Neurobiology – Block 3
Neurophysiology # 12/4
Spinal control of skeletal muscle
activity - II
Activation of the gamma fibres
γ activation
lenghtening
Consequence of the gamma activation is the same as
that of lenghtening the muscle: increased stretching of
the central part of the intrafusal fiber
Muscle receptors can be activated in different ways - I
In case of passive stretching of the muscle:
•
Bag receptors sense the change of the length of the muscle
•
Chain receptors sense the length of the muscle
•
Tendon (Golgi) receptors show low sensitivity towards
passive stretching
In case of passive stretching of the muscle
along with intrafusal contraction the
consequences of passive stretching and
gamma stimulation are the same, the
changes are summed!
Muscle receptors can be activated in different ways - II
In case of extrafusal muscle contraction:
•
Due to the decreased length of the muscle the discharge of
the muscle receptors ceases, temporarily there is no
information about the length of the muscle and about the
changes of the length
•
Sensitivity of the tendon (Golgi) receptors towards active
stretching is high
In case of extrafusal muscle contraction
accompanied by intrafusal contraction:
•
During the decrease of the muscle length the discharge of the
muscle receptors does not cease, because the simultaneous
gamma activity keeps their stretching, consequently there is
information about the muscle length and its changes
•
This example reflects physiological activation
Activity of gamma motor neurons
(Kandel - Principles of neural science, Chapter 36, Figure 36-10)
•
Activity in the fusiform system is set at different levels for different types
of behavior.
•
During activities in which muscle length changes slowly and predictably
only static gamma motor neurons are active.
•
Dynamic gamma motor neurons are activated during behaviors in which
muscle length may change rapidly and unpredictably.
Sensory fibers from muscle
(Kandel - Principles of neural science, Chapter 36, Table 36-1)
• Apart from type Ia (primary), II (secondary) and type Ib (originating from
Golgi tendon organs) afferents, pressure sensitive and pain/heat sensitive
sensory fibers also arise from the muscle and reach the spinal dorsal horn.
Basic spinal reflexes as building blocks of complex
movements
• stretch reflex
“... simple reflexes, stereotyped movements elicited by
the activation of receptors in skin or muscle, are the
basic units for movement... ...complex sequences of
movement can be produced by combining simple
reflexes.” (Sherrington, 1906)
• flexor reflex
• „group II” reflex
Many coordinated movements can be produced in the
absence of sensory information. For example, in a
variety of species locomotor patterns can be initiated
and maintained in the absence of patterned sensory
input. (Thomas Graham Brown, 1911)
• inverse myotatic
reflex
Contemporary view: reflexes are integrated with
centrally generated motor commands to produce
adaptive movements.
Stretch reflex
• It continously maintains the length of the muscle (negative feed-back)
• Synonymes: proprioceptive reflex, miotatic reflex, stretch reflex, tendon reflex
• Physical investigation: transient elongation of a muscle by hitting its tendon
with a reflex hammer will evoke contraction in the muscle (e.g., knee-jerk)
• Electrophysiological investigation of the reflex is also possible. (e.g. H-reflex:
electrical stimulation of the tibial nerve evokes contraction in the soleus
muscle that can be detected and measured via EMG)
(Kandel - Principles of neural science, Chapter 36, Figure 36-13)
• The stretch reflex is monosynaptic and segmental. Due to the segmental
nature, loss or alteration of a certain reflex will give us information about the
level of the spinal cord injury / problem.
(Kandel - Principles of neural science, Chapter 36, Table 36-2)
• Contraction of the agonist muscle during the stretch reflex is accompanied by
relaxation of the antagonist muscle(s) of the same joint. This phenomenon is
called reciprocal innervation: collaterals of primary afferents originating from
muscle spindles of the agonist muscle will innervate inhibitory (Ia) interneurons
that in turn will inhibit motoneurons supplying the antagonist muscle(s).
Flexor reflex
• Synonimes: skin reflex, withdrawal reflex, exteroceptive reflex, defense
reflex
• Physical examination: applying a strong but not harmful stimulus (e.g.,
scratching the skin with the pointed end of the shaft of a reflex-hammer) on
a certain skin area will evoke contraction in the underlying muscle.
• Sensory information arriving to the spinal dorsal horn shows extensive
rostrocaudal divergence (intersegmental irradiation)
• The flexor reflex is polysynaptic and shows reciprocal innervation.
• The flexor reflex includes a crossed extensor reflex component: e.g. in case
of reflex-evoked flexion of the lower limb the contralateral limb will be
extended.
Comparison of the features of the stretch and flexor
reflexes
Inverse myotatic reflex – Ib afferent fibers
•
They originate from the tendon (Golgi) receptors
•
The Ib inhibitory interneuon also recieve convergent input from
muscle spindles, joint and cutaneous fibers and descending
pathways
•
They characteristically inhibit ipsilateral extensor muscles and
facilitate ipsilateral flexor muscles
•
The reflex action of Ib afferent fibers depends on the behavioral
state of an animal
•
Virtual aim is to prevent overstretching of the muscle
•
Main function is to regulate the active stretch of the muscle
(Kandel - Principles of neural science, Chapter 36, Figure 36-7)
„Group II” reflex
•
They originate from the chain receptors
•
Spinal cord connections are uncertain
•
They characteristically facilitate both the flexor and the extensor
muscles
•
Main aim is the preservation of the body position and
configuration
Modulation and coordination of spinal reflex actions
The strength of a spinal reflex can be modulated by changes in
transmission in the reflex pathway
(Kandel - Principles of neural science, Chapter 36, Figure 36-8)
Inhibitory interneurons play special roles in coordination of reflex
actions
(Kandel - Principles of neural science, Chapter 36, Figure 36-5)