Motor Systems Flashcards
Where do alpa motor neurons live?
• Cell bodies reside in the ventral horn of the spinal cord and brainstem cranial motor nuclei
What are the higher motor centers in the cortex and brainstem?
• Vestibular nucleus ○ Anticipatory motor control • Reticular formation ○ Rhythmic motor output (locomotion) • Superior colliculus ○ Eye movements • Motor cortex ○ voluntary motor control
What does the cerebellum act as (general)?
- Feedback error correction circuit
* Computes short and long-term (learned) corrections to the errors in the circuits from the other motor systems
What role (general) does the basal ganglia play?
• Roles in movement initiation and action selection as well as roles in motor learning, reinforcement, motivated behavior
Alpha motor neurons (LMNs) in the ventral horn of the spinal cord are organized somatotopically. What does that mean?
- Lateral musculature is innervated by laterally situated motor neurons and medial musculature is innervated by medially situated motor neurons
- That allows for symptoms of ascending or descending motor loss
What is the definition of a motor unit?
• Alpha motor neuron and the muscle fibers it innervates
- more muscle fibers than nerves so one nerve will often innervate a ton of muscle fibers
- leads to different characteristics of fatigueability and force between the different motor units
How does the gradual recruitment of force help you hold an egg?
• The fact that smaller alpha motor neurons innervate smaller motor units allows the nervous system to recruit less movement for a given task
• Gradients from small to large motor units exist, generating graded forces
• They can be recruited systematically by higher motor centers, resulting in a gradual increase in force
*to grip a fragile object you recruit the smaller motor units
Describe the physiology of graded motor unit recruitment
- Small neurons have high input resistances because of fewer channels in the membrane
- V=IR
- A smaller current is needed to cause a voltage increase sufficient to stimulate an EPSP
- Thus, less NT is needed to fire the smaller motor neurons than the larger ones
- “size principle”
What are the three basic types of muscle fibers that differ in fatigability?
• Tonic muscle fibers
○ Super low fatigability, mostly in spindles and in extraocular muscles
○ Generate isometric tension, shorten very slowly
• Slow twitch muscle fibers
○ Low fatigability with lots of myoglobin and mitochondria
• Fast twitch oxidative fibers
○ Activate quickly and have lots of mitochondria so fatigue moderately slowly
• Fast twitch glycolytic fibers
○ Activate quickly
○ Fatigue rapidly as they rely on anaerobic glycolysis ATP generation
○ Less mitochondria
The size principle of motor neurons matches up how with the type of muscle fibers?
- One motor unit is all the same type of muscle fiber
- Small neurons innervate slow motor units (posture)
- Large neurons innervate the fast-twitch glycolytic fibers
- Medium neurons innervate the fast twitch oxidative fibers
What is a muscle spindle?
• Technically, a muscle spindle is a speciaal type of muscle fiber
○ Intrafusal muscle fiber
• Run in parallel with the main extrafusal muscle fiber
• Stretch of the muscle spindle is communicated to the spinal cord through group Ia and II sensory afferents
○ Large, fast axons that have the modality of stretch
• Ia sensory afferents contact alpha motor neurons in spinal cord and form the DTRs or stretch reflexes
• Remember that muscle spindles are contractile
What motor neurons innervate muscle spindle cells?
- Gamma motor neurons
- Alpha and gamma neurons fire together during voluntary contraction
- This allows a maintenance of stretch receptivity (even a shorter muscle will have the same(ish) stretch receptivity)
What is the GTO?
- Golgi tendon organ
- Collagen structures at the junction of a muscle and a tendon and are innervated by and signal bia Ib sensory afferents that wind around and within the collagen strands
- Situated in series with the muscle and tendon (not parallel like muscle spindles)
- Are preferentially sensitive to muscle tension since passive stretch lengthens the muscle before straning the tendon
- During muscle contraction force increases the tension on collagen strands and pinches the intertwined afferent fibers causing them to fire
- GTOs preferentially detect muscle tension rather than passive stretch
- REGULATE muscle force
Describe the neuronal process behind the patellar tendon DTR
• Hammer tap stretches the muscle
• Stimulates activity in the Ia sensory axons (fast and fat)
• This reports the stretch of muscle spindles to both cortex and the relay to alpha motor neurons in the spinal cord that contract the stretched muscle
○ Synergist muscle
• Considered monosynaptic but the Ia spindle afferents branch and diverge in the spinal cord to contact large populations of motor neurons which innervate populations of muscle fibers to create the reflex motion
• Thus it produces large contraction and maintains faitfulness by not depending on one or a few neurons
Besides contraction of the synergist muscle, what muscles are coordinated to act by the type Ia afferents in the reflex arc?
- Type Ia afferents directly contact alpha motor neurons in the spinal cord of the homogenous/syergist muscle leading to contraction
- IN ADDITION the type Ia afferents stimulate inhibitory neurons in the spinal cord to RELAX the opposing muscle
- Reciprocal innervation, happens simultaneously
What is flexor-extensor coupling?
- Type 1b afferents innervating the GTOs directly contact inhibitory and excitatory interneurons in the spinal cord
- Not a jerk that is produced, but a protection of the syngergist muscle by contracting the antagonist