Chapter 13: Spinal Cord Flashcards
Three types of muscles referring to locations
Axial muscles: trunk movement, maintains posture (most medial)
Proximal muscles: shoulder, elbow, pelvis, knee movement, important for locomotion
Distal muscles: hands, feet, digits (fingers and toes) movement (most lateral)
What are the three main roles of the spinal cord?
1) Carrie’s motor information from the brain to the periphery
2) Carries sensory input from periphery to the brain
3) mediates reflexes for nody
What is the anatomy of the spinal cord?
- dorsal root-sensory (afferents)
- dorsal root ganglion- contains somas of sensory afferents entering the cord
- ventral root - motor (efférents)
- spinal nerves - mixed sensory and motor roots
What’s e the three divisions of the gray matter?
Dorsal horn
Intermediate gray
Ventral horn
What’s are the types of neurons you can find in gray matter?
Ventral horn: mainly motor neurons whose axons exit the spinal cord
Dorsal horn: mainly neurons responding to sensory inputs
Intermediate gray: sensory neurons, motor neurons and interneurons
Define a motor unit
One alpha motor neuron and all the muscle fibers it innervates
What is a motor neuron pool?
The collection of alpha motor neurons that innervates a single muscle
What are the two types of lower motor neurons in the spinal cord? What do they innervate and trigger?
1) alpha motor neurons: - innervate extrafusal muscle
- trigger the generation of force in the muscle
2) gamma motor neurons: - innervate intrafusal muscle fibers of the muscle spindles
LMN vs UMN
- lower motor neuron (LMN) - provide direct input into muscle contraction, split into Alpha and Gamma motor neurons, cell body in ventral horn, death leads to ALS (paralysis)
- upper motor neuron (UMN) - provide input into LMN, cell body in cortex, death leads to spasticity
What does the descending spinal pathway command?
Modulates spinal reflexes or directly drive motor output
What can you say about the lateral pathways?
- voluntary movement of distal musculature
- under direct cortical control
- innervate distal musculature
What are the lateral pathways?
Corticospinal tract
Rubrospinal tract
What can you say about ventromedial pathways?
- control of posture, locomotion, orienting and balance
- under brainstem control
- innervate axial and proximal musculature
Why are the ventromedial pathways?
- tectospinal tract
- vestibulospinal tract
- pontine reticulospinal tract
- medullary reticulospinal tract
What’s the the two mechanisms for the force of muscle contraction by lower motor neurons? Explain each one
- By varying firing rate of motor units
——-> 1 AP in the alpha motor neuron causes an end plate potential (epp) strong enough to generate one postsynaptic AP in the muscle. 1 AP in the muscle causes a twitch. Increasing the AP frequency causes twitch summation and eventually sustained contraction - Recruitment of synergistic motor units: large motor units (fast twitch) generate more force but tire easily
Small motor units(slow twitch) generate less force for endurance
What are the three sources of input for alpha motor neurons?
- sensory input from muscle spindles
- input from upper motor neurons in the brain
- input from spinal inter neurons
What do muscle spindles detect?
Changes in muscle length
Where is the muscle spindle located?
In parallel with extrafusal muscle fibers
What wraps around intrafusal muscle fibers? What do they do?
1a afferents inform CNS about muscle length
What composes a muscle spindle?
1a afferents
Intrafusal muscle fibers
Which motor neurons innervate intrafusal muscle fibers?
Gamma motor neurons
What do gamma motor neurons do?
They adjust the sensitivity of the muscle spindle, reflex length when muscle contracts so it is not limp.
What does stretching the muscle entail?
It stretches the 1a nerve, activating alpha motor neurons
What happens when extrafusal muscle fibers contract?
Activity in 1a axon stops- spindle becomes limp and no longer sensitive to stretch, until gamma motor neuron resets length of spindle
What resets spindle length?
Gamma motor neurons activity contracts intrafusal fibers
Which afferents are linked with muscle tension?
1b afferents in Golgi tendons
Explain reciprocal inhibition
The contraction of one set of muscles accompanied by the relaxation of their antagonist muscles
Explain the process of t reciprocal inhibition
1a axon projects from muscle spindle to either an inhibitory inter neuron then the alpha motor neuron or directly the alpha motor neuron. In the case Of only the alpha motor neuron, there is contraction of bicep extrafusal fibers. In the case of the triceps, which is the antagonist muscle, there will be a “reciprocal inhibition” thanks to the effect of the inhibitory interneuron
Explain ipsilateral and contralateral flexor crossed-extensor reflex
Ipsilateral: excite flexors and inhibit extensors (bending leg that is hurt)
Contralateral: inhibit flexors, excite extensors (extending leg so you don’t fall over)
Differences between Muscle Spindles and Golgi Tendons
Muscle Spindles: send info about stretch (change in length), in intrafusal fibers, 1a afferent axons, in parallel, fires more in passive stretch, no activity when actively contracted
Golgi tendons: Muscle Spindles: send info about tension (change in force), 1b afferent axons, in series, fires less in passive stretch, massive firing activity when actively contracted
2 Descending Spinal Tracts and their functions
- Lateral Pathways: voluntary movement of distal muscles, under cortical control
- Ventromedial Pathways: posture, locomotion, under brain stem control
Explain Corticospinal Tract
Carrying motor information
Decussate in caudal medulla
Longest axon in CNS
Damage leads to impaired Babinski reflex
Rubrospinal tract
Originates in red nucleus of mid brain
Decussate in pons
Function is reduced in humans, coricospical tract takes over
