Neurophysiology 8 Flashcards
What are the somatosensory pathways?
Sensory pathwaysconsist of the chain of neurons that are responsible for the perception of sensations, from receptor organ to cerebral cortex.
Somatosensory stimuli activate a chain of neurons which starts with the peripheral first-order (1°) afferent and ends in the cerebral cortex.
Each somatosensory pathway is named after a major tract or nucleus in the pathway.
Describe the anatomical features within a somatosensory pathway
Within each somatosensory pathway,
1. The 1° afferent neuron synapses with 2° afferent neuron(s) in a spinal cord or brain stem nucleus.
2. The 2° afferent may synapse with 3° afferent neurons in the spinal cord or may ascend to synapse with 3° afferent neurons in the thalamus.
3. There is a decussation (i.e., axons crossing the midline to the opposite side of the spinal cord or brain stem) in each somatosensory pathway below the level of the thalamus
4. All somatosensory pathways include a thalamic nucleus.
5. The thalamic neurons send their axons in the posterior limb of the internal capsule to end in the cerebral cortex.
6. The ultimate destination of these sensory stimuli is the somatosensory cortex
Differentiate the types of peripheral somatosensory axons
- Group I and II 1° afferent axons
Form the muscle/tendon receptors
Carry body proprioceptive information
Have the largest diameter and the thickest myelin. - Type C 1° afferent axons
Form free nerve endings
Carry dull pain, deep pain, crude touch or warm/hot information
Are the smallest 1° afferent axons
Are unmyelinated - Type Aδ 1° afferent axons
Form free nerve endings
Carry sharp pain or cool/cold information
Thinly myelinated
Larger than the Type C axons - Type Aβ1° afferent axons
Form encapsulated endings in skin and joints or hair follicle endings.
Are myelinated
Have diameter less than Group I afferents and greater than the Type Aδ1° afferent axons.
Describe the somatotopic organization of neurons
Somatosensory neurons are topographically (i.e. spatially) organized so that adjacent neurons represent neighboring regions of the body or face.
This organization is preserved by a precise point-to-point somatotopic pattern of connections from the spinal cord and brain stem to the thalamus and cortex.
Consequently, within each somatosensory pathway there is a complete map (spatial representation) of the body or face in each of the somatosensory nuclei, tracts, and cortex.
Describe the Functional Neuroanatomy of the Somatosensory Cortex
The primary somatosensory cortex (S1) andthe secondarysomatosensory cortex (S2) are in the parietal lobe of the brain.
The primary somatosensory cortex receives sensory information from the ventral posterior lateral nucleus (VPL) of the thalamus via the internal capsule.
The S2 region is a larger area associated with S1 that helps to process the sensory information delivered to S1.
The S1 cortex receives the peripheral sensory information but requires the S2 cortex to store, process, and retain it.
The S2 region has links to the hippocampus and amygdala; this helps to receive information from our environments and make decisions on how to approach them by integrating our prior stored experience, factoring our current appraisal of the stimulus, and finally forming a reaction using other areas of the brain based on the former two aspects to aid higher-order processing and problem-solving.
The S1 region would be associated with identifying the aspects of touch, such as shape, size, and texture.
The S2 region would be associated with spatial and tactile memory associated with sensory experiences
What is the function of the dorsal column - medial lemniscal pathway?
Also referred to as the dorsal (posterior) column pathwayor the medial lemniscal pathway).
Carries and processes discriminative touch and proprioceptive information from the body.
It is important to keep in mind that within the medial lemniscal pathway, the afferents carrying discriminative touch information are kept separate from those carrying proprioceptive information up to the level of the cerebral cortex
Outline how a stimulus travels in the dorsal column - medial lemniscal pathway?
Foot is stimulated by touch…
The APs ascend the spinal cord via the central process of the 1° afferent in the fasciculus gracilis of the posterior column until they reach the medulla.
In the medulla, the APs initiate the release of neurotransmitter from the 1° afferent axon terminals onto 2° afferents within the gracile nucleus.
The 2° afferent generates APs that are conducted by its axons which decussate to form the medial lemniscus.
These APs are conducted by the 2° afferent axon contralateral to their site of origin and contralateral to the foot where the stimulus was applied.
The APs ascend to the thalamus where they initiate the release of neurotransmitter from the 2° afferent axon terminals.
They release neurotransmitters onto the 3° afferents in the ventral posterior lateral nucleus (VPL) of the thalamus.
The APs generated by the 3° VPL afferents are conducted by their axons, which travel in the posterior limb of the internal capsule, to the parietal lobe.
These APs initiate the release of neurotransmitter from the 3° afferent axon terminals onto cortical neurons and initiate the higher-order processing of the stimulus information generated by the Meissner corpuscle
The fibers within the dorsal column pathway are joined in the brain stem by fibers mediating sensation from the head.
Touch and proprioception are relayed mostly via the main sensory and mesencephalic nuclei of the trigeminal nerve
Describe the Ventrolateral Spinothalamic Tract
Also known as the neospinothalamic pathway.
Fibers from nociceptors and thermoreceptors synapse on neurons in the dorsal horn.
Aδ fibers terminate primarily on neurons in laminae I and V, whereas the dorsal root C fibers terminate on neurons in laminae I and II.
The synaptic transmitter secreted by afferent fibers subserving fast, mild pain is glutamate, and the transmitter subserving slow, severe pain is substance P.
The axons from these neurons cross the midline and ascend in the ventrolateral quadrant of the spinal cord, where they form the ventrolateral spinothalamic tract.
Fibers within this tract synapse in the VPL.
Other dorsal horn neurons that receive nociceptive input synapse in the reticular formation of the brain stem (spinoreticular pathway) and then project to the centrolateral nucleus of the thalamus
Outline how a stimulus travels in the Ventrolateral Spinothalamic Tract
Foot stimulated by a pin prick…
The APs enter the spinal cord via the central process of the 1° afferents to initiate the release neurotransmitter from the 1° afferent axon terminals onto 2° afferents within the posterior marginal nucleus.
The 2° afferent generates APs that are conducted by its axon, which decussates in the anterior white commissure of the spinal cord.
The crossed 2° afferent axons form the lateral component of the spinothalamic tract.
The APs conducted by the crossed 2° afferent axon are contralateral to their site of origin and contralateral to the foot where the stimulus was applied.
The APs ascend to the thalamus where they initiate the release of neurotransmitter from the 2° afferent axon terminals
They release neurotransmitters onto the 3° afferents in the VPL nucleus of the thalamus.
The APs generated by the 3° VPL afferents are conducted by their axons, which travel in the posterior limb of the internal capsule, to the posterior lobule of the parietal lobe.
These APs initiate the release of neurotransmitter from the 3° afferent axon terminals onto cortical neurons and initiate the higher-order processing of the stimulus information generated by the free nerve ending.
