Spinal Cord Transmission

Question 1

Spinal Cord Transmission

Ascending & Descending Tracts (Gen. Overview)

What information ascends, descends the spinal cord? What is a tract?

Answer 1

-Sensory information ascends the spinal cord, beginning in the periphery of the body and traveling up through the spinal cord, brain stem, cerebellum, and brain.
-There are more sensory pathways than motor pathways, and that’s because we have temperature, vibration, and pain sensors

-Motor information descends the spinal cord and travels to our skeletal muscles

-Tract refers to a bundle of axons within the CNS. A bundle of axons outside the CNS are called nerves.

Question 2

Motor and Descending Pathways

Answer 2

Pyramidal Tracts: Primary voluntary movement
-Lateral corticospinal tract
-Anterior corticospinal tract

Extrapyramidal Tracts (Accessory tracts): Movement that we do not usually have a knowledge of. Ex: fine-tuning our motor commands
-Rubrospinal tract
-Reticulospinal tract
-Olivospinal tract
-Vestibulospinal tract

Question 3

Sensory and Ascending Pathways

Answer 3

Dorsal Column Medial Lemniscus: Located in the dorsal part of the cord. These transmit information from pressure sensors in our skin (if we’re holding onto something, or our hands are in the air). Touch sensors

Anterolateral System: Pain signals. Typically follow one of two pathways
-Lateral spinothalamic tract
-Anterior spinothalamic tract
Spinothalamic tract terminology corresponds to the fact that the pain is going into the cord, relayed through the thalamus, and then out to the parietal cortex where it can be sorted out (where is it coming from?)

Question 4

Rexed’s Laminae- Lamina I

Answer 4

-Numbered starting at the most dorsal portion of the dorsal horn
-Lamina I (Lamina Marginalis): Transmits fast, sharp pain via myelinated nociceptors that fall into the category of A-delta fibers.
Fast pain comes in through the dorsal rootlet, into the dorsal horn, where it has synapses in Lamina I. From there, the sensory information crosses over to the other side of the cord and then ascends the anterolateral pathway

Question 5

Rexed’s Laminae- Lamina II, III, ~V

Answer 5

-Substantia Gelatinosa
-Synapses for slow pain conduction are located here. Sometimes slow pain signals will also synapse at lamina V
-Slow pain is typically routed through nociceptors that are non-myelinated (C fibers).

Once information is received in the substantia gelatinosa (and sometimes lamina V) it jumps over to the other side of the cord and ascends via the anterolateral pathway

Question 6

Lamina 1-VI

Mechanoreceptors

Answer 6

-Also have mechanoreceptors that relay information to these areas of the grey matter in the cord.
-Mechanoreceptors are pressure sensors

Question 7

Rexed’s Laminae-Lamina VII, VII, IX

Answer 7

-These laminae make up the anterior horn.
-This is where our large motor neuron cell bodies sit, and they can be elicited to send an action potential if the stimulus is strong enough

Question 8

Lamina X, Anterior White Commisure

Answer 8

Lamina X- An area of the grey matter where signals are relayed to the other side of the cord

AWC- The area of white matter in the spinal cord where information is relayed to the other side

Question 9

Names & function of the 5 main spinal tracts

Answer 9

Typically the name tells us where the pathway is located or what function it performs

Spinocerebellar Tract: Sensory information that goes up the spinal cord to the cerebellum

Dorsal-Column Medial Lemniscal System (DCML): Major pressure sensory pathway that sits in the dorsal column of the cord. The medial lemniscal portion of the name refers to an area of the brainstem that the information passes through

Spinothalamic Tract/Anterolateral: Pain

Corticospinal Tracts/ Pyramidal tracts: Signal originates in the cerebral cortex (motor cortex) and passes through the spine on the way to the skeletal muscles

Extrapyramidal Tracts: Primarily accessory motor pathways

Question 10

Dorsal Column Medial Lemniscus

Fibers? Sensory Information? Two routes this info can take

Answer 10

-Major sensory pathway for everything other than pain
-Capable of very fast sigal propagation
-Variety of a-fibers: alpha, beta, delta, gamma
- Fine vibration, fine pressure
- Crosses over at the medulla in the medial lemniscus
The path to getting to the medula follows one of two routes:

Touch sensation coming into the cord-> enters through the posterior rootlets of the doral horn and enters the grey matter of the cord. The information that enters the grey matter of the cord often stays there. This usually involves lateral inhibition or modulation of the activity in the cord

The other route that the sensory information takes is up to the brain through a pathway in the dorsal column (if the information needs to ascend all the way to the brain)

Question 11

Dorsal Column Pathways: Fasciculus Gracilis

Answer 11

The further up the cord you get, the larger the dorsal column becomes.
-Lower extremity sensory information is fed into the fasciculus gracilis. As we ascend the cord, more bundles are added to the lateral side of the dorsal column

A tickle on the foot –> information is passed through the dorsal root –> dorsal root ganglia –> into the dorsal column–> ascends the same side of the cord that it entered on–> crosses over at the lower medulla–> medial lemniscus –>relayed to the ventrobasal complex of the thalamus –> internal capsule; a route that the information takes on the way to the parietal lobe

Question 12

Dorsal Column Pathways: Fasciculus Cuneatus

Answer 12

-Higher up the cord. This is where the sensory information from the upper extremities is fed into the cord

A tickle on the arm –> information is passed through the dorsal root –> dorsal root ganglia –> into the dorsal column–> ascends the same side of the cord that it entered on–> crosses over at the lower medulla (lemniscal decussation)–> medial lemniscus –>relayed to the ventrobasal complex of the thalamus –> internal capsule; a route that the information takes on the way to the parietal lobe

Question 13

Parietal Lobe layout

Which areas of the parietal lobe receive what information?

Answer 13

-Topographical layout
-Most anterior portion of the parietal lobe receives sensory information from lower extremities
-Immediately posterior (more midline) to that is the trunk sensory area
-Posterior to the trunk sensory area is the upper extremities
-The inferior, lateral borders of the parietal lobe receive sensory information for the face

Question 14

Creepy Homunculus

Answer 14

The amount of area that you have processing sensory information in the brain is proportional to the number of sensory receptors

We have tons of receptors in our hands. These very sensitive sensors allow us to read braile.

We have a lot of receptors in the face

We have a low density of pressure sensors in trunk

Question 15

Descending Motor Pathway

Pyramidal Tracts-Primary Pathway

Why are these called pyramidal tracts?

Answer 15

-Two separate tracts, primary and secondary. 80% of motor signals travel through the primary route
-Pass through the pyramids of the medulla

-Primary descending pathway begins in the primary motor cortex –> internal capsule (right outside thalamus) –> upper medulla –> medullary pyramids (anterior brainstem) –> lower medulla –> crosses over at pyramidal decussation –> lateral corticospinal tracts down the cord until at the appropriate level –> activation of motor neurons in anterior horn

Question 16

Midbrain? Pons? Medulla? Pyramids? Pyramidal Decussation?

Answer 16

-The ridges are the pyramids
-The “cross-hatch” pattern is where the pyramidal decussation lies. These are strands of neurons crossing over, or bridging the gap, between the left and right pyramids

Question 17

Pyramids? Decussation? Pons?

Question 18

Anterior Corticospinal Tract

Pyramidal Tracts- Secondary Pathway

What is different about this pathway? How much information gets lost?

Answer 18

-Significantly smalled than the lateral corticospinal tracts.
-Responsible for ~17% of our motor function
2-3% of information does not crossover at all

Crossover happens at the level of the cord where the tract needs to activate a motor neuron

Begins in the primary motor cortex –> internal capsule (right outside thalamus) –> upper medulla –> medullary pyramids (anterior brainstem) –> lower medulla –> pyramidal decussation (information passes through the decussation, does not cross over here)–> continues down the anterior corticospinal tract on the same side of the body in which the signal originated –> crosses over at the level of the motor neuron where the message needs to be communicated

Question 19

Fast Pain- Gen overview

Which portion of pathway? Runs parallel to? NE? Alt. Name?

Answer 19

-Lateral pathway of the anterolateral pathway
-A delta fibers, heavily mylenated
-Glutamate is always neurotransmitter, acts very quickly
-Runs parallel to the DCML, reaches the parietal lobe allowing for detailed localization of pain
-Also called neospinothalamic tract

Question 20

Slow Pain- Gen Overview

Answer 20

-Anterior portion of the anterolateral pathway

-Slow, non-myelinated fibers (C fibers)

-Primary NE is substance P, can also use CGRP (Calcitonin G-related peptide) or glutamate (but not fast here)

-“Everything else” other than fast pain. Heat & vibration
-Stimulus travels up to the brainstem, but does not make it past that. Poor localization of pain because it does not reach the parietal lobe. Body has a difficult time pinpointing where the pain is occuring

-Paleospinothalamic Tract

Question 21

Pain Transmission Pathway

Fast Pain: Anterolateral/Spinothalamic Tracts

Pathway?

Answer 21

Enters the cord at the level of the painful stimulus (primary ascending) –> synapses in lamina I –> crosses over at the anterior white commisure via the secondary ascending neuron
–> ascends the lateral portion of the anterolateral pathway, passing through the brainstem, ventrobasal complex of thalamus, internal capsule, and finally arriving in the parietal lobe

Question 22

Pain Transmission Pathway

Slow Pain: Anterolateral/Spinothalamic Pathway

Pathway? Emotional centers? Reticular Formation?

Answer 22

-Enters cord at level of the stimulus (primary ascending)–> synapses in laminas II, III, sometimes V–> crosses over at the AWC (secondary ascending) –> ascends the cord in the anterior portion of this pathway.
The majority of this information terminates in the brainstem at the reticular formation (swaft of tissue) or immediately after leaving the brainstem

-Chronic, slow pain typically affects the emotional centers in the brain. The emotional centers in the brain are located very close to the middle of the brain, or right around where the brainstem connects with the diencephalon

Question 23

Descending Motor Accessory Pathways

Extrapyramidal Tracts

Answer 23

1. Vestibulospinal tract: Maintain balance, focus our eyes
2. Olivospinal: Don’t really have anything to say
3. Reticulospinal tract: Maintaning a certain level of muscle tone. Muscles aren’t entirely relaxed, all of the time. Underlying activity
4. Rubrospinal: Monitoring and adjusting voluntary movements

Question 24

Descending Pain Suppression System (DIC)

Answer 24

-Inhibitory in nature
-Operates in the background and helps the body deal with pain after the ascending signal has arrived at the brain
-Three neurons in the DIC

-The first order descending neuron originates from either the periaqueductal grey (around the midbrain) or the periventricular nuclei (located in front of 3rd ventricle)

-When excited, the first order descending neuron will release Enkephalins right in the middle of the pons (Raphe magnus nucleus)

-Enkephalins excite the second order descending neuron inside of the RMN. Second order descending neuron releases serotonin (5-HT) in the dorsal horn of the spinal cord. 5-HT is inhibitory in the spinal cord

-5-HT can excite a third order descending neuron (very small) that also excretes enkephalins, but in the cord enkephalins are an inhibitory neurotransmitter

-The enkephalin receptors in the cord are located ON the pain nociceptors of the pain sensing neuron. The pain sensing neuron has dendrites out in the periphery

-Pain signal travels into the dorsal root–> dorsal rootlets –> dorsal part of the cord –> enkephalin is being released and binding to the nociceptors –> inhibits that receptor causing pain to be diminished

-Another neuron that contains enkephalin receptors is the second order neuron in the ascending pathway
-Depending on the type of pain, the synapse will be in laminate I, II, III

Question 25

Enkephalins, Enkephalin Receptors, and DBS

What is it? Where are they located? Where are they E, I

Answer 25

Endogenous Morphine analog
Opiate receptors = enkephalin receptors

Enkephalin is released in the raphe magnus nucleus (the first synapse in the DIC)- Stimulatory at this point

Enkephalin is also released in the dorsal horn (inhibitory here)
Enkephalin receptors are within the cord ON nociceptors

If we were to implant an electrode into the periaquaductal gray or the periventricular nuclei, that would generate an inhibitor pain signal that could reduce the amount of pain we perceive. Why? DIC. If activated strongly enough, it can surpass pain within the body. The presence of this system gives anesthesia a target for their drugs (dulls pain perception)

Question 26

Ascending Pain Pathway & DIC

Overview of both, and how they interact, what gets shut down here?

Answer 26

1st order ascending nociceptor senses pain –> pain travels up the spinal nerve, through the dorsal root, through the dorsal rootlets–> synapses in the dorsal horn–> 2nd order ascending neuron hops over to the other side of the cord via the AWC–> travels up the anterolateral pathway

-Slow pain will terminate in brain stem
-Fast pain will travel through the brain, thalamus, internal capsule, to the parietal lobe

Our descending pain suppression system starts in either the periaquaductal grey or periventricular nuclei –> 1st order descending neuron (enkephalin neuron) –>synapses in the RMN of the pons, releases enkephalins which are excitatory–> 2nd order descending (serotonergic neuron) travels down toward the dorsal horn, releases 5-HT–> stimulates 3rd order descending neuron–> 3rd order descending releases enkephalins in the synapse in the dorsal horn.
Shuts down the presynaptic and post synaptic side of the synapse

Question 27

Things that cause a nociceptor to depolarize

What causes pain?

Is pain a survival mechanism? Meditation can do what?

Answer 27

Pain is a survival mechanism that tells us when we’re doing something stupid.
Meditation and massive self-control over the mind can do things like deaden our pain completely, or allow us to lift a car off of a kid

-Damage (crush injury, cuts)
-H+ ions (acid)
-Lactic acid build up in the muscle
-Hyperkalemia (causes cells to depolarize) (Dialysis patient example)
-Histamine (inflammation)
-5-HT in the periphery
-Prostaglandins (cannot generate an action potential, but causes more sensitivity to pain)
-Bradykinins

Question 28

Chronic Pain

What are some drug classes that can help? SSRI, TCA

Answer 28

-Serotonin is recycled and reused by the neuron (serotonin reuptake system)
-If serotonin is what stimulates the 3rd order descending neuron to release enkephalins, then inhibiting the serotonin reuptake process should help reduce pain
-Some tricyclic antidepressants (have been around 50-60yrs) also inhibit serotonin reuptake. One main side effect of this class are drugs is drowsiness, which can actually be beneficial in chronic pain management. Because pain is stimulating the brain, it is harder for these people to sleep

Question 29

Lateral Inhibition

Answer 29

Pressure sensor in the periphery takes the DCML pathway (this is the portion of the information that stays in the grey matter of the cord)–> this runs paralell to the 1st order ascending nociceptor

When neurons are close to together, they are able to talk to each other (mechanism unknown at the moment, probably uses a neurotransmitter & receptor). Neighboring neurons have the ability to shut down neurons nearby.

When there is a pressure sensor running parallel to a nociceptor, the pressure sensor has the ability to shut down some of the pain response.

This is how acupuncture works

Question 30

Receptors in the pain system

2 main receptors? How do they function? What blocks them?

Answer 30

-Glutamate is the primary fast pain neurotransmitter and is almost always excitatory
-1st order nociceptor releases glutamate in response to Ca++ fluxing into the cell. Ca++ interacts with glutamate storage vesicles, vesicles fuse with the cell wall and dump their contents into the synapse

-The 2nd order ascending neuron will need to have receptors on it in order to receive messages from the nociceptor
1. Primary glutamate receptor is the AMPA receptor –> they open in response to glutamate–> causes ion channel next to the glutamate receptor to open and allow Na+ through the cell wall. (This occurs due to an increase in Na+ permeability in the target cell)
2. NDMA-r; glutamate receptor that is attached to an ion channel. This receptor is large, so allows Na & Ca++ through the cell wall (Ca++ is primary). This receptor is a little slower than the AMPA receptor
-The NDMA-r needs two things in order to open:
1. Glutamate and 2. Prior depolarization

Question 31

Basic Overview- Spinal Reflex Pathways

Names, Function

Answer 31

1. Stretch Reflex
2. Tendon Reflex
   -Stretch or tension reflexes; embedded in the muscles in our extremities
3. Withdrawal Reflex
4. Crossed Extensor Reflex
   -Responses to pain

Question 32

Basic Spinal Wiring Schematic- General overview

What types of sensors? Where are they located?

Answer 32

-All of these reflex pathways are going to consists of some sensory component and then a way to elicit the reflex

-Reflexes that are dependent on skeletal muscle contraction or relaxation will need to have a way for the sensory reflex arc to influence motor function

-We have pain sensors out in the periphery. These pain sensors can elicit a reflex

-Tension receptors are located in the skeletal muscle or found within the tendons of skeletal muscle. Can sense the amount of tension within a muscle.

Consider the sensor a “spring,” if you pull apart the muscle, the spring will have increased tension

This gives our CNS a picture of what’s happening in the skeletal muscle

Question 33

Interneurons

What are they? Function? Bridge between what?

Answer 33

-Sensory information can travel into the dorsal horn via it’s regular pathway. Sometimes sensory information projects directly into the anterior horn to elicit a reflex, other times we need the help of an intermediary neuron referred to as an interneuron
-Considered a bridge between the sensor itself and the motor neuron that needs to be communicated with

-Can be excitatory or inhibitory
-Reflexes can be bilateral and interneurons are utilized for crossover of the sensory information

Question 34

Stretch Reflex

Goal? Reflex 1, 2. Interneurons? How to test for this clinically?

Answer 34

-Goal: Keep your muscles at a constant length. This reflex is engaged to keep our posture constant
Applicable to weight bearing muscle. Usually involve the leg muscle

Ex: Pushing in your forehead; this causes you quadricep muscles to stretch out. Without this reflex, your body will allow itself to be pushed backward

Reflex 1: With this reflex engaged, the quadricep muscles stretch, contract, then shorten back to their original length, allowing us to keep our posture constant.

Typically one sensory neuron is needed to achieve this reflex. The sensory neuron can synapse directly onto the motor neuron in the anterior horn of the cord without the help of an interneuron

**Reflex 2: **Involves an antagonistic muscle and an inhibitory interneuron. Antagonistic muscle in this case is the hamstring muscle. This muscle relaxes via an inhibitory connection, allowing the leg to straighten

How to test for this clinically?
Goal of this test is to determine if the sensors have a complete circuit with the other portions of the pathway.
To elicit this reflex, tap lightly on the ligament inferior to the patella. Striking that ligament causes contraction (lengthening) of the quadricep muscle, and relaxation of the hamstring muscle. Striking the ligament does not directly engage the muscle spindle. Foot will also twitch
Quadriceps= extensor muscle

Question 35

Stretch Reflex- Complete Circuit

Answer 35

1. Stretching stimulates the muscle spindle (stretch receptor)
2. Sensory neuron is excited
3. Sensory information travels to spinal cord. Some information travels to the brain, some passes over to the anterior horn to activate a motor neuron
4. Motor neuron is excited
5. Effector muscle (where the original muscle spindle is located) contracts to relieve the stretching, antagonistic muscle relaxes

Question 36

Tendon Reflex

Goal? Where are the sensors? Reflex 1, 2, bicep example

Answer 36

-Protective in nature. Consists of stretch receptors that are embedded in the tendons of our skeletal muscle capable of sensing large amounts of tension within the tendon
-Golgi tendon sensors detect heavy loads on the muscle by measuring tension.

Usually involves both reflexes

Reflex 1: Ceases contraction (relaxation) under a heavy load in order to prevent tendon tears or ripping the muscle out of their insertion point in the bone

Reflex 2: Contracts antagonistic muscles in order to speed up retraction from the heavy load.

There are ways to get around this reflex, but not totally sure of the mechanism. Ex: lifting a 5000lb car off of a child

Biceps/triceps Ex:
Reflex 1: Inhibits bicep with inhibitory neuron
Reflex 2: Excite triceps with excitatory neuron

Question 37

Tendon Reflex- Complete Circuit

Answer 37

1. Increased tension stimulates the golgi tendon sensor
2. Sensory neuron is excited, travels through the dorsal horn
3. Interacts with both and inhibitory and excitatory interneuron.
   -Inhibtory interneuron is responsible for inhibiting the motor neuron that attaches to the muscle under the heavy load
   - The excitatory interneuron causes reflex activation of the antagonistic muscle group

Question 38

Flexor/Withdrawal Reflex

Goal? Muscle type used? Interneurons? Where are the cell bodies?

Answer 38

Goal: Withdraw from painful stimuli to avoid injury. This is usually done with the flexor muscles

-This reflex involves only one side of the cord. There are no interneurons bridging crossover.

-Involves multiple levels of the cord, ~ two levels above and two levels below, via ascending and descending interneurons. The cell bodies of these interneurons reside in the lateral dorsal area of the white matter “ Tract of Lissaur”

Ex; Stub your toe–> reflex is to pull your limb away from whats doing the injury–> this activates the flexor muscle (hamstring in this case) and also relaxes the antagonistic muscle/extensors (quadriceps) to speed up the process of withdrawing

Question 39

Flexor Reflex- Complete Circuit

Answer 39

1. Painful stimuli
2. Sensory neuron excited–> info travels to the cord
3. Ascending and descending interneuron transmit information two levels above and two levels below stimuli
4. Motor neuron excited–> flexor muscles contract to pull away from painful stimuli
5. Antagonistic muscle relaxes

Question 40

Crossed Extensor Reflex

Answer 40

-Involves withdrawing from pain, but also stabilizing with the other side of the body if there our weight is shifting. Allows us to withdraw from pain, but not fall over

-We have ascending and descending interneurons that run through the tract of lissaur, and interneurons that allow communication to the other side of the cord

Ex: Stub right toe or run into furniture –> we will need to plant our left leg to stabilize –> extensor muscles in the left leg contract, flexor muscles in this leg relax, straightening the leg, and giving us a stable base.
The affected limb will contract the flexor muscle group, and we will see relaxation in the antagonistic muscle group (extensor muscles)

Question 41

Crossed Extensor Reflex- Complete Circuit

Answer 41

1. Painful stimuli on one side of the body
2. Sensory information sent to multiple levels of the cord via ascending and descending interneurons
3. Motor information leaves the spinal cord
   4a. Flexor muscles of affected limb contract, extensor muscles relax allowing us to withdraw the limb
   4b. Extensor muscles contract, flexor muscles relax allowing for us to stabilize