Canonical Question
Neuro – Pathways
Master answer
- Somatic senses: touch, proprioception, pain, temperature
- Somatosensory fibers:
- Non-myelinated fibers (type C): slowest; sense burning pain, hot temperature
- Small myelinated fibers (type Aδ): faster; sense sharp pain, gross touch, cold temperature
- Large myelinated fibers (type A-α; A-β): fastest; sense proprioception, vibration, fine touch
SOMATOSENSORY PATHWAYS
- Carry somatosensory input up spinal cord to brain
- Consist of 4-neuron relay
- 1st order neuron/afferent sensory neuron: has sensory receptors, converts stimuli into impulse
- 2nd order neuron: cell body in spinal cord or brainstem, synapses with 3rd-order neuron
- 3rd order neuron: cell body in thalamus, sends signal to somatosensory cortex
- 4th order neuron/cortical neuron: cell body in sensory cortex
- Includes medial lemniscal/posterior pathway, spinothalamic/anterolateral pathway
MEDIAL LEMNISCAL PATHWAY
- Carries information about fine touch, proprioception
- Large myelinated fibers of 1st order neurons run to spinal cord
- Neurons run through posterior/dorsal funiculus of spinal cord
- Via cuneate fascicle for arms, chest
- Via gracilis fascicle for trunk, legs
- 1st, 2nd order neurons synapse in medulla
- 1st synapse
- 2nd order neurons run to medial lemniscus, decussate; run through pons, midbrain to the thalamus
- 2nd, 3rd order neurons synapse in thalamus
- 2nd synapse
- 3rd order neurons run to sensory cortex in parietal lobe
- 3rd, 4th order neurons synapse in sensory cortex
- 3rd synapse
- Some 1st order neurons synapse with interieurons at posterior horn
- Axons run to anterior horn, synapse directly with motor neuron
- Important for reflexes
SPINOTHALAMIC (ANTEROLATERAL) PATHWAY
- Carries information about pain, temperature, crude touch
- Small/non-myelinated fibers of 1 order neurons run to spinal cord
- Small myelinated fibers: sharp pain, cold temperature
- Non-myelinated fibers: hot temperature, burning pain, crude touch
- 1st, 2nd order neurons synapse in posterior horn of spinal cord/1st synapse
- Small myelinated fibers: enter through dorsal root, bend upwards, travel through two vertebral segments
- Non-myelinated fibers: follow same pathway but synapse with interneurons first, AKA before reaching posterior horn
- 2nd order neurons decussate, cross to anterior horn through central canal
- Neurons then carried through one of two tracts to thalamus
- Lateral tract: carries information for pain, pressure, temperature through lateral funiculus
- Anterior tract: carries information for crude touch through anterior funiculus
- 2nd, 3rd order neurons synapse in thalamus/2nd synapse
- 3rd order neurons run to sensory cortex in parietal lobe
- 3rd, 4th order neurons synapse in sensory cortex /3rd synapse
SOMATOSENSORY RECEPTORS
- Perceive general somatic senses. Includes:
- Mechanoreceptors: for touch
- Proprioceptors: for proprioception
- Thermoreceptors: for temperature
- Nociceptors: for pain
| MECHANOSENSORS | Meissner/ tactile corpuscles | – Sensitive to light touch – Encapsulated; located in dermis of hairless skin – Fast adapting; small receptive fields |
| Merkel (tactile) discs | – Sensitive to pressure – Non-encapsulated; located in epidermis of hairless skin – Slow adapting; small receptive fields | |
| Ruffini (bulbous corpuscles | – Sensitive to skin stretching – Encapsulated; located in dermis of all skin – Slow adaptingl big receptive fields | |
| Pacinian (lamellar) corpuscles | – Sensitive to vibration – Encapsulated; located deep in dermis/ subcutaneous tissue of all skin – Fast adapting; big receptive fields | |
| PROPRIOCEPTORS | Muscle spindle | – detect when muscle stretched – Located throughout perimysium, AKA connective tissue around muscle cells |
| Golgi tendon organ | – Detect when tendon stretched – Located in tendons close to muscle insertion | |
| Joint receptors | – Detect joint position, motion – Located in joint | |
| THERMORECEPTORS | – Transient receptor potential channels mediate sensations – At extremely cold/hot temperaturs, nociceptors take over | |
| NOCICEPTORS | Thermals | sense extremely cold/hot temperatures |
| Mechanical | sense excess pressure/deformation | |
| Polymodal | Sense combination of both |
Exam appearances
| Exam | Exact wording | Relationship | Success |
|---|---|---|---|
| 2022B Q03 | Briefly outline the major somatosensory pathways of the body (excluding cranial nerves) | historical_member | — |