Cerebral Cortex

• Research on the structure and function

of the brain reveals that there are both
specialized and diffuse areas of function
• Motor and sensory areas are localized
in discrete cortical areas called domains
• Many higher mental functions such as
memory and language appear to have
overlapping domains and are more
diffusely located
• Broadmann areas are areas of localized
function
Cerebral Cortex - Generalizations
• The cerebral cortex has three types of
functional areas
– Motor areas / control voluntary motor function
– Sensory areas / provide conscious awareness of
sensation
– Association areas / act mainly to integrate
diverse information for purposeful action
• Each hemisphere is chiefly concerned with
the sensory and motor functions of the
opposite (contralateral) side of the body
 Motor Areas

• Cortical areas controlling motor functions lie in the posterior part of the
frontal lobes
• Motor areas include the primary motor cortex, the premotor cortex,
Broca’s area, and the front eye field
 Primary Motor Cortex
• The primary motor cortex is located in
the precentral gyrus of the frontal lobe
of each hemisphere
• Large neurons (pyramidal cells) in these
gyri allow us to consciously control the
precise or skill voluntary movements of
our skeletal muscles
Pyramidal cells
• These long axons, which
project to the spinal
Dendrites
cord, form the massive
voluntary motor tracts
called the pyramidal, or
corticospinal tracts
• All other descending
motor tracts issue from
brain stem nuclei and
consists of chains of two,
three, or more neurons
 Pyramidal Tracts

• The lateral corticospinal tract consists of the

long axons of the pyramidal cells located
within the primary motor cortex
Motor
Somatotopy
• Body is represented
spatially in the primary
motor cortex of each
hemisphere
• Most of the neurons in
these gyri control
muscles in body areas
having the most
precise motor control
• The areas with the
most control (face,
tongue, and hands)
 Motor
Somatotopy
• Motor innervation is
contralateral; left primary
motor controls right side of
body
• A given muscle may be
controlled by several
cortical neurons recruited
for several specific actions
 Motor
Somatotopy
• Damage to the
localized areas of the
primary motor cortex
paralyzes the muscles
controlled by this area
• If the lesion is in the
right hemisphere, the
left side will be
paralyzed
• Only voluntary control is
lost as the muscles can
still contract reflexively
Premotor Cortex
• The premotor cortex
controls motor skills
of repetitive or
patterned nature
(typing or piano)
• The premotor cortex
coordinates the
movement of several
muscle groups to act
simultaneously or
sequentially
Premotor Cortex
• The premotor cortex
sends activating
impulses to the
primary motor cortex
• Also influences
motor actively more
directly by supplying
about 15% of
pyramidal tract fibers
• A memory bank of
skilled motor
activities
Premotor Cortex
• This area appears to
involved with motor
planning
• It controls voluntary
actions that depend
on sensory feedback
Premotor Cortex
• Damage to the premotor
area results in the loss
of the motor skills in that
region
• Muscle strength and the
ability to perform the
discrete individual
movements are not
hindered
• Neurons relearning the
skill would require
practice
Broca’s area
• The area has long
been considered to
be present in only one
hemisphere (usually
left)
• A special motor
speech area that
directs the muscles of
the tongue, throat,
and lips in articulating
words
Broca’s area
• Recent PET scans
indicates that Broca’s
area and a similar
area in the opposite
hemisphere become
active as we prepare
to speak
• The areas may be
involved with planning
speech and other
voluntary motor
activities
Frontal Eye Field
• This cortical region
controls the voluntary
movements of the
eyes
• Engaged when we
look quickly at
something, as in
moving our eyes to
follow a moving target
DESCENDING PATHWAYS FROM THE MOTOR CORTEX
SOME CORTICAL NEURONS PROJECT AXONS THAT
SYNAPSE ON SPINAL MOTOR NEURONS
 Descending (Motor) Tracts

• The pyramidal tracts are also called the direct

pathways because their axons descend without
synapsing from the pyramidal cells of the primary
motor cortex all the way to the spinal cord
 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)
 The 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
The Direct (Pyramidal) System

Figure 12.34a
 Descending (Motor) Tracts

• The lateral (pyramdial) and anterior corticospinal

tracts are the major motor pathways concerned with
voluntary movement, particularly precise or skilled
movement
Spinal Cord Descending Tracts
Corticospinal Tract
Origin: Cerebral Cortex
Brodmann Area 4 (Primary Motor Area, M I)
Brodmann Area 6 (Premotor Area, PM )
Brodmann Area 3,1,2 (Primary Somesthetic Area, S I)
Brodmann Area 5 (Anterior Portion of Sup. Parietal Lobule)
Corona Radiata
lnternal Capsule, Posterior Limb
Crus Cerebri, Middle Portion
Longitudinal Pontine Fiber
Pyramid - pyramidal decussation
Corticospinal Tracts:
- Lateral (crossed) - 85%
- Anterior (Not crossed) - 15%
Termination: Spinal Gray (Rexed IV-IX)
 Corticobulbar tract
• Cortical projections to red nucleus and
reticular formation
• Cortical projections from frontal eye fields
to gaze centers
• Corticonuclear projections to V, VII, IX-X,
and XII
Bilateral – 5, 7 (upper
face), ambiguus

Contralateral – 7
(lower face); 12
 Descending (Motor) Tracts

• The remaining tracts originate in different subcortical

motor nuclei of the brain stem
• These tracts are lumped together as the
extrapyramidal tracts
 Indirect (Extrapyramidal)
System
• Includes the brain stem, motor nuclei, and all
motor pathways not part of the pyramidal system
• This system includes the rubrospinal,
vestibulospinal, reticulospinal, and tectospinal
tracts
• These motor pathways are complex and
multisynaptic, and regulate:
– Axial muscles that maintain balance and posture
– Muscles controlling coarse movements of the
proximal portions of limbs
– Head, neck, and eye movement
Indirect (Extrapyramidal)
System

Figure 12.34b
 Extrapyramidal (Multineuronal)
Pathways
• Reticulospinal tracts – maintain balance
• Rubrospinal tracts – control flexor muscles
• Superior colliculi and tectospinal tracts
mediate head movements
Tract Pathway Function

Corticospinal From the motor cortex to The major function of this

tract
lower motor neurons in pathway is fine voluntary
the ventral horn of the motor control of the limbs.
spinal cord The pathway also controls
voluntary body posture
adjustments.
Corticobulbar From the motor cortex to Involved in control of facial
tract
several nuclei in the and jaw musculature,
pons and medulla swallowing and tongue
movements.
(Medial)

(Lateral)
Difference between pyramidal and extrapyramidal
tracts
Pyramidal tracts Extrapyramidal tracts
• Newer system Older system
• Origin Origin
• Cerebrum Basal ganglia and cerebellum
• Skilled movements Posture and axial movements
 Upper Motor Neurons
• Upper motor neurons are motor neurons
that originate in motor region of the
cerebral cortex or the brain stem and carry
motor information down to the final
common pathway.
 Lower Motor Neurons
• These include alpha and gamma motor neurons.
• Alpha motor neurons have their cell bodies in
their CNS. Their axons course through cranial
and spinal nerves and terminate on the motor
end plates of skeletal muscle fibers
• Gamma neurons also have cell bodies within the
CNS. Their axons pass through cranial and
spinal nerves to innervate the intrafusal muscle
fibers.
 Lower Motor Neurons
• These are the only neurons that innervate
the skeletal muscle fibers, they function as
the final common pathway, the final link
between the CNS and skeletal muscles.
• Axons are located both in the cranial and
spinal nerves.
 SOMATIC MOTOR SYSTEM

upper motor neuron

UMN Brain Stem
Descending
Pathway

VOLUNTARY Final Common Pathway Rubrospinal Tract

CONTROL Tectospinal Tract
lower motor neuron Vestibulospinal Trac
Reticulospinal Tract
LMN
Pyramidal Tract AUTOMATIC CONTROL

REFLEX
EFFECTORS
skeletal muscle
Upper Motor Neuron (UMN) vs. Lower Motor Neuron (LMN)
Syndrome
UMN syndrome LMN Syndrome

Type of Paralysis Spastic Paresis Flaccid Paralysis

Atrophy No (Disuse) Atrophy Severe Atrophy

Deep Tendon Reflex Increase Absent DTR

Pathological Reflex Positive Babinski Sign Absent

Superficial Reflex Absent Present

Fasciculation and Absent Could be

Fibrillation Present
 Babinski reflex - an UMN sign
• Adult response - plantar flexion of the big toe and
adduction of the smaller toes
• Pathological (Infant) response - dorsoflexion
(extension) of the big toe and fanning of the other toes
• Indicative of upper motor neuron damage
 Paralysis/Paresis
• Hemiplegia : Paralysis to one side of the
body. A lesion of corticospinal tract in the
internal capsule results in a hemiplegia
• Monoplegia : Paralysis of a single limb
• Paraplegia and Quadriplegia: If the
spinal cord damage occurs at cervical
level, then all four limbs will be paralyzed
(quadriplegia). If the damage occurs
below the cervical enlargement, then only
the legs are paralyzed (paraplegia).