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 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.
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)
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.
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.

BASAL NUCLEI
Basal Ganglia Functions
Compare proprioceptive information and
movement commands.
 Sequence movements.
 Regulate muscle tone and muscle force.
 May be involved in selecting and inhibiting
specific motor synergies.

Basal Ganglia Functions
Basal ganglia are vital for normal
movement but they have no direct
connections with lower motor neurons.
 Influence LMNs:

Through planning areas of cerebral cortex.
Pedunculopontine nucleus of midbrain.
Basal Ganglia Functions
Basal nuclei set organism’s level of
responsiveness to stimuli.
 Extrapyramidal disorders are associated
with basal nuclei pathology:

Negative symptoms of underresponsiveness:
Akinesias
i.e. Parkinson disease
Positive symptoms of over-responsiveness:
Choreas, athetoses, ballisms
i.e. Huntington’s chorea
Basal Nuclei Components
Corpus striatum
 Substantia nigra (within the midbrain)
 Subthalamic nuclei (diencephalon)
 Red nucleus (?)
 Claustrum (?)
 Nucleus accumbens (?)

Corpus Striatum

Composed of caudate nucleus + lentiform
nucleus:
Striatum = caudate nucleus + putamen.
Pallidum = globus pallidus.
Putamen + globus pallidus = lentiform
nucleus.
Controls large subconscious movements
of the skeletal muscles.
 The globus pallidus regulates muscle tone.

Corpus Striatum
Corpus Striatum
(Telencephalon)
Striatum
Caudate Nucleus
Pallidum
Putamen
Globus Pallidus
Substantia Nigra Subdivisions
 Dorsal
pars compacta:
Has melanin containing neurons and
dopaminergic neurons.
 Ventral
pars reticularis:
Has iron-containing glial cells.
Has serotonin and GABA (no melanin).
Substantial Nigra
Substantia Nigra
Mesencephalon
Dorsal
Pars compacta
Ventral
Pars reticularis
Melanin containing neurons
Dopaminergic neurons
Iron-containing glial cells
Serotonin and GABA
Input Nuclei

Striatum
Caudate nucleus
Putamen
Nucleus accumbens

Receive widespread input from:
Neocortex
Intralaminar nuclei
Substantia nigra
Dorsal raphe nucleus
Input Nuclei

Striatum projects to:
Globus pallidus
Substantia nigra
Pars reticularis
Via gabaminergic fibers
Motor and sensory cortices project to
putamen.
 Association areas of all lobes project to
caudate nucleus.

Output Nuclei
Globus pallidus (medial part)
 Substantia nigra:

Pars reticularis
Ventral pallidum
 Fibers project to:

VA/VL nuclei
Mostly inhibitory
General Core Circuit
Cerebral cortex to:
 Striatum to:
 Globus pallidus to:
 Thalamus to:
 Portions of motor cortex to:
 Upper motor neurons

Thalamic Fasciculi
 Ansa
lenticularis:
Consists of fibers from dorsal portion of
globus pallidus.
Loops under internal capsule.
To VA/VL complex.
Thalamic Fasciculi
 Lenticular
fasciculus:
Consists of fibers from ventral portion
of globus pallidus.
Passes across the internal capsule.
To VA/VL complex.
Dopamine Neuronal System
Consists of nigrostriatal fibers
 From pars compacta of substantia nigra
 To striatum
 Dopaminergic

Direct Basal Ganglia Circuit

Motor cortex projects to putamen:
Excitatory (glutamate)

Putamen projects to output nuclei (globus
pallidus internus and substantia nigra
reticularis):
Inhibitory (GABA and substance P)
Basal Ganglia Connections
Red = excitatory; Black = Inhibitory
Motor areas of cerebral cortex
Ventrolateral
thalamus
Putamen
Globus pallidus
externus
Output
nuclei
PedunculoPontine
nuclei
Reticulospinal and
Vestibulospinal
pathways
Subthalamic
nuclei
Substantia nigra
compacta
Lateral
Activation
pathways
Direct Basal Ganglia Circuit

Output nuclei project to motor thalamus
(VA-VL) and pedunculopontine nuclei:
Inhibitory (GABA)

Ventrolateral (VA-VL) thalamus projects to
motor cortex:
Excitatory

Therefore:
Increasing input to putamen increases activity in
corticofugal fibers
Direct Basal Ganglia Circuit
Pedunculopontine nuclei project to
reticulospinal and vestibulospinal
pathways.
 Stimulation of pedunculopontine nuclei
elicit rhythmical behaviors such as
locomotor patterns.

Indirect Basal Ganglia Circuit

Motor cortex to putamen:
Excitatory (glutamate)

Putamen to globus pallidus externus:
Inhibitory (GABA and enkephalins)

Globus pallidus externus to subthalamic
nuclei:
Inhibition (GABA)
Indirect Basal Ganglia Circuit

Subthalamic nuclei to output nuclei
(substantia nigra reticularis)
Excitatory (glutamate)

Output nuclei to VA-VL complex (motor
thalamus)
Inhibitory (GABA)
Indirect Basal Ganglia Circuit

VA-VL complex to motor cortex:
Excitatory

Therefore: decrease in corticofugal
pathways.
Input from Substantia Nigra
Compacta

Projects to putamen:
Excitatory (dopamine)
Two kinds of receptors in basal ganglia
circuit:
D1: facilitates activity in direct pathway
D2: inhibits activity in indirect pathway