2. Somatosensory Pathways & Thalamus Flashcards Preview

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Flashcards in 2. Somatosensory Pathways & Thalamus Deck (38)
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1
Q

In what two types of nerves (broadly speaking) is sensory information carried to the CNS

A
  1. Spinal Nerves
  2. Cranial Nerves
    (Afferent Fibres in Each Case)
2
Q

What are the two subdivisions of the sensory system?

A
  1. Somatosensory System

2. Special Senses

3
Q

How is somatosensory information carried to the CNS?

A

Peripheral information carried from trunk/limbs in spinal nerves, or from the head in one cranial nerve (trigeminal, V).

4
Q

What is the somatosensory system

A

System comprising peripheral receptors and processing centres to produce the somatic sensory modalities. System reacts to diverse stimuli through the receptors, causing impulses to flow via sensory (afferent) nerves, through tracts in the spinal cord, and into the brain for processing

5
Q

List the somatosensory modalities and any sub-types.

A
  1. Touch (Discriminative & Non-Discriminative)
  2. Temperature
  3. Proprioception (Conscious & Unconscious)
  4. Nociception
6
Q

Differentiate between discriminative and non-discriminative touch?

A

i. Discriminative (density of receptors increase, able to discriminate between single point or multiple point touch. E.g. finger v shoulder)
ii. Non-Discriminative Compass points on skin of finger tips v skin on shoulder.

7
Q

What is proprioception?

A

Sense of the relative position of neighbouring parts of the body and strength of effort being employed in movement. It is provided by proprioceptors in muscle spindles of skeletal striated muscles and in joints. Essentially, a computation of the degree of contraction of muscle fibres allow this awareness.

8
Q

Differentiate between conscious and unconscious proprioception?

A
Conscious proprioception(CP):
Sensory information from from receptors in limbs and trunk is carried by peripheral nerves then the spinal cord (via faciculus cuneatus and faciculus gracillus) to the brainstem where it crosses over to the contralateral cerebral cortex; here precise information about the position and orientation of limbs and trunk is perceived and organized. (for schematic of this, see above) This information is utilized to facilitate complex motor activity, e.g. running/catching a frisbee, fighting Defects anywhere along the pathway are manifest as stumbling, knuckling; however, the routine gait and posture may be normal (depending on status of UCP...see below)

Unconcious Proprioception(UCP):

Sensory information from receptors in the limbs and trunk are transmitted via peripheral nerves to the spinocerebellar tracts where the information terminates on the ipsolateral cerebellum. This information is utilized to coordinate basic posturing during sitting, standing and simple gait activities
Defects anywhere along the pathway may be manifest as postural deficits, ataxia

9
Q

What is nociception?

A

Sensing of harmful stimuli such as intense chemical, mechanical or thermal stimulation.
Nociception utilises nociceptors or “pain receptors

10
Q

Outline the Organisation of Sensory Pathways

A

1st Order Neurons
• Pressure, vibration, tickling (from pacinian corpuscle, via large myelinated)
• Free nerve endings = pain, temperature (via small myelinated)

2nd Order Neurons
• Cell body lies in grey matter of the spinal (spinal nerve) or the brainstem (trigeminal).
• Axons pass through the ascending tracts of the cord and cross over (decussate).
• Most axons then project to the terminate in thalamus (aka gateway to cerebral cortex)

3rd Order Neurons/Thalamus
• Third order neurone projects from here to cerebral cortex (primary somatosensory cortex)
• Parietal lobe strip the primary destination for most of these axons.
• Right & left thalami control flow of sensory information to cortex.
• Joined across midline (interthalamic adhesion)
• Form lateral walls of third ventricle
• Largest part of diencephalon (between brainstem/cerebral hemispheres)
• All sensory information to brain synapses in the thalamus before going to the cortex (smell is a partial exception)
• Cranial nerve II (optic) connects directly to the thalamus
• Other cranial nerves send fibres to the thalamus from lower (brainstem) nuclei

11
Q

Describe the gross anatomy of the thalamus.

A
  • Internal capsule (white matter made up of axons squeezing through the grey matter of the thalamus)
  • Thalamic Nuclei
12
Q

What are the two types of spinal roots?

A

Dorsal

Ventral

13
Q

Where does the the spinal roots join? What properties does this structure have?

A

Join at spinal nerve trunk which is a mixed nerve

14
Q

What type of fibers are contained in the anterior and posterior rami and from what structure are they derived?

A

The anterior and posterior rami contain mixed fibres derived from the the spinal nerve trunk, which in turn is derived from the dorsal horn (sensory) and ventral horn (motor).

15
Q

Which is thicker the anterior or posterior rami? Why?

A

Anterior ramus thicker as carries more info from ant trunk and whole of limbs

16
Q

What is the white matter of the spinal cord made up of?

A

Consists of longitudinally running nerve fibres

17
Q

How are the nerve fibers of the white matter of the spinal cord organised?

A

Organised into a series of Ascending and Descending Tracts

18
Q

Name the 3 ascending pathways (tracts) of the spinal cord?

A

– Dorsal Columns
– Spinothalamic Tract
– Spinocerebellar Tract

19
Q

What sensory information is transmitted through the Dorsal Column Pathway?

A

Discriminative (fine) touch
Vibratory sense
Conscious proprioception

20
Q

Describe the location and course of the first order neurones of the Dorsal Column Pathway?

A

Enter the cord & ascend ipsilaterally in the gracile fasciculus (lower limb, lower trunk) and the cuneate fasciculus (upper limb, upper trunk) to medulla oblongata and synapse (nucleus gracilis or nucleus cuneatus)
(The cell bodies of somatic sensory afferent fibers lie in ganglia throughout the spine)

21
Q

Describe the location and course of the second order neurones of the Dorsal Column Pathway/Medial Lemniscus?

A

Second order axons cross and ascend as contralateral medial lemniscus (or trigeminal lemniscus) to the thalamus (VPL, VPM nucleus)

22
Q

Describe the location and course of the third order neurones of the Dorsal Column Pathway/Medial Lemniscus?

A

Third order neurones project from thalamus to postcentral gyrus ‘somatotopically’

23
Q

What are the possible effects of lesions affecting the Dorsal Column Pathway?

A

Lesions result in loss of fine tactile sensation and interference with proprioception
e.g. Sensory ataxia (ataxia = loss of co-ordination)

24
Q

Where is the primary somatosensory area of the sensory cortex located?

A

In the Post-central Gyrus of the parietal lobe

25
Q

Describe the pathological principles behind sensory ataxia?

A

Important factors to maintain posture:
o Visual information, and
o Proprioceptive information (via dorsal column system)
Normal person can maintain their balance when standing with their eyes closed

With a dorsal column lesion: can maintain balance with the eyes open (but begins to totter when asked to close their eyes) - Romberg’s sign
• Damage to dorsal columns may result from
1. Vit B12 deficiency (degeneration of the cord)
2. Multiple sclerosis (demyelination)
3. Diseases of peripheral nerves (after infections sometimes)
4. Tertiary syphilis (tabes dorsalis)

26
Q

What is Romberg’s sign?

A

Patient can maintain balance with the eyes open (but begins to totter when asked to close their eyes) - Romberg’s sign

27
Q

What sensory information is transmitted through the Spinothalamic Pathways?

A

Pain
Temperature,
Non-discriminative (crude/gross) touch
Pressure

28
Q

Describe the location and course of the first order neurones of the Spinothalamic Pathway?

A

First order fibres enter the cord from peripheral receptor, carry impulse via the dorsal horn of grey matter (*may travel up one or two segments before synapsing with second order neurone)

29
Q

Describe the location and course of the second order neurones of the Spinothalamic Pathway?

A

Second order axons cross the midline (almost immediately) and ascend as the contralateral spinothalamic tract (STT) to the thalamus (VP nucleus)

30
Q

Describe the location and course of the third order neurones of the Spinothalamic Pathway?

A

Third order neurones project from thalamus to postcentral gyrus ‘somatotopically’

31
Q

Describe the differences in the lamination of the dorsal and spinothalamic ascending tracts

A
  • In dorsal columns, fibres from the lower part of the body (lower sacral) ascend closest to the midline. As fibres from higher levels enter, they layer themselves progressively in a lateral direction so that the uppermost (cervical) fibres lie most lateral
  • In the spinothalamic tracts, fibres are layered progressively so that those from the lower part of the body lie on the outer rim, and those from the upper body in the inner rim

(i.e. Lateral/medial orientation of inferior/superior fibers is reversed between dorsal and spinothalamic tracts.)

32
Q

Where do fibres carrying pain and temperature tend decussate?

A

Pain temperature fibres cross almost immediately upon entering the spinal cord and ascend contralaterally.

33
Q

Where do fibres carrying proprioceptive/fine touch tend decussate?

A

Proprioceptive/fine touch fibres ascend ipsilaterally and cross in medulla

34
Q

How will lesions of higher brainstem or cerebral hemisphere affect the different somatosensations?

A

Lesions of higher brainstem or cerebral hemisphere lead to loss of all sensation on contralateral side

35
Q

How will a hemisection of the spinal cord affect the different modalities of somatosensation? What is this type of lesion known as?

A

Hemisection of Cord (Brown-Sequard syndrome) (Tumour, gunshot, etc)
• Immediate cross over of spinothalamic pathway sensory pathways upon entering the cord.
• The dorsal column pathway does not cross over immediately.
• Therefore; Contralateral loss of pain & temperature senses beginning a few segments below lesion
• Ipsilateral loss of fine tactile & limb position sense at lesion level and below

36
Q

What modalities of somatosensation does the Spinocerebellar Pathway transmit?

A

Unconscious propriception

Also some pain/pressure information

37
Q

What course does the spinocerebellar pathway take? How does this course affect the transmission of the information?

A
  • Do not project to the thalamus/cerebral cortex, and hence does not reach conscious awareness
  • MOST proprioceptive information (tendons, muscles and joints) is transmitted directly to the cerebellum (two-neurone pathway)
  • Provides the cerebellum with minute- by-minute feedback on what the muscles are doing
  • Spinocerebellar pathways remain IPSILATERAL!
  • Note: Posterior tract does not cross, anterior tract ‘double-crosses’!
38
Q

What would be the effect of cerebellar lesions of somatosensation?

A

Cerebellar lesions tend to produce ipsilateral malfunctioning (versus cerebral lesions)