#15 Flashcards
(26 cards)
The alpha (lower) motor neuron
At the lowest level of the neuromotor hierarchy are the alpha motor neurons, the socalled final common pathway to muscle contraction. Located in the ventral horn of the
spinal cord, alpha motor neurons are capable of elaborate dendritic processing engaged by multiple descending systems.
Alpha motor neurons controlling distal
muscles receive more X
Those controlling more axial and proximal
muscles receive more Y
pyramidal (i.e., lateral
corticospinal tract) input.
extrapyramidal (e.g.,
vestibulospinal, reticulospinal, tectospinal)
input.
The motor unit
This is the ‘essential’ muscle control system, defined as a
single alpha motor neuron and the muscle fibers innervated
by it.
each individual muscle fiber is innervated by a
single motor neuron, but a single motor neuron may innervate several muscle fibers.
Larger motor units
Smaller motor units
can generate more force
are associated with fine
motor control of e.g. fingers and facial muscles
Rate Code
: Increases in rate of action potentials of a motor neuron cause increases in muscle force (from single twitch to fused
tetanic contraction). Generally, this allows as
much as a 4-fold increase in force.
Size Principle
With increasing strength of
input, motor neurons in a motor pool are
recruited from smallest (fewer ion channels) to
largest (most ion channels)
Small neurons innervate X
* Medium neurons innervate Y
* Large neurons innervate Z
X slow-twitch fibers
(for postural control)
Y fast-twitch, fatigue-resistant fibers (for normal movements)
Z fast-twitch,
fatigable fibers (for bursts of energy)
A motor pool is defined as
the collection of all motor neurons that innervate a single
muscle.
Though the extrafusal fibers are responsible for X, intrafusal
muscle spindles embedded within them facilitate maintenance of Y
X skeletal muscle contraction
Y muscle tone: the
continuous and passive partial contraction of extrafusal fibers and their resistance to
passive stretch.
The muscle spindle contains two types
of stretchable fibers:
Type Ia and Type II spindles?
- Nuclear chain fibers: The cell nuclei are aligned in a single row.
Deformation (stretch) signals information about static length of
the muscle. - Nuclear bag fibers: Cell nuclei are jumbled together in the middle.
Deformation encodes the rate of change (velocity) of muscle length
Type Ia spindle afferents wrap around both kinds of fibers, and therefore provide info to the CNS about muscle length and velocity.
Type II spindle afferents only innervate the nuclear chain fibers, and so signal muscle immediate length only.
1) When a muscle is stretched suddenly, the embedded muscle spindle stretches and
the X fires more strongly
2) Spindle Xs enter the spinal cord (medial bundle) and terminate on the Y going back to the extrafusal fibers of the stretched motor
unit – this is the basis for the Z reflex
X Ia afferent
Y alpha (α) motor neuron
Z monosynaptic stretch
Monosynaptic stretch reflex
- Stretch of the muscle spindle intrafusal fibers
triggers an action potential in a 1a afferent neuron - The 1a afferent neuron synapses with the alpha
motor neuron of the same muscle and excites it - 2 neurons, 1 synapse
Gamma (γ) motor neurons
Within the ventral horn are also found gamma (γ)
motor neurons, mixed in with the alpha (α) motor
neurons. The axons of gamma motor neurons end in the contractile end portions of muscle spindles. Their function is important: they keep the muscle spindles taut.
Nuclear chain fibers and nuclear bag fibers are both directly innervated by
gamma
motor neurons (γ motor) at their tapered end portions
X are activated by higher brain centers during voluntary movements. This causes a contraction
and stiffening of the end portions of the intrafusal muscle fibers.
Gamma motor neurons
Making a voluntary movement requires co-activation of both
alpha and gamma
motor neurons from higher centers
Gamma motor neurons do more than maintain muscle spindle sensitivity to muscle stretch. Importantly, the gamma motor system also
adjusts muscle tone, the general resting level of tension in each muscle, necessary for maintaining posture and for “kick-starting” quicker movements (partially contracted muscles can attain full contraction
more quickly, requiring less neuronal firing and thus time)
How does gamma motor system adjust muscle tone
A steady level of alpha motor neuron activity is maintained by the muscle spindle Ia afferents (stretch reflex). Muscle tone is fine-tuned by tonic discharge of gamma
motor neurons activated by brainstem extrapyramidal pathways. As explained above, supraspinal activation of the gamma motor neuron leads to stretching of the muscle spindle, which further activates the alpha motor neuron, partially contracting the muscle and increasing tone.
Small interneurons called propriospinal neurons (“proper spinal” aka “local
circuit neurons”) yoke together sensory and motor neurons in networks called
central
pattern generators (CPGs). Every alpha and gamma motor neuron is engaged by many
propriospinal interneurons
Intermediate zone: heavily populated by
propriospinal interneurons
CPGs can generate patterns of
rhythmic activity for stereotyped movements, like locomotion, even in the absence of external
feedback or supraspinal control.
Spinal cord intermediate zone contains central pattern generators (CPGs)
responsible for:
- Responsible for complex stereotypic movements
involving multiple muscle groups - Composed of short-axon propriospinal neurons
- Input: sensory fibers, supraspinal neurons (e.g.,
corticospinal system) - Output: (mostly inhibitory) to alpha and gamma motor neurons
E.g., - Control of gait (walk, run, jog, skip)
- Coordination of upper and lower limbs by
propriospinal fibers
Propriospinal interneurons in the spinal cord are often organized as
sets of excitatory and
inhibitory interneurons, arranged to inhibit one another reciprocally:
Pontine and medullary reticular formations contain interneurons
which interact with motor neurons in the brainstem tegmentum.
Brainstem CPGs regulate stereotypic movements of the head; e.g.
- swallowing
- respiration
- coughing
- vomiting
- chewing
- saccadic eye movements
- et al
