Circle of Willis, which ensures a backup supply in the event of a blockage. This is a good example of how evolution "protects" key functions: it is sometimes referred to as functional redundancy. You will have noticed that most transatlantic aircraft have 4 engines. They only need 3, but in case of a malfunction, they carry a spare: a little like a spare wheel on long journeys, except that in the case of the Circle of Willis and the jet engines, they are in use all of the time.The details of the branching of the arteries differs between mammals, and some species like sheep and cows have a set of vessels that lie on the top of the brain a little like a hair net. This allows for better temperature regulation, which we presumably don't need? Here is a section of text that captures the challenge (in red) of the nomenclature and "language" of anatomists (and a diagram to help!)
There are several important structural and functional differences between pial arteries on the surface of the brain and smaller parenchymal arterioles. First, pial arteries receive perivascular innervation from the peripheral nervous system also known as “extrinsic” innervation, whereas parenchymal arterioles are “intrinsically” innervated from within the brain neuropil (see Perivascular Innervation). While parenchymal arterioles have only one layer of circumferentially oriented smooth muscle, they possess greater basal tone and are unresponsive to at least some neurotransmitters that can have large effects on upstream vessels (e.g., serotonin, norepinephrine). Lastly, pial vessel architecture forms an effective collateral network such that occlusion of one vessel does not appreciably decrease cerebral blood flow. 

So let's try and navigate our way through this challenging language and extract the key points, since it gets worse when we come to the nervous system, next! To summarise the circulatory systems: the brain is supplied with blood by two pairs of major arteries which are on a linked, circular network (like the Circle Line on the London Underground). This ensures that even in the event of a blockage, the brain is still able to function.

Let's now look at the nerve fibres, and we begin with a very nice schematic diagram from wikipedia, setting out the incoming (afferrent) and outgoing (efferrent) nerve types. Signals come from peripheral (on the outskirts) receptors (molecules that act like eyes and ears) and are processed by the brain. In return, signals are sent from the brain to the tissues (efferent). If you haven't come across these words before (and why should you!), whenever there is an "e" at the front it means moving away (escape, exit): an "a" at the front often means moving to, something, such as arrive or against (not perfect perhaps, but it might help remember which is which in an exam). 

 

There are a large number of nerves, all with challenging names (the superior cervical ganglia, the sphenopalatine, otic, or trigeminal ganglion), but here we will look at a couple of examples that explain how they function, without being put of by the nomenclature. Consider the eyes and ears. Both organs (like many others in mammals) are duplicated. The ear is superbly adapted to capturing sound and then "dividing" it up into its different components. You must have seen a record producer building up a sound-track by fading in different instruments and voices, well consider the ear as a receiver of sound waves that are "mixed" to produce a perfectly balanced sound. Ears are designed to function in air, so when you go underwater the sound waves are now "changed" or modulated by the water and so the ear doesn't work properly. 

You can read more about the eyes and the ears on the internet by simply searching on Google for "ear (or eye) biology", as you can see from the diagram, the ear is a complicated receiver and transducer of sound waves. The details are fascinating, but again you can find this out for yourselves. What I want you to  notice is the tube labelled "auditory nerve". This is the connection from the ear to the brain and is agreat example of how the brain works in receiving signals. The sound waves (such as the sound of a police siren, or some music) are captured and processed (this is what we often call transducing a signal). The brain then makes sense of the signal. Just think of the brain compared with a computer. You can input information into a computer (or your mobile 'phone) via the keyboard (touch), by visual inputs (camera), speaking (audio). The computer uses different file types to make sense (transduce) these inputs. So too does the brain. It has different types of receptors, some for sound, some for sight etc. However, just like the computer, it uses electronics to send the signals around the body to the brain. 

The same situation applies to the visual organs: otherwise known as the eyes. As you can see form the diagram (RHS), the eye is another complex receiver, this time incorporating a bio-lens, which acts just like a convex lens that you might have come across in Physics. However, unlike the beautifully polished lenses that we associate with Isaac Newton, the human lens is more like a modern contact lens. It is flexible, slimy and it is connected to the roof and foot of your eye through a set of small muscles (called ciliary muscles) that allow for precision focusing. Sometimes, the lens and the muscles don't function correctly, and we need to add an artificial, extra lens in the form of specs or contact lenses. There are some other features of the eye worth mentioning here. First the retina is a complex array of cells that capture light of different intensities (you may want to look up rods and cones, otherwise known as photoreceptors). The blood vessels that feed the eye, float in a bath of fluid called vitreous humour. Remember from your Classics lessons, in vitro, in glass? That makes sense, but sense of humour? Well humour is an old Latin word that means liquid or mositure and the ancient scholars believed that mood was determined by the balance of different fluids in the body, bile, phlegm etc. Hence the expression developed that relates humour to mood. I must also mention that if you are lucky enough to own an iPad, you will perhaps know they have a retinal recognition feature. Like the grooves in your fingerprints, the pattern of blood capillaries in your eye is unique. By shining a beam of infra-red light (similar to your TV remote) at your eye, an image is returned (a reflection) minus the energy taken up by your blood vessels. This digital image is unique and can be stored in your device to make sure you alone can access the device.

Finally then just what are nerve cells, or neurons? In simple terms, blood vessels are like the plumbing in a house, while the neurons are like your electrical supply. In fact, they are possibly better described as the cable network that services your TV or your internet. A neuron looks like a tadpole: it has a head (the cell body) and a tail (the axon), and at the end of the tail, the insulating cable is open and the conducting fibres, just like copper wire, fray outward. These terminals connect to the target tissue (the ear or the eye, or your muscles, for example) and the signal can be transmitted. The transmission of nerve impulses, are best thought of as the bleeping electrical traces on the cardiac monitors, so well used on TV programmes like Casualty. Without getting into too much detail, this is where Chemistry meets Physics and gives birth to Biology (and not forgetting the mathematical equations that describe the events). The movement of ions like sodium, potassium and chloride, through membrane associated pores (called ion channels) transport the electrical charges along the axons and at the tips of the neurons, which we call synapses: two neurons communicate through the action of neurotransmitter chemicals such as Acetylcholine in conjunction with the enzyme acetylcholinesterase (neurotransmitters need to reach thresholds and then need to be removed by the enzyme to keep the system dynamic). 

I think we made it! The brain is able to coordinate our thoughts and actions through a communication network of neurons, supplied with the nutrients that we extract from our food, through a separate network of blood vessels. The signals, sound touch, light etc that are received by our organs of perception are processed into a set of specific "file types" and we are able to act by running, relaxing, jumping for joy etc. all though this amazing system in our heads. What I haven't told you is just how much we know (or perhaps how little) about the way all of this works. As Professor Richard Feynman, the great Physicist said: If I understand it, I can create it. I think we are some way off this prospect! However, I hope I have simplified some of the mysteries of the Brain and I hope some of you will want to become Neuroscientists, something I would like to do, if I only I had the Brains!