Ch 22 & 23 - Motor Systems I & II

Question 1

define alpha motoneurons

Answer 1

they contact muscle fibers. they are the final common pathway by which the brain and spinal cord influence motor output. innervate extrafusal fibers (striated muscle fibers)

Question 2

define motor unit

Answer 2

a single alpha motoneuron and all the muscle fibers it innervates. smaller motor units are found in muscles involved in fine motor control.
- increased tension results from more motor units being recruited, and by each unit firing more rapidly.

Question 3

gamma motoneuron

Answer 3

innervates intrafusal muscles coming from the ventral horn (intrafusal fibers are contractile muscles)

Question 4

motoneuron pool

Answer 4

motoneurons collecting together in the spinal cord to innervate upper limb musculature

• more medial is more rostral
• more lateral is more caudal

Question 5

muscle spindle

Answer 5

intrafusal muscle fibers running parallel with skeletal muscles; alpha 1a fibers; stretch receptors.

• core of muscle spindle is composed of intrafusal fibers attached at both ends to the extrafusal fibers.
• sensory afferents conduct length and velocity information from the spindle to the spinal cord by 1a fibers
• muscle stretch causes an increase in the discharge of the spindle receptors to reflect the dynamic component of muscle lengthening and to provide a static component reflecting the new muscle length.

Question 6

golgi tendon organ

Answer 6

detect muscle tension and are found in tendon rather than muscle.

• very sensitive
• serve as tension transducers and protective devices; info from the golgi tendon organ travels to the CNS via group 1b afferents which are myelinated but somewhat slower

Question 7

myotatic reflex

Answer 7

• the knee tap
• a rapid stretch of the muscle activates 1a afferents producing a monosynaptic muscle contraction.
• sensory nerve makes contact with motor nerve which is unique

Question 8

flexor reflex

Answer 8

• excites flexor motor neurons and inhibits extensor motor neurons
• interneuron with filled cell body is inhibitory
• unfilled cell body is excitatory
• by interposing neurons between motor neurons, you have the opportunity to amplify or diminish the signal.

Question 9

crossed extensor reflex

Answer 9

• maintains posture by exciting extensors on the side opposite flexor excitation
• when you flex one side you automatically extend on the other side to keep from falling over.

Question 10

CPGs

Answer 10

central pattern generator
they are local networks of neurons that produce stereotypical, repetitive output such as the sequences typical of locomotion. they are controlled by input from the brainstem reticulospinal and vestibulospinal tracts.
- flexible movement results from modulation of CPGs by descending pathways like the tectospinal, rubrospinal, and corticospinal tracts.
the basal ganglia play a major role in selecting which CPGs are activated and dopaminergic influences are impt.

Question 11

basal ganglia

Answer 11

subcortical telencephalic nuclei that communicate with the cortex.