1. Spinal Cord

Sensory information enters CNS through Dorsal Roots
Activation of the muscles happens through the Ventral Roots
The central axons of the Dorsal Root Ganglion Cells form a neural map of the body surface when they terminate in the Spinal Cord
- This orderly somatotopic distribution of inputs from different portions of the body surface is maintained throughout the entire Ascending Somatosensory Pathway

2. Descending Pathways

<aside> 💡 Ascending Pathway see: 4. Ascending Pathway

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Signals travel from the Primary Motor Cortex to the Spinal Cord via Descending Pathways
- Axons of projection neurons (Pyramidal Tract Neurons) originating from the Motor Cortex run all the way down to their respective signal segments
The major Descending Pathway is:
- Axons of Pyramidal Tract Neurons project through all the way down:
  1. Midbrain
    1. Cerebral Peduncle
  2. Pons/Medulla
    1. Pyramid
  3. Medulla - Spinal Cord Juncture
    1. Pyramid Descussation
    2. Lateral Corticospinal Tract
  4. Spinal Cord
    1. Lateral Corticospinal Tract
    2. Lateral Column
- Connects to Motor Neurons in the Ventral Horn (Lateral Motor Nuclei)
- Only 1 synapse between Motor Cortex and Motor Neurons in the Spinal Column
  - Modulation can only occur in the Spinal Cord or at the Cortical level

Located in Broadman Area 4
- Electrode stimulation in this area is extremely efficient to cause movement
Threshold of stimulation for movements lowers as getting closer to Central Sulcus
Movements bringing hand to the mouth or bringing an object close to the visual mid line are well represented and encoded in the activities of neurons in the Primary Motor Cortex
Somatotopy presents in both Primary Motor Cortex and Somatosensory Cortex
- Motor Cortex has much less precise body mapping than Somatosensory Cortex
Body mapping in Motor Cortex is called Fractured Somatotopy because:
1. Overlapping areas where extensor or flexor activities can be activated
2. Multiple representation of the same body region
3. No strict internal Somatotopy within this general region representing a body region
- Single axon in the Cortical Spinal Tract terminates in multiple columns of Lower Motor Neurons in the Ventral Horn

Magnetic stimulation of Motor Cortex and Spinal Cord induces EMG activity
- Non-invasive stimulation allows selective activation, through the skull, of Projection Neuron sending motor commands to the spinal Motor Neurones
- Applied directly at Motor Cortex level or at Spinal Cord Level, same Projection Neuron is activated
Stimulus causes an artefact in EMG trace
- In both cases the stimulation at of the Motor Cortex is followed by a deflection in the EMG after a longer delay compared to the delay observed for Spinal Cord stimulation
- Reflects the different distances the signal travels from the stimulation site to the muscle
  - Cortical stimulation takes longer to affect the muscles than Spinal Cord stimulation
- This method would be useful to diagnose at which level a Spinal injury has occurred

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<aside> 💡 Post-spike Facilitation

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Used to reveal the effect of individual Cortical neurones on muscle activity
Method:
- Spikes of a neuron in the Motor Cortex are recorded
- Simultaneously, the EMG of the thumb muscle is recorded
- The trace shows the cumulative EMG recording after the occurrence of a number of spikes
  - the cumulative average of EMG activity after 9000 spikes-triggered EMG sections shows effect of Cortical neuron
  - the neuron’s contribution is apparent as averaging reduces contributions caused by other neurons
SNR due to Spike-triggered Averaging is facilitated revealing the contribution of a single neuron to the EMG

<aside> 💡 Rearrangement of Somatotopic Maps after lesions is possible

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