Somatosensory Pathways

Question 1

What are the dorsal columns and how are they divided? What is important to remember about them?

Answer 1

• Sensory nerve fibres mediating “discriminative touch” perception (Ab diameter) project into the dorsal fasciculi (also known as posterior columns or dorsal columns). These are large tracts of white matter in the dorsal spinal cord.
• The dorsal or posterior columns are divided into the fasciculus gracilis and fasciculus cuneatus.
• Fasciculus gracilis (in English gracile tract) contains afferents from the lower limb and genitalia.
• Fasciculus cuneatus (cuneate tract) contains afferents from the upper limb.
• Remember that fibres in the dorsal columns are not post-synaptic; they are branches of the primary afferent fibres in the peripheral nerves. (connected directly to peripheral nerve with cell body in dorsal root ganglion)
• Dorsal column fibres ascend the spinal cord on the same side they enter it.

See lecture notes for pictures

Question 2

What do the dorsal fasciculi project to? What are these known as?

Answer 2

• The dorsal fasciculi project to the cuneate and gracile nuclei (nucleus cuneatus & nucleus gracilis, DCN) in the lower medulla of the brainstem.
• These nuclei are together known as the dorsal column nuclei. They are columns of cells extending from the top of the spinal cord several mm rostrally into the medulla.
• The first synapse in the discriminative touch pathway is in the dorsal column nuclei.

Question 3

Where is the first synapse in the discriminative touch pathway?

Answer 3

• The first synapse in the discriminative touch pathway is in the dorsal column nuclei.

Question 4

Where do post-synpatic fibres emerge from and where do they decussate? What are these and what do they form?

Answer 4

• Post-synaptic nerve fibres leave the base of the cuneate and gracile nuclei and cross over (decussate) to the opposite side of the neuraxis. These are called internal arcuate fibres. They form the medial lemniscus - a tract which travels to the somatosensory thalamus in the brain.

Question 5

What does the medal lemniscus consist of?

Answer 5

• The medial lemniscus consists of second order fibres in the somatosensory pathway. The medial lemniscal fibres travel to the somatosensory thalamus in the brain. N.B. The dorsal columns and the medial lemniscus are sometimes grouped together as the dorsal column-medial lemniscal or ‘DCML’ pathway.

Question 6

How does cutaneous afferents from the face travel? How do post-synaptic fibres travel?

Answer 6

• Cutaneous afferents from the face travel in cranial nerves and enter the trigeminal nucleus (situated rostrally in medulla). Post-synaptic fibres from the trigeminal nucleus decussate and run alongside the medial lemniscal fibres from the body. The face afferents end in the VPM thalamus (ventro-postero-medial nucleus), which is the thalamic relay for sensation from the face

Question 7

Where are the cell bodies of the trigeminal nerve located?

Answer 7

• The cell bodies of the trigeminal nerve are in the trigeminal ganglion, which is the equivalent of a dorsal root ganglion. The Trigeminal nerve is discussed further in the cranial nerve lectures
• *called ‘closed’ because below lowest point of 4th ventricle

Question 8

What is the somatosensory thalamus and what does it do?

Answer 8

• The somatosensory thalamus is the relay station for information from the body to reach the cortex.: All inputs to the somatosensory cortex (except for inputs from other cortical areas) must come via the thalamus.
• Cortex instructs the thalamus to pass on any inputs from one particular chosen skin area (for example the right hand) but to ignore or block stimuli from any other part of the body (cortical selective attention)

Question 9

How is the face represented in the thalamic nuclei?

Answer 9

• In the thalamic nuclei, the face is represented medially (VPM) and the upper limb more laterally (VPL), and then the lower limb most laterally (VPL) . This organised body map is called a homunculus. Axons from the thalamus to somatosensory cortex twist round 180 degrees, so that in the somatosensory cortex the leg is medial-most, the arm is in the middle and the head and face most lateral.

Question 10

What is the primary somatosensory cortex?

Answer 10

• The primary somatosensory cortex is a strip of cortex running medio-laterally just posterior to the central sulcus (sulci are grooves or fissures in the cortical surface)

Question 11

What is the cortical map?

Answer 11

• The primary somatosensory cortex has a complete map of the body surface but with a gross distortion of the relative size of different body areas.
• Areas of skin with the highest density of touch and proprioceptive receptors occupy the largest areas of the cortical map.
• The hands, lips & tongue have the highest density of touch receptors and have the largest areas in the homunculus.
• There is plasticity in this map of the body surface. If a limb is amputated the area devoted to that limb shrinks and the area devoted to surrounding parts of the body expands.
• The somatosensory cortex is a map. It tells you where something is on your body and its touch and texture. It does NOT tell you about its temperature or whether it is painful. These modalities are processed in other cortical regions.

Question 12

What areas have the largest representations in the somatosensory cortex?

Answer 12

• The parts of our body that we use to discriminate shape and texture (hands and lips) have the largest representations in the somatosensory cortex.

Question 13

How do you test the function of the CSML pathway to the cortex?

Answer 13

• The patient is screened for any abnormalities in their abilities in the tests below.
• Fine touch/stereognosis tested by asking the patient to recognise common objects hidden within a cloth bag using touch alone.
• Vibration sense tested using a tuning fork placed along a bony prominence of the desired corresponding spinal level(s) to be tested.
• Barognosis tested by asking subject to determine the approximate weight of an object placed in the hand.
• Graphesthesia can the subject recognise writing on the skin by touch.
• Kinaesthesia & proprioception tested using the subject’s ability to detect an externally imposed passive movement, or the ability to reposition a joint to a predetermined position.
• Proprioception can also be assessed using Romberg’s test.
• In the Romberg test, the patient is stood up and asked to close their eyes. A loss of balance is interpreted as a positive Romberg sign.

Question 14

What is Romberg’s test?

Answer 14

• The Romberg test assesses proprioception from leg muscles and joints. This is the ability to sense the position of our limbs and thus our position in space.
• The standing patient is asked to stand upright and close his or her eyes. An instability (wobble) followed by a loss of balance is interpreted as a positive Romberg’s test.
• This inability to balance with eyes closed is called sensory ataxia: it often occurs with damage to the dorsal columns

Question 15

What is pain and how is it activated? What receptors are responsible for this?

Answer 15

• Pain is a “neuronal program” that is normally activated by tissue damage
• The receptors specific for tissue damage called nociceptors
• Nociceptors are free nerve endings found at the ends of Ad (small myelinated) and C (unmyelinated) nerve fibres
• Free nerve endings have no connective tissue around the axon terminal

Question 16

How is pain sensation normally activated?

Answer 16

• Requires a noxious (tissue damaging) stimulus.
• Has protective function (minimises injury, promotes recovery).
• Has acute (fast pain) and chronic (slow pain) components

Question 17

What is fast pain? What is normal pain?

Answer 17

• Fast pain (Ad afferents) triggers withdrawal reflexes to minimise injury. Fast pain goes away after the initial injury to be followed by ‘slow pain’ (C fibres) which may continue until the wound has healed
• Normal or Nociceptive pain can be treated by conventional analgesics eg NSAIDs, steroid anti-inflammatory agents, opiates

Question 18

What are polymodal noiceptors? What does tissue damage release? What does this do? What does this inflammation cause?

Answer 18

• Because they produce generator potentials to several different chemicals as well as physical deformation of the ending, free nerve endings are referred to as polymodal nociceptors.
• Tissue damage releases pro-inflammatory chemicals into the extracellular space. These include bradykinin, prostaglandins and serotonin.
• These chemicals and others in the inflammatory fluid around the ending open special sodium channels in the endings (known as TTX resistant channels) that stay open for prolonged periods; this leads to a long lasting depolarisation of the ending and thus tonic generation of action potentials.
• Thus inflammation produces long lasting pain

Question 19

What will a painful stimulus activate? Where do these receptors project?

Answer 19

• A painful stimulus will activate touch and wide dynamic range receptors as well as nociceptors. These receptors project in the anterior spinothalamic tract. This ends in the ventroposterolateral (VPL) nucleus of the thalamus along with the medial lemniscus.
• Because it projects to the thalamus and somatosensory cortex the anterior spinothalamic tract enables us to localise pain on our body surface

Question 20

Where do afferents from nociceptive C fibres and A-delta fibres terminate?

Answer 20

• Afferents from nociceptive C fibres and A fibres terminate in laminae I of the dorsal horn.
• Cells in lamina I project across the midline (decussate) in the spinal cord and ascend in the contralateral anterolateral part of the cord as the lateral spinothalamic tract.

Question 21

Where does the lateral spinothalamic tract project to?

Answer 21

• This tract does NOT project to the VPL but branches profusely in the brainstem where it connects to the periaqueductal grey (PAG).
• It also connects to the thalamic mediodorsal nucleus (MDvc), which connects to the anterior cingulate cortex.
• It also connects to the ventromedial thalamic group (VM) which send thalamocortical fibres to the cortex of the insula
• The lateral spinothalamic tract projects to several parts of the forebrain. The sensation and unpleasant nature of pain is mediated by the projection to the cingulate and insula cortices of the limbic system. Pain also produces arousal via the periaqueductal grey (PAG) of the midbrain and thalamus.

Question 22

What does co-activation of the anterior and lateral spinothalamic tracts allow for?

Answer 22

• Co-activation of the anterior and lateral spinothalamic tracts by a nociceptive stimulus enables you to localise the pain (anterior) and assess its unpleasantness (lateral

Question 23

What is the anterior cingulate cortex?

Answer 23

• The anterior cingulate cortex which appears to mediates our sensation of pain is part of the the Limbic System. This includes a group of cortical and subcortical nuclei found on the medial aspect of the frontal, parietal and temporal lobes.

Question 24

What is the insula and what does it do?

Answer 24

• Hidden inside the lateral sulcus is a fold of cortex known as the insula.
• The functions of the insula are still not fully understood but imaging techniques show that it is active when someone is in pain.
• It may contribute to the perception of pain, but how this is divided up between cingulate cortex and insula is not clear.

Question 25

What is referred pain and what causes it?

Answer 25

• Pain from internal organs is difficult for the brain to comprehend as it has no body schema or ‘map’ of the interior of the body to project the pain on to.
• Normally the pain is felt in the region of the body surface that has its afferent input in the same spinal segment (dermatome) that the active visceral pain fibres terminate.
• Note that stomach injury can be felt in the back. It is something of a mystery why injury to the liver or gallbladder should be felt in the shoulder!

Question 26

How might patterns change help diagnose the location of a lesion in the nervous system?

Answer 26

• Patient A: Brown-Sequard (spinal lesion)
• Patient B: stroke (cortical lesion)
• Patient C: diabetic neuropathy (pns lesion)
  See picture in lecture notes!

Question 27

What does the anterior spinothalamic tract do? What does it contain? Where does it project and how does this allow it to function?

Answer 27

• This tract is said to convey ‘crude touch’. But what exactly does this mean?
• We know the anterior spinothalamic tract contains axons with wide dynamic range receptive fields. These fibres have a convergent input from several different types of receptor, so cannot be used to identify a particular type of stimulus.
• However the receptive fields of the fibres are small, and the anterior spinothalamic tract projects to the somatosensory thalamus and cortex, which means this pathway enables you to know where a stimulus is.
• It is clear that information in this tract can be combined in the brain with information in the lateral spinothalamic tract. The latter tract tells your brain if something is painful, the former tells your brain where the pain is.
• But what sensations are transmitted by the anterior spinothalamic tract if the lateral spinothalamic tract is silent???
• One detail rarely mentioned in textbooks or the web is that the anterior spinothalamic tract contains a large number of fibres with receptive fields on the genitals and surrounding areas.
• Bilateral anterolateral chordotomy for the relief of pain, which severs the anterior spinothalamic tracts, abolishes erection, ejaculation and orgasm in the male, orgasm in the female, as well as sexual arousal in both.
• It is therefore possible that the anterior spinothalamic tract mediates the affective, particularly pleasurable aspect of tactile stimulation, although this has not been definitely proven.