Motorcontrol 1
2nd motor neuron is a neuron that lives either on the
brainstem or in de spinal cord but mainly in the spinal cord.
The spinal cord is a non-segmented continuous structure.
Dorsal horn:
Afferent/sensory
Neuron in spinal ganglion
Ventral horn:
Efferent/motor
Neuron in CNS
The anterior horn contains motor neurons. The anterior
rootlets consist of axons that originate in the anterior horn.
The anterior rootlets coalesce into the anterior root.
The posterior horn contains sensory neurons. The posterior
rootlets consist of axons that terminate in the posterior
horn. The posterior rootlets coalesce into the posterior root.
A spinal nerve is the fusion of an anterior and posterior spinal root at the
level of the intervertebral foramen.
In between dorsal horn and ventral horn is the intermediary grey which is
the local integration center.
Motor neuron:
In ventral horn of spinal cord
Similar position in brainstem
Axon of the motor neuron:
Leaves spinal cord through ventral root.
Ventral root fuses with dorsal root into spinal nerve.
o Trunk: ventral branch of spinal nerve innervates muscle.
o Limb: first spinal nerves exchange axons in plexus, peripheral nerves emerge, these
innervate muscles.
A nerve is a bundle of axons, surrounded by connective tissue (no neurons, no nerves in CNS).
1ste motor neuron projects to the 2nd motor neuron by cortico-bulbospinal tract (bundle of axons)
and the 2nd motor neuron projects to the muscle.
1st Motor neuron is upper motor neuron:
In motor cortex.
Pyramidal projection neuron.
Connects to the 2nd motor neuron.
2nd motor neuron is a-motor neuron is lower motor neuron.
In brainstem or ventral horn of spinal cord.
Connects to the striated muscles.
Muscle
Extensor
Each skeletal muscle consists of striated muscle fibers. Straited muscle fibers reach from tendon to
tendon (from begin to end of the muscle). Agonistic muscles:
Work together (M. biceps (flexor) & M. brachialis (flexor))
Antagonistic muscles:
Work against each other (M. biceps (flexor) & M. triceps (flexor))
1
,For smooth movement a very close cooperation between the muscles acting on a specific joint is
necessary. Contraction of an agonist requires relaxation of the antagonist (Sherrington’s law of
reciprocal innervation).
Many muscles act over more than one joint.
Hence the cooperation of muscles across the various moving joints is a very complex problem. Both
the extrapyramidal sub-system (basal ganglia) and the cerebellar sub-system address the
coordination of the skeletal musculature.
Each adult muscle fiber is innervated by exactly 1 axon. This axon originates from an a-motor neuron:
the 2nd or lower motor neuron located in the ventral horn of the spinal cord.
Between the a-motor neuron and muscle fiber is a very large specialized synapse called (3):
Motor end plate (mep) One mep/muscle fiber
Myoneuronal junction Neurotransmitter: Acetylcholine
Neuromuscular junction
The motor end plate is a very effective synapse every motor neuronal action
potential will cause an action potential in the muscle fiber:
Large surfaces
High density of postsynaptic receptors
A muscle fiber action potential (AP) results in a twitch:
Muscle fiber twitch lasts 25-200 msec.
Muscle fiber AP lasts 5 msec.
Next muscle fiber AP can be evoked before muscle fiber twitch
is over. A train of AP will tetanise (complete contraction) the
muscle.
Tetanus physiology:
Sustained muscle contraction evoked when the motor nerve that innervates
a skeletal muscle emits action potentials at a very high rate.
The frequency of the a-motor neuron action potentials determines the
amount of contraction.
Motor unit is a motor neuron and all the muscle fibers it innervates:
Size varies from 1 to 100dreds of muscle fibers.
All muscle fibers in a motor unit are always located within the same
muscle.
Small units precise control.
2
, Motor neuron pool is a group of motor neurons together
innervating all the muscle fibers of a single muscle.
Coherent cluster in ventral horn.
Location of (somato) motor neurons:
Spinal cord:
Medial somatomotor column:
o Entire length of spinal cord
o Postural musculature
Lateral somatomotor column:
o Only in the intumescences
o Limb musculature
Brainstem:
Medial and lateral somatomotor column:
o Oculomotor, trochear, and abducent nuclei: muscles for eye movement.
o Hypoglossal nucleus: tongue muscles.
o Accessory nucleus (C1-C5): M. trapezius & M. sternocleidomastoideus.
Branchiomotor column (bulbo-spinal somatomotor system):
o Motor trigeminal nucleus: muscles for mastication.
o Facial motor nucleus: mimical muscles of face.
o Ambiguus nucleus: muscles for swallowing.
The size of a motor neuron is proportional to the number of muscle fibers it innervates
(large motor unit, large motor neuron). The smaller a motor neuron is, the easier it is
excited. Smaller motor units allow for more precise control of muscle contraction.
The size principle of motor neuron recruitment. Imagine 1 axon synapsing upon all motor neurons in
a motor neuron pool:
Low frequency train of action potentials
o Only small neurons become excited
o Only small motor units are activated
Progressive increase of frequency
o Progressively larger motor neurons become excited as well.
o Progressively larger motor units are activated as well.
The increase of the frequency of the input into the motor neuron pool results in increasing force
exerted by the muscle.
Progressive increase of drive automatically recruits the motor NEURONS from small to large.
Consequently progressive increase of drive automatically recruits the motor UNITS small to large.
Increasing the drive results in progressive increase of the force exerted by the muscle.
(Size principle of motor neuron recruitment of Henneman).
Types of muscle fiber:
Slow (dark)
Intermediate
Fast (light)
All fibers in a motor unit are the same type. The motor neuron dictates the muscle fiber type.
Small motor unit: Large motor unit:
Small, slow but fatigue-resistant Large, fast, but very fatiguable muscle
muscle fibers. fibers.
Small motor neuron. Large motor neuron
3
2nd motor neuron is a neuron that lives either on the
brainstem or in de spinal cord but mainly in the spinal cord.
The spinal cord is a non-segmented continuous structure.
Dorsal horn:
Afferent/sensory
Neuron in spinal ganglion
Ventral horn:
Efferent/motor
Neuron in CNS
The anterior horn contains motor neurons. The anterior
rootlets consist of axons that originate in the anterior horn.
The anterior rootlets coalesce into the anterior root.
The posterior horn contains sensory neurons. The posterior
rootlets consist of axons that terminate in the posterior
horn. The posterior rootlets coalesce into the posterior root.
A spinal nerve is the fusion of an anterior and posterior spinal root at the
level of the intervertebral foramen.
In between dorsal horn and ventral horn is the intermediary grey which is
the local integration center.
Motor neuron:
In ventral horn of spinal cord
Similar position in brainstem
Axon of the motor neuron:
Leaves spinal cord through ventral root.
Ventral root fuses with dorsal root into spinal nerve.
o Trunk: ventral branch of spinal nerve innervates muscle.
o Limb: first spinal nerves exchange axons in plexus, peripheral nerves emerge, these
innervate muscles.
A nerve is a bundle of axons, surrounded by connective tissue (no neurons, no nerves in CNS).
1ste motor neuron projects to the 2nd motor neuron by cortico-bulbospinal tract (bundle of axons)
and the 2nd motor neuron projects to the muscle.
1st Motor neuron is upper motor neuron:
In motor cortex.
Pyramidal projection neuron.
Connects to the 2nd motor neuron.
2nd motor neuron is a-motor neuron is lower motor neuron.
In brainstem or ventral horn of spinal cord.
Connects to the striated muscles.
Muscle
Extensor
Each skeletal muscle consists of striated muscle fibers. Straited muscle fibers reach from tendon to
tendon (from begin to end of the muscle). Agonistic muscles:
Work together (M. biceps (flexor) & M. brachialis (flexor))
Antagonistic muscles:
Work against each other (M. biceps (flexor) & M. triceps (flexor))
1
,For smooth movement a very close cooperation between the muscles acting on a specific joint is
necessary. Contraction of an agonist requires relaxation of the antagonist (Sherrington’s law of
reciprocal innervation).
Many muscles act over more than one joint.
Hence the cooperation of muscles across the various moving joints is a very complex problem. Both
the extrapyramidal sub-system (basal ganglia) and the cerebellar sub-system address the
coordination of the skeletal musculature.
Each adult muscle fiber is innervated by exactly 1 axon. This axon originates from an a-motor neuron:
the 2nd or lower motor neuron located in the ventral horn of the spinal cord.
Between the a-motor neuron and muscle fiber is a very large specialized synapse called (3):
Motor end plate (mep) One mep/muscle fiber
Myoneuronal junction Neurotransmitter: Acetylcholine
Neuromuscular junction
The motor end plate is a very effective synapse every motor neuronal action
potential will cause an action potential in the muscle fiber:
Large surfaces
High density of postsynaptic receptors
A muscle fiber action potential (AP) results in a twitch:
Muscle fiber twitch lasts 25-200 msec.
Muscle fiber AP lasts 5 msec.
Next muscle fiber AP can be evoked before muscle fiber twitch
is over. A train of AP will tetanise (complete contraction) the
muscle.
Tetanus physiology:
Sustained muscle contraction evoked when the motor nerve that innervates
a skeletal muscle emits action potentials at a very high rate.
The frequency of the a-motor neuron action potentials determines the
amount of contraction.
Motor unit is a motor neuron and all the muscle fibers it innervates:
Size varies from 1 to 100dreds of muscle fibers.
All muscle fibers in a motor unit are always located within the same
muscle.
Small units precise control.
2
, Motor neuron pool is a group of motor neurons together
innervating all the muscle fibers of a single muscle.
Coherent cluster in ventral horn.
Location of (somato) motor neurons:
Spinal cord:
Medial somatomotor column:
o Entire length of spinal cord
o Postural musculature
Lateral somatomotor column:
o Only in the intumescences
o Limb musculature
Brainstem:
Medial and lateral somatomotor column:
o Oculomotor, trochear, and abducent nuclei: muscles for eye movement.
o Hypoglossal nucleus: tongue muscles.
o Accessory nucleus (C1-C5): M. trapezius & M. sternocleidomastoideus.
Branchiomotor column (bulbo-spinal somatomotor system):
o Motor trigeminal nucleus: muscles for mastication.
o Facial motor nucleus: mimical muscles of face.
o Ambiguus nucleus: muscles for swallowing.
The size of a motor neuron is proportional to the number of muscle fibers it innervates
(large motor unit, large motor neuron). The smaller a motor neuron is, the easier it is
excited. Smaller motor units allow for more precise control of muscle contraction.
The size principle of motor neuron recruitment. Imagine 1 axon synapsing upon all motor neurons in
a motor neuron pool:
Low frequency train of action potentials
o Only small neurons become excited
o Only small motor units are activated
Progressive increase of frequency
o Progressively larger motor neurons become excited as well.
o Progressively larger motor units are activated as well.
The increase of the frequency of the input into the motor neuron pool results in increasing force
exerted by the muscle.
Progressive increase of drive automatically recruits the motor NEURONS from small to large.
Consequently progressive increase of drive automatically recruits the motor UNITS small to large.
Increasing the drive results in progressive increase of the force exerted by the muscle.
(Size principle of motor neuron recruitment of Henneman).
Types of muscle fiber:
Slow (dark)
Intermediate
Fast (light)
All fibers in a motor unit are the same type. The motor neuron dictates the muscle fiber type.
Small motor unit: Large motor unit:
Small, slow but fatigue-resistant Large, fast, but very fatiguable muscle
muscle fibers. fibers.
Small motor neuron. Large motor neuron
3
