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Upper motor neuron versus lower
motor neuron
• Lower motor neurons are the
anterior gray horn cells (alpha
motor neuron) in the spinal
cord and the motor neurons of
the cranial nerve nuclei
situated in the brain stem.,
which innervates the muscles
directly.
• The lower motor neuron are
under the influence of the
upper motor neurons.
•
Upper motor neurons are
the neurons in the higher
centers of the brain, which
control the lower motor
neurons.
Spinal Reflexes
• Somatic reflexes mediated by the spinal cord
are called spinal reflexes
• These reflexes may occur without the
involvement of higher brain centers
• The brain can facilitate or inhibit them
Extrafusal and intrafusal fibers of Muscle
•Every striated muscle has
encapsulated muscle fibers
scattered throughout the
muscle called muscle
spindles.
The extrafusal muscle
fibers are innervated by
Alpha motor neuron
The intrafusal muscle
fibers are innervated by
Gamma motor neurons
Anatomy of Muscle Spindle
• The contractile region of the intrafusal muscle fibers are
limited to their ends as only these areas contain actin and
myosin filaments
• These regions are innervated by gamma () efferent fibers
Primary sensory
endings are
Stimulated by
both the rate and
degree of stretch.
Secondary sensory
endings are
stimulated only by
degree of stretch
The Stretch Reflex
• Exciting a muscle spindle occurs
in two ways
– Applying a force that
lengthens the entire muscle
– Activating the  motor
neurons that stimulate the
distal ends of the intrafusal
fibers to contact,
• thus stretching the midportion of the spindle
(internal stretch)
Stretch reflex (monosynaptic)
Golgi tendon reflex
When muscle tension increases moderately during muscle contraction, GTO receptors
are activated and afferent impulses are transmitted to the spinal cord
The Crossed Extensor
Reflex
•
The reflex occurs when sensory
receptors are excited by painful
stimulus.
•
There is a rapid lifting of the
affected foot (ipsilateral
withdrawal reflex )
•
while the contralateral response
activates the extensor muscles of
the opposite leg (contralateral
extensor reflex) to support the
weight shifted to it
.
Effects of upper motor versus lower
motor neuron lesion
upper
lower
Muscle tone
Hypertonia
Hypotonia
Type of paralysis
Spastic type
Flaccid type
Deep tendon reflex
Increase
Absent
Wasting of muscle
(atrophy)
No wasting
present
Electrical activity
Normal
Absent
Muscles affected
Groups of muscles
Individual muscles
Fasciculation
Absent
Present