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MOTOR SYSTEMS

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ACTION POTENTIAL MEASUREMENT, NERVE-TRUNK PROPERTIES

EXTRACELLULAR RECORDING: Non-invasive recording of action potentials by placing electrodes outside the neuron, on the skin.

AXON DIAMETER: Many properties of nerves vary according to axon diameter:

COMPOUND ACTION POTENTIAL: An extracellularly stimulated action potential that has graded stimulus intensity. It only occurs with artificial stimulation.

CLASSIFICATION OF NERVE TRUNKS:

BELL-MAGENDIE LAW: Afferent nerves enter over the dorsal Root, and efferent nerves leave through the Ventral Root.

MEASURING CONDUCTION VELOCITY: Conduction velocity of a motor axon cannot be measured directly, because it would include the time for the potential to cross neuromuscular junction and to conduct through the muscle membrane. If you want to just measure conduction through the nerve itself:


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SPINAL CORD, SPINAL CORD REFLEXES

MOTOR UNIT: An alpha-Motor Neuron, plus all of the muscle cells it innervates.

FORCE TRANSDUCTION: Twitch Tension in a muscle is increased by increasing the frequency with which alpha-motor neurons are fired.

SLOW -vs- FAST TWITCH MUSCLE
SLOW TWITCH FAST TWITCH
alpha-Motor Neuron Diameter Small Large
Conduction Velocity Fast Slow
Muscle Cells innervated (motor unit size) Fewer cells innervated, and with a smaller diameter More cells innervated, with a larger diameter
Twitch Tension Small tension Large tension
Contraction Speed Slow speed of contraction Rapid contraction
Extracellular spike size (magnitude) Small Large
Metabolism Oxidative (lots of mitochondria) Glycolytic (few mitochondria)
Capillary Supply High Low
Resistance to fatigue High resistance to fatigue Easily fatigued
Muscle Color Red (from mitochondria) White
Functional Adaptation Generate small forces over a long period of time -- Endurance Generate large forces for a brief time -- Sprint

MUSCLE SPINDLE: Intrafusal muscle fibers are innervated by afferent nerves that send signals back to the CNS about the muscle's contractility. The muscle spindle is arranged in parallel with extrafusal muscle.

GOLGI TENDON ORGAN: The muscle receptor responsible for the Inverse Stretch Reflex.

STRETCH REFLEX: Contraction of a muscle elicited by stretch of the muscle, such as the patellar tendon reflex.

INVERSE STRETCH REFLEX: Once you stretch a muscle past a certain threshold, the rigidity will "melt away" and the muscle will stretch easily. Past the threshold, the muscle is undergoing the inverse stretch reflex.

WITHDRAWAL (FLEXION) REFLEX: Remove hand from a burning stove, for example.

RECIPROCAL INNERVATION: IA Afferents from an extensor muscle will synapse with an interneuron that inhibits the opposing (i.e. flexor) muscle.

CROSSED EXTENSOR REFLEX: The contralateral reflex for the withdrawal reflex. Step on a tack, and you withdraw one foot while extending the other.

MUSCLE TONE DISORDERS: Hypotonia and Hypertonia arise from disorders in the sensitivity of alpha-Motor Neurons.

SPINAL MUSCULAR ATROPHY (SMA):


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CENTRAL MOTOR SYSTEMS

Categories of Descending Systems:

General Properties of Descending Systems:

THE BABINSKI REFLEX: Test for UMN Lesions.

THE MOTOR (PRE-CENTRAL) CORTEX: BRODMANN'S AREA 4

Secondary Cortical Motor Areas:


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EYE MOVEMENTS AND VESTIBULAR SYSTEM

SACCADES: Extremely rapid movement of eyes, creating an instantaneous change in gaze, from one location in space to a different location.

SMOOTH PURSUIT: Eye movements involved in maintaining fixation on a moving target while the head is stationery.

VESTIBULO-OCULAR REFLEX (VOR): Maintaining gaze on a fixed object while moving your head. The direction of eye movement will be equal and opposite to that of the head, i.e. the eyes will turn medially.

SEMICIRCULAR CANALS: Detect head turns and keep them in balance; detect angular acceleration.

OTOLITH ORGANS:

VESTIBULO-POSTURAL REFLEX: Compensation for turning the body to the right or the left. Example: TURN BODY RIGHT, and two responses happen

NYSTAGMUS REFLEX: The reflexive response of eye movements to continual rotation in one direction. SPIN BODY TO THE RIGHT, and the eyes will do the following:

BARANY CHAIR TEST: Rotate a person in a chair really fast to the right for about 30 sec; stop the chair, and then test for leftward acceleration reflexes of the eyes.

INNER NUCLEAR OPHTHALMOPLEGIA: Lesion in the MLF, as often occurs with Multiple Sclerosis.

UNILATERAL LABYRINTHECTOMY: Unilateral lesion of VIIIth Nerve.

CALORIC STIMULATION: Clinical test for functionality of semicircular canals.


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THE CEREBELLUM AND BASAL GANGLIA

CEREBELLAR FIBERS AND CELL TYPES:

VESTIBULOCEREBELLUM: Archicerebellum.

SPINOCEREBELLUM: Paleocerebellum

CEREBROCEREBELLUM: Neocerebellum

BASAL GANGLIA: Components

PARKINSON'S DISEASE: A lesion of the Dopamine-containing Nigrostriatal Tract, producing a dopamine deficit.

HUNTINGTON'S CHOREA: Lesion of the Corpus Striatum, involving GABA and Enkephalin neurons

HEMIBALLISMUS: Lesion of Subthalamic Nucleus, ruins the GPi inhibitory neurons that project to the thalamus ------> over excitation.


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Copyright 1999, Scott Goodman, all rights reserved