Module E-04 Flashcards
what does the Muscle spindle detect?
the length and rate of change of
the length of the muscle
What does Golgi Tendon organ detect?
muscular tension (related to contraction)
Where are the cell bodies of these muscle afferents located?
the ganglia of spinal dorsal roots or of select cranial nerves (e.g., CN V)
Why do neurons with larger diameter axons have faster conduction velocities?
larger axons bear lower intracellular electrical resistance
Type of Fiber:Ia
Myelination:?
Receptor:?
Sensitive to:?
Myelination: Yes
Receptor: Primary Spindle Endings
Sensitive to: length and rate of change of length
Type of Fiber:Ib
Myelination:?
Receptor:?
Sensitive to:?
Myelination:Yes
Receptor: Golgi Tendon Organ
Sensitive to: Tension
Type of Fiber:II
Myelination:?
Receptor:?
Sensitive to:?
Myelination:Yes
Receptor: Secondary Spindle endings
Sensitive to: Length
Type of Fiber:II
Myelination:?
Receptor:?
Sensitive to:?
Myelination:Yes
Receptor: Non -Spindle Endings
Sensitive to: Deep Pressure
Type of Fiber:III
Myelination:?
Receptor:?
Sensitive to:?
Myelination:Yes
Receptor: Free Nerve Ending
Sensitive to: Pain, chemicals, Temperature
Type of Fiber:IV
Myelination:?
Receptor:?
Sensitive to:?
Myelination:NO
Receptor: Free Nerve Ending
Sensitive to: Pain, chemicals, Temperature
What is a myotatic reflex?
A reflex whereby stretching of a muscle provokes contraction of the same muscle
A _________ connection between __ afferents and _____ motor neurons mediates the myotatic reflex.
monosynaptic; Ia ; alpha
What is Inverse myotatic reflex ?
– A reflex whereby heightened tension in the muscle
provokes relaxation of the same muscle.
A __________ connections between ___ afferents and _______ motor neurons mediates the inverse myotatic reflex.
polysynaptic; Ib; alpha
What is the purpose of gamma motor neuron innervation of muscle spindle fibers?
adjusts sensitivity of the spindle to stretch
Muscles involved in fine movements (e.g. of the hand) have a _______ distribution of spindles than those involved in coarse movements
denser
What are the Components of a Muscle Spindle?
1) Small group (i.e., 2 – 12) of fine intrafusal muscle fibers.
2) Myelinated sensory axons (large diameter) with receptive endings on the central (noncontractile) regions of intrafusal fibers.
3) Myelinated motor axons (small diameter) that innervate the contractile regions adjacent to the central regions of the intrafusal fibers.
Types of Intrafusal Fibers
1) Nuclear bag- numerous large nuclei located in a central bag region
a) Dynamic nuclear bag fiber
b) Static nuclear bag fiber
2) Nuclear chain Fiber- many nuclei arranged in a row
What type change are Dynamic nuclear bag fibers sensitive to?
both changes in length but also to rates of change
What type change are Static nuclear bag fibers sensitive to?
changes in length
What type change are Nuclear chain Fibers sensitive to?
changes in length
What are the 2 afferent fibers of the intrafusal FIbers?
1) Group Ia - large , rapid conduction, relay dynamic and static properties
2) Group II - smaller , less rapid conduction, relay only static properties
Where on the intrafusal fiber are Group Ia fibers found?
encircle the central (equatorial) zone of each nuclear bag and nuclear chain fiber
Where on the intrafusal fiber are Group II fibers found?
communicate with all nuclear chain and static nuclear bag fibers ONLY (NO dynamic)
Motor Innervation of Intrafusal Fibers
1) Dynamic Aγ (Myelinated gamma) motor neurons synapse on dynamic nuclear bag fibers on the contractile portion
2) Static Aγ (Myelinated gamma) motor neurons synapse on static nuclear bag and nuclear chain fibers on the contractile portion.
Motor Innervation of Extrafusal Fibers
myelinated and large Group Aα (A alpha) fibers
What is Physiological Role of the Muscle Spindle?
• Participates in stretch (myotatic) reflexes
• Provides proprioceptive information to the CNS
• Regulates muscle contraction under the influence
of descending motor pathways and afferent inputs
Describe the pathway of the Stretch reflex
1) the muscle spindle is suddenly stretched b
2) Ia afferents conduct action potentials towards excitatory synapses on alpha motor neurons of the stretched muscle and its synergists.
3) These muscles contract immediately.
4) Collaterals of the excited Ia afferents also excite interneurons that inhibit firing of the alpha motor neurons of the antagonist muscle, contributing to a process called reciprocal innervation.
During Sustained Stretch what happens to the firing rate of Muscle spindle
muscle spindle afferents fire tonically rather than phasically
What happens to Muscle Spindle Activity during Alpha Motor Stimulation
muscle shortens briefly. This shortening may briefly „unload‟ muscle spindles making it inactive, and afferent
discharge ceases, leading to reduced activity of alpha motor neurons. ( not important mechanism as alpha and
gamma motor neurons tend to be co-activated by upper
motor neurons)
What happens when Alpha and Gamma Motor Neurons are co-activated?
Both extrafusal and intrafusal muscle fibers contract.
enables the muscle spindle to measure muscle length continuously
Where are Golgi tendon organs located?
between tendon and extrafusal muscle fibers
Golgi tendon organ fibers are in ________ with extrafusal fibers , whereas muscle spindle fibers are in ________ with them
series; parallel
Describe the afferent innervation of the Golgi Tendon organ
- innervated by large , myelinated Ib fiber
- Within the capsule of the tendon organ, the Ib fiber branches and loses its myelin sheath
- the naked sensory nerve endings interweaves among chains of collagen fibers
How are the Ib fibers activated?
1) Muscular contraction distorts the receptive endings, 2)opening membrane-spanning stretch-sensitive cation channels,
3) the rate of action–potential firing thus increases
Physiological role of Golgi tendon Fibers
At great tensions, it mediates inhibition of over-active alpha motor neurons. This distributes tension among
the extrafusal fibers in a muscle.
The Golgi tendon organ is thus part of a reflex (inverse myotatic) that relaxes the homonymous and synergistic muscles and excites the antagonist muscles.
Also continuously active to facilitate rhythmic motor activity (e.g., walking)
Describe the synaptic pathway started by Golgi tendon organ
- High tension in the muscle vigorously activates Ib afferents
- Ib afferents activate interneurons that inhibit alpha motor neurons communicating with overactive extrafusal fibers
- Reduced activity in alpha motor neurons relaxes overactive extrafusal fibers, thus reducing chances of injury and/or redistributing load to less strained extrafusal fibers
- At lower muscular tensions, the apparatus exerts excitatory influences that aid in rhythmic muscular contraction
What are the Muscle efferents
1) Aα (alpha motor)
- myelinated
- large-diameter axons ( high conduction velocities)
- innervate extrafusal muscle fibers
- receive inputs from central and peripheral sources
2) Aγ (gamma motor)
- myelinated
- small-diameter axons (lower conduction velocities)
- innervate intrafusal fibers forming the muscle spindles
- receive inputs from central and peripheral sources
Which has larger cell body alpha or gamma motor neuron
Alpha motor neuron
What are Renshaw cells?
Inhibitory Interneurons that are innervated collateral axons from A alpha motor fibers
Location of motor nuclei of distal and proximal muscles in the ventral horn
Proximal (axial) muscle motor neuron nuclei are more medially located and distal muscles are more laterally located
Location of motor nuclei of flexor and extensor muscles in the ventral horn
nuclei controlling flexor muscles lie dorsal to the motor nuclei controlling extensor muscles
What are the Input to Lower Motor Neurons?
o Muscle spindle afferents
o Local interneurons
o Brainstem and cortical neurons (i.e.upper motor neurons)
What is a motor unit?
an alpha motor neuron and the extrafusal muscle fibers that it directly activates
Where are motor nuclei located?
1) ventral (anterior) horn of the spinal cord.
2) The brainstem
The more Precise the movement, the ______ muscle fibers are in the motor units
fewer
3 types of Motor Units
1) Type I
2) Type IIA
3) Type IIB
Type I Motor Unit aka
Slow twitch
Type IIA Motor Unit aka
Fast Twitch
Type IIB Motor Unit aka
Fast Twitch
Describe Type I Motor units
low tension; fatigue resistant; aerobic;
innervated by relatively small lower motor neuronal axons
Describe Type IIA Motor units
large tension; fatigue resistant (relatively); some aerobic capacity; innervated by relatively large lower motor neuronal axons
Describe Type IIB Motor units
large tension; rapidly fatiguing; anaerobic; innervated
by relatively large lower motor neuronal axons
What are 2 factors that control contraction, ie increases force produced by muscle?
- The firing rates of the motor neurons
2. The number of motor units recruited
Damage to which fibers causes Hypotonia
either the la afferent pathway or the lower motor neurons will reduce muscle tone, yielding hypotonia and hence flaccid paralysis
What cause Lower Motor Neuron Syndrome?
Arises with damage to cells derived from the basal
plate that communicate with somatic muscle cells
Features of Lower Motor Neuron Syndrome
– Paresis/paralysis – Hyporeflexia – Hypotonia – Fasciculations (aberrant twitches) – Fibrillations ( aberrant electrical responses) – Muscular wasting/atrophy
