Ascending Pathways

Question 1

Identify examples of sensory receptors, and their modality as well as whether they are rapidly or slowly adapting.

Answer 1

Hair follicle receptors: Touch

Merkel endings: Pressure + low freq vibration

Meissner corpuscles:
Light touch

Pacinian corpuscles: Vibration, JPS

Ruffini endings: Skin stretch, pressure, JPS

Nociceptors: Pain

Muscle spindles: Muscle length, proprioception

Golgi tendon organs: JPS

Question 2

State whether the following is slowly, or rapidly adapting:
Hair follicle receptors

Merkel endings
Meissner corpuscles
Pacinian corpuscles
Ruffini endings
Nociceptors
Golgi tendon organs

Answer 2

Merkel endings (rapidly) 
Merkel endings (slowly) 
Meissner corpuscles (rapidly) 
Pacinian corpuscles (rapidly) 
Ruffini endings (slowly)
Nociceptors (free nerve endings) 
Golgi tendon organs (slowly adapting)

Question 3

Identify the main types of peripheral nerve fibers.

Answer 3

MEYELINATED
1)
Group Aα (axons from skin), Group I (axons from muscles)
largest diameter
Fastest conduction
Sensory receptors: proprioceptors of skeletal muscle

2) 
Group Aβ (axons from skin) 
Group II (axons from muscles) 
second largest diameter
second fastest conduction
Sensory receptors: mechanoreceptors of skin

3) 
Group Aδ (axons from skin) 
Group III (axons from muscles) 
Second smallest diameter
Second slowest conduction
Sensory receptors: Pain, temperature

UNMYELINATED
4) 
Group C (axons from skin) 
Group IV (axons from muscles) 
Smallest diameter
Slowest conduction
Sensory receptors (temperature, pain, itch)

Question 4

Describe the correlation between degree of myelination and speed of salutatory transmission.

Answer 4

Degree of myelination increases with the speed of saltatory conduction.

This is because:

1) Myelin is such an effective insulator that once myelinated, the membrane can no longer act as a capacitor, and it no longer accumulates charge:

Since the neuron is at a negative membrane potential, it’s got a lot of agitated negative ions that don’t have a positive ion nearby to balance them out. Like charges repel, so the negative ions spread out as far from each other as they can, to the very outer edges of the axon, near the membrane. This then attracts positive ions outside the cell to the membrane as well. We then end up with thin layers of negative ions inside of the cell membrane and positive ions outside the cell membrane. However, where myelin wraps around the cell, it provides a thick layer between the inside and the outside of the cell. Fewer negative ions gather at those points because it is further away from the positive charges. Now there are parts of the axon that are still negative, but contain proportionally far fewer negative ions. This means that as the action potential comes rushing by, it is easier to depolarize the areas that are sheathed, because there are fewer negative ions to counteract.

2) As AP travels down the membrane, sometimes ions are lost as they cross the membrane and exit the cell. The presence of myelin makes this escape almsot impossible, and so helps to preserve the action potential

Question 5

Explain the concept of lateral inhibition.

Answer 5

Each sensory neuron has a receptive field. Dendrites are dense at the center and diffuse at the periphery. Stimuli at the center of the field activate more dendrites than at the periphery and so cause faster firing. However, there is some overlap of receptive fields, which means that for a given stimulus which is central for area A, but simultaneously peripheral for area B, both neurons A and B will be stimulated, though there will be quicker firing of neuron A.

Lateral inhibition further refines this discrimination between areas A and B. Lateral inhibition relies on reciprocal inhibition (through inhibitors interneurones synapsing with the lateral branches of sensory neuron) between two adjacent neurons where the extent of inhibition from each one is linked to the stimulus point on the neuronal receptive field overlap. This increases the contrast between the area receiving input and the one not receiving input. This lateral inhibition increases tactile acuity at a cortical level. The primary sensory cortex sends projections to the primary motor cortex, where it influences motor output through the descending motor pathways.

Question 6

Graph AP frequency at cortex, with and without lateral inhibition.

Answer 6

Refer to slide 9 in lecture “Ascending Pathways)

Question 7

Describe the general organisation of the general somatosensory system.

Answer 7

1) Primary afferent is a first order neuron and terminates in spinal cord or brain stem
2) The second order neuron projects to the thalamus
3) The third order neuron projects to the somatosensory cortex
- Most conscious sensory tracts follow this pattern, and this somatic sensory info ends up at the somatosensory cortex

Question 8

Where do the cervical spinal nerves come out, relative to their corresponding vertebrae ?

Answer 8

“All cervical nerves except C8 emerge above their corresponding vertebrae, while the C8 nerve emerges below the C7 vertebra”

Question 9

Identify the main pathways of the spinal cord, along with the modalities for each.

Answer 9

Dorsal column – medial lemniscal pathway (two sets of dorsal columns, Gracilis, and Cuneatus) (for conscious proprioception, light touch)

Spinothalamic (pain and temperature) (AKA spinal lemniscus)

Spinocerebellar (unconscious proprioception, helps coordination) including ventral (from Golgi organs) and dorsal (from muscle spindles) spinocerebellar tracts

Corticospinal (motor, main descending pathway)

Question 10

Identify the main modalities to test in a clinical sensory testing, why, and how to test each.

Answer 10

• JPS (because exclusively goes through dorsal columns), by stabilising joint and moving up or down
• Pain and temperature (because exclusively lateral spinothalamic tracts), by using sharp/dull ends of neurotip or cold tuning fork
• Light touch (help localise lesion, though not one pathway so not useful for knowing which part of spinal cord might be affected)
• Vibration sense (food screening for several pathologies, though not one particular pathway), by using 125 hz tuning fork

Question 11

Why is the dorsal column- medial lemniscal named like this ?

Answer 11

Its name arises from the two major structures that comprise the DCML. In the spinal cord, information travels via the dorsal (posterior) columns. In the brainstem, it is transmitted through the medial lemniscus.

Question 12

Describe the dorsal column-medial lemniscal pathway.

Answer 12

1) The first order neurones (with a dorsal root axon of Aβ group) carry sensory information regarding touch, proprioception or vibration by ascending ipsilaterally from the peripheral nerves to the medulla oblongata. There are two different pathways which the first order neurones take:

Signals from the upper limb (T6 and above) – travel in the fasciculus cuneatus (the lateral part of the dorsal column). They then synapse in the nucleus cuneatus of the medulla oblongata.
Signals from the lower limb (below T6) – travel in the fasciculus gracilis (the medial part of the dorsal column). They then synapse in the nucleus gracilis of the medulla oblongata.

2) The second order neurones (arcuate fibers) begin in the cuneate nucleus or gracilis. The fibres receive the information from the preceding neurones, and delivers it to the third order neurones in the thalamus.

Within the medulla oblongata, these fibres decussate (cross to the other side of the CNS). They then travel in the contralateral medial lemniscus to reach the thalamus (in the VPL nucleus)
(in brainstem, DCML pathway is mainly medial)

3) The third order neurones transmit the sensory signals from the thalamus to the ipsilateral primary sensory cortex of the brain. They ascend from the ventral posterolateral nucleus of the thalamus, travel through the internal capsule and terminate at the sensory cortex.

Question 13

Describe the effect of a lesion in half of the spinal cord below the point of decussation of the fibers of the dorsal column-medial lemniscal pathway. A lesion higher than the point of decussation ?

Answer 13

Lower than point of decussation: Affect JPS in ipsilateral side
Higher than point of decussation: Affect contralateral JPS

Question 14

Describe Anterior spinal artery syndrome in the different pathways.

Answer 14

“-Retained proprioception and vibratory sensation due to intact dorsal columns

• Complete motor paralysis below the level of the lesion due to interruption of the corticospinal tract
• Loss of pain and temperature sensation at and below the level of the lesion due to interruption of the spinothalamic tract”

Question 15

Show, on a transverse section of the spinal cord, where each of the main tracts resides.

Answer 15

Refer to slide 18 in lecture “Ascending Pathways”

Question 16

What region of the spinal cord does a section of spinal cord with two dorsal columns come from ?

Answer 16

Two dorsal columns (means contributions from arms and legs): Cervical area

Question 17

Identify conditions that may affect the dorsal spinal cord.

Answer 17

• Subacute combined degeneration, due to vitamin B12 deficiency, often due to pernicious anemia (unable to absorb vitamin B12), causing loss of myelin and death of nerve cells in both dorsal and pyramidal corticospinal tracts
• HIV myelopathy (can cause dorsal column deficit)
• Tabes Dorsalis (syphilis tertiary form)
• Multiple sclerosis (can affect any part of CNS, including dorsal columns)

Question 18

Where in brain does sensory information end up ?

Answer 18

The primary somatosensory cortex (parietal lobe) and the somatosensory association cortex

Question 19

Define homunculus.

Answer 19

“A schematic representation of the volume of cerebral cortex dedicated to motor or sensory with respect to each corporal region” (lips, face, hands, all have relatively large representation in cortex).

Question 20

Explain the importance of the thalamus for sensory pathways.

Answer 20

En route to the cortex (connected to cortex via peduncles), all
sensory information
(except some olfaction)
passes through the thalamus. The thalamus also receives motor information. It also sends off sensory fibers to sensory cortices.

The thalamus has reciprocal connections to all cortical regions, and can relay information, receive feedback and modulate cortical activity.

Question 21

Identify the main Dorsal column - medial lemniscal pathway cortical functions. What is required for these to work ?

Answer 21

Fine tactile and proprioceptive information is used:

1) To determine the shape of an object (without sight) (i.e. stereognosis)
- Proprioception from the finger joints and fine discriminating touch provides a very accurate model of any object that is handled

2) To determine texture of an object (without sight)
- Texture is determined by vibration and slip receptors as well as fine discriminating touch. Note pleasure can be associated with this pathway

Requires the somatosensory association area in the parietal lobe is functioning normally.

Question 22

Identify possible pathologies interfering with Dorsal column-medial lemniscal pathway cortical functions.

Answer 22

Astereognosis (loss or lack of the ability to understand the form and nature of objects that are touched (stereognosis), a form of tactile agnosia)

Question 23

Describe the spinothalamic pathway.

Answer 23

The anterolateral system consists of two separate tracts:

Anterior spinothalamic tract (AKA Paliospinothalamic tract, crude touch and pressure)
Lateral spinothalamic tract (AKA Neospinothalamic tract, pain and temperature)

1) The first order neurones (anterolateral system, both lateral and anterior spinothalamic tracts) arise from the sensory receptors in the periphery. They enter the spinal cord, ascend 1-2 vertebral levels, and synapse at the tip of the dorsal horn – an area known as the substantia gelatinosa.
2) The second order neurones carry the sensory information from the substantia gelatinosa to the thalamus. After synapsing with the first order neurones, these fibres decussate early within the spinal cord, and then form two distinct tracts:

Crude touch and pressure fibres – enter the anterior spinothalamic tract.
Pain and temperature fibres – enter the lateral spinothalamic tract.
Although they are functionally distinct, these tracts run alongside each other, and they can be considered as a single pathway. They travel superiorly within the spinal cord, synapsing in the thalamus (Neospinothalamic tract terminates in the ventral posterior lateral nucleus (VPL), whilst Paliospinothalamic tract terminates in the dorsomedial (DM) and intra laminar areas).

3) The third order neurones carry the sensory signals from the thalamus to the ipsilateral primary sensory cortex of the brain. They ascend from the ventral posterolateral nucleus of the thalamus, travel through the internal capsule and terminate at the sensory cortex

Question 24

Which types of axon are present in the Neospinothalamic, and Paliospinothalamic tracts ?

Answer 24

Neospinothalamic- Aδ fibres

Palispinothalamic- C fibers

Question 25

How discrete are the lateral, and anterior spinothalamic pathways ?

Answer 25

Lateral- discrete (Neospinothalamic tract terminates in VPL of thalamus, which is somatotopic, so there is locational discrimination along this pathway)

Anterior- generalised (Paliospinothalamic tract terminates in the dorsomedial (DM) and intra laminar areas, which only provide a generalised location for pain)

Question 26

Describe the function of the anterolateral system.

Answer 26

• These fibres provide us with the pain basis for avoidance behaviours that protect us from harm.
• They are linked to a strong memory forming circuit via the hippocampus (memory) and amygdala (emotion)
• Prolonged stimulation of the paleo-spinothalamic pathway (C fibres and slow dull pain) is said to be one of the hardest pains to bear and can affect mood and outlook.
• Synapse with brainstem nuclei to increase arousal – pain prevents sleep
• Stimulates (indirectly) hypothalamic autonomic circuitry – pain makes you sweat and feel sick
• Positively associated with limbic system – emotional component

Question 27

Describe the effect of a lesion in half of the spinal cord below the point of decussation of the fibers of the spinothalamic pathway, of the JPS, and of the spinocerebellar pathways. A lesion higher than the point of decussation ?

Answer 27

If have lesion of half spinal cord above the point of decussation, will affect contralateral pain and temperature. A lesion in the same place will affect ispliteral JPS (because only decussate in the medulla), so fibers have not crossed over. A lesion anywhere in the spinal cord will affect ispilateral unconscious proprioception (spinocerebellar pathways do NOT cross).

Question 28

What kind of spinal cord pathology can are the spinothalamic tract fibers susceptible to ?

Answer 28

Spinothalamic tract fibres cross early, susceptible to central cord pathology (e.g. Syringomyelia, cyst in the middle of the spinal cord, especially in the cervical region, result in wasting of the hand, suspended sensory level of pain and temperature loss often in a cape-like distribution affecting arms/shoulders due to pathology affecting fibers crossing in the spinal cord, but JPS in preserved because dorsal columns are preserved)

Question 29

Describe the main effects of anterolateral tract lesions.

Answer 29

Lesions in the neo part of the anterolateral tract cause decreased perception of pain and temperature on the contralateralside of the body, always one or two dermatomes below the level of the lesion.
This may be used as a surgical procedure (cordotomy) for terminal disease pain, pain returns after about 1 year.

Question 30

What is the function of the spinocerebellar pathways ?

Answer 30

Unconscious proprioception

The cerebellum functions to gather information about the movements of the body (using inputs from eyes, from vestibular system), and compare them with planned movements of the body. It then issues real time correctional information to achieve smooth accurate movements.

Question 31

Describe the spinocerebellar pathway.

Answer 31

Fibers are lateral, and do not cross, from periphery (from muscle spindle proprioceptors) to ispilateral cerebellum.

Question 32

How is the white matter of the spinal cord divided ?

Answer 32

The e white matter of the spinal cord is divided up into tracts of axons carrying typified information

Question 33

Name the group of conditions which affects pathways to the cerebellum. Give examples of symptoms.

Answer 33

Inherited conditions that affect pathways to the cerebellum: Spino-cerebellar ataxias (e.g. nystagmus, vertigo)

Question 34

What kind of lesion does the following suggest:

• Cape-like distribution of pain and temperature sensory loss
• Spinothalamic loss on one side, and JPS loss on other

Answer 34

• Cape-like distribution of pain and temperature sensory loss: Central cord lesion
• Spinothalamic loss on one side, and JPS loss on other: half cord lesion