Anatomy and pathology of the nervous system is understood by directly visualizing it. This is best accomplished by handling the brain (or model of the brain as the case may be) and dissecting or taking it apart for direct examination. The purpose (for the clinician) of understanding neuroanatomy and neurophysiology is to be able to use that knowledge to solve clinical problems. The first step in solving a clinical problem is anatomical localization. So, if one cannot directly inspect the patient's brain, how is this localization accomplished? The "window" to the patient's brain is the neurological examination. The neuro exam is a series of tests and observations that reflects the function of various parts of the brain. If the exam is approached in a systematic and logical fashion that is organized in terms of anatomical levels and systems then the clinician is lead to the anatomical location of the patient's problem.
So you're sitting in a bus when you see a baby smile sunnily and gurgle at his mother. Your automatic response? You smile too. You're jogging in the park, when you see a guy trip over his shoelaces and fall while running. Your knee jerk reaction? You wince. Even though you're completely fine and unscathed yourself. Or, to give a more dramatic example; you're watching Titanic for the umpteenth time and as you witness Jack and Rose's final moments together, you automatically reach for a tissue and wipe your tears in whole hearted sympathy ( and maybe blow your nose loudly, if you're an unattractive crier like yours truly).
And here the question arises- why? Why do we experience the above mentioned responses to situations that have nothing to do with us directly? As mere passive observers, what makes us respond at gut level to someone else's happiness or pain, delight or excitement, disgust or fear? In other words, where is this instinctive response to other people's feelings and actions that we call empathy coming from?
Science believes it may have discovered the answer- mirror neurons.
In the early 1990s, a group of scientists (I won't bore you with the details of who, when and where) were performing experiments on a bunch of macaque monkeys, using electrodes attached to their brains. Quite by accident, it was discovered that when the monkey saw a scientist holding up a peanut, it fired off the same motor neurons in its brain that would fire when the monkey held up a peanut itself. And that wasn't all. Interestingly, they also found that these motor neurons were very specific in their actions. A mirror neuron that fired when the monkey grasped a peanut would also fire only when the experimenter grasped a peanut, while a neuron that fired when the monkey put a peanut in its mouth would also fire only when the experimenter put a peanut in his own mouth. These motor neurons came to be dubbed as 'mirror neurons'.
It was a small leap from monkeys to humans. And with the discovery of a similar, if not identical mirror neuron system in humans, the studies, hypotheses and theories continue to build. The strange thing is that mirror neurons seem specially designed to respond to actions with clear goals- whether these actions reach us through sight, sound, smell etc, it doesn't matter. A quick example- the same mirror neurons will fire when we hop on one leg, see someone hopping, hear someone hopping or hear or read the word 'hop'. But they will NOT respond to meaningless gestures, random or pointless sounds etc. Instead they may well be understanding the intentions behind the related action. This has led to a very important hypothesis- the 'action understanding' ability of mirror neurons.
Before the discovery of mirror neurons, scientists believed our ability to understand each other, to interpret and respond to another's feeling or actions was the result of a logical thought process and deduction. However, if this 'action understanding' hypothesis is proved right, then it would mean that we respond to each other by feeling, instead of thinking. For instance, if someone smiles at you, it automatically fires up your mirror neurons for smiling. They 'understand the action' and induce the same sensation within you that is associated with smiling. You don't have to think about what the other person intends by this gesture. Your smile flows thoughtlessly and effortlessly in return.
Which brings us to yet another important curve- if mirror neurons are helping us to decode facial expressions and actions, then it stands to reason that those gifted people who are better at such complex social interpretations must be having a more active mirror neuron system.(Imagine your mom's strained smile coupled with the glint in her eye after you've just thrown a temper tantrum in front of a roomful of people...it promises dire retribution my friends. Trust me.)
Then does this mean that people suffering from disorders such as autism (where social interactions are difficult) have a dysfunctional or less than perfect mirror neuron system in some way?
Some scientists believe it to be so. They call it the 'broken mirror hypothesis', where they claim that malfunctioning mirror neurons may be responsible for an autistic individual's inability to understand the intention behind other people's gestures or expressions. Such people may be able to correctly identify an emotion on someone's face, but they wouldn't understand it's significance. From observing other people, they don't know what it feels like to be sad, angry, surprised or scared.
However, the jury is still out on this one folks. The broken mirror hypothesis has been questioned by others who are still skeptical about the very existence of these wonder neurons, or just how it is that these neurons alone suffered such a developmental hit when the rest of the autistic brain is working just dandy? Other scientists argue that while mirror neurons may help your brain to understand a concept, they may not necessarily ENCODE that concept. For instance, babies understand the meaning behind many actions without having the motor ability to perform them. If this is true, then an autistic person's mirror neurons are perfectly fine...they were just never responsible for his lack of empathy in the first place.
Slightly confused? Curious to find out more about these wunderkinds of the human brain? Join the club. Whether you're an passionate believer in these little fellas with their seemingly magical properties or still skeptical, let me add to your growing interest with one parting shot- since imitation appears to be the primary function of mirror neurons, they might well be partly responsible for our cultural evolution! How, you ask? Well, since culture is passed down from one generation to another through sharing, observation followed by imitation, these neurons are at the forefront of our lifelong learning from those around us. Research has found that mirror neurons kick in at birth, with infants just a few minutes old sticking their tongues out at adults doing the same thing.
So do these mirror neurons embody our humanity? Are they responsible for our ability to put ourselves in another person's shoes, to empathize and communicate our fellow human beings? That has yet to be determined. But after decades of research, one thing is for sure-these strange cells haven't yet ceased to amaze and we definitely haven't seen the last of them. To quote Alice in Wonderland, the tale keeps getting "curiouser and curiouser"!
Examination of the cranial nerves allows one to "view" the brainstem all the way from its rostral to caudal extent. The brainstem can be divided into three levels, the midbrain, the pons and the medulla. The cranial nerves for each of these are: 2 for the midbrain (CN 3 & 4), 4 for the pons (CN 5-8), and 4 for the medulla (CN 9-12).
It is important to remember that cranial nerves never cross (except for one exception, the 4th CN) and clinical findings are always on the same side as the cranial nerve involved.
Cranial nerve findings when combined with long tract findings (corticospinal and somatosensory) are powerful for localizing lesions in the brainstem.
Cranial Nerve 1
Olfaction is the only sensory modality with direct access to cerebral cortex without going through the thalamus. The olfactory tracts project mainly to the uncus of the temporal lobes.
Cranial Nerve 2
This cranial nerve has important localizing value because of its "x" axis course from the eye to the occipital cortex. The pattern of a visual field deficit indicates whether an anatomical lesion is pre- or postchiasmal, optic tract, optic radiation or calcarine cortex.
Cranial Nerve 3 and 4
These cranial nerves give us a view of the midbrain. The 3rd nerve in particular can give important anatomical localization because it exits the midbrain just medial to the cerebral peduncle. The 3rd nerve controls eye adduction (medial rectus), elevation (superior rectus), depression (inferior rectus), elevation of the eyelid (levator palpebrae superioris), and parasympathetics for the pupil.
The 4th CN supplies the superior oblique muscle, which is important to looking down and in (towards the midline).
Cranial nerves 5, 6, 7, and 8 are located in the pons and give us a view of this level of the brainstem.
Cranial Nerve 6
This cranial nerve innervates the lateral rectus for eye abduction. Remember that cranial nerves 3, 4 and 6 must work in concert for conjugate eye movements; if they don't then diplopia (double vision) results.
The medial longitudinal fasciculus (MLF) connects the 6th nerve nucleus to the 3rd nerve nucleus for conjugate movement.
Major Oculomotor Gaze Systems
Eye movements are controlled by 4 major oculomotor gaze systems, which are tested for on the neurological exam. They are briefly outlined here:
Saccadic (frontal gaze center to PPRF (paramedian pontine reticular formation) for rapid eye movements to bring new objects being viewed on to the fovea.
Smooth Pursuit (parietal-occipital gaze center via cerebellar and vestibular pathways) for eye movements to keep a moving image centered on the fovea.
Vestibulo-ocular (vestibular input) keeps image steady on fovea during head movements.
Vergence (optic pathways to oculomotor nuclei) to keep image on fovea predominantly when the viewed object is moved near (near triad- convergence, accommodation and pupillary constriction)
Cranial Nerve 5
The entry zone for this cranial nerve is at the mid pons with the motor and main sensory (discriminatory touch) nucleus located at the same level. The axons for the descending tract of the 5th nerve (pain and temperature) descend to the level of the upper cervical spinal cord before they synapse with neurons of the nucleus of the descending tract of the 5th nerve. Second order neurons then cross over and ascend to the VPM of the thalamus.
Cranial Nerve 7
This cranial nerve has a motor component for muscles of facial expression (and, don't forget, the strapedius muscle which is important for the acoustic reflex), parasympathetics for tear and salivary glands, and sensory for taste (anterior two-thirds of the tongue).
Central (upper motor neuron-UMN) versus Peripheral (lower motor neuron-LMN) 7th nerve weakness- with a peripheral 7th nerve lesion all of the muscles ipsilateral to the affected nerve will be weak whereas with a "central 7th ", only the muscles of the lower half of the face contralateral to the lesion will be weak because the portion of the 7th nerve nucleus that supplies the upper face receives bilateral corticobulbar (UMN) input.
Cranial Nerve 8
This nerve is a sensory nerve with two divisions- acoustic and vestibular. The acoustic division is tested by checking auditory acuity and with the Rinne and Weber tests.
The vestibular division of this nerve is important for balance. Clinically it be tested with the oculocephalic reflex (Doll's eye maneuver) and oculovestibular reflex (ice water calorics).
Cranial nerves 9,10,11, and 12 are located in the medulla and have localizing value for lesions in this most caudal part of the brainstem.
Cranial nerves 9 and 10
These two nerves are clinically lumped together. Motor wise, they innervate pharyngeal and laryngeal muscles. Their sensory component is sensation for the pharynx and taste for the posterior one-third of the tongue.
Cranial Nerve 11
This nerve is a motor nerve for the sternocleidomastoid and trapezius muscles. The UMN control for the sternocleidomastoid (SCM) is an exception to the rule of the ipsilateral cerebral hemisphere controls the movement of the contralateral side of the body. Because of the crossing then recrossing of the corticobulbar tracts at the high cervical level, the ipsilateral cerebral hemisphere controls the ipsilateral SCM muscle. This makes sense as far as coordinating head movement with body movement if you think about it (remember that the SCM turns the head to the opposite side). So if I want to work with the left side of my body I would want to turn my head to the left so the right SCM would be activated.
Cranial Nerve 12
The last of the cranial nerves, CN 12 supplies motor innervation for the tongue.
A 6th nerve palsy may be a "false localizing sign". The reason for this is that it has the longest intracranial route of the cranial nerves, therefore it is the most susceptible to pressure that can occur with any cause of increased intracranial pressure.
There are loads of survival guides out there to help medical students adapt well to university life but which ones should you be taking notice of? I’ve put together a list of my top 6 must reads - I hope you find them useful.
1. BMJ’s Guide for Tomorrow's Doctors
If you don’t read anything else, read this. It covers everything from the pros and cons of using the library to essential medical websites (check out number 6 on the list :D).
2. Money Matters
Ok, this isn’t the most exciting topic but definitely a stress you could do without. The Money Saving Expert gives some great advice on how to make money and manage your finances.
This guide includes 4 simple but essential study tips relevant throughout your years at university.
4. Dos and Don’ts
Some great advice from Dundee University here on the dos and don’ts of surviving medical school.
5. Advice to Junior Doctors
Karin shares some of her hospital experiences and gives advice to junior doctors.
6. Looking after yourself
To get the most out of university it’s important that you look after yourself. The NHS provide some great tips from eating healthily on a budget to managing stress during exam time.
If you know of any other useful survival guides or would like to create your own please send them across to me email@example.com.
The management of pain is a key element of the care of all patients-- ICU or not. Often providers have little understanding of the concepts and medications of pain management. This episode serves as an introduction.