Exam 3 - Spinal Cord Pathways Flashcards
Which spinal pathways take up more space in the spinal column? Why?
The ascending/afferent/sensory pathways
There is lots of sensory information that is being relayed from around the body
What is a tract?
A bundle of axons in the spinal cord
In the PNS they are called nerves
What are the two motor/efferent/descending pathways? What information does each send?
- Pyramidal tracts (corticospinal) - majority of voluntary motor movement
- Extrapyramidal tracts - usually involuntary, outside of pyramidal system and coordinate complex tasks
What pathway occupies the dorsal column of the spinal cord? What information is sent here? Where is it going?
- Dorsal column medial lemniscus - transmits pressure and sensory information from sensors in our skin to the parietal lobe
Where are pain signals sent from the spinal cord?
Anterolateral System - contains an anterior and lateral spinothalmic tract
Why are the pain tracts called spinothalamic?
These pain signals are routed to the spine and then up to the thalamus for processing.
What are Rexed’s laminae?
They are designated sections of the gray matter in the spinal cord where different information is processed. Numbered I - X.
Where is lamina I? What is it’s name? What information arrives here? How is it sent?
- The most dorsal horn of the gray matter
- Lamina Marginalis
- Fast/sharp pain
- Sent by A delta fibers and crosses over to the lateral spinothalamic pathway
What is the name for laminae II and III? What information arrives here? How is it sent?
- Substantia Gelatinosa
- Slow pain
- C fibers (non-myelinated) - moves over and ascends via anterior spinothalamic pathway
These sometimes also have a connection with lamina V
What laminae do mechanoreceptors send information to?
- Laminae I through VI
What lamina acts as a cross over point for signals? What white matter does the same?
- Lamina X
- Anterior White Commissure
Which lamina contatins lots of large motor neurons?
Lamina VIII
What fibers are in the dorsal column lemniscus pathway (DCML)? What information is sent through these? Where is the cross over point for this pathway?
- A fibers - alpha, beta, gamma, delta
- Sensory information, vibrations, pressure
- Cross over at lower medulla
What two pathways can touch/sensation information be sent when it arrives to the cord?
- Move into the dorsal horn of the gray matter (stays there)
- Jump over to the dorsal column and ascend to the brain
What is the trend in size of the dorsal column? Why?
- Dorsal column is larger and increases laterally ascending the cord
- Ascending the cord adds more and more sensory information from our upper body and limbs
What part of the dorsal column recieves sensory information from the legs?
Fasiculus Gracilis
How to remember: Gracilis is leg muscle
What part of the dorsal column recieves sensory information from the upper extremities?
Fasiculus Cuneatus
Describe the pathway that sensory information is sent from the lower extremities to the brain?
- Information enters the dorsal root
- Ascends the fasiculus gracilis of the dorsal column
- Crosses over at the lower medulla oblongata
- Passes through the medial lemniscus of the pons
- Passes through ventrobasal complex of thalamus
- Cross through internal capsule
- Arrives at parietal lobe of cerebral cortex
Where in the parietal lobe does sensory information from around the body arrive? How does a stroke effect these regions?
Postcentral gyrus
* Lower extremites at the top and then pathways arrive further back as you move up the body.
* The region of the parietal lobe where the stroke occurs determines what region is effected
What is the homunculus? How does amount of surface area that a region occupies relate to sensation?
- A visual description of where sensory information is processed in the parietal lobe.
- The more sensory receptors in that area, the more space they occupy in the brain
Where body regions have high and low density of sensory receptors?
- Face, hands, and legs have high density of sensors
- Trunk has low density of sensors
What is the major motor output pathway of our corticospinal tracts? Describe the pathway.
Primary Pathway (80 % of our motor function)
*Cortex to internal capsule to pyramids of medulla
*Signal crosses over at the pyramidal decussation
*Descends the latereal corticospinal tracts
*Arrives at cell bodies in the gray matter
Where are the pyramids found? What happens here?
- Found in the anterior medulla
- Motor information descends here
- Cross over happens at the pyramidal decussation for the lateral corticospinal tract
Describe the appearance of the pyramids and pyramidal decussation?
- Pyramids are ridges on anterior medulla
- Pyramidal decussation can be visualized as neurons are crossing over between pyramids
What tract does the secondary corticospinal tract utilize? Describe the realtionship of the area of cord it covers compared to the primary tract.
- The anterior corticospinal tract
- This area is much smaller because not as much information is being sent through them (17 % of motor function)
Where does cross over happen in the secondary corticospinal tract?
Crosses over at the level of the cord where the information needs to be communicated.
What pathway is the smallest portion of motor output? What is special about how these are sent?
2 % of motor function and information does not cross over at any point.
What type of nerves send fast pain information? What is the fiber type? What spinal tract do these signals use?
- Noiceceptors (free nerve endings)
- A delta
- The lateral spinothalamic tract
What is the neurotransmitter used in the fast pain signal pathway? What is important to know about this?
- Glutamate
- It is excitatory
- It causes very fast transmisson of signals between neurons
What path does fast pain signals travel to the brain?
Fast pain is sent to the parietal lobe in a parallel system to the DCML pathway
-after entering lamina I and cross over at the AWC, the signal travels via lateral spinothalamic tract, ascends to thalamus, ventrobasal complex, and to distinct regions of the parietal lobe
What is the difference in pain localization between fast and slow pain?
Fast pain relays detailed location of where pain is coming from
Slow pain has much poorer localization of pain
Where does both fast and slow pain signals cross over in the cord before arriving at the anterolateral tract?
The anterior white commissure
What is the alternate name for the fast pain pathway?
Neospinothalamic tract
Neo = new
What is the alternate name for the slow pain pathway?
Paleospinothalamic tract
Paleo = old
What fibers transmit slow pain?
C fibers
What neurotransmitters are used for slow pain? What pathway do these signals use?
- Substance P (primary)
- CGRP (calcitonin gene related peptide)
- Glutamate - acts slower in this pathway
Anterior spinothalamic tract
What specific types of information do slow pain fibers sense?
Thermoreceptors and vibration
Why do slow pain signals have poor localization?
Because most of the signals do not go up to the parietal lobe, they terminate in the brainstem or thalamus
Which pain pathway involves our emotional centers? How?
- Slow pain pathway
- Our emotional centers are near the thalamus and brainstem where slow pain signals terminate
What is the portion of the brain stem where a lot of slow pain signals terminate?
Reticular formation
What tract helps use with eye fixation and balance?
Vestibulospinal extrapyramidal tract
What type of tract is the olivospinal tract?
Extrapyramidal
What tract is responsible for maintain musle tone?
Reticulospinal extrapyramidal tract
We always have a baseline of tone in our muscles even at rest
What extrapyramidal tract coordinates voluntary movement?
Rubrospinal extrapyramidal tract
Where is the primary/1st order descending neuron located in the desending inhibitory complex (DIC)? Where are these nuclei located anatomically?
In the periventricular nuclei (anterior to third ventricle) or periaqueductal gray (midbrain)
What neurotransmitter does the 1st order descending neuron release in the DIC? Where does this happen? Is this excitatory or inhibitory?
Enkephalin
Excitatory
Raphe magnus nucleus (pons)
Where does the 2nd order descending neuron in the DIC originate and end? What neurotransmitter does it release?
Originates a RMN and terminates in dorsal horn of spinal cord
Releases serotonin (5-HT)
Where is the 3rd order neuron located in the DIC? What neurotransmitter does it release? Excitatory or inhibitory?
- Cell body is in white matter outside of dorsal horn of spinal cord with axon reaching into gray matter between 1st and 2nd order ascending pain pathways (lamina I,III, or III)
- Enkephalin
- Inhibitory
What type of compound is enkephalin?
Endogenous morphine analog
Our pain receptors are enkephalin receptors
What neurons does enkephalin in the 3rd order descending neuron affect?
Both the presynaptic 1st order pain neuron (noiceceptor) and the postsynaptic 2nd order neuron.
They both have enkephalin receptors on them at the synapse.
What are the different things that noiceceptors sense as pain? There were 8 things mentioned in lecture.
- Physical damage - cuts, crushing
- Acid (H+) - lactic acid from exercise
- K+ - can depolarize cells which leads to pain signal
- Usually acidity and hyperkalemia occur together
- Histamine - swelling and inflammation
- Serotonin
- Acetylcholine
- Prostaglandins - augment pain sensation, don’t generate AP themselves
- Bradykinin
“Please Always Allow High School Kids Peanut Butter”
What treatments are there for pain that work on the 2nd order descending neuron in the DIC?
- SSRI - increases amount of serotonin in synapse = more activation of 3rd order enkephalin neuron (paxil, prozac)
- TCA - also has serotonin reuptake inhibition
What side effect of TCA is desirable for people with chronic pain?
Drowsiness - can improve sleep for people with chronic pain
Describe lateral inhibition in the spinal cord?
DCML sensory neurons are parallel to noiceceptors. The DCML signals can either ascend the dorsal column or stay in the gray matter of the dorsal horn. The pressure signals in the dorsal horn can inhibit the pain signals in the 2nd order ascending neurons in the gray matter as well as in the periphery before the cord. This inhibition is probably mediated by neurotransmitters.
Ex: putting pressure on an injury can help alleviate the sensation of pain, accupuncture
How is glutamate released from a neuron?
Ca++ influx into the neuron
What is the primary receptor of glutamate in the pain transmission system? How does it work?
- AMPA receptor
- Ionotropic receptor that opens when bound to glutamate and allows Na+ ions in
Describe the NDMA receptor in the pain transmission system?
- Iontropic to glutamate
- Primarily allow Ca++ in, some Na+ in
- Slower than the AMPA receptor because must have glutamate binding as well as depolarization to move the intracellular Mg++ that is blocking the inner pore
Why does Mg++ block the NDMA receptor?
The area near the cell wall is negative and attracts the positively charged Mg++
Depolarization (positive charge) pushes the Mg++ away
How do we obtain NDMA receptors?
We are not born with many, they populate over time with growth
What can block the NDMA receptors?
- Ethanol
- Lead poisoning
- Ketamine
- Nitrous
- Tramadol - very weak narcotic (not a good drug), has some serotonin reuptake inhibition also
ELK NT
How are the pain receptors in the synapse effected by chronic pain?
Chronic pain typically causes more AMPA and NDMA receptors to be populated on the postsynaptic cell wall leading increased pain sensitivity
What is the pain receptor that mediates GABA?
Kainate receptor
What is the difference in myelination in the ascending neurons between 2nd order slow and fast signals?
- Fast - always myelinated
- Slow - may or may not be myelinated
Describe the structure and function of enkephalin receptors (opiod receptors)?
- They are GPCRs that are connected to K+ channels
- Activation causes loss of K+ and hyperpolarizes the neurons (inhibition of pain signals)
- These receptors are on both the 1st order and 2nd order ascending pain neurons
Describe the structure and funtion of alpha 2 receptors? What are some examples of drugs that act here?
- GPCR that also causes loss of potassium that leads to cell hyperpolarization
- This occurs on both 1st and 2nd order ascending pain neurons
- Alpha 2 agonists include: xylazine, clonidine, and precedex
- These drugs lead to relaxation and suppression of the CNS without the “high” of opiods
How do volatile anesthetics affect CNS activity?
They cause a generalized increase in K+ conductance leading to loss of conciousness and pain inhibition
Describe COX-2’s role in pain sensation in the neuron?
COX-2 is expressed more with increased synaptic activity (more pain). This causes an increase in PG receptor expression which increases the sensitivity to pain in both pre and postsynaptic neurons.
COX-2 inhibitors block this pathway
Describe what iNOS is and how it affects pain in the neuron?
- iNOS = inducable nitric oxide synthase
- Increases sensitivity to pain in the synapse by increase prescence of NO
How can magnesium supplements aid in pain control? What is a side effect?
- Can increase the blockage of NMDA-R intracellularly
- Not absorbed well in the GI tract and in large doses can cause GI upset
List the 4 spinal reflex pathways
- Stretch
- Tendon
- Withdrawl
- Crossed Extensor
What three things can activate a spinal reflex?
- Pain sensor
- Tendon stretch sensor
- Muscle spindle
What type of neuron is used to bridge between neurons on the same side of the cord, as well as to illicit cross communication from one side to the other?
Interneurons
What is the goal of the stretch reflex?
To keep our posture constant
Keep our muscles at a constant length
What does the stretch reflex do?
It causes reflex activation (contraction) of the muscle group being stretched, as well as reflexive relaxation of the antagonistic muscles
Describe the stretch reflex pathway through the spinal cord?
- There is a complete pathway from muscle spindle to motor neuron to cause contraction
- There is one interneuron that causes inhibition (relaxation) of the antagonistic muscles
- This only utilizes one side of the cord
How do test for the stretch reflex?
By striking the ligament under the patella which causes lengthening of the muscle
What is the goal of the tendon reflex?
To prevent pulling of the muscles away from the bone when under heavy loads
What and where are the sensors for the tendon reflex?
Golgi tendon sensors in the muscle tendons
Describe what happens when the tendon reflex is illicited?
- Under heavy loads the reflex will cause relaxation of the muscle to prevent tearing
- It will also cause reflexive contraction of the antagonistic muscle group.
There are ways around this (heroic acts)
Describe the tendon reflex pathway through the spinal cord?
The signal runs into an excitatory and inhibtory interneuron in the cord
The inhibitory interneuron causes reflexive relaxation of the effector muscle
The excitatory interneuron causes excitation/contraction of the antagonistic muscle
Involves only one side of the cord
What is the goal of the flexor reflex? What are the sensors?
To withdraw the limb from painful stimuli to prevent injury
Pain and pressure sensors
Also called the withdraw reflex
Describe the flexor reflex signaling in the cord?
Stimuli causes reflexive activation/contraction of flexor muscles via an excitatory neuron.
Ascending and descending interneurons are excited to activate more muscles.
Usually there is inhibition/relaxation of the antagonistic muscle as well to speed up the withdraw
Only invovles one side of the cord
How does the flexor reflex activate more muscle groups to speed up the withdraw from pain? What is the name of the location where this occurs?
Uses ascending and descending interneurons to recruit more muscle groups (usually 2 levels above and 2 levels below)
The ascending and descending interneuron cell bodies are located in the white matter just outside the of the dorsal horn in a space called the Tract of Lissaur
What is the goal of the crossed extensor reflex? What sides of the cord does this involve?
To provide balance when painful stimuli is encountered while in movement.
Involves both sides of the cord
Describe the signaling in the cord for crossed extensor reflex?
- Uses interneurons to contract flexors/relax extensors in the muscles of painful extremity
- Uses ascending/descending neurons to activate more muscle groups since this is pain related
- Uses interneurons to cross over the cord to cause contraction of extensors/relaxation of flexors in the opposite leg to create a stable base
Uses the most interneurons
