Transcript neuro 13 descending tracts student

DESCENDING TRACTS
Fiber Types

A Fibers:
Somatic, myelinated.
Alpha (α):
Largest, also referred to as Type I.
Beta (β):
Also referred to as Type II.
Gamma (γ):
Delta (δ):
Smallest, referred to as Type IV.
Fiber Types


B Fibers:
Lightly myelinated.
Preganglionic fibers of ANS.
C Fibers:
Unmyelinated.
Found in somatic and autonomic systems.
Also referred to as Type IV fibers.
Fiber Types

Sensory fibers are either:
A-α or A-β fibers:
Conduction rate = 30-120 m/sec.
A-δ fibers:
Conduction rate = 4-30 m/sec.
C fibers:
Conduction rate is less than 2.5m/sec.
Fiber Types

Nociceptors and thermoreceptors are
related to C fibers or A-δ fibers.
Generalizations: Motor Paths

Typical descending pathway consists of a
series of two motor neurons:
Upper motor neurons (UMNs)
Lower motor neurons (LMNs)
Does not take into consideration the
association neurons between UMNs and
LMNs
Upper Motor Neurons
Are entirely within the CNS.
 Originate in:

Cerebral cortex
Cerebellum
Brainstem

Form descending tracts
Lower Motor Neurons
Begin in CNS.
 From anterior horns of spinal cord.
 From brainstem cranial nerve nuclei.
 Made up of alpha motor neurons (A-α).
 Make up spinal and cranial nerves.

UMN Classification

Classified according to where they
synapse in the ventral horn:
Medial activation system:
Innervate postural and girdle muscles
Lateral activation system:
Associated with distally located muscles used
for fine movements
Nonspecific activating system:
Facilitate local reflex arcs
Pyramidal System

Characteristics:
Upper motor neurons:
75 – 85% Decussate in pyramids.
Remainder decussate near synapse with
lower motor neurons.
Most synapse with association neurons in
spinal cord central gray.
Pyramidal System

Components:
Corticospinal Tract
Corticobulbar Tract
Corticospinal Tract Divisions

Lateral corticospinal tract:
Made up of corticospinal fibers that have
crossed in medulla.
Supply all levels of spinal cord.

Anterior corticospinal tract:
Made up of uncrossed corticospinal fibers
that cross near level of synapse with LMNs.
Supply neck and upper limbs.
Corticospinal Tract Pathway
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Corticospinal Tract (Tectum)
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Corticospinal Tract (Pons)
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Corticospinal Tract (Sup. Med.)
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Corticospinal Tract (Inf. Med.)
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Lateral Corticospinal Tract
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Anterior Corticospinal Tract
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Corticospinal Tract Functions

Add speed and agility to conscious
movements:
Especially movements of hand.

Provide a high degree of motor control:
(i.e., movement of individual fingers)
Corticospinal Tract Lesions
Reduced muscle tone
 Clumsiness
 Weakness
 Not complete paralysis
 Note: complete paralysis results if both
pyramidal and extrapyramidal systems are
involved (as is often the case).

Corticobulbar Tract
Innervates the head
 Most fibers terminate in reticular formation
near cranial nerve nuclei.
 Association neurons:

Leave reticular formation and synapse in
cranial nerve nuclei.
Synapse with lower motor neurons.
Corticobulbar Tract (Sup. Tectum)
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Corticobulbar Tract (Inf. Tectum)
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Extrapyramidal System
Includes descending motor tracts that do
not pass through medullary pyramids or
corticobulbar tracts.
 Includes:

Rubrospinal tracts
Vestibulospinal tracts
Reticulospinal tracts
Rubrospinal Tract
Begins in red nucleus.
 Decussates in midbrain.
 Descends in lateral funiculus (column).
 Function closely related to cerebellar
function.
 Lesions:

Impairment of distal arm and hand movement.
Intention tremors (similar to cerebellar lesions)
Rubrospinal Tract: (Sup. Tectum)
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Rubrospinal Tract: (Inf. Tectum)
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Rubrospinal Tract: (Inf. Medulla)
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Rubrospinal Tract
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Vestibulospinal Tract

Originates in vestibular nuclei:
Receives major input from vestibular nerve:
(CN VIII)
Descends in anterior funiculus (column).
 Synapses with LMNs to extensor muscles:

Primarily involved in maintenance of upright
posture.
Vestibulospinal Tract
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Reticulospinal Tract
Originates in various regions of reticular
formation.
 Descends in anterior portion of lateral
funiculus (column).
 Thought to mediate larger movements of
trunk and limbs that do not require balance
or fine movements of upper limbs.

Reticulospinal Tract
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