Spinal Cord & PNS (VI)

Question 1

What are the spinal ganglia? They’re also mostly called….. because they’re mostly in the …..?

Answer 1

• A cluster of nerve bodies positioned along the spinal cord at the dorsal and ventral roots of a spinal nerve
• Dorsal root ganglia, in the dorsal root.

Question 2

How many peripheral nerves do we have? How many cranial nerves, how many spinal nerves?
name the groups of spinal nerves!

Answer 2

From up to down:
* We have 12 cranial nerve pairs
* we have 31 spinal nerve pairs:
1. 8 cervical nerve pairs
2. 12 thoracic nerve pairs
3. 5 lumbar nerve pairs
4. 5 saccral nerves pairs
5. 1 coccygeal pair of nerves.

Question 3

How long is the spinal cord, what’s its diameter and at how many cm can we see a new nerve?

Answer 3

45cm
1.5cm diameter
A spinal nerve pair every 1cm

Question 4

What meninges are on the spinal cord?

Answer 4

Just like the brain:

three layers of meninges: the dural sac, the arachnoid mater, and the pia mater.

Question 5

Can a spinal cord injury be fatal? why?

Answer 5

Yes. We have the respiratory system that depends on the cervical nerves of the SC!

Question 6

How can we identify the start of the PNS? But what if we can’t see the skeletal structures?

Answer 6

Where the foramen magnum begins - this represents the end of the brainstem and the start of the spinal cord. But in terms of nervous tissue, wherever the pyramidal decussation occurs, which we can physically see, is just before the start of the PNS. Just after this decussation at the pyramids, the nerves exit through the foramen magnum.

Question 7

The Spinal Cord is protected by what?

Answer 7

1. vertebral canal (which has the vertebral body, the spinous process, the intervertebral disk, foramen, etc.)
2. ligaments
3. meninges
4. CSF in subarachnoidal space
5. EPIDURAL SPACE between the dural sac and the bone. Unlike the brain which doesn’t have a space between the dura mater and the skull. Contains fatty tissue, blood vessels, and nerves, and is the site where* epidural anesthesia *is administered for pain relief during childbirth and certain surgeries.

Question 8

In which parts of the spinal cord do we see enlargements?

Answer 8

• cervical enlargement: neurons for the upper limb
• lumbo-sacral enlargement: neurons for the lower limb

Question 9

Does each spinal nerve have an efferent and an afferent? What are they?

Answer 9

Yes. Sensory afferent and motor efferent

Question 10

Dermatomes correspond to what segmentation of the nervous system?

Answer 10

Spinal nerves

Dermatome = territory innervated
by the fibers originating from one
pair of spinal nerves. Dermatomes correspond to the nerves even if we can’t see it at first glance.

Question 11

What is a plexus, and what are the two main plexuses in our PNS?

Answer 11

plexus = several nerves that fuse together
- 1) brachial plexus (between neck and shoulder, at back)
- 2) lumbosacral plexus (where my SI pain is!)

Question 12

How can you identify which nerve might be pinched/damaged?

Answer 12

The pain will project through the whole corresponding dermatome

Question 13

Where does the spinal cord end caudally? show it on the picture. What’s below that point? And what would you see on a saggital or horizontal view?

Answer 13

Between vetebrae L1 - L2.
Below that, the spinal cord’s roots (bundles of nerve fibers ) extend out like a horse’s tail “cauda equina”.

Question 14

Where is CSF collected from in a lumbar puncture?

Answer 14

Below the spinal cord, between the L4-L5 vertebrae

Question 15

Where are the 3 exits of CSF? Show them on the pictures. And which exit lets it go to the spinal cord?

Answer 15

1. Foramina of Monro: CSF flows from the lateral ventricles of the brain –> foramen monro –> the third ventricle.
2. Aqueduct of Sylvius: through which CSF flows from the third to the fourth ventricles of the brain.

**3. Foramina of Luschka and Magendie: **The final exit points for CSF are the foramina of Luschka and Magendie. These are small openings in the fourth ventricle of the brain that allow CSF to exit and flow into the subarachnoid space, which surrounds the brain and spinal cord.

Question 16

So during the third week of development the neural tissue morphs into the neural plate, and then folds into the neural tube with the neural crest cells around it. There’s also the notochord which forms underneath it. Which part is what part of the NS?

Which part becomes the ventricles of the brain?

Answer 16

• Neural plate –> neural tube --> CNS. SC is the lowest part
• Neural crest cells –> peripheral NS + glia.
• Notochord –> vertebrae
• The lumen of the neural tube–> ventricles + the central canal

Question 17

What’s the difference between glial cells in the CNS and PNS?

Answer 17

In the CNS (including SC) we have Oligodendrocytes which build the myelin sheath, whereas in the PNS (nerves) it’s the schwann cells that build the myelin sheath.

Question 18

What could be the consequential difference between lesion of a nerve in the PNS and a neuron in the CNS?

Answer 18

In the PNS, schwann cells can rebuild axons… But the oligodendrocytes cannot do this in the CNS, so no axon regrowth in the brain or SC.

Question 19

In a transverse cut of the spinal cord, what is in the white matter and what is in the grey matter?

Answer 19

White matter is the descending and ascending axons
Grey matter has the cell bodies of the motoneurons, sensory neurons and interneurons

Question 20

Dorsal roots contain axons of what? And the dorsal root ganglions contain?

What about the ventral roots? They contain what?

Answer 20

Dorsal root has sensory neuron axons,
and dorsal root ganglion has sensory neuron cell bodies.

Ventral has the axons of all motorneurons (both somatic and autonomous). The cell bodies of the motoneurons are not in the white matter or any ganglion, but in the grey matter of the ventral root.

Question 21

What do we call the “parts”/”regions” of the white and grey matter of the SC?

What kind of function is each mainly dedicated to?

And is this division consistent along the length of the SC?

Answer 21

IN ALL OF THE SC,
White matter has:
* dorsal columns
* lateral columns
* ventral columns

Grey matter has:
* dorsal horn - sensory ascending
* ventral horn - motor descending

IN THE T1-L3 region:

There is also the lateral horn which has sympathetic cell bodies of motor autonomous sympathetic system.

Question 22

Which system’s cell bodies are outside the spinal column? Ascending or descending?

Answer 22

Sensory, in the dorsal root ganglion. Ascending

Question 23

What is meant by segmental, inter-segmental and supra-segmental circuits?

Answer 23

• Segmental: Neural circuits operating within specific segments of the NS (the SC is segmented), responsible for local sensory input input and motor control within that segment.
• inter-segmental: connecting multiple different segments of the SVC
• supra-segmental: neural circuits at a higher level of organisation, involving brain regions beyond segments –> higher order processing, cognition and integration of information.

Question 24

What are the two main types of ascending somatosensory pathways, and what are they for? There’s another ascending pathway that’s not one of the two main. What is it?

Answer 24

• Dorsal column (medial lemniscus) pathway - for touch proprioception
• Anterolateral (spinothalamic) pathway - pain, crude touch, temperature
• Spinocerebellar pathway - relays unconscious proprioceptive information from the lower limbs and trunk of the body to the cerebellum

Question 25

**Outline the details of the trajectories of the fine touch and prioprioception pathway… **

Answer 25

*Dorsal Column-Medial Lemniscus Pathway:
*

1. Sensory Receptors: Sensory receptors in the skin, muscles, and joints detect tactile, proprioceptive (position sense), and vibratory sensations.
2. First-Order Neurons:
   Sensory signals are transmitted from the sensory receptors through peripheral nerves to the dorsal root ganglia (DRG) located in the spinal cord.
   Cell bodies of the first-order neurons reside in the DRG.
   The first-order neurons have peripheral axons that enter the spinal cord through the dorsal roots.
3. Ascending Projection in the Spinal Cord:
   Once inside the SC, the axons of these neurons immediately ascend without extensive interaction in the grey matter. Peripheral axons of the first-order neurons ascend ipsilaterally (on the same side) through the dorsal columns of the spinal cord.
   In the spinal cord, the axons synapse with second-order neurons in the gracile nuclei (lower body) and cuneate nuclei (for upper body), depending on whether the signal originated from the lower or upper body, respectively. G is lower in the alphabet than C - remember this way.
4. Medulla Oblongata:
   In the medulla oblongata, the second-order neurons cross the midline (decussate) and form the medial lemniscus. Crosses only after synapse
5. Midbrain and Thalamus:
   The medial lemniscus ascends through the brainstem and reaches the thalamus, specifically the ventral posterolateral nucleus (VPL).
   In the VPL nucleus, the second-order neurons synapse with third-order neurons.
6. Sensory Cortex:
   Third-order neurons project from the thalamus to the primary somatosensory cortex (postcentral gyrus) in the parietal lobe. The S1 then interprets and processes the sensory information, allowing for conscious perception of touch, proprioception, and vibration.

Question 26

Outline the trajecotry of the pain and temperature pathway…?

Answer 26

Anterolateral Spinothalamic Pathway:

1. Sensory Receptors: Sensory receptors detect pain, temperature, and crude touch sensations.
2. First-Order Neurons:
   Similar to the dorsal column-medial lemniscus pathway, sensory signals are transmitted from the receptors to the DRG via peripheral nerves. Unlike the dorsal column-medial lemniscus pathway the axons of these neurons **immediately enter the grey mater of the SC. **
3. Ascending Projection in the Spinal Cord:
   The axons of pain and temperature fibers synapse with second-order neurons in the dorsal horn, grey matter
4. Decussation in the Spinal Cord:
   Early on, here in the spinal cord, the second-order neurons decussate (cross the midline) and ascend contralaterally (on the opposite side) within the anterolateral tract (spinothalamic tract). ALTHOUGH IT ENTERED DORSALLY, THIS TRACT’S ASCENDING OCCURS in THE FRONT (anterior) OF THE SC, SINCE ITS DECUSSED HERE, as well as the lateral side - hence, why its called “anterolateral” pathway. But dorsal medial lemniscus pathway stays dorsal and only crosses at the medulla level.
5. Ascending Projection in the Brainstem:
   The anterolateral tract ascends through the brainstem, passing through the reticular formation and the brainstem nuclei. The tract reaches the thalamus, specifically the ventral posterolateral nucleus (VPL).
6. Thalamus and Sensory Cortex:
   In the VPL nucleus, the second-order neurons synapse with third order neurons and go to the S1.

Question 27

So we know that both of the two main ascending sensory pathways have three neurons. How many synapses do they have and where are they?

Answer 27

Dorsal medial lemniscus pathway: Synapse 1 = gracile or nuneate nuclei in the medulla oblongata. Synapse 2 = VPL of the thalamus

Anterolateral: Synapse 1 = dorsal horn, just before decussating. Synapse 2: VPL of thalamus

Question 28

where are the spinocerebellar tracts in the cross section of the SC?

Answer 28

Super lateral, at the edge!

Question 29

What are the two types of descending pathways?

And what are the 2 neurons of the descending motor pathways

Answer 29

• 1) Through Pyramidal Tracts = corticospinal tract/CST
  2) Brainstem - indirect cortico-spinal pathways that dont go through pyramids. *tectospinal pathway, rubrospinal, vestibulospinal etc.
• Upper motorneuron: Brain –> ventral horn
• Lower motor neuron: ventral horn –> muscle

Question 30

Show which tracts are which on the cross section? ON this photo

https://quizlet.com/330581579/chapter-5-cross-section-of-the-spinal-cord-showing-motor-and-sensory-tracts-diagram/

Answer 30

Answers in link https://quizlet.com/221990525/unit-3a-the-spinal-cord-ascending-and-descending-pathways-diagram/

Question 31

Can you give an example of segmental circuit and intersegmental circuit?

Answer 31

segmental: knee jerk reflex

Intersegmental: ‘crossed extensor reflex’ - When you step on a pin, you activate muscles in other leg to maintain equilibrium (the sensory neurons synapse with interneurons. The interneurons, which connect different segments of the spinal cord, form the inter-segmental circuit. rossed extensor reflex: The interneurons in the spinal cord activate motor neurons that control the muscles of both legs.

Question 32

3 Functions of the spinal cord?

Answer 32

**1. Connects the PNS to the brain **Sensory functions: somatic and visceral information from sensory neurons in the spinal nerves are transmitted to the brain Motor functions: brain commands are transmitted through somatic and autonomic motoneurons in the spinal nerves
2. Acts as a reflex center
Somatic or autonomic, stretch reflex, flexor reflex, muscle tone regulation,…..
3. Contains sensorimotor networks for movements

Question 33

what is the consequence of a lateralised spinal cord injury?

Answer 33

Ipsilateral effects:
1. Hypotonic paralysis
2. Spastic paralysis +* loss of tactile/vibration/proprio. sensation* (dorsal column medial lemniscus pathway only crosses in the lower medulla - stay ipsilateral throughout the SC)
Contralateral effects:
3. On the other side: Loss of pain/temperature/crude touch (because the spinothalamic pathway crosses immediately to the other side in the spinal cord)

Question 34

A complete injury to the spinal cord at these points cause what?
- cervical injury (quadriplegic)
- thoracic injury (paraplegia)
- lumbar injury
- saccral & coccyx?

Answer 34

• cervical: loss of sensory and motor function in all four limbs and the torso. If it’s lower, it may also affect breathing
• thoracic: the loss of sensory and motor function in the lower limbs.
• lumbar: affects the sensory and motor function in the lower limbs. The upper body, including the arms and hands, remains unaffected by a lumbar injury.
• saccral & coccyx: lower back pain, weakness or paralysis in the legs, numbness or loss of sensation in the lower body, and problems with bowel and bladder control (autonomous problems).

Question 35

What is the function of the lateral horn?
Draw it and where can we find it?

Answer 35

The** sympathetic** division of the autonomic nervous system - responsible for the “fight or flight” response and regulates various bodily functions in response to stress or danger. The lateral horn plays a crucial role in mediating these autonomic responses by coordinating the sympathetic outflow. Here are a few examples of the functions controlled by the lateral horn:

• Regulation of heart rate and blood pressure: The sympathetic preganglionic neurons in the lateral horn innervate the cardiovascular system, influencing heart rate, blood vessel constriction, and blood pressure. Stimulation of these neurons can lead to increased heart rate and vasoconstriction, preparing the body for action.
• Sweating and temperature regulation: The lateral horn contains sympathetic preganglionic neurons that control sweating and thermoregulation. When activated, these neurons can increase sweating to dissipate heat and regulate body temperature.
• Pupil dilation: The sympathetic preganglionic neurons in the lateral horn innervate the muscles that dilate the pupils (pupillary dilator muscles). Stimulation of these neurons results in the dilation of the pupils, enhancing visual acuity in preparation for potential threats.
• Control of digestive functions: Although the parasympathetic division primarily regulates digestion, the sympathetic division, including the lateral horn, can modulate digestive processes. Sympathetic stimulation can reduce digestive activity

We can find it in mostly** thoracic regions** (also in upper lumbar, and sacral region)

Question 36

what creates:
quadriplegia, paraplegia, hemiplegia, monoplegia

Answer 36

• quadriplegia: big (both sides) injury to cervical spinal cord
• paraplegia: thoaric SC injury
• hemiplegia: brown sequard syndrome - lateralised injury to SC
• monoplegia(one limb paralysis) : injury to one single spinal nerve root

Question 37

What are the corticospinal tracts and what are their trajectories?

Answer 37

• CSTs carry motor info from the motor cortex to the spinal cord.
• The upper motor neurons of the corticospinal tract leave the motor cortex and descend to the brainstem, entering the midbrain in large fiber bundles called the cerebral peduncles.
• The tract continues down into the medulla where the fibers form two bundles, known as the pyramids, which create visible ridges on the exterior surface of the brainstem.
• Lateral Corticospinal Tract: At the base of the pyramids, about 90% of the fibers in the corticospinal tract decussate, or cross to the other side of the brainstem in a bundle called the pyramidal decussation. The decussating fibers will enter the spinal cord on the opposite side of the body.
• ventral or Anterior Corticospinal Tract: The other 10% of the fibers will continue into the spinal cord on the same side of the body where they originated as part of the and only cross over when they reach the level of the spinal cord where they will synapse on a lower motor neuron.
• Both CSTs synapse with the lower/secondary motor neuron cell body in the ventral horn of the SC, contralaterally to where everything began.
• It is thought that the two types of corticospinal tract fibers have slightly different specializations, with the lCST controlling the movement of more distal muscles like those of the hands, and the aCST controlling the movement of more proximal muscles like those of the trunk.

Question 38

What happens when upper motor neurons are damaged, and what about when lower motor neurons?

Answer 38

• When upper motor neurons sustain damage, –> We still have lower motor neurons and their direct circuit loops –> we get spasticity (muscle stiffness) or spastic paralysis may occur muscles are tight and hard, and might even jerk periodically in what is called a spasm
• damage to lower motor neurons may result in weakness, loss of muscle, - flaccid paralysis. Nothing can take place anymore.