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Excellent Network Courses Department of Neurology Affiliated hospital of Jiangsu University Motor Descending (Motor) Pathways Descending tracts deliver efferent impulses from the brain to the spinal cord, and are divided into two groups • Direct pathways equivalent to the pyramidal tracts • Indirect pathways, essentially all others Motor pathways involve two neurons (upper and lower) Pyramidal System concerned with voluntary movement of the skeletal m. and is composed of two orders of neurons: upper motor neuron lower motor neuron upper motor neuron lower motor neuron effector Upper Motor Neurons • • upper motor Lie in the motor area of cerebral cortex neuron Connect directly or through interneurons to lower motor neurons Lower Motor Neurons • • Include cranial motor nuclei and motor neurons in anterior horn of spinal cord. Their axons leave the CNS and extend through peripheral nerves to supply skeletal muscles The pyramidal system consists of corticospinal tract corticonuclear tract lower motor neuron effector Descending (Motor) Pathways Upper motor neurons • Cell bodies in the cerebral cortex Lower motor neurons • Cell bodies located in a motor nucleus of the brainstem or spinal cord Definition: Upper motor neuron • It refers to the nerves that connect from the brain to the spinal cord. It relays information to the lower motor neurons. • When the upper motor neurons are damaged the result is muscle spasticity and very strong automatic reflexes, such as the knee jerk reaction. Definition: Lower motor neuron • An efferent neuron which has its body located in the anterior gray column of the spinal cord or in the brainstem nuclei, and its axon passing by way of a peripheral nerve to skeletal muscle. • Also known as final common pathway. Effects of Upper and Lower Motor Neuron Damage UMN damage LMN damage Supranuclear paralysis Infranuclear paralysis Paralysis Spastic Flaccid Muscle tone Increased↑ Decreased↓ Tendon reflexes Increased↑ Decreased↓ Pathological reflexes Positive (+) Negatively (-) Amyotrophy No present Distinctly 1. Direct (Pyramidal) System • Direct pathways originate with the pyramidal neurons in the precentral gyri • Impulses are sent through the corticospinal tracts and synapse in the anterior horn • Stimulation of anterior horn neurons activates skeletal muscles • Parts of the direct pathway, called corticobulbar tracts, innervate cranial nerve nuclei • The direct pathway regulates fast and fine (skilled) movements Corticospinal tract Upper motor neuron is the Giant pyramidal cells (Betz cells) and other pyramidal cells in superior 2/3 of precentral gyrus and ant. part of paracentral lobule Upper motor neuron Corticospinal tract Decussation of pyramid Lateral corticospinal tract Lower motor neuron Anterior corticospinal tract 1. Corticospinal tract Lower motor neuron is the Motor neurons of anterior horn of spinal cord Lateral corticospinal tract Motor neurons of anterior horn Anterior corticospinal tract 2. Corticonuclear Tract Giant pyramidal cells and other pyramidal cells in lower 1/3 of precentral gyrus upper motor neuron Corticonuclear Tract Genu of internal capsule lower motor neuron Brain stem Cranial motor neuclei Cranial nerves Skeletal m. of the head ,neck and face. The Corticonuclear tract send out fibers to end Bilateral: nucleus of oculomotor n., nucleus trochlear n., motor nucleus of trigeminal n., nucleus of abducent n., ambiguus nucleus, nucleus of accessory n., superior part of facial nucleus. facial nucleus Contralateral: inferior part of facial nucleus hypoglossal nucleus hypoglossal nucleus Corticonuclear tract facial nucleus Hypoglossal nucleus and inferior part of facial nucleus receive only the fibers from the contralateral corticonuclear tract. hypoglossal nucleus The injury of the unilateral corticonuclear tract can usually cause the paralysis of the contralateral glossal m. and facial m. below the palpebral fissure. • The paralysis caused by the injury of the upper motor neuron is called the supranuclear paralysis. • The paralysis caused by the injury of the lower motor neuron is called the infranuclear paralysis. Dept of Neurology, Medical College, Jiangsu University 16 Supranuclear paralysis of facial n. ---- the paralysis of facial m. in opposite side below the pelpebral fissure Infranuclear paralysis of facial n. ---- the paralysis of facial m. of same side Supranuclear paralysis of hypoglossal n. ---- when tongue extend, the apex of tongue inclined to the opposite side Infranuclear paralysis of hypoglossal n. ----when tongue extend, the apex of tongue inclined to the same side 2. Indirect (Extrapyramidal) System • Includes the brainstem, motor nuclei, and all motor pathways not part of pyramidal system • Issue motor commands as a result of subconscious processing • Stimulate, facilitate, or inhibit LMN • This system includes the vestibulospinal, rubrospinal, reticulospinal, and tectospinal tracts 2. Indirect (Extrapyramidal) System These motor pathways are complex and multisynaptic, and regulate: • Axial muscles that maintain balance and posture • Muscles controlling coarse proximal portions of limbs • Head, neck, and eye movement movements of the 2. Indirect (Extrapyramidal) System Rubrospinal tracts • Controls muscle tone and movements of distal muscles of the upper limbs Vestibulospinal tracts • Carry subconscious regulation of balance and posture Reticulospinal tract • Carries subconscious regulation of reflex activity Tectospinal tracts • Carries subconscious regulation of visual and auditory reflex stimulus Upper motor neurons travel in several pathways through the CNS Tract Pathway Function Corticospinal tract From the motor cortex to lower motor neurons in the ventral horn of the spinal cord The function of this pathway is fine voluntary motor control of the limbs. The pathway also controls voluntary body posture adjustments. Corticobulbar tract From the motor cortex Involved in control of facial and to several nuclei in the jaw musculature, swallowing and tongue movements. pons and medulla From the superior Involved in involuntary to lower adjustment of head position in response to visual information. motor neurons Tectospinal tract/ Colliculospinal tract colliculus Upper motor neurons travel in several pathways through the CNS Tract Rubrospinal tract Pathway Function Involved in involuntary From red nucleus to lower adjustment of arm position in response to balance motor neurons information. Vestibulospinal tract From vestibular nuclei, which processes stimuli from semicircular canals to lower motor neurons Reticulospinal tract From reticular formation Regulates involuntary motor activities and assists in balance. to lower motor neurons Involved in control of facial and jaw musculature, swallowing and tongue movements. Spinal Cord Trauma: Paralysis Paralysis – loss of motor function Flaccid paralysis • severe damage to the ventral root or anterior horn cells • Lower motor neurons are damaged and impulses do not reach muscles • There is no voluntary or involuntary control of muscles Spinal Cord Trauma: Paralysis Spastic paralysis • only upper motor neurons of the primary motor cortex are damaged • Spinal neurons remain intact and muscles are stimulated irregularly • There is no voluntary control of muscles Spinal Cord Trauma: Transection • Cross sectioning of the spinal cord at any level results in total motor and sensory loss in regions inferior to the cut • Paraplegia – transection between T1 and L1 • Quadriplegia–transection in the cervical region Example: Poliomyelitis • Destruction of the anterior horn motor neurons by the poliovirus • Early symptoms – fever, headache, muscle pain and weakness, and loss of somatic reflexes • Vaccines are available and can prevent infection Example: Amyotrophic Lateral Sclerosis (ALS) • Lou Gehrig’s disease – neuromuscular condition involving destruction of anterior horn motor neurons and fibers of the pyramidal tract • Symptoms – loss of the ability to speak, swallow, and breathe • Death occurs within five years • Linked to malfunctioning genes for glutamate transporter and/or superoxide dismutase