Spine and Sensory Pathways

Question 1

1. what do dorsal and ventral mean above the midbrain-diencephalic junction?
2. what do dorsal and ventral mean below the midbrain-diencephalic junction?
3. what are axons bundled into and by?
4. what are fascicles bundled into and by?

Answer 1

1. dorsal = superior; ventral = inferior
2. dorsal = posterior; ventral = anterior
3. into fascicles by perineurium
4. into nerves by epineurium

Question 2

1. what are spinal roots?
2. what do dorsal spinal roots contain? Where are the cell bodies of these axons found?
3. what do ventral spinal roots contain? Where are the cell bodies of these axons found?
4. what is the grey matter of the spinal cord divided into?
5. what do these dorsal regions of grey matter contain?
6. what do these ventral regions of grey matter contain?
7. what do the lateral regions of grey matter contain?
8. what emerge from spinal nerve? What do they supply?

Answer 2

1. bundles of axons that connect the spinal nerve to its spinal cord segment.
2. sensory axons. Cell bodies are found within dorsal root ganglion, that are found within the dorsal root
3. motor axons. Cell bodies are found within the spinal cord
4. horns
5. dorsal horns contain cell bodies and axons of inter neurons, and afferent fibres from sensory neurons
6. ventral horns contain cell bodies of somatic motor neurons
7. laternal horns contain the cell bodies of autonomic neurons
8. rami. The dorsal ramus carries information that supplies muscles and sensation to the human back. The ventral ramus carries information that supplies muscles and sensation of anterolateral regions

Question 3

1. What do nerves from ventral rami tend to form?
2. what is the purpose of these structures?
3. what is a dermatome?
4. what is a myotome?

Answer 3

1. plexuses
2. more effective branching
3. the area of skin supplied by one spinal nerve
4. the area of muscle supplied by one spinal nerve

Question 4

1. describe the process of neural tube development
2. how is the neural crest formed?
3. what does the neural crest give rise to? (5)
4. what do cranial neural crest cells do?
5. what are somites?
6. what is the notochord?

Answer 4

1. central region of trilaminar disc ectoderm becomes the neural plate
   neural plate begins to invaginate. Folds towards each other to form a tube. Neural tube is surrounded by surface ectoderm
2. cells at the neural fold undergo epithelial mesenchymal transition and migrate in the gaps between the newly forming tissues.
3. cranial and sensory ganglia and nerves, schwann cells, pigment cells, adrenal medullary cells and dentine/cranial bones, muscles and connective tissues
4. migrate to the pharyngeal arches, to give rise to bone, muscle, teeth, cranial nerves and connective tissue associated with structures developing in each arch
5. mesodermal structures that give rise to the musculoskeletal structures of the spine
6. supporting structure that is a precursor to the vertebral column and an important signalling centre.

Question 5

1. Name the 3 basic regions of the newly developing brain
2. what does these further divide into?
3. what are the adult derivatives of these structures?
4. what do the alar plates of the spinal cord give rise to?
5. what do the basal plates of the spinal cord give rise to?

Answer 5

1. prosencephalon, mesencephalon and rhombencephalon
2. procencephalon divides into telencephalon and diencephalon
   rhombencephalon divides into metencephalon and myencephalon
3. telencephalon - cerebral hemispheres
   diencephalon - thalamus and basal ganglia
   mesencephalon - midbrain
   metencephalon - pons and cerebellum
   myencephalon - medulla
4. dorsal roots
5. ventral roots

Question 6

1. in which regions do the following develop?
   a) lateral ventricles?
   b) 3rd ventricle?
   c) 4th ventricle
2. what is found within the C shape of the lateral ventricles?
3. which structures encircle the lateral ventricles?
4. what type of matter are the thalamus and basal ganglia?
5. what is the name given to the white fibres that pass through the thalamus between the cerebral hemispheres and brainstem?

Answer 6

1a) telencephalon
b) diencephalon
c) rhombencephalon

2. thalamus and basal ganglia
3. fornix and hippocampus
4. grey (cell bodies)
5. internal capsule

Question 7

which cranial nerves are associated with the following pharyngeal arches:

1. first
2. second
3. third
4. fourth

• are these cranial nerves sensory, motor or mixed?

Answer 7

1. trigeminal
2. facial
3. glossopharyngeal
4. vagus

• mixed cranial nerves

Question 8

define the following:

1. conus medullaris
2. filum terminale
3. cauda equina
4. lumbar cistern
5. in which regions of the spinal cord are there enlargements and why?

Answer 8

1. tapered, lower end of the spinal cord. Found at the level of L1/L2
2. strand of fibrous tissue extending down from the conus medullaris. Modification of pia mater
3. bundle of spinal nerves emerging from L2-5 and S1-5. They continue to travel in the vertebral canal, exiting at their appropriate vertebral foramen
4. enlargement of subarachnoid space between conus medullaris and inferior end of subarachnoid space (around S2) filled with cauda equina, fillum terminale and CSF. It is the site of a lumbar puncture
5. in the cervical and lumbosacral regions. This is die to an increased number of lower motor neurons to supply the limbs.

Question 9

1. what sensory modalities to dorsal columns carry?
2. describe the 2 first order neuron pathways of dorsal columns relating to upper limbs and lower limbs
3. where do dorsal columns decussate. Following decussation, where do they travel?
4. What does dysfunction of dorsal columns lead to?

Answer 9

1. fine touch, vibration and proprioception
2. signals from upper limbs travel in fasiculus cuneatus (lateral dorsal column) and synapse onto the nucleus cuneatus of the medulla
   signals from the lower limbs travel in the fasiculus gracialis (medial dorsal column) and synapse onto the nucleus gracialis of the medulla
3. medulla. Travel in the medial lemniscus to the thalamus
4. sensory ataxia

Question 10

1. what sensory modalities to the spinothalamic tracts carry?
2. what do the anterior spinothalamic tracts carry?
3. what do the lateral spinothalamic tracts carry?
4. what is the path of first order neurons?
5. what is the path of second order neurons?
6. what is the path of third order neurons?
7. where do these tracts decussate?

Answer 10

1. crudely localised touch, pain and temperature
2. crude touch and pressure
3. pain and temperature
4. periphery to spinal cord (substantia gelatinosa)
5. substantia gelatinosa to thalamus
6. thalamus to cerebrum
7. second order neurons decussate within the spinal cord

Question 11

1. what sensory modalities do spinocerebellar tracts carry?
2. via which tracts is info from the lower limbs carried?
3. via which tracts is info from the upper limbs carried?
4. where do these tracts decussate?

Answer 11

1. unconscious proprioception
2. anterior and posterior spinocerebellar tracts
3. cuneocerebellar and rostraspinocerebellar tracts
4. they do not decussate. info is carried to ipsilateral cerebellum

Question 12

what is the pathway and the role of the following:

1. spino-olivary tracts
2. spinotectal tracts
3. spinoreticular tracts

Answer 12

1. project to accessory olivary nuclei and cerebellum. Involved in movement and co-ordination associated with balance
2. project to superior colliculi . Involved in reflex turning of head and eyes to cutaneous stimulation
3. project to reticular activating system. Involved in arousing consciousness through cutaneous stimulation

Question 13

describe the basic process of an action potential

Answer 13

1. signals arrive via synaptic transmission from presynaptic neurons and are received by dendrites
2. electronic potentials pass from dendrites to soma
3. in the soma, negative and positive electrotonic potnetials are summed
4. sum of electrotonic potentials reach axon hilliock. If the net electrical potential exceeds threshold at the axon hillock, an action potential is generated and is propagated down the axon.

Question 14

1. what is the resting membrane potential of a neuron?
2. what is the charge inside and outside the neuron?
3. what are the relative concentrations of sodium and potassium inside and outside the neuron?
4. how is the resting membrane potential generated? (3)
5. what does the nernst equation consider?
6. what does the goldmann equation consider?
7. why is the resting membrane potential close to but not exactly the equilibrium potential for potassium?

Answer 14

1. -70mV
2. negative inside, positive outside
3. high Na and low K outside
   low Na and high K inside
4. low protein permeability - traps negatively charged proteins inside the cell
   active transport via Na/K+ pump - leaves behind a negative charge
   high potassium permeability - potassium moves out of cell along concentration gradient. leaves behind a negative charge. K then moves back into cell along electrical gradient until equilibrium is established. This high K+ permeability drives Vm towards Ek
5. equilibrium potential of an ion.
6. valence, concentration and permeability of multiple ions
7. high K+ permeability drives it closely to Ek, however there is some Na permeability which drives it slightly towards ENa.

Question 15

1. what is the state of ion channels during resting potential?
2. what is the voltage of threshold?
3. what happens to ion channels when threshold is reached?
4. at what membrane potential is repolarisation triggered?
5. what happens to ion channels during repolarisation?
6. what happens to ion channels during hyperpolarisation?

Answer 15

1. Nav and Kv channels closed; K+ leak channels open
2. +55mV
3. Nav channels open causing the influx of sodium which drives Vm towards ENa
4. +40mV
5. Kv channels open, and Nav channels become inactivated. Potassium efflux drives Vm back towards Ek
6. Kv channels remain open to establish resting membrane potential. Inactivation gate of NaV channels slowly opens (refractory period)

Question 16

1. how are subthreshold stimuli encoded?
2. how are suprathreshold stimuli encoded?
3. what is saltatory conduction?
4. what is conduction velocity dependent upon? (2)

Answer 16

1. magnitude of graded response
2. frequency of AP firing
3. in myelinated axons, action potentials appear to “jump” from node to node; this increases conduction velocity
4. axon diameter - conduction velocity is directionally proportional to axon diameter (this is because there is less resistance facing the ion flow)
   myelination

Question 17

1. what is multiple sclerosis?
2. how does teterodoxin affect the action potential?
3. how do local anaesthetics work? On which fibres do they work best? How can their action be prolonged?

Answer 17

1. Autoimmune disease whereby T cells attack myelin sheaths
2. blocks Nav channels. People die of respiratory paralysis
3. reversibly block Nav. Work best on small diameter fibres associated with pain sensation (A delta and C fibres). Effects can be prolonged by administering with vasoconstrictors

Question 18

1. define pain
2. describe 4 reasons why pain is good
3. describe 2 reasons why pain is bad
4. what is somatic pain?
5. what is visceral pain?
6. what is acute pain?
7. what is chronic pain?

Answer 18

1. an unpleasant sensory and emotional experience associated with actual or potential tissue damage. A subjective response
2. protects body from harm. teaches us to avoid harmful stimuli. forces to rest an injured part of the body. during sleep, it makes us move to prevent bedsores and skeletal strain
3. serves no useful function in many instances. persists even when tissues have healed
4. a type of nociceptive pain that is also referred to as skin pain, tissue pain, or muscle pain. Superficial - skin, a sharp breif pain. Deep - muscles or tissues. An aching pain that lasts longer
5. pain from organs. dull, aching pain that is poorly localised
6. pain that resolves when injury heals, is well defined, and adequately treatable
7. persistent pain that has a long duration and ill defined onset. No apparent biological function and is difficult to treat

Question 19

1. describe the basic pain pathway
2. what type of nerve endings are nociceptors?
3. which afferent fibres are associated with:
   a) somatic sensation
   b) pain sensation
   4 which fibres carry info from:
   a) thermal and mechanical nociceptors
   b) polymodal nociceptors

5a) what is first pain?
5b) by which fibres is it transmitted by?
6a) what is second pain?
6b) by which fibres is it transmitted by?

7a) what is congenital analgesia?
7b) which genes are mutations causing congenital analgesia found?

Answer 19

1. nociceptors > primary afferent > dorsal horn > spinothalamic tracts > higher brain centres
2. free nerve endings
   3a) Aɑ and Aβ
   3b) Aδ and C
   4a) Aδ
   4b) C

5a) sharp, easily localised pain with a rapid onset and short duration
5b) Aδ
6a) dull, poorly localised pain with a slow onset and short duration
6b) C

7a) congenital insensitivity to pain associated with recurrent injury but otherwise normal sensory responses
7b) Nav channels on first order nociceptive afferents - unable to become depolarised thus transmit pain action potential

Question 20

1. In which spinal cord lamina are neurons innervated by Aδ and C fibres found? What are these lamina otherwise known as?
2. where do second order neurons take information?
3. which spinal column carries pain?
4. which brain structure acts as a relay centre, and is associated with pain perception?
5. which structures are associated with the behavioural and emotional responses to pain?
6. which brain structure is associated with alertness of pain?
7. which brain structure is associated with localisation of pain?
8. which cranial nerves carry nociceptive signals from the face and head

Answer 20

1. I and II. Lamina II is also known as the substantia gelatinosa
2. laminae IV-VI (via relay neurons)
3. spinothalamic tracts
4. thalamus
5. hypothalamus and limbic system
6. reticular formation
7. somatosensory cortex
8. 5,7,9,10

Question 21

1. what is referred pain?

2. what is the explanation for referred pain?

Answer 21

1. pain is located away from or adjacent to the organ involved
2. few neurons in dorsal horn are specialised solely for transmission of visceral pain. Nociceptive afferents from internal afferents and skin enter the spinal cord and target overlapping populations of spinal neurons. This results in perception of cutaneous stimulation when a visceral nociceptor is activated

Question 22

1. what is “phantom limb”?
2. what is “phantom limb pain”
3. what is the explanation for phantom limb sensation and pain

Answer 22

1. sensation that a missing limb is still attached to the body and moving appropriately
2. chronic pain in a phantom limb
3. SOMATOTOPIC REMAPPING - another part of the body takes over the region of the somatosensory cortex that was previously occupied with the removed limb. Sensation in this part of the body can result in perceived sensation in the phantom limb

Question 23

1. describe gate theory
2. what is hyperalgesia?
3. what is allodynia?
4. how are hyperalgesia and allodynia induced?

Answer 23

1. inhibitory interneurons in the spinal cord innervate projection neurons
   nociceptive innervation of this interneuron is inhibitory (therefore activates projection neurons via disinhibition); somatosensory innervation of this interneuron is excitatory.
   Co-activation of somatosensory and pain fibres reduces the excitation of the projection neurons. Hence why pain can be reduced by mechanical stimulation (e.g rubbing)
2. increased sensitivity/reduced threshold to pain from a stimulus that normally provokes pain
3. pain from a stimulus that does normally provoke pain
4. inflammatory mediators such as histamine, prostaglandins and substance P sensitise peripheral nociceptors

Question 24

describe the descending endogenous analgesic pathways that exogenous opiods exploit

Answer 24

descending pathways from periaqueductal grey and raphe nuclei use endogenous opioids, acting on opiod receptors, which decreases neurotransmitter release from nociceptive primary afferents