Upper motor neurons originate in the motor cortex or brainstem and carry signals to lower motor neurons in the spinal cord. Betz cells are large upper motor neurons that connect the motor cortex to the spinal cord. Lower motor neurons are located in the spinal cord or brainstem and connect upper motor neurons to muscles. They release acetylcholine at the neuromuscular junction to trigger muscle contraction. Damage to lower motor neurons causes muscle weakness and atrophy due to inability to contract muscles.

1. Concept of upper and lower motor
neurons
NEUROPHYSIOLOGY

3. UPPER MOTOR NEURON
 Upper motor neurons are motor neurons that
originate either in the motor region of the cerebral
cortex or in the brain stem and carry motor
information down to the lower motor neurons
 The main effector or motor neurons (efferent
neurons) for voluntary movement lie within layer
of the primary motor cortex and are a type of
giant pyramidal cell called Betz cells
 The cell bodies of these neurons are the largest
in the brain, approaching nearly 0.1mm
in diameter.

5. Betz cells
 Betz cells (also known as pyramidal cells of
Betz) are giant pyramidal cells (neurons) located
within the fifth layer of the grey matter in
the primary motor cortex
 These neurons are the largest in the central
nervous system, sometimes reaching 100 μm in
diameter
 Betz cells are upper motor neurons that send
their axons down to the spinal cord via
the corticospinal tract, where
in humans they synapse directly with anterior
horn cells, which in turn synapse directly with
their target muscles

8. UPPER MOTOR NEURON
 The upper motor neuron descends in the spinal
cord to the level of the appropriate spinal nerve
root
 At this point, the upper motor neuron synapses
with the lower motor neuron, each of whose
axons innervate a fiber of skeletal muscle.
 These neurons connect the brain to the
appropriate level in the spinal cord, from which
point nerve signals continue to the muscles by
means of the lower motor neurons.

9. UPPER MOTOR NEURON
 The neurotransmitter glutamate transmits the
nerve impulses from upper to lower motor
neurons, where it is detected
by glutamatergic receptors.

11. LOWER MOTOR NEURON
 Lower motor neurons (LMNs) are motor neurons located
in either the anterior grey column, anterior nerve
roots (spinal lower motor neurons) or the cranial
nerve nuclei of the brainstem and cranial nerves with
motor function (cranial nerve lower motor neurons)
 All voluntary movement relies on spinal lower motor
neurons, which innervate skeletal muscle fibers and act as
a link between upper motor neurons and muscles
 Cranial nerve lower motor neurons control movements of
the eyes and tongue, and contribute to chewing,
swallowing and vocalization Damage to the lower motor
neuron can lead to flaccid paralysis.

12. CLASSIFICATION
 Classification
 Lower motor neurons are classified based on the
type of muscle fiber they innervate:
 Alpha motor neurons (α-MNs) innervate extrafusal
muscle fibers, the most numerous type of muscle fiber
and the one involved in muscle contraction.
 Beta motor neurons (β-MNs) innervate intrafusal
fibers of muscle spindles with collaterals to extrafusal
fibers (type of slow twitch fibers).
 Gamma motor neurons (γ-MNs) innervate intrafusal
muscle fibers, which together with sensory afferents
compose muscle spindles. These are part of the
system for sensing body position (proprioception)

13. LOWER MOTOR NEURON
 Glutamate released from the upper motor
neurons triggers depolarization in the lower motor
neurons in the anterior grey column, which in turn
causes an action potential to propagate the
length of the axon to the neuromuscular
junction where acetylcholine is released to carry
the signal across the synaptic cleft to the
postsynaptic receptors of themuscle
cell membrane, signaling the muscle to contract.

15. CONSEQUENCE OF LESION
 Damage to lower motor neurons, lower motor
neuron lesions (LMNL) cause muscle atrophy,
decreased strength and decreased reflexes in
affected areas. These findings are in contrast to
findings in upper motor neuron lesions. LMNL is
indicated by
abnormal EMG potentials, fasciculations, paralysi
s, weakening of muscles, and
neurogenic atrophy of skeletal muscle.) are all
pathologies associated with lower motor neuron
dysfunction.