6. Sensory Pathways 2

Question 1

Anterolateral systems is composed of….

Answer 1

1. The SPINOTHALAMIC PATHWAYS which run in ANTERIOR and LATERAL tracts of the anterolateral system. These two pathways are also known as the NEOSPINOTHALAMIC (discriminating) and PALIOSPINOTHALAMIC (undiscriminating) pathways
2. The SPINOMESENCEPHALIC (spinal cord to midbrain)
3. the SPINORETICULAR (spinal cord to reticulum) pathways

Question 2

What does the thalamus do?

Answer 2

Connects reciprocally with every part of the cortex
Receives input from all of the cortex
Acts as “gatekeeper” to the cortex for sensory information
Is able to control the arousal and somnolence of the cortex.

Question 3

What does the mesencephalon/midbrain do?

Answer 3

• Involved in reflexes associated with hearing and vision
• Involved with the fine control of skeletal movement through the
red nucleus
• Involved in habituation and motivation via dopaminergic centers
• Contains cerebral peduncles and so accommodates most of the ascending and descending information
• Contains the periaquaductal grey matter which can modulate
pain

Question 4

What does the reticulum do?

Answer 4

A largely unstructured dense mass of axons and dendrites which carries information between different areas of the brainstem. Modulation of these complex interconnections can be used to modulate the passage of signals such as pain.
Nuclei in the reticulum modulate the cortex via the thalamus

Question 5

What is the VPL?

Answer 5

Ventral posterior lateral nucleus

Question 6

Difference between neospinothalamic and paliospinothalamic tract?

Answer 6

Both apart of the anterolateral system.

The Neospinothalamic tract which terminates in the ventral posterior lateral nucleus (VPL) is mainly composed of Aδ fibres.
The Paliospinothalamic tract which terminates in the dorsomedial (DM) and intra laminar areas is composed of C fibres.

As the VPL is somatotopic, there is locational discrimination along this pathway
WHEREAS the dorsomedial (DM) nucleus and intralaminar areas only provide a generalised location for pain.

Question 7

What is the origin of the spinothalamic and spinomesencephalon tracts in the grey matter?

Answer 7

Originate mainly in laminae 1 and 5.

*Note these laminae are points of pain modulation from descending pathways *

Question 8

Where do the spinoreticular tracts originate in the grey matter?

Answer 8

Originate diffusely in the intermediate zone and ventral horn laminae 6-8

Question 9

What are the Rexed’s laminae?

Answer 9

A system of ten layers of grey matter

Question 10

Function of anterolateral system?

Answer 10

1. Pain basis for avoidance behaviours that protect us from harm
2. Linked to strong memory forming circuit via hippocampus and amygdala
3. Prolonged stimulation of the paleo-spinothalamic pathway (c fibres and slow dull pain)_ is very painful.
4. Arousal from sleep due to pain as it synapses with brain nuclei to increase arousal
5. Stimulates (indirectly) hypothalamic autonomic circuitry. –>Pain makes you sweat and feel sick
6. Positively associated with the limbic system (has an emotional component)

Question 11

What is the hippocampus?

Answer 11

Elongated ridges on the floor of the lateral ventricles

Centre of emotion, memory and ANS

Question 12

What is the amygdala?

Answer 12

Almond-shaped mass of grey matter in each hemisphere involved in experiencing emotions

Question 13

What is clinical sign of a lesion in the anterolateral tract?

Answer 13

In the NEO part of the tract:

• Decreased perception of pain and temperature on the CONTRALATERAL side of the body.
• Always 1/2 dermatoms below the level of the lesion (Lissauers tract)
• Not usually paraestesia but when they do it’s a searing shooting or burning pain

Question 14

What is cervical cordotomy?

Answer 14

in which the anterolateral portion of the cord is lesioned using electrical current
for terminal disease pain, pain returns after about 1 year.

Question 15

How do sensations reach the brainstem from the head?

Answer 15

Via trigeminal nerve branches

TRIGEMINAL PATHWAY

Question 16

What are the modalities of the trigeminal pathway?

Answer 16

Pain
Temp
Discriminatory touch
Proprioception

Question 17

What are the 4 trigeminal brainstem nuclei?

Answer 17

Mesencephalic nucleus of V
Chief sensory nucleus of V
Motor nucleus of V
Spinal nucleus and tract of V

Question 18

Name the 3 branches of the trigeminal nerve and the structure they pass through to reach the head?

Answer 18

Ophthalmic nerve through SUPERIOR ORBITAL FISSURE
Maxillary nerve though FORAMEN ROTUNDUM
Mandibular nerve though the FORAMEN OVALE

Question 19

What does the ophthalmic nerve detect?

Answer 19

Sensory from upper face, orbit and eye

Question 20

What does the maxillary nerve detect?

Answer 20

Sensory from midface and upper teeth

Question 21

What does the mandibular nerve detech

Answer 21

Sensory from lower teeth
Motor to muscles of mastication
Proprioception from muscles of mastication

Question 22

What is the function of the:
Mesencephalic nucleus of V
Chief sensory nucleus of V
Motor nucleus of V
Spinal nucleus and tract of V

Answer 22

Mesencephalic nucleus of V: Proprioception from mouth/jaw

Chief/main//principal pontine sensory nucleus of V: Discriminating touch, vibration and conscious proprioception from face

Motor nucleus of V: Controls muscles of mastication

Spinal nucleus and tract of V: Pain and temperature sensation (A-delta and C fibres) from face, back of tongue, pharynx, larynx and ear

Question 23

What is the pathway of nerves From the Chief/main/principal Sensory Trigeminal Nucleus?

Answer 23

Primary axons all synapse in the brainstems
trigeminal nucleus.
From the principal trigeminal nucleus there are two pathways
1. Carries TOUCH and VIBRATION sensation from the FACE via the trigeminal lemniscus to the CONTRALATERAL VPM (thalamus) From here this pathway projects somatotopically to the somatosensory CORTEX
2. Carries TOUCH and VIBRATION sensation from the MOUTH via the ipsilateral POSTERIOR TRIGEMINOTHALAMIC tract to the THALAMUS

Question 24

What is the pathway of nerves after the mesencephalic nucleus?

Answer 24

Primary axons all synapse in the brainstems
trigeminal nucleus
From the mesencephalic nucleus unconscious proprioceptive information is passed to the cerebellum for processing

Question 25

What is the pathway of the nerves from the spinal trigeminal nucleus?

Answer 25

The spinal trigeminal pathway is a homolog of the anterolateral system

These fibres travel ipsilaterally down the cord to the spinal trigeminal nucleus (C1) in the spinal trigeminal tract, where they synapse with the 2nd order neurons
The 2nd order neurons then cross the midline and ascend contralaterally in the trigeminal lemniscus as the trigeminothalamic

Other afferents travelling with them branch off:

1. Into the reticulum forming the trigeminoreticular pathway
2. As the trigeminomesencephalic pathway which terminates on the PAG and is involved with face pain modulation

The 2nd order neurons synapse with tertiary neurons in the VPM lobe of the thalamus.

Question 26

What is meant by lemniscus?

Answer 26

A lemniscus (Greek for ribbon or band) is a bundle of secondary sensory fibres in the brainstem. The medial lemniscus and lateral lemniscus terminate in specific relay nuclei of the diencephalon.

Question 27

How do lesions of the trigeminal pathway present?

Answer 27

SENSORY: loss of Touch; Pain; Temperature; Joint proprioception (conscious) ; Vibrations

MOTOR: loss of control of muscles of mastication; tensor tympani muscle. Note it’s the
Facial muscle that controls the muscles of facial expression not the trigeminal
Because of the precise divisions of the trigeminal system, specific loss of sensation/motor ability is diagnostic for where the lesion is.

Question 28

What is trigeminal neuralgia?
Cause?
Association?
Treatment

Answer 28

DEFINITION: Fairly common form of trigeminal dysfunction, manifests as intense shooting sharp pain in the areas innervated by the trigeminal nerve often in response to light touch
CAUSE: By intense quasi-ectopic firing of the nerve either proximal to or within the brainstem induced by normally low frequency action potentials.

ASSOCIATION: With de-myelination of the nerve (MS causes inflammatory de- myelination) which can lead to reverberation firing in the axon, or axon other forms of damage.

TREATMENT: with voltage sensitive sodium channel blockers (phenytoin) can help.

Question 29

What is the function of the spinocerebellar pathway?

Answer 29

The cerebellum functions to gather information about the movements of the body, and compare them with planned movements of the body. It then issues real time correctional information to achieve smooth accurate movements.

Question 30

What are the two types of information fed into the cerebellum in the spinocerebellar pathway?

Answer 30

1. Joint proprioception, cutaneous, muscle spindle and Golgi tendon organ information, which provide real time information about the positions of the limbs.
2. All of section 1, but integrated with descending modulatory and reflex arc messages, which provides information about how the body is moving and how the CNS proposes to move the limbs

Question 31

What is the pathway of the Non-integrated proprioceptive information in the spinocerebellar pathway?

Answer 31

This type of information enters the cerebellum at the INFERIOR peduncle via TWO ipsilateral pathways:
1. The POSTERIOR SPINOCEREBELLAR tract in which sensory axons from the LOWER limbs enter the
dorsal horn and synapse in a vertical column of cells known as CLARKES COLUMN. Post synaptic neurons in Clarkes Column ascend to the cerebellum as the posterior Spinocerebellar tract, and enters via the INFERIOR PEDUNCLE.
2. The CUNEOCEREBELLAR tract in which sensory axons from the UPPER limbs and NECK enter the dorsal horn, ascend ipsilaterally to the accessory CUNEATE nucleus and synapse onto the secondary neurons in the pathway. These secondary neurons then project in the cuneocerebellar tract and enter the cerebellum via the INFERIOR PEDUNCLE

Question 32

What is the pathway of the

integrated proprioceptive and motor information in the spinocerebellar pathway?

Answer 32

This type of information enters the cerebellum at either the SUPERIOR or BOTH the superior
and INFERIOR peduncles via two pathways:
1. The ANTERIOR SPINOCEREBELLAR tract in which sensory axons from the LOWER LIMBS enter the dorsal horn and synapse on a group of interneurons known as SPINAL BORDER CELLS found in the anterolateral parts of the anterior horn. Also synapsing onto these cells are COLLATERALS from DESCENDING MODULATORY MOTOR OUTPUT AND LOCAL REFLEX ARCS. From these cells arise the ascending anterior Spinocerebellar tract which crosses to the CONTRALATERAL side of the spinal cord. This pathway enters the cerebellum via the contralateral superior peduncle, whereupon the fibres re-cross back to the ipsilateral side. The cerebellum uses this information to measure the stability of the lower limbs and can influence descending modulatory motor pathways.
2. The ROSTRAL SPINOCEREBELLAR tract in which sensory axons from the UPPER LIMBS and NECK enter the dorsal horn, project to the anterolateral portion of the anterior horn where they synapse as before on spinal border cells. Axons arising from SPINAL BORDER CELLS at this level form the rostral Spinocerebellar pathway which enters the cerebellum ipsilaterally via BOTH the superior and inferior peduncles.

Question 33

What is friedreichs ataxia?
Cause?
Results in?

Answer 33

Inherited disease where spinocerebellar tract becomes increasingly ineffective

Caused by multiple repeats of a gene for the protein Frataxin which is responsible for iron metabolism in mitochondria

Results in:

1) progressively uncoordinated arm and leg movements
2) wide based ‘reeling’ gait
3) intention tremor