Lecture 9 Flashcards
Define and list characteristics of motor units and motor neuron pools.
Motor unit: composed of a single motor neuron and the muscle fibers it innervates
Composed of extrafusal fibers
Each motor unit has small and large motor neurons
Motor Neuron Pools: group of motor neurons that innervate fibers within the same muscle
Compare Small and large motor neurons in terms of their fiber number, threshold, and force generated
Small motor neurons: innervate a few muscle fibers, have lowest thresholds (so they fire first), and they generate the smallest force
Large motor neurons: Innervate many muscle fibers, have the highest threshold (so they fire last), and generate the largest force
Define recruitment
Recruitment: refers to the increase in tension of muscle contractions by the activation of additional motor units (size principle)
Describe the organization of interneurons and motor neurons in the spinal cord
Interneurons connect the dorsal sensory neurons to the ventral motor neurons that exit the ventrolateral portion of the spinal cord
What are interneurons mostly responsible for? Describe them in terms of their size, level of excitability and what type of activity they are capable of
spinal cord integrative function
They are small, highly excitable, and capable of spontaneous activity
Interneurons are mostly inhibitory but few are excitatory. State the neurotransmitter that both of these types of interneurons use.
Inhibitory interneurons use GABA
Excitatory interneurons use Glutamate
Where are renshaw cells found and what neuron branches do they recieve?
Renshaw cells: inhibitory cells in the anterior horns of the spinal cord that receive collateral branches from alpha motor neurons (may be supplied by more than 1 alpha MN and may synapse on multiple MNs)
Describe the function of Renshaw cells when inducing “lateral inhibition” and “recurrent inhibition”
They transmit inhibitory signals to surrounding motor neurons, to cause lateral inhibition and enhances the “fluidity of limb movement”
They transmit inhibitory signals to the same motor neuron, to cause recurrent inhibition
Where are muscle spindles found and what 2 structures do they connect to? describe these structures.
Muscle spindles are found in the muscle and are connected to a 1a fiber and a gamma motor neuron
The 1a fiber runs through the dorsal root ganglion and into the anterior horn of the spinal cord
The gamma motor neuron connects directly to the anterior horn of the spinal cord
State a similarity between nuclear bag and nuclear chain fibers. What do these 2 detect and what group of gamma efferents are they innervated by?
Both nuclear bag and nuclear chain fibers are intrafusal fibers
Nuclear bag: detect the rate of change in muscle length
Innervated by group Ia afferent and dynamic gamma efferents
(They feature multiple nuclei that are located in a central “bag-like” configuration)
Nuclear chain fibers: detect static change in muscle length
Innervated by group II afferents and static gamma efferents
(They feature multiple nuclei arranged in a single row throughout the receptor area)
(more numerous than nuclear bag fibers)
Explain the circuitry of a muscle spindle. Also describe it’s function.
Muscle spindles send sensory information back into the motor neuron that innervates the muscle the spindle is within
Muscle spindles detect changes in muscle length and send these signals back to the motor neuron
Distinguish between dynamic stretch reflex and static reflex. (include if these are transmitted by primary or secondary nerve endings)
Dynamic reflex: is elicited by rapid stretch/unstretching of a muscle and it opposes the sudden changes to muscle length
the signal is transmitted from primary nerve endings
Static reflex: causes the degree of muscle contraction to remain relatively constant
(prevents jerkiness of body movements (aka dampening))
Transmitted by both primary and secondary endings
For the golgi tendon organ, describe it’s arrangement (include how many muscle fibers are attached to each golgi tendon organ) and state its main function.
The Golgi tendon organ is arranged in series with extrafusal fibers that pass through an encapsulated sensory receptor
10-15 muscle fibers are attached to each golgi organ
It’s main function is to detect muscle tension
(The golgi tendon reflex is opposite of the stretch reflex)
Describe the flexor reflex, withdrawal reflex, and crossed extensor reflex.
Flexor/withdrawal reflex: basically, if I am contracting my bicep in order to move my hand off of a hot stove, my bicep motor neurons are excited while my triceps motor neurons are inhibited simultaneously
Withdrawal reflex: a polysynaptic reflex that is designed to prevent/reduce exposure to damaging stimuli
Crossed extensor reflex: flexion on the ipsilateral side that occurs simultaneously with extension of the contralateral side
This reflex is more prevalent in the lower extremities (makes sense if you want to pick you foot up after stepping on a nail but want to avoid falling down)
Compare the functions of the premotor, supplementary motor, and motor cortex.
Premotor cortex: Generates a plan for movement and transfers this plan to the primary motor cortex
Supplementary Motor Cortex: programs complex motor sequences and conducts mental rehearsal of them
Motor Cortex: generate the impulses that cause/control motor movements (execute)
