NERVOUS SYSTEM - LECTURE 2 Flashcards
Basics of the spinal cord and spinal column
The spinal chord (which includes all of the nervous system under the brain) is a complex structure that needs to be surrounded by flexible material, because it needs to be over a metre long but still flexible so we can move our body and function properly. With this said, the spinal chord still needs to be anchored in place for structure
The spinal column includes all of the bones and nervous system material that sits below the brain
The occipital bone (part of the skull) sits on top of the spinal column
The spinal chord arises from the medulla, and neurons come off from the spinal chord on either side of the body
The spinal chord is arguably more complex than the brain - so it is important to take time to figure out how it functions
What are the different regions of the spinal column
From top to bottom:
- Cervical (8 nerves)
- Thoracic (12 nerves)
- Lumbar (5)
- Sacral (5)
- Coccyx (1 nerve)
Describe the cervical vertebral levels
There are 8 paired cervical vertebral levels (where one level of the spinal cord and vertebrae is)
There are 7 cervical bones
There are eight vertebral levels because there are 8 nerves -the first nerve comes out between the occipital bone of the skull and the first cervical vertebrae (all of the others sit underneath the cervical bones)
‘Cervix’ refers to how there is narrowing from the skulls/nervous system above before the shoulders
At around nerve 6-7, there is a cervical enlargement. This is important because there is a large volume of neurons coming from the periohery/hands and arms so must be accommodated for
What is the cervical enlargement
At around cervical nerve 6-7, there is a cervical enlargement. This is important because there is a large volume of neurons coming from the periohery/hands and arms - large surface area - so must be accommodated for
Describe the thoracic vertebral levels
Inferior to the cervical vertebrae , there are 12 paired thoracic vertebral levels - there are twelve thoracic bones and twelve thoracic nerves (one nerve for each rib)
There is no particular enlargement because most information is coming back from sensory perspective from our trunk
Describe the Lumbar vertebral levels
More inferiroly to the thoracic vertebrae, there are 5 paired lumbar vertebral levels
This includes 5 lumbar vertebrae and 5 lumbar nerves
There is an enlargement called the thoracolumbar enlargement at about the T-11/12 to L1 nerves, due to neurons/information coming in from the legs
At about Lumbar level 1 (where the nerve coming out), there is termination of the spinal cord
Where does the spinal cord end
At about lumbar (nerve) level 1
From this point on, instead of a solid spinal chord, there is a structure made from individual nerves called the cauda equina
Where does the spinal cord end
At about lumbar (nerve) level 1
From this point on, instead of a solid spinal chord, there is a structure made from individual nerves called the cauda equina
What is the cauda equina
After termination of the spinal cord, instead of a solid spinal chord, there is a structure made from individual nerves called the cauda equina that goes down (rather than a large neural structure)
In the cauda equina, individual fibres go down and exit through the various foramen (openings) of the spinal chord
How is the spinal cord anchored
The spinal cord is anchored by the film terminale, which is a bundle of spinal cord coverings that becomes a theread and anchors the spinal cord in the coccyx (it means that when we bend over our vertebrae don’t flex - as we can’t bend over that much)
Describe the sacral nerves
Below the lumbar nerves are 5 sacral nerves (even though the sacurm is a fused group of bones, there are 5 outlet for the nerves)
Describe the coccygeal nerves
Furthermore, there is the coccyx - really just an extension of the sacrum, which has one coccygeal nerve
How many paired nerves are there
31 (paired as they go to the left and right)
Describe the spinal segmental level
Taking a slice through the spinal cord transversely that shows on level of the spinal cord
What nerves/nerve fibres come in and go out from the spinal cord
Sensory, motor, and autonomic nerves/fibres (those that control visceral organs) are located within bundles of neurons called the spinal nerve that come out form the spinal cord
Where do the motor vs sensory nerves arise from or leave through
Regarding the sensory nerves, they enter the spinal cord via the dorsal root , and then fan out into very small things celled (dorsal) rootlets, that are dorsal to the spinal chord. As such, these rootlets are called dorsal rootlets
Regarding the motor nerves, they come out of the spinal cord via the ventral roots from ventral rootlets, which is a fan like structure of nerve fibres.
What surrounds the spinal cord
Surrounding the spinal chord there are multiple distinct layers, all of which are there to protect the brain
On the outside, is the dura mater (note that mater means mother - in this case protection) - which is very tough and can barely be penetrated by a scalpel, it is designed to protect the spinal chord
Underneath/inside the dura mater, there is arachnoid mater - is a shimmery structure like a ‘spiders web on a dewy morning’, also for protection
Pia mater - a soft layer, that in reality is indistinguishable with the neural structure (you couldnt get a scalpel and divide the two as they are tightly adhered to the outside of the spinal cord
Describe receptors
There are a series of receptors (a way of housing the end of a neuron) within the body - there are 12-13 different types of them - and they act different when a force is acted on it (based on the structure, e.g. certain receptors measure tension changes in muscles, and some will detect change of pressure based on clothes, some receptors are activated consistently, and some only for change).
Meissner’s corpuscle - Touch receptors
Paccinian corpuscle - Pressure receptors
What are two key receptors
Meissner’s corpuscle - Touch receptors
Paccinian corpuscle - Pressure receptors
Describe how discriminaotry information reaches the spinal cord
Just beneath our skin we have these receptors, and in the middle of them is a neuron, which will send its axon back to the spinal cord via the dorsal root.
Within the dorsal root, there is an area called the dorsal root ganglia, which is where the cell body of the neuron is housed.
The neuron axon carries on past the ganglion and goes into one of two regions - one into the dorsal funiculus (big bundle of white matter, which goes straight up the spinal cord to the brain brain stem without synapsing) or into a particular area in the centre/grey matter of the spinal cord called the nucleus propius (where some discriminative sensation neurons synapse with a single neuron, which will join the other neuron pathway in the dorsal funiculus to the brain - this concept is called convergence, which allows all the information to go to the right location but for the spinal cord to not be too big), where the neuron then ascends towards the brain
Describe descriminitive neurons
They are said to be encapsulated by the receptor and hence encapsualted neurons
The neurons are surrounded by myelin - a fatty substance that helps the efficiency of the neurons that allows them to fire frequently, more specifically at about 50m/s (very fast)
This is because you need to understand what is happening in the periphery quickly
We refer to these neurons as pseudounipolar neurons
Some of the neurons can be more than a metre long, but some are shorter
Importantly, different neurons go to the two different areas, and there are many neurons coming from the skin and going to both areas
Describe how non-discriminative information enters the spinal cord
These neurons (free nerve endings), enter the spinal cord via the same dorsal root, however, they go to a region of the grey matter of the spinal cord called the substancia gelatenosa (the anterior section)
The spinal cord can use the pain and temperature information and undergo reflexes
Describe the non-discriminitive neurons
The neuron body is also in the dorsal root ganglion
Importantly, they are unencapsulated - they have a low threshold for firing if something painful is going on
Furthermore, they are unmyelinated, therefore is slower - conduct at roughly 1m/s
They activate a response at both the spinal cord and brain level
For these reasons, they aren’t made to be fired all the time and we don’t need to fire them as much
Describe a dermatome
A dermatome is a map related to sensory information and tells us roughly where in the body each of the nerves that go back to the spinal cord come from (e.g. if you stroke the thumb the nerve is going back in cervicallevel 6, middle finger is c7, and little finger is c8)
Therefore we know roughly speaking where the neurons are localised within the spinal cord that have particular functions
This is useful if someone is experiencing issues in certain areas, and dermatomes are the bases of neurology and understanding the nervous system involves using these maps
