Lecture 29
PHYSIOLOGY OF THE SPINAL CORD,
BRAINSTEM AND CEREBELLUM
The three main principles of the sensorimotor function are:

Motor output is guided by sensory input.

The system that controls motility is hierarchically and interactionally
organized. This system coordinates sequences of simultaneous complex
movements of many muscles controlled by the central sensorimotor programs.

Learning and experience may change the nature of sensorimotor control.
After learning a new motor skill, control is shifted to a lower sensorimotor level.
GENERAL ORGANIZATION OF THE SOMESTHETIC
INPUT
SOMATOSENSORY MECHANISMS
The sensations of the body are detected by several somatic senses. The
subdivisions of the somatic sensory system are:
exteroceptive sensation (cutaneous senses)
proprioceptive sensation (kinesthesia)
visceroceptive sensation
angioceptive sensation
Exteroceptive senses are:
touch
pressure
vibration
heat
cold
pain
Proprioceptive senses are:
tension of the muscles
tension of the tendons
angulation of the joints
deep pressure from the bottom of the feet
Visceroceptive and angioceptive sensation involves:
pain
fullness
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heat or burning sensations
SKIN RECEPTORS ACCORDING TO THE TYPE OF STIMULUS
The Pacinian corpuscles function exclusively as touch, vibration and pressure
receptors. However, other types of receptors may also function as touch and
pressure receptors. Also, no solid evidence has been found to link pain, warmth or
cold with any single type of receptor. A single receptor may act as both a
mechanoreceptor and thermoreceptor or nociceptor. Only the pattern of firing may
distinguish different modalities.
1. Mechanoreceptors
2. Thermoreceptors
3. Nociceptors
4. Proprioceptive receptors: They are found mostly in muscles, tendons and joints.
We are seldom aware of their input. The position (proprioceptive) sense can be
divided into:
*
static position sensors indicating the position of body parts;
*
kinetic sensors detecting the rate of movement
MUSCLE SPINDLES
The muscle spindles are kinesthetic receptors composed of a spindle-like
capsule of connective tissue parallel to the muscle fibers. They contain a few short
and very slender striated muscle fibers called intrafusal muscle fibers. They are
attached to the extrafusal muscle fibers. In the central region of the intrafusal fiber,
there are few or no actin and myosin filaments. This area is the receptor portion of
the muscle spindle.
The intrafusal fibers and spindles may be divided into:
nuclear bag fibers containing a large number of cell nuclei aggregated into
an expanded bag in the central portion of the receptor area.
nuclear chain fibers, thinner and shorter than the nuclear bag fibers.
The muscle spindles contain two sets of afferent nerve endings:
1. annulospiral nerve endings (primary), wrapped around the muscle fibers. Their
afferent fibers are large and myelinated. The primary nerve endings innervate both
the nuclear bag fibers and the nuclear chain fibers.
2. flower-spray nerve endings (secondary); their afferent fibers are thin and they
are located on the side of the primary ending. The secondary nerve fibers innervate
only the nuclear chain fibers.
There are two types of responses of the muscle spindle: the static and the
dynamic. The static response is elicited when the spindle is stretched slowly. Both
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the primary and the secondary nerve endings are activated. The number of nerve
impulses from both fibers is the same and may last many minutes. The nuclear
chain fibers are probably activated since both types of nerve endings innervate
them.
The dynamic response appears when the spindle is extended quickly. Only
the primary ending is activated and fires rapidly as long as the spindle is
expanding. If the expansion stops, the firing ceases. The nuclear bag fibers are
probably involved in this response.
The muscle spindles also contain a motor nerve terminal from the small
(gamma) motoneurons located in the spinal cord. These gamma fibers may be
divided into gamma-d (dynamic) and gamma-s (static) fibers. The gamma-d fibers
excite the nuclear bag fibers and the dynamic response of the fiber is enhanced.
The gamma-s fibers stimulate the nuclear chain fibers and enhance the static
response of the fiber.
The muscle spindle may be excited in two ways:
lengthening of the whole muscle which stretches the middle portions of the
intrafusal muscle fibers.
or, by the contraction of the end portions of the intrafusal muscle fiber
induced by gamma innervation.
The muscle spindles fire continuously at a certain rate. The stretching of the
spindle increases the rate of firing, its shortening decreases the rate.
GOLGI TENDON ORGANS
About 1 mm long, they are encapsulated sensors located in all tendons.
About 10 - 15 nerve fibers are associated with each Golgi tendon organ. They are
sensitive to the rate of tension and have a high threshold. The impulses from these
organs pass through the nerve fibers to the spinocerebellar tracts or locally to a
single inhibitory interneuron which inhibits the motoneurons. They respond to
increases of muscle tension and protect the tendon and muscle from excessive
stretching. They adapt very slowly and may even fire continuously. The Golgi
tendon organ has both dynamic and static responses.
PACINIAN CORPUSCLES
The pacinian corpuscles are in the muscle fascias; also around joints and
ligaments. They mediate sensitivity to joint movements and rotation. Their number
is small. They are stimulated mainly when the joint moves suddenly. They are
innervated by a large myelinated A fiber. They show a rapid adaptation.
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SOMESTHETIC INPUT THROUGH THE SPINAL CORD
All impulses from the receptors first enter the peripheral afferent nerve fiber,
then the spinal ganglion, then the dorsal roots and finally the spinal cord. The
surface of the body is divided into dermatomes, more-or-less parallel bands on the
skin that correspond to the embryonic distribution of nerve fibers. Each dermatome
is innervated by one segment of the spinal cord. In humans, they are irregular in
shape and they overlap. In the spinal cord, the afferent fibers branch and form two
systems:
1. DORSAL COLUMN-MEDIAL LEMNISCAL SYSTEM
Medial branches of the dorsal root fibers form the dorsal column-medial
lemniscal system. The nerve fibers of this system are large and myelinated. This
system is uncrossed at the spinal level. The first synapse is in the dorsal column
nuclei of the medulla (nucleus gracilis, nucleus cuneatus). The fibers of these
nuclei cross the midline in the medulla and form a bundle of nerve fibers called the
medial lemniscus going toward the ventrobasal complex of the thalamus. The
transmission of nerve impulses in this system is very rapid, precise and has a high
degree of spatial organization. Finally, it reaches a discrete area of the cortex. The
divergence of nerve impulses into various brain areas is small and depends on the
intensity of the stimulus.
The modalities transmitted by this system are:
- touch and pressure sensation requiring precise localization and gradation of the
stimulus;
- vibratory sensations and signals of movement against the skin;
- position of body parts as perceived by the kinesthetic receptors. The detection of
the position of body parts depends on the joint receptors. Their input is analyzed
and integrated in the thalamus by two types of neurons:
* those activated when the joint is at full rotation;
* those activated when the joint is at minimal rotation.
2. SPINOTHALAMIC SYSTEMS
The lateral branches of the afferent nerve fibers terminate in the dorsal horns
and form the anterior and lateral spinothalamic system. Neurons of the dorsal horns
send fibers across to the other side. These fibers then travel up the spinal cord
through the lateral and ventral columns. The fibers of this system are thin.
Some of them terminate at all levels of the brain stem, from the medulla to
the posterior thalamus. Others join the medial lemniscus and end in the ventrobasal
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complex of the thalamus. The degree of spatial organization is not very high but
this system can transmit a broad spectrum of sensory modalities. The velocity of
transmission is low, the gradation of intensities is not very accurate, and the ability
to transmit repetitive signals is poor.
These systems transmit:
pain;
thermal sensation;
crude touch and pressure;
tickle and itch;
sexual sensations.
All input from all pathways terminates in the ventrobasal complex of
thalamus. The thalamic neurons send fibers from the thalamus to the cerebral
cortex.
Sensory input from the lateral and ventral spinothalamic tracts also
terminates in the reticular formation, the central gray of the midbrain, the
cerebellum, the thalamus and the cerebral cortex. Each area serves a specific
function. Thus, the cerebellum uses sensory input for subconscious control of
motor functions; the thalamus performs crude analysis of sensory input; the
cerebral cortex provides detailed analysis of sensory input.
MOTOR FUNCTIONS OF THE SPINAL CORD
The descending pathways of the spinal cord (the corticospinal pathway in
particular) reach the alpha and gamma motoneurons of the ventral horns or
interneurons in the same area. The alpha motoneurons are more numerous than
gamma motoneurons. They are the final common path for motor movement.
However, interneurons are thirty times more numerous than all anterior
motoneurons. Alpha motor neurons may be divided into two classes:
 larger cells have fast conduction velocities, they innervate fast-twitch,
high-force but fatigable muscle fibers;
 smaller alpha motoneurons are slower, innervate slow-twitch, low-force,
fatigue-resistant muscle fibers,
During voluntary movement, mainly the lateral (crossed) corticospinal tract
from the motor area of the cerebral cortex (pyramidal pathway) excites the spinal
motoneurons. The corticospinal tract terminates mainly on the interneurons.
The motoneurons are also innervated, directly or indirectly, by:
 the reticulospinal pathway from the reticular formation of the brain stem;
 the rubrospinal pathway from the nucleus ruber;
 the (lateral) vestibulospinal tract;
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 monosynaptic and polysynaptic segmental inputs.
Motor neurons represent the final common path for all these pathways. In
the ventral horns, there are small motoneurons forming the Renshaw cell inhibitory
system. They are innervated by the collateral branches of the motoneuron axons
and are responsible for the feedback innervation of the neighboring motoneurons.
SPINAL REFLEXES
Beside the mediation of voluntary movements, the spinal cord can perform
several reflex functions. Spinal reflexes are involuntary motor responses to certain
sensory stimuli. They may be classified into:
muscle - stretch reflexes
superficial reflexes
The Muscle Stretch Reflex
The muscle stretch reflex is also called the tendon reflex, or myotatic reflex.
An adequate stimulus to elicit this reflex is the passive stretching of the muscle.
During neurological examination, this is achieved by the tapping of the tendon,
directly or through the examiner's finger. The receptor is the muscle spindle. The
reflex is initiated by the primary nerve endings in the spindle and terminated by the
secondary fibers and by the Golgi tendon reflex. It is a monosynaptic reflex
mediated by the motoneurons controlling the same muscle that has been stretched.
The dynamic stretch reflex is elicited by a strong signal from the primary
endings of the muscle spindles when the muscle is suddenly stretched. An
instantaneous muscle contraction follows. Then, a static stretch reflex follows
which is weaker but maintains the muscle contraction as long as the muscle is kept
in the stretched state.
The muscle spindles are also involved in the inhibitory stretch reflex. When
the muscle is passively shortened, muscle inhibition follows. The stimulation of the
muscle spindles also prevents some types of oscillation and jerkiness of body
movements. The irregular signal from the motoneurons causes a relatively smooth
contraction due to the response of the muscle spindles.
Examples of the stretch reflexes:
Biceps reflex localized in the spinal cord segments C5-6;
Triceps reflex localized in C6-8;
Quadriceps reflex(knee jerk) localized in L2-4;
Gastrocnemius (ankle jerk, Achilles' tendon reflex) localized in L5-S2.
These reflexes normally participate in the regulation of muscular contraction
and posture. The contraction of the muscle is terminated by a local feedback
mediated by inhibitory interneurons (recurrent collateral inhibition). The
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interneurons performing this function are called the Renshaw cells. The Golgi
tendon reflex is also involved in the termination of the stretch reflex.
Superficial Reflexes
The superficial reflexes are also called polysynaptic spinal reflexes,
withdrawal reflexes or flexor reflexes. They are elicited by a painful or tactile
stimulation of the skin and involve groups of muscles. During neurological
examination, these reflexes are elicited by a light touch with a cotton swab or a
blunt stick.
Examples of superficial reflexes are:
Epigastric from the spinal segments Th6-9
Mediogastric from Th9-11
Hypogastric from Th11 - L1
Plantar from L5-S2, normal response is flexion of toes
Some superficial reflexes only appear in pathological situations. One of them is the
Babinski reflex that is elicited like plantar reflex, but the toes are extended, or
fanned.
The interneuron pool in the spinal cord mediates the flexor reflexes and the
shortest circuit has at least three to four interneurons. The diverging circuits spread
the excitation to the necessary muscles and inhibit the antagonist muscles, too.
After the stimulus is over, a repetitive afterdischarge may be produced. The pattern
of the response depends on the sensory nerve that is being stimulated ("local
sign").
Reciprocal Inhibition
The agonistic muscles are those acting together in one direction. On the
other hand, the antagonistic muscles act one against another. The antagonistic
muscles are innervated in a reciprocal way. When one muscle is contracted, the
antagonistic muscle must relax. In reality, both agonist and antagonist muscles are
always contracted to some degree. This is called cocontraction. Reciprocal
innervation also depends upon the inhibitory interneurons. Simultaneous
contraction of both extensors and flexors results in a rigid limb, a condition termed
spasticity. The crossed extensor reflex appears about 0.2 - 0.5 sec after the flexor
reflex. The opposite limb is extended. The afterdischarge of the motoneurons is
prolonged due to the activity of the reverberatory circuits among the interneurons.
Basic locomotor actions
Rhythmic stepping movements of a single limb are often observed in the
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limbs of animals with a transacted spinal cord, even when the lumbar spinal cord
has been severed. There is a central pattern generator controlling alternating
movements such as walking. Each limb has its own pattern generator. There is
oscillation between the flexor and extensor muscles with a rebound excitation of
the antagonists. Reciprocal stepping of opposite limbs that also appears is
accomplished by reciprocal innervation.
Diagonal stepping of all four limbs, the "Mark Time Reflex" may also be
observed in an animal with a sectioned spinal cord. This reflex is a manifestation
of reciprocal innervation along the spinal cord.
The scratch reflex is elicited by a tickle sensation or itching. It involves a
position detection and a to-and-fro scratching movement. An oscillation circuit is
necessary to accomplish this reflex. A large number of muscles (up to 19) are
involved in this reflex.
The Postural Reflexes
In a decerebrated animal, the positive supportive reaction holding the body
in an upright position is elicited by pressure on the food pad; It causes the limb to
extend (positive supportive reaction). It involves a system of interneurons similar
to the flexor and cross-extensor reflexes. Pressure on one side of the foot causes
extension in the same direction (magnetic reaction). The righting reflexes may also
be demonstrated in cats and dogs with a severed spinal cord. When placed on their
side, they make incoordinated movements in an attempt to stand up (cord righting
reflex). These righting reflexes are therefore mediated and integrated by the spinal
cord.
SPINAL SHOCK
Transection of the spinal cord is followed by a period of spinal shock during
which all spinal reflexes are depressed. The spinal motoneurons are
hyperpolarized. In humans, this period may last several weeks. Its cause is
uncertain.
FUNCTIONS OF THE BRAIN STEM
THE RETICULAR FORMATION
The medulla is a source of three descending pathways:
 from red nucleus
 from vestibular nuclear complex
 from reticular formation
The reticular formation constitutes anatomically the core of the brain stem. It
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is composed of interneurons in a meshwork of axons. Its role is the integration of
data from widespread sources, from sense organs, the cerebral hemispheres and the
cerebellum. Inputs into this system are heterogeneous, output fibers are
widespread.
The functions of the reticular formation may be divided into the descendent
and ascendent part.
The descendent part involves:
* the motor function and stabilization of posture.
* maintenance of homeostasis (respiratory center)
* pneumotaxic center executing the turnover of inspiration into expiration.
The ascendent part is very complex and involves:
*
alert wakefulness
*
rousing from sleep
*
alerting and focusing of attention
*
perceptual association
The ascendent function of RF shall be discussed together with the
mechanisms of sleep.
THE DESCENDENT FUNCTION OF THE RETICULAR
FORMATION
The control of motility, the descendent function, is just one of many
functions of the reticular formation. Descendent fibers from the reticular formation
go into the spinal cord and modify motor activity and sensory input. The
reticulospinal pathway controls the gamma motor system, and, therefore, muscle
tonus. Ascendent fibers go into the diencephalon and then into the cerebral cortex
where they influence the level of consciousness.
The reticular nuclei controlling antigravity muscles are divided into two
portions:
 the pontine reticular nuclei that send the pontine (medial) reticulospinal
tract to the spinal cord; these fibers excite the antigravity muscles;
 the medullary reticular nuclei that project via the medullary (lateral)
reticulospinal tract sending inhibitory signals to the antigravity muscles.
The medullary nuclei get input from the collaterals of the corticospinal
tract, the rubrospinal tract and other pathways. The damage to the
medullary reticular nuclei is followed by an excessive effect of the
pontine nuclei followed by an increased tension of antigravity muscles
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(decerebrate rigidity, spastic rigidity).
The gamma motoneurons, called also fusiform neurons, innervate the muscle
spindles. Their axon diameter is small. They participate in two motor functions:
1. First, they control of the body posture also called body hold system. These
neurons obtain their direct input from the facilitatory and inhibitory regions of the
reticular formation, from the vestibular nuclei and from the nucleus ruber, and
indirectly from the cerebellum, basal ganglia and from the cerebral cortex. The
nucleus ruber has a predominantly inhibitory influence on the α- and γmotoneurons of the extensor muscles and excitatory influence on the flexor
muscles. The fibers originating in the Deiters' nucleus and lateral RF activate the
extensor muscles and inhibit the flexor muscles. The muscle tone is maintained, in
particular the muscle tone of the antigravity muscles.
2. The second function of the gamma motoneurons involves the control of the
movement. About 31% of all motor fibers innervating the muscle belong to the
gamma system. Whenever the alpha motoneurons are activated to elicit movement,
the gamma motoneurons are activated simultaneously. This is called coactivation
of both systems. The purpose of coactivation is to hold the muscle spindle
contracted. This contraction prevents the muscle spindle from opposing the muscle
contraction. Second, it maintains the damping function of the muscle spindle
regardless of change in the muscle length.
FUNCTIONS OF THE CEREBELLUM
The cerebellum is a dumbbell shaped mass set across the back of the pons. It
contains a medial constricted part (vermis) and the right and left cerebellar
hemisphere. It forms the roof of the 4th ventricle. The main anatomical divisions of
the cerebellum are:
the anterior lobe;
the posterior lobe;
the flocculonodular lobe (the oldest, vestibular cerebellum).
Gray matter of the cerebellum
The cerebellar cortex is formed by gray matter. The vermis located between
the two hemispheres controls the muscle movements of the axial body, the neck,
the shoulders and the hips. There is a topographical representation of the body in
the vermis.
The cortex of the hemisphere is divided into:
the intermediate zone concerned with the control of muscular contractions in
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the hands, fingers, feet and toes;
the lateral zone that controls the overall planning of sequential motor
movements. This zone is associated with the association areas of the cerebral
cortex, such as the premotor cortex and the somatic and sensory association areas.
Within the white matter of the cerebellum are found the deep cerebellar nuclei: the
fastigial nucleus (in the vermis), the globose n., the emboliform n., the dentate n.
(in the hemisphere). The dentate is the most lateral nucleus.
The Cerebellar Cortex
The cerebellar cortex has numerous transverse sulci parallel to each other.
The cells of the cerebellar cortex are arranged in layers. The structure of these
layers is the same in all parts of the cortex and in all vertebrates. The size of the
cerebellum depends on the complexity of movements.
The three layers of the cortex are:
- the molecular layer on the surface that is formed by dendrites and fibers coming
from deeper cortical layers. It contains a small number of cells, basket cells,
stellate cells and Golgi cells.
- the middle layer that is one cell thick and contains Purkinje cells. These cells
have large flask-like bodies. Their dendrites extend in a sagittal plane to the outer
surface of the cortex. They are innervated with climbing fibers that come from the
inferior olivary nucleus of the medulla and other brain areas. There is
approximately one climbing fiber for every 10 Purkinje cells, and about 300
synapses are on each Purkinje cell.
- the granule cell layer that is formed by closely packed cells. It is innervated with
mossy fibers coming from all parts of the brain, mainly from the brain stem and the
spinal cord. Each granule cell has 3-5 short dendrites with mossy fibers attached
and sends an axon to the molecular layer where it is divided into two branches,
parallel fibers, which then pass through the dendrites of the Purkinje cells and
synapse with each cell on the way. There is one Purkinje cell for every 500 - 1000
granule cells; 80,000 to 200,000 parallel fibers synapse with each Purkinje cell.
The cerebellar cortex receives input from the spinal cord, the inferior olive,
the vestibulum, and the sense organs of the head, the retina and the cochlea, and
several areas of the cerebral cortex including the motor area. The relay nuclei of
the cortico-pontine fibers are in the pontine nuclei. The names of the main
pathways to the cerebellum are:
- the corticopontocerebellar tract
- the olivocerebellar tract
- the vestibulocerebellar tract
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- the reticulocerebellar fibers
- the dorsal and ventral spinocerebellar tracts that transmit information mainly
from muscle spindles, less from other receptors.
The efferent pathways from the cerebellum connect the cerebellum to the
brain stem, to several midline structures of the thalamus, to the ventrolateral and
ventromedial nucleus of the thalamus and further to the cerebral cortex, the red
nucleus and reticular formation.
Primary Intrinsic Circuits
The signals that come from the cerebral cortex go to the agonist muscles and
their collaterals go to the cerebellum. The fibers afferent to the cerebellar cortex
synapse with the granule cells. From the granule cells, the impulses go to the
dendrites of the Purkinje cells and to the dendrites of the basket and stellate cells.
The Golgi cells, basket cells and stellate cells located in the cerebellar cortex are
all inhibitory cells with short axons. They are responsible for lateral inhibition of
the Purkinje cells. The Purkinje cells innervate the deep cerebellar nuclei and have
an inhibitory effect on them. The Purkinje cells fire continuously, as do the deep
nuclear cells. The collaterals of the mossy and climbing fibers excite the deep
nuclei. The activity of the neurons in the deep nuclei predicts the next movement.
Their spikes are complex, oscillatory, lasting up to one second.
Functions of the Cerebellum
The cerebellum is involved in a number of functions:
 non-motor functions involve the control of emotions, integration of the
sensory input, learning and memory, attention, higher cognition, ability
to plan and schedule activities, time estimation, and possibly even in
states such as schizophrenia and autism. Adult and child cerebellar
patients have difficulty modulating their emotions, they react either too
much or not at all.
 motor control, functioning in cooperation with other centers such as the
spinal cord, the reticular nuclei and the cerebral cortex. The cerebellar
cortex is divided into narrow sagittal zones with different functions,
about 1 mm wide and more than 100 mm long. Each zone controls a
particular motor mechanism. The signals from the periphery indicate the
actual position of the body part, how rapid the movement is and in what
direction. The cerebellum then calculates where the limb will be in the
coming fraction of a second.
The Control of Postural and Equilibrium Movements
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The flocculonodular lobe and portions of the vermis are involved in the
control of body equilibrium, in particular during rapid changes in the head and
body positioning. Selective ablation of the flocculonodular lobe in dogs abolishes
the syndrome of motion sickness that is a consequence of excessive and repetitive
labyrinthine stimulation.
The vermis is also involved with the orientation in space. It controls the
axial and girdle muscles. Its lesions induce trunk ataxia, staggering and diminished
or absent response to rotational stimulation of the labyrinths.
Control of Voluntary Muscle Movements
The circuitry for voluntary muscle movement is independent of the circuitry
controlling body balance. The intermediate zone of the cerebellar cortex and the
globose and emboliform nuclei controls the planning and coordination of
movements of the distal parts of the limbs. Their lesions induce dysmetria,
intention tremors and the inability to perform rapidly changing movements. The
main function of the cerebellum is to turn on the agonist muscles and
simultaneously turn off the antagonist muscles at the onset of movement. At the
end of the movement, the turn off signal is sent to agonists and the turn on signals
to the antagonists.
The distal limb movements are controlled by an input of direct information
from the motor cortex conveying the intended plan of movement. The feedback
information from the peripheral parts of the body then reports the actual movement
result. Following this, the nucleus interpositus sends corrective output signals back
to the motor cortex through the relay nuclei of the thalamus; also, through the
magnocellular portion of the red nucleus to the rubrospinal tract, and further to the
most lateral motor neurons in the anterior horns which control the distal parts of
the limb. Smooth, coordinated movements of the fingers and hands are assured by
this mechanism.
The ventral spinocerebellar tract transmits the signals intended for muscles
back to the cerebellum ("efference" copy). This signal is then integrated with the
signals arriving from muscle spindles. Similar signals also go to the inferior olivary
nucleus.
The Timing and Sequencing of Movements
The lateral zone of the cerebellar hemisphere and the associated dentate
nucleus controls the timing and sequencing of movements. The cerebellum plans
the next sequential movement. In humans, this area is highly developed. The
response chunking hypothesis postulates that individual programs controlling
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behavior are joined into higher order "chunks" of behavior.
The lateral cerebellar hemispheres communicate with the premotor and
sensory portions of the cerebral cortex. There is a two-way communication
between these two areas. The plan of the sequential movements is transmitted from
the cerebral cortex to the lateral zones of the cerebellar hemispheres. Also, the
lateral hemispheres predict ahead of time how far different parts of the body will
move. Thus, the successive movements may begin in time and the progression of
the movements is smooth.
The Damping of the Movements
Almost all movements of the body are pendular and have a tendency to
overshoot. The cerebellum may stop the pendular movements and the "intention
tremor".
Control of Ballistic Movements
Many rapid movements of the body occur so rapidly that feedback control is
impossible. These are called ballistic movements. Their entire movement is
preplanned and has to run a certain distance and stop. The cerebellum controls the
execution of these movements.
Learning in the Cerebellum
Most motor acts have to be learned. This is accomplished by repetition.
Then, the movements become more precise and automatic. During the process of
motor learning, the sensitivity levels of the cerebellar circuits and the Purkinje cells
in particular are adjusted.
If a new movement is learned, the intended movement usually does not
match the achieved movement. This difference is detected by the inferior olivary
complex activated both from the corticospinal tract and from the sensory endings
in the muscles. The inferior olivary nucleus is the comparator testing how the
actual performance matches the intended performance. Under resting conditions
the climbing fibers fire once a second. If there is a mismatch, the climbing fibers
originating in the inferior olive are either activated to a maximum of 4 discharges
per second, or inhibited down to zero discharges per second.
This "error signal" is transmitted to the Purkinje cells and alters the longterm sensitivity of the Purkinje cells. Each discharge of the climbing fibers causes
long-lasting depolarization of the dendritic tree of the Purkinje cells followed by a
strong output spike from the Purkinje cells. The error signal changes the sensitivity
of the Purkinje cells to the discharges from the parallel fibers coming from the
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granule cells. When the movement is learned as desired, the error signal is not sent.
Cerebellar Damage
The destruction of small portions of the cerebellum rarely leads to
abnormalities in motor function. A lesion must involve the deep nuclei to cause
serious dysfunction. The consequences of the lesions may be classified into the
following groups:
 Hypotonia, disappearance of the muscle tonus.
 Dysmetria and ataxia are the most important signs. Dysmetria is the term
describing the overshooting of movements with a subsequent
overcompensation. Incoordinate movements resulting from dysmetria are
called ataxia.
 The intention (action) tremor is a consequence of dysmetria. It is
characterized by overshooting causing a tremor of the fingers toward the
end of the movement. Cerebellar nystagmus is a tremor of the eyeballs
during their fixation on a certain feature in the visual field. Dysarthria is
the lack of coordination of the muscles involved in the speech, located in
the larynx, the mouth and the respiratory system. Vocalization is
jumbled.
 Adiadochokinesia or dysdiadochokinesia is a defect of the sequencing of
movements. It appears when the damaged cerebellum cannot predict
movement. The progression of movements is lost. The hypotonia of
muscles is caused by a loss of the deep cerebellar nuclei.
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