Ch. 13e Tracts
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Pathway of information
Sensory fibers enter the spinal cord via the dorsal root. The root connects to the dorsal gray horn, where a sensory fiber synapses with an internueron. From here it can go to another area in the spinal cord or the brain. If action is warranted the corresponding motor nueron is stimulated in the anterior gray horn.
What is a “tract?”
A group of axons in the CNS
Spinal tract decussates, so right side of the brain controls the left side of the body and Vice versa
Spinal tract does not decussate
Ascending (afferent tracts): First-order neurons
Detects the stimulus, transmits the signal to the spinal cord or brainstem, synapes in dorsal gray horn.
Ascending (afferent tracts): Second-order neurons
continues up the spinal cord to the thalamus,synapses with next order neuron
Ascending (afferent tracts): Third-order neurons
Cell body is in thalamus, axon travels up to the sensory region of the cerebral cortex
Spinothalamic tract: What signals does it carry?
signals for pain and temperature feeling
Spinothalamic tract: What is its course?
- First-order: End in posterior gray horn near entry-point. Synapses,
- Second-order: Decussates through anterior gray commissure to other side of spinal cord. Travels up the anterolateral region of white columns to the thalamus
- Third-order: From thalamus to cerebral cortex Neurons are contralateral to point of origin.
Dorsal column tract: (what does it do?)
Proprioception: Sense limb positions and fine touch discrimination
Dorsal column tract: What is its course?
- 1st order: Enters spinal cord and ascends the Fasciculus gracilis (below T6) or the Fasciculus cuneatus (above T6). Synapses in medulla oblongata at either the respective nucleus gracilis or nucleus cuneatus
- 2nd order: The neurons of both tracts cross in the medulla and run together as the medial lemniscus tract. The axons synapse in the thalamus
- 3rd order: neurons leave the thalamus and synapse in sensory portion of the cerebral cortex
The cerebral cortex processes the information. If a response is needed by a skeletal muscle then a descending pathway is stimulated. This path must decussate again in order to stimulate the original side of the body
Descending tracts carry what kind of nerve fibers?
Motor, so it stimulates muscles, glands, blood vessels, digestive organs
Describe an upper motor neuron
Cell body is in the cerebral cortex or brainstem, and axon ends at the brainstem or spinal cord.
Describe a lower motor neuron
Cell body is in the brainstem or spinal cord and the axon goes the rest of the way to the muscle or spinal cord.
What is the descending tract that you need to know?
Corticospinal tract: What does it do?
Controls skeletal muscle contraction
Corticospinal tract: What is its course?
Upper motor neuron: The axons descend from the motor part of the cerebral cortex. Axons that decussate in medulla will control distal skeletal muscles. Axons that decussate at their point of exit in the spinal cord will control proximal muscles. Synapse with lower motor neuron occurs at point of exit.
What are the 4 important properties of a reflex?
- Requires stimulation
Describe the stretch reflex
Stretched muscles stimulate contraction to mantain a position. This happens when you “nod off” and your head flips back
Describe the patellar tendon reflex
A muscle can automatically contract if its tendon is tapped. The patellar tendon reflex is a result of the tendons over the patella having only one synapse between the sensory and motor neurons, causing the quick and involuntary contraction of the muscles attached to it.
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