BIOLOGY 2C03 Lecture Notes - Muscle Spindle, Stretch Reflex, Rigor Mortis

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28 Jan 2013

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Outline of Lecture 36 (03-05 A; Shadmehr)
Motor System I – Spinal Motor System
I. Major components of motor system
Parietal cortex: visual and proprioceptive processing gives positional info
Motor cortex: compute forces needed to cause desired motor action
Brainstem: tells spinal cord how to maintain balance during movement
Spinal cord: effects movement, reflex stability, and relays sensory info
Cerebellum: multijoint movements, posture, and motor learning
Basal ganglia: learning and stability of movements, emotional aspects of movement
II. Motor system diseases
A) How spinal cord injury results in paralysis
- Initial injury in small region growth of damaged area hemorrage of blood vessels
cyst, swelling, and cell death
- Glu release neuron excitotoxicity and glia apoptosis
- Paralysis can be partially overcome with direct electrical stimulation from neural prosthetic
B) Other diseases/conditions
- Polio: virus kills motoneurons directly, post-polio syndrome includes motor unit enlargement
followed by muscle breakdown
- ALS: slow degeneration of motorneurons and motor cortex
- Motor stroke: spinal motoneurons not affected, but central control is weak
- Rigor mortis: stiff muscles after death, no ATP available for myosin detachment
III. Properties of muscle fibers and muscle movement
- Extrafusal vs intrafusal fibers and relationship to α and γ motoneurons
- There is an optimum force of contraction with respect to muscle length (i.e. not monotonic!)
- Rapid limb movements follows a 3-phase pattern (agonist, antagonist, agonist) directed by
descending commands
- Types of muscle fibers: Type I slow, Type IIa fast fatigue resistant, Type IIx fast fatiguing
- Different motor units are composed of different ratios of fiber types
- Muscle fiber type is a result of myosin isotype expr; the type can change; fast type is default
- Grading of muscle force: recruitment of more motor units, frequency of activation; later ones
are usually faster and more forceful
IV. Role of afferent neurons in motor control
- Afferent neuron fiber types
- Ia and II: muscle spindles measures length change; used for proprioception; Ia spindle afferent
excites MTNs of agonist mm. and inhibits MTNs of antagonist mm. which provides the
stretch reflex
- Ib: Golgi tendon measures force change; inhibit α-MTN of same mm.
- Stretch reflex: short-loop response caused by Ia afferents, followed by long-loop response via
DC-ML/CST pathways under voluntary control (but loss of this control in Parkinson’s disease or
ipsolateral stroke)
- In absence of afferent input, accurate motor control depends strictly on visual input
Omitted from this outline: review of major CNS divisions, spinal cord segments, muscle fiber structure and
motor units, twitch and tetanus
Summary of major ideas
- Timecourse of spinal cord paralysis
- Types of muscle fibers, and functional consequences of motor unit organization
- Afferent fiber types, and role in stretch reflex
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