Lecture 10: Motor Pathway Flashcards
Upper Motor Neuron
a motor neuron contained entirely within the CNS (no part of it leaves the CNS)
Lower Motor Neuron
a neuron with the cell body in the CNS projecting its axon into the PNS to innervate something such as a skeletal muscle fiber
What do neurons of the lateral corticospinal tract innervate?
limb muscualture
What do the neurons of the anterior corticospinal tract innervate?
axial musculature
Rexed’s Laminae Layers I-VI
Intermediate Sensory Neurons
Rexed’s Laminae Layers VII-VIII
Local Circuit, Autonomic and Commisural neurons.
Rexed’s Lamina Layers IX
Lower Motor Neurons
What composes a motor unit?
one motor neuron plus the muscle fibers that it innervates
How are the muscles fibers of a motor unit distributed?
Widely and evenly within a single muscle so as to assure a smooth contraction of said muscle.
Describe slow fatigue resistant muscle fibers.
- Generate smaller forces that are sustained for a longer amount of time.
- Motor neurons tend to be small in size and thus the ratio of motor neurons to fibers is also small.
- Rich in myoglobin, capillary beds, contain many mitochondria, and function primarily via aerobic metabolism.
- Type I Muscle
Describe fast fatiguable muscle fibers.
- Generate large contraction forces but fatigue very quickly.
- Have lesser amounts of myoglobin, fewer mitochondria, and less dense capillary supply.
- Innervated by comparatively few motor neurons and thus have a high muscle fiber to neuron ratio.
- Primarily glycolytic (non aerobic) metabolism.
- Type II muscle.
Describe fast fatigue resistant muscle fibers.
- Somewhere in between the two.
ex. Soleus
What composes the muscle spindle?
small, intrafusal muscle fibers contained in a CT sheath that lie in parallel to the extrafusal muscle fibers which make up the bulk of the muscle
What is the purpose of the parallel arrangement of the muscle spindle?
So that when the muscle is stretched, like when you lengthen your biceps, the intrafusal muscle fibers of the spindle are also stretched since they are connected in parallel with the extrafusal muscle fibers.
What happens when a muscle undergoes mechanical stretch?
- Specialized nerve fibers (annulospiral endings) encircle the intrafusal fibers and these sensory neurons possess membrane channels that are sensitive to mechanical stretch, thus firing off an AP.
- The AP travels via a 1A sensory nerve back tot he dorsal horn of the spinal cord where it connects directly to an alpha motor neuron which in turn fires an AP to contract the extrafusal muscle. Since the muscle spindle is connected in parallel with the extrafusal muscle, as it contracts and shortens, so does the muscle.
- Simultaneously, the muscle spindle is served by a gamma motor neuron that fires incorrect with the alpha motor neuron in order to cause contraction of the spindles intrafusal fibers.
- Accordingly, the muscle spindle monitors and regulates throughout full extension and flexion of the muscle so that the process will be smooth with relatively constant muscle tone.
What type of motor neuron serves extrafusal fibers?
alpha motor neuron
What type of motor neuron serves intrafusal fibers?
gamma motor neuron (adjusts the tension of the spindle)
-reflects positiv bias or gain in muscle tone
What type of sensory (afferent) nerves serve the muscle spindle?
1A and II
Describe the intrafusal muscle fibers.
- Sensory organs that have cross striations similar to those observed in skeletal muscle fibers.
- Are much smaller than skeletal muscle fibers in both length and diameter.
- Are attached to one or more muscle fibers by their ECM.
What are the tow forms of intrafusal fibers and how are they classified?
- Nuclear Chain and Nuclear Bag
- Classified based on the location of their nuclei. However, we will treat them the same.
Describe the Type II Afferent nerve from the muscle spindle.
- Encodes fiber length information in the frequency of its action potentials.
- Decreasing length decreases the frequency of APs and vice versa.
Describe the Type 1A Afferent nerve from the muscle spindle.
- Encodes both length and velocity information on the frequency of its APs.
- The frequency of APs increases dramatically during a rapid increase in muscle fiber length and voce versa.
- When length is static, the frequency of APs in the nerve fiber reflect the length of the muscle fiber.
- The change in both length and frequency of APs indicates velocity.
Describe the Golgi Tendon Organ
- Designed primarily to monitor muscle force and pretest the muscle and tendons against generating excessive and potentially harmful forces.
- Comprised of encapsulated afferent nerve endings located at the junction of the muscle and tendon.
Describe the nerve endings found in the golgi tendon organs.
-They are terminals of 1b sensory afferent nerves that synapse on inhibitory local circuit neurons in the spinal cord that in turn inhibit the alpha motor neurons that are connected to the same muscle as the golgi tendon organ. Inhibition of the AP from the alpha motor neuron cause the muscle to relax.
Describe the negative feedback by the golgi tendon organ.
Stretch of the golgi tendon organ stimulates an inhibitory circuit neuron to inhibit the LMN of one pathway causing its muscle to relax while simultaneously stimulating an excitatory LMN of the pathway of the opposing muscle causing it to contract.
Describe an UMN Lesion.
Weakness: YES, weak muscle. Atrophy: NO Reflexes: INCREASED Muscle Tone: INCREASED Fasciculations: NO Clonus: YES Babinski: POSITIVE
Describe an LMN Lesion.
Weakness: YES, muscle weak. Atrophy: YES Reflexes: DECREASED Muscle Tone: DECREASED Fasiculations: YES Clonus: NO Babinski: NEGATIVE
Vestibulospinal Tract
- Fibers originate from the vestibular nuclei.
- Terminates, along white the reticulospinal tract, at the cervical and thoracic spinal cord levels to innervate the neck and trunk muscles.
- Provides info coming from the vestibular formation to control coordinated movements of the neck and trunk muscles.
Reticulospinal Tract
- Fibers originate from the reticular system located diffusely in the brainstem.
- Terminates, along white the vestibulospinal tract, at the cervical and thoracic spinal cord levels to innervate the neck and trunk muscles.
- Provides info coming from the vestibular formation to control coordinated movements of the neck and trunk muscles.
Rubrospinal Tract
- Fibers originate in the red nucleus.
- Travels closely with the corticospinal tract.
- Participates in the control of arm muscles.
Colliculospinal (Tecto-) Tract
- Fibers originate from the superior colliculus.
- Terminates largely at the cervical spinal cord level to innervate neck muscles to provide information to coordinate head movements with eye movements.
Babinski Sign
- Normally if the lateral sole of the foot is stroked with a blunt object, flexion of the big toes and sometimes others will result.
- The babinski sign (extension of the big toe and fanning of others) occurs when there is damage to the UMNs or corticospinal tract.
- Exists normally in newborns but disappears by age 3-6 months.
What do clinicians use the muscle stretch reflex for?
To assess the integrity of the upper and lower motor neurons.
Clonus
Characterized by repetitive plantar extension of the foot when it is forcefully stretched upwards.
ALS
Affects both upper and lower motor neurons so presentation is mixed and dependent upon which system is more several affected.
