Lecture 9 Flashcards
Define and list characteristics of motor units
Composed of a single motor neuron and the muscle fibers it innervates; composed of extrafusal fibers
Define and list characteristics of motor neuron pools
Group of motor neurons that innervate fibers within the same muscle
List the functions of interneurons
Small and highly excitable, capable of spontaneous activity, responsible for most of spinal cord integrative function; most are inhibitor, where GABA is neurotransmitter; excitatory ones use glutamate as neurotransmitter
Describe the anatomy and function of Renshaw cells
These are inhibitory cells in anterior horns of spinal cord that receive collateral branches from alpha motor neurons
Transmit inhibitory signals to surrounding motor neurons; this results in lateral inhibition and enhance fluidity of limb movement
Transmit inhibitory signals to same motor neuron, which results in recurrent inhibition (negative feedback)
List and compare types of muscle sensors
Muscle Spindle (group Ia and II afferents): arranged in parallel with extrafusal fibers and detect both dynamic and static changes in muscle length
Golgi Tendon Organ (group Ib afferents): arranged in series with extrafusal fibers and detect muscle tension
Pacinian Corpuscles (group II afferents): detect vibration
Free Nerve Endings (groups III and IV afferents): detect noxious stimuli
Describe the anatomy of a muscle spindle (including types of motor and sensory neurons associated with the spindle)
Consists of 3-12 intrafusal fibers; innervated by small gamma motor neurons (group II afferents), encapsulated within a sheath to form muscle spindle, and run parallel to extrafusal fibers; with finer movements, the number of muscle spindles required increases; central region of spindle has no contractile fibers and functions as a sensory receptor; sensory fibers originate from central region and stretching of central region of intrafusal fiber stimulates sensory fibers; detects changes in muscle length
Includes alpha motor neurons, gamma motor neurons, Ia sensory fibers, II sensory fibers,
Describe and compare the three types of gamma motor neurons
A-gamma: 5 micrometers, supply small intrafusal fibers in middle of muscle spindle
Gamma-Dynamic: Excite nuclear bag intrafusal fibers
Gamma-Static: Excite nuclear chain intrafusal fibers
Can adjust sensitivity of muscle spindle
Describe the functions of a muscle spindle
Dynamic stretch reflex, static reflex, and prevents jerkiness of body movements
Describe how a myotactic reflex works
- Stretching of muscle stretches group Ia afferent fibers
- Group Ia afferents synapse directly on alpha-motor neurons of same muscle
- Muscle contracts and decreases tension on muscle spindle
- Synergistic muscles are activated and antagonistic muscles are inhibited
Describe the structure, circuitry, and function of the golgi tendon organ
Encapsulated sensory receptor through which muscle tendon fibers pass; arranged in series with extrafusal fibers; 10-15 muscle fibers attached to each Golgi organ; organ stimulated by contracting or stretching of muscle; detects muscle tension; golgi tendon reflex is opposite of the stretch reflex
Circuitry: Type Ib afferent -> inhibitory interneuron -> anterior motor neuron
List and compare the functions of the premotor, supplementary motor, and motor cortex
Premotor and supplementary motor cortices generate a plan for movement (transfer plan to primary motor cortex); signals generated here cause more complex patterns of movement than the more discrete pattern generated by the primary motor cortex, anterior part of the premotor cortex develops a “motor image” of the total muscle movement that is to be performed; supplementary motor cortex programs complex motor sequences and is responsible for mental rehearsal for a movement
What are the outputs of the posterior motor cortex?
Posterior motor cortex sends signals to primary motor cortex -> basal nuclei and thalamus -> primary motor cortex
What is the dynamic stretch reflex?
Signals transmitted from primary nerve endings, elicited by rapid stretch or unstretch, opposes sudden changes to muscle length
What is the static stretch reflex?
Transmitted by both primary and secondary endings; causes degree of muscle contraction to remain relatively constant
What are nuclear bag fibers?
Detect rate of change in muscle length, innervated by group Ia afferents and dynamic gamma efferents; multiple nuclei located in a central “bag-like” configuration in receptor area
What are nuclear chain fibers?
Detect static change in muscle length, innervated by group II afferents and static gamma efferents; more numerous than nuclear bag fibers; multiple nuclei arranged in a single row; aligned in chain throughout receptor area
What do alpha motor neurons give rise to?
A-alpha fibers
Describe an upper motor neuron
Descends the spinal cord to level of the appropriate spinal nerve, synapses with lower motor neuron or interneuron, and the neurotransmitter from upper to lower motor neurons is glutamate, which binds to glutamatergic receptors
Originate in cerebral cortex, cerebellum, and brainstem
Describe a lower motor neuron
Typically found in anterior gray portion of SC or cranial nerve nuclei of brainstem, terminate at effector with acetylcholine as neurotransmitter; cranial nerves are unique lower motor neurons
Begin in CNS from anterior horns of spinal cord and from brainstem cranial nerve nuclei
What is recruitment?
Refers to the increase in tension of muscle contractions by the activation of additional motor units (size principle)
Describe small motor neurons
Innervate a few muscle fibers, lowest thresholds, fire first, and generate smallest force
Describe large motor neurons
Innervate many muscle fibers, highest thresholds, fire last, and generate largest force
Describe Ia inhibitory interneurons
Inhibit antagonistic muscle, signal from muscle spindle, and avoid spasticity
Describe Ib inhibitory interneurons
Autogenic inhibition reflex, golgi tendon activated with stretch, which connects with Ib to inhibit same muscle, and are responsible for smooth contraction and protects muscle from tears
Describe excitatory interneurons
Involved in spinal reflex to remove extremity from damaging stimulus; place foot on a tack, sensory information must be translated into motor output, sensory group III afferents send signal to excitatory interneurons that send ipsilateral and contralateral signals
Describe A fibers
Somatic and myelinated; include the subtypes alpha (largest, type I), beta, gamma, and delta (smallest, type IV)
Describe B fibers
Lightly myelinated, preganglionic fibers of ANS
Describe C fibers
Unmyelinated, found in somatic and autonomic systems; also referred to as type IV fibers
What does the medial activation system innervate?
Innervates the postural and girdle muscles
What is the lateral activation system associated with?
Associated with distally located muscles used for fine movements
What does the nonspecific activating system facilitate?
Facilitates local reflex arcs
What brain areas control gamma fibers?
Bulboreticular region of brain stem, cerebellum, basal nuclei, cerebral cortex
What is after-discharge?
It is the result of persistent neural activity of polysynaptic circuits that prevents the muscle from relaxing for some time after the stimulus stops
