Week 4: Somatic Sensory Pathway

Question 1

what are first order neurons ?

Answer 1

the neurons that conduct information from the sensory receptors and bring that information to the central nervous system

Question 2

the cells that the first order neurons synapse with in the central nervous system are called ?

Answer 2

second order neurons. these are found in the brainstem or spinal cord and their axons always project towards the thalamus.

Question 3

in the thalamus is the location of ?

Answer 3

the third order neurons. these are the cells in the thalamus that receive information from the second order neurons and then through their axons, transmit that information to the primary somatosensory area of the brain: postcentral gyrus.

Question 4

the sensory path is found in two regions of the central nervous system…?

Answer 4

the posterior column of the spinal cord and the medial lemniscus of the brain

Question 5

posterior column-medial lemniscus pathway to the cortex ?

Answer 5

1. stimulus detection: sensory receptors detect touch, vibration, pressure, and proprioception.
2. first order neuron: sensory information is transmitted by the first order neuron, which enters the spinal cord through the dorsal root and ascends in the cuneate fasiculus for the upper body or the gracile fasiculus for the lower body.
3. synapse with second order neuron: the axon of the first order neuron synapses with second order neuron in the cuneate nucleus or gracile nucleus in the medulla oblongata.
4. decussation and medial lemniscus: the second order neuron decussates to the opposite side of the medulla and goes through the medial lemniscus to the thalamus.
5. synapse with third order neuron: the second order neuron continues to the thalamus where the second order neuron synapses with the third order neuron.
   projection to somatosensory cortex: the third order neuron transmits information to the primary somatosensory cortex/postcentral gyrus in the parietal lobe.

Question 6

anterolateral pathway to the cortex ?

Answer 6

1. stimulus detection: sensory receptors detect stimuli such as pain, temperature, tickle, and itch.
2. first order neurons carry sensory information from the receptors to the dorsal root into the cns.
3. synapse with second order neuron: the first order neuron synapses with the second order neuron in the dorsal horn of the spinal cord. the second order neuron immediately decussates to the opposite side of the spinal cord and occurs at the anterior white commissure.
4. the second order neuron ascends in the anterolateral tract through the brainstem, on its way to the thalamus.
5. synapse with third order neuron: upon reaching the thalamus, the second order neuron synapses with a third order neuron.
6. projection to the primary somatosensory cortex: the third order neuron axon projects from the primary somatosensory cortex in the postcentral gyrus of the parietal lobe where the information is processed.

Question 7

trigeminothalmic pathway to the cortex ?

Answer 7

1. stimulus detection: sensory receptors in the face detect stimuli such as touch, pressure, vibration, pain, cold, warmth, itch, tickle. there are two basic things that can happen.
2. the first order neurons are located in the trigeminal ganglion. these neurons detect the stimulus from the receptors in the face and transmit it through their axons through the trigeminal nerve to the second order neurons.
3. first order neuron can synapse with second order neurons in pons or in the medulla oblongata depending on the type of stimulus. the second order neuron will then decussate to the opposite side of the cns.
4. the second order neuron ascends in the trigemenothalmic tract into the thlamus.
5. once the second order neuron reaches the thalamus, they synapse with the third order neuron and then the axons project into the primary somatosensory cortex in the parietal lobe of the brain for the information to be processed.