9 Flashcards
The spinal cord is formed of……
These neurons are
The spinal cord is formed of 31 segments cord
Anterior Motor Neurons
The neurons are of two types:
1) Alpha motor neurons: give rise to large type (A ) fibers, these fibers branch many times and innervate the large skeletal muscle fibers.
2) Gamma efferent neurons: give rise to small type (A ) fibers, these fibers innervate the intrafusal skeletal
muscle fib.
Interneurons
Interneurons
Receptor Function of the Muscle Spindle
It is a capsulated structure
present in the fleshy part of the muscle
Formed of 8 10 intrafusal muscle fibers
that are:
The central part of intrafusal fibers is called sensory area.
* The ends of intrafusal fibers are contractile
Structure of muscle spindle
There are two types:
There are two types:
I-Nuclear bag fibers: large number of nuclei are concentrated in the central part of the stimulated area.
II-Nuclear chain fibers: Having nuclei which spread in a chain throughout the receptor area.
Innervation of muscle spindle
(Afferents)
a-Primary endings
- Thick myelinated A fibers.
-Supply both nuclear bag and nuclear chain fibers. -They discharge mainly when the stretch is going on. -They are the receptor for dynamic stretch reflex.
Secondary endings:
-Myelinated nerve fibers A .
-Supply only the nuclear chain fibers.
-They discharge with maintained stretch.
-They are responsible for static
stretch reflex.
Innervation of muscle spindle (Efferents)
The intrafusal fibers are supplied by gamma efferent
motor fibers A .
Impulses sent by these fibers lead to contraction of
the peripheral contractile parts of intrafusal muscle
fibers and stretching of the non contractile part
leading to stimulation of both primary and secondary endings
Mode of stimulation of muscle spindle
1- When the whole muscle is stretched.
2- When the peripheral parts of intrafusal muscle fibers contract and the central receptor area is
to stretched.
Dynamic and Static Responses of the Neuronal circuit of the stretch reflex Muscle Spindle
Static Response:
Slow stretch both the 1ry and the 2ry endings in proportion to the degree of stretching, These impulses continue for several minutes. poorlyadapting
Dynamic Response:
Reflex arc
Stimulus: stretch of the muscle. Receptor: Muscle spindle (nuclear bag and nuclear chain fibers)
Afferent : Primary and secondary endings
Center : A.H.C
Efferent : Alpha motor neuron to the ordinary
muscle fibers.
Response: Muscle contraction
Functions of the muscle spindle
1- Damping” Function of the Dynamic and Static Stretch Reflexes
prevent oscillation or jerkiness of body movements by adjusting the power of contraction via increasing or decreasing the power of stretch reflex
–This effect can also be called a signal averaging function
2- Muscle Spindle System Stabilizes Body Position During Tense Action
excitation of the appropriate muscle spindles by signals from the bulbo-reticular facilitatory region of the brain stem stabilizes the positions of the major joints.
•This occur by increasing the tone of agonists and antagonists around the joints.
3- Role of the Muscle Spindle in Voluntary Motor Activity
1- The pathway through which the higher centers adjust the stretch reflex by changing the sensitivity of muscle spindle.
2- Alpha-gamma co-activation:
30% of motor fibers are of gamma type which:
A- Potentiate the voluntary movement (reflex muscle contraction is added to voluntary contraction).
B- Keep the sensitivity of muscle spindle during voluntary contraction.
4- Muscle tone: Mechanism:
Static stretch reflex.
mild maintained contraction of the skeletal muscle during rest.
Muscle tone
Mechanism: –Static stretch reflex:
–This is because the natural length
Distribution: Present in all muscles of the body but
more prominent in antigravity muscles, which are
extensors of the lower limbs , trunk and neck ,
flexors of the upper limbs and elevators of the jaw.
Supra spinal centers affecting stretch reflex (and Muscle Tone)
•Facilitatory centers:
Inhibitory centers:
•Facilitatory centers:
1- primary motor area
(area 4).
2- Neo-cerebellum.
3- Facilitatory reticular formation.
4- Vestibular nucleus.
•Inhibitory centers:
1- pre motor area
(area 6).
2- Paleo-cerebellum.
3- Inhibitory reticular formation.
4- Red nucleus & basal ganglia
Why Muscle tone is maintained without fatigue ?
This is due to:
a- Alternate activity of different motor units.
b- Muscle fibers involved in muscle tone are slow (red) type which is adapted for slow prolonged contraction without fatigue.
Abnormalities of muscle tone:
A-Increased (hypertonia) in:
B-Decreased (hypotonia) in:
A-Increased (hypertonia) in:
1- UMNL which give Clasp-knife sapsticity.
2- Parkinsonism which give lead-pipe rigidity.
B-Decreased (hypotonia) in:
1- LMNL.
2- Neocerebellar syndrome.
