NERVOUS SYSTEM - LECTURE 3 Flashcards
Describe basics of the brain developments:
One of the challenges with the development of the brain is that it changes very rapidly during development
At bout week 4 the three vescicle form is apparent
Then there is a five vesicle stage (at week 4-5)
Then there is development of the forebrain and development of the subcortical structures until we get to the full brain
(note that week 4 is when it really starts to develop)
Describe the basic structure of the spinal cord at a segmental level
There is butterfly shaped matter within the middle of the spinal chord that is grey matter (cell bodies)
Everything else is white matter - the fibres/connections between cells
What are the four distint areas within the white matter of the spinal cord
Dorsal funiculus (includes the gracile and cuneate fasciculus), the lateral funiculus (or motor/lateral columns), and the ventral funiculus
There is also the anterior/ventral white commisure
There are a lot of other white matter areas which are also important
Describe the dorsal funiculus
It is a structure/bundle of white matter in the dorsal side of the spinal chord that has the primary purpose of carrying discriminative sensation information.
It can be broken down into two different regions - which have a humunculur map of the body:
Gracile fasciculus - a fasciculus (bundle of white matter) that is the more medial section of the dorsal funiculus. This fasciculus sends discriminatory sensation information from the legs/feet to the brain
Cuneate fasciculus - a fasciculus (bundle of white matter) that is the more lateral section of the dorsal funiculus. This fasciculus sends discriminatory sensation information from the trunk/torso, arms and legs to the brain.
Regarding both of these regions, nerve fibers will go to the corresponding area of the dorsal funiculus where they will head to the brain directly
As mentioned in previous lectures, there will also be other fibers that will come in and enter the nucleus proprias in the grey matter of the spinal chord, synapse to another nerve, and this nerve will go back to the dorsal funiculus to join the other nerve, going up to the brain
Describe the motor/lateral columns/funiculus
A lateral funiculus that caries (motor) information from the brain down into the spinal chord and information can be dispersed to the periphery (where something else happens like muscle movement)
Describe the ventral funiculus
A ventral funiculus that functions to carry pain and temperature sensory information in the spinal chord to the brain
Note that the information comes in and a nerve synapses in the substancia gelantinosa, but then the next nerve crosses the anterior/ventral white fissure and enters the ventral funiculus
Describe the anterior/ventral white commisure
This is an anteior bundle of white matter that carries information across the sides of the spinal cord - nerve fibres form the substancia gelatanosa cross through it to get to the ventral funiclus on the other side
Describe what are the key regions in the grey matter of the spinal cord
Within the grey matter of the spinal chord, there is a laminar structure, and each of the laminae are specialised, do different jobs, and have different cells in them
The lammallae include the substancia jelatanosa, the nucleus proprius, the intemrediate area, and the low motor neuron pool
Describe the substantia Gelatanosa
the posterior section of the grey matter, there is a see-through substance (jelly like substance). This is where the free nerve fibres synapse
Describe the nucleus proprius
An area ventral to the substantia jelatanose that is where discriminative sensation/proprioseptive information enters the spinal cord
Describe the intermediate area
The part of the grey matter in the spinal chord that is most central, and controls visceral organs and automated movements like posture (therefore don’t have to think of it)
Describe the lower motor neuron pool
It is a ventral region of large neurons that is communicated to by the motor neurons within the motor columns coming from the brain, and then the low motor neurons send a fibre out towards muscle so that it can be activated
Describe what happens to non-discriminative information when it enters the spinal cord
Free nerve fibres enter the dorsal root and enter the substantia gelantinose (the dorsal part of the spinal cord). These fibres are for pain and temperature.
Importantly, they have opioid receptors on them - this is why we treat pain with opioids, as it blocks the activity of these fibres and as such, they do not fire (very important for everyday medicine)
In the substancia gelatanosa, the neuron will synapse, and the next neuron will go around to the opposite side of the spinal chord, through a structure called the anterior/ventral white commisure (commisure meaning a bundle of white matter that carries information from left to right), and into the ventral funiculus
Importantly this means that unlike for the discriminaotry sensory information, where the information only crosses over at the medulla, the pain and temperature information changes side immediately - most things in the spinal chord will not do this, and they will only cross over at the brainstem rather than the individual spinal segmental level
Describe the anterior/ventral white commisure
The anterior/ventral white commisure is an area of the spinal chord where the
Describe the dorsal column-medial lemniscal system
It is the pathway of discriminative sensation from the periphery towards the brain
Describe what occurs in the dorsal column
The dorsal column refers to the fibres that go from the spinal cord to the gracile and cuneate nuclei.
Starting with an encapsulated sensory nerve ending (e.g. from the right arm), the fibre will go to the dorsal root ganglion where the neuron cell body will be housed
It will then go up into the spinal cord, more specifically the dorsal funiculus (in the gracile or cuneat funiculus)
It will then go up all the way until it reached the brains stem (medulla)
In the brainstem, which is where the dorsal funiculus is split into nuclei - the gracile and cuneate nuclus, the nerve fibre will synapse with another cell body, and this neuron fibre will cross over to the other side of the brain stem
The place where this crossover happens is called the internal arcuate fibres
Before synapsing, this neuron is an example fo a neuron called the first order neuron - it is the very first neuron coming from the periphery
Describe the medial lemniscus pathway
The medial lemniscus refers to the fibres that go from the medulla to the somatosensory cortex (lemniscus is another term for white matter)
Discriminative senosry fibres from the dorsal column will the continue up through the pons and midbrain to a particular area of the thalamus (don’t need to know atm)
There they will synapse with other neurons, and then these fibres will pass through a structure of white matter called the internal capsule, and then will continue up and enter the primary sensory cortex (post central gyrus), where there is a humuncular representation
The purpose of having a synapse in the thalamus is critically important - the thalamus is the gateway of the brain. This is because the thalamus decides what the cortex should pay attention to. This why when you feel two different levels of pain, you will focus on the one that is more significant (this is dealt with and priorities in the thalamus
What is the purpose of synapsing sensory information (at least discriminitive) in the thalamus
The purpose of having a synapse in the thalamus is critically important - the thalamus is the gateway of the brain. This is because the thalamus decides what the cortex should pay attention to. This why when you feel two different levels of pain, you will focus on the one that is more significant (this is dealt with and priorities in the thalamus
Describe the spinothalamic tract
Information from free nerve endings (e.g. in the hand) - regarding pain the temperature - will have its cell body in the dorsal root ganglion, synapse in the substancia gelatanosa, will decasate (cross sides) in the anterior white commissure, and then it will turn its direction up into what we call the ‘(lateral) spinothlamic tract’
It will go up into the medulla and past the pons, and then it will go into the thalamus, where it will synapse with another neuron, go out into the internal capsule, and up into the humuncular region corrsepodning in the sensory cortex
Describe brown sequired syndrome
Brown sequired syndrome - where you injure one side of your spinal chord (outside doesn’t look to bad but inside is affected)
When this happens, there can be different affects depending on where the lesion/problem occurs
Describe the two types of sensory loss
With brown-sequired syndrome, if we get a lesion in the pons and end up damaging both the pain and temperature and discriminatory information pathways on the right side (ie for the left side of our body), we would lose sensation for both for our left side of our body - although the right side would be fine for both
This is called an associative sensory loss (both lost on the same side)
On the other hand, if we were to have a lesion on the left side before the brain stem, and the pain and temperature information for the left side had already crossed, we would lose the discirminative information senses for the left side (which would not have crossed yet) but not the pain and temperature information for the left side (as on the opposite side).
However, we would have not lost the discriminative information for the right side (as it would still be on the right), but we would have lost the pain and temperature information on the right.
This is called a dissociative sensory loss (loss of opposite sides)
Describe associative sensory loss
When discriminative and non-discriminative sensation is lost on the same side
Describe dissociative senosry loss
When discriminative and non-discriminative sensation is lost on the opposite sides
Describe monosynaptic reflexes
Some encapsulated fibres will go through the dorsal root and go straight to the lower motor neuron pool, where they will synapse (with a lower motor neuron), and move off to a muscle
This creates a reflex arch, which means that the brain doesn’t know what is happening at the periphery, everything is happening at the spinal segmental level.
This particular reflex is called the monosynpatic/myotactic reflex (reflex = something happens automatically, monosynaptic because there is only one synapse)
This is why when you walk you do not need to think about shortening and flexing muscles or why when you tap the knee and it jerks. It allows for fast muscle movements/
Describe polysynpatic reflex
We also have fibers that synapse from the free nerve endings in the substantia gelatanosa (the pain and temp neurons synapse with them) and will send a fibre to the lower motor neurons as well, synapse, and go towards and activate muscle and bring about withdrawal when we experience pain
This is referred to as the polysynaptic/withdrawal reflex (polysynaptic as two synapses)
