Contents:
1. Muscle, muscle spindle, motor unit and
reflexes
2. Voluntary movement: Organization
and major pathways
3. Extrapyramidal system of movement
- Basal ganglia
- Cerebellum
Motor systems: Reflexes, pyramidal and extrapyramidal
system, cerebellum
Literatur:
Dudel et al., Neurowissenschaft (Springer)
Reichert, Neurobiologie (Thieme)
Kandel et al., Principles of Neural Science (McGraw Hill)
Kahle, Taschenatlas der Anatomie, Band 3: Nervensystem und
Sinnesorgane (Thieme)
Greenstein and Greenstein, Color Atlas of Neuroscience (Thieme)
Muscle, muscle spindle, motor unit and reflexes
Organization of muscles
Skeletal muscle
→ Muscle fiber (fused cells, surrounded
by a plasma-membrane (sarcolemma))
→ myofibril (surrounded by
sarcoplasmic reticulum)
→ sarcomere
(functional motor unit)
1.5 – 3.5 µm
Sliding filament
hypothesis
(A.F. Huxley et al., 1950s)
Contains thick and thin
filaments (myosin,
F-actin)
Z-disk
Sensing muscle tension
Muscle spindles: encapsulated sensory receptors
located within muscles
signal changes in the length of the muscle (stretch sensor)
Anatomy: encapsulated muscle fibers located parallel to the muscle fibers
large-diameter sensory endings in the middle noncontractile portion
and small sensory endings at contractile portions (blue),
small-diameter motor endings at the polar portions (gamma motor
neurons; red)
Muscle, muscle spindle, motor unit and reflexes
(Kahle, Taschenatlas der Anatomie Bd. III)
Basic motor unit: a-motoneuron in the
ventral horn + innervated muscle fibers
Most of efferent tracts (orange) do not directly end at
a-motoneurons but at interneurons (black) →
complex integration
Muscle, muscle spindle, motor unit and reflexes
Peripheral tracts (blue) may directly end at a-
motoneurons
(Kahle, Taschenatlas der Anatomie Bd. III)
Recruitment of motoneurons
(Campbell, Biologie)
Basic motor unit: a-motoneuron in the ventral horn + innervated muscle
fibers
Muscle, muscle spindle, motor unit and reflexes
Tetanic contraction (physiologic tetanus):
Sustained muscle contraction evoked
when the motor nerve that innervates a
skeletal muscle emits action potentials at a
very high rate (interval < 75 ms)
SpeedStrength
Coordinated work of muscles
Muscles pull but cannot push → hinge requires at least two antagonist muscles
Muscle, muscle spindle, motor unit and reflexes
Reflex pathways
What are reflexes?
Involuntary coordinated patterns of muscle contraction and relaxation
elicited by peripheral stimuli
traditionally seen as automatic, stereotyped movement
modern view: can be modified via supraspinal signals
Test: complete transection of the spinal cord from the brain
Receptors in muscles: stretch reflexes
Cutaneous receptors: withdrawal reflexes
Muscle, muscle spindle, motor unit and reflexes
Stretch reflex
Receptors in muscles (spindle)
Most studied and most simple
reflex: Contraction of a
muscle when the
muscle is lengthened
Experiment by Sherrington
(beginning of 20th century):
reflex was abolished by cutting
either dorsal or ventral root
Involves monosynaptic pathway
Heteronymous innervation
Reciprocal innervation
Muscle, muscle spindle, motor unit and reflexes
(Kandel, Principles of Neural Science)
Flexion-withdrawal reflex
Cutaneous receptors
Polysynaptic pathway
Reciprocal innervation
Crossed-extension reflex
Muscle, muscle spindle, motor unit and reflexes
Modification of reflexes
Supraspinal contacts to alpha or
gamma motoneurons
„Spinal shock“
Muscle, muscle spindle, motor unit and reflexes
Voluntary movement: Organization and major pathways
Control of subcortical motoric centers by the cortex
primary motor cortex (area 4) →
lowest intensity of stimulation
elicits movement
premotor cortex (area 6)
46
Motor homunculus
Major pathway of voluntary motor information: Corticospinal tract
(“Pyramidenbahn”)
Massive bundles of fibers (approx. 1 million of
axons)
Originates from primary motor cortex, premotor
cortex and somatosensory cortex
Somatotopic organization
Main tract crosses at the
medulla (pyramidal
decussation) and
descends in the lateral
column (Tractus
corticospinalis lateralis)
Most end at interneurons
between dorsal and
ventral horn
Pyramidal decussation
Voluntary movement: Organization and major pathways
Majority (70-90%) of the axonal fibres cross at the
pyramidal decussation
Remaining uncrossed fibers descend in the ventral
column (Tractus corticospinalis anterior) and cross
at the position of their endings
Tractus corticospinalis
anterior
Tractus corticospinalis
lateralis
Control more
distal
limb muscles
→important for
goal-directed
movement
Control of posture by integrating
visual, vestibular and
somatosensory information
Voluntary movement: Organization and major pathways
Corticospinal tract usually ends at Zona intermedia between ventral and
dorsal horn at interneurons
Minority directly contact motoneurons at the ventral horn (in most cases
motor neurons that innervate flexors of the distal limb) → direct control
by the corticospinal tract
Voluntary movement: Organization and major pathways
Phylogenetically old extrapyramidal
motor system
multisynaptic chain of neurons
In the narrower sense: group of nuclei
with a high iron content (basal
ganglia)
In a broader sense: together with the
cerebellum as an integration center
Function: regulation of involuntary
movements for keeping posture and
trained movements
Acts as a „servo-mechanism“ for
voluntary movement and provide feed-
back loops
→ required for „smooth“ movement
Extrapyramidal system of movement
Basal ganglia
Striatum (putamen and caudatum)
Pallidum
Nucleus subthalamicus
Nucleus ruber
Substantia nigra
Belong to different parts of the
brain:
telencephalon (Striatum),
diencephalon (Pallidum, N.
subthalamicus)
brain stem (N. ruber, S. nigra)
Extrapyramidal system of movement
Connections of the basal ganglia (schematic representation)
Reciprocal coupling between - Striatum and S. Nigra
- Pallidum and N. subthalamicus
Nigrostriatal pathway
Extrapyramidal system of movement
Lesions in the basal ganglia lead to characteristic disturbances of movement:
e.g., Lesion of the N. subthalamicus:
Hemiballism – involuntary (often violent) movement of the limbs
e.g., Degeneration of the Striatum:
Chorea Huntington – involuntary movement, cognitive impairment (dementia)
genetic disease
Extrapyramidal syndromes
Parkinson‘s diseaseMost frequent disease of the motoric system
(incidence: 1-5% of 70-90 year old
people)
paucity of spontaneous movement
(„Maskengesicht“), increased muscle
tone (rigidity), characteristic tremor at rest
(„Schüttellähmung“)
Extrapyramidal system of movement
Functional structure:
Extrapyramidal system of movement
Cerebellum: Integration unit of the extrapyrimadal system
Latin: „little brain“, constitutes 10% of the brain volume but 50% of its neurons
Neurons are arranged in highly regular manner as repeating units → basic
circuit modules
Major output: premotor and motor cortex, basal ganglia of the brain stem
Many parallel convolutions called folia („leaves“)
Connected to the brain stem via pedunculi cerebellaris („Kleinhirnstiele“)
(Fischer et al. (1998) Neuron 20: 847-854)
Cells of the cerebellar cortex
Neurons are highly ordered and organized in repeat units → „modules“
Only five types of neurons:
- Stellate neurons (“Sternzellen”)
- Basket neurons (“Korbzellen”)
- Purkinje neurons
- Golgi neurons
- Granule cells (“Körnerzellen”)
Inhibitory
neurons
Excitatory
neurons
Extrapyramidal system of movement
Molecular layer: cell bodies of stellate and basket cells, axons of granule cells
(oriented as parallel fibers along the “folia”), dendrites of Purkinje cells
(oriented perpendicular to the parallel fibers)
Purkinje cell layer: single layer of Purkinje cell bodies, axons project to white
matter
Granule layer: many granule cells and some Golgi neurons
Organized in
3 layers:
Organization of the cerebellar cortex
Extrapyramidal system of movement
Input connections of the cerebellar cortex
Two main types of inputs:
- input from mossy fibers - input from climbing fibers
Extrapyramidal system of movement
1. Axons of granule cells travel to
molecular layer and excite as
“parallel fibers” many Purkinje
neurons in the same transverse
plane
2. Basket cells and stellate cells
make inhibitory contacts to
Purkinje neurons, thus
producing an inhibitory side-
loop
Connections within the cerebellar cortex
Extrapyramidal system of movement
Output connections of the cerebellar cortex
Axons of the Purkinje neurons project
into the white matter (deep nuclei of the
cerebellum) and provide the (entirely
inhibitory) output of the cerebellar
cortex (mediated by GABA)
Extrapyramidal system of movement
Incoming signals from mossy fibers
(from spinal cord and brain stem)
synapse at dendrites of granule and
golgi cells
Parallel fibers of granule cells excite
only one row of Purkinje cells
→ Enhancement of contrast
Golgi cells are much larger than granule cells and
have dendrites filling a large volume in all directions →
are excited in a larger volume and locally inhibit (via
short axons) neighboring Purkinje cells
Principles of function of the cerebellar cortex I
Golgi cells
Extrapyramidal system of movement
Inhibitory interneurons
Climbing fiber
Mossy fiber
Neurons of
subcortical nuclei
Principles of function of the cerebellar cortex II
Extrapyramidal system of movement
Neurons of the subcortical nuclei are
excited by axon collaterals of
climbing and mossy fibers but
inhibited by Purkinje neurons.
Thus, the output to the descending
motor systems is modulated by the
cortical side loop.
Principles of function of the cerebellar cortex II
Extrapyramidal system of movement
Neurons of the subcortical nuclei are
excited by axon collaterals of
climbing and mossy fibers but
inhibited by Purkinje neurons.
Thus, the output to the descending
motor systems is modulated by the
cortical side loop.
Neighboring purkinje cells
receive signal of the
same granule cell via the
parallel fiber with
increasing delay
(speed of conductance of a
parallel fiber: 0.2 m/s;
corresponds to about 0.1
ms for every neighboring
Purkinje cell)
→ Temporal correlation of signals can be determined and movement
(activation of muscle fibers) can be segmented (there are often difficulties
with cerebellar lesions)
Principles of function of the cerebellar cortex III
Extrapyramidal system of movement
e.g. vestibulo-ocular reflex: ensures that the eyes can fix a target when turning the
head.
When wearing prismatic glasses, the reflex turns after a learning phase.
Destruction of the vestibulocerebellum prevents this adaptation
The cerebellum in motor learning
Extrapyramidal system of movement
The cerebellum in motor learning
Simple model system: Rabbit eyelid conditioning
tone
puff
Conditioned
Stimulus (CS)
Unconditioned
Stimulus (US)
Mossy fiber
Climbing fiber
Cerebellar
cortex
Deep nuclei of the
cerebellum
(Interpositus nuclei)
(from: Ohyama et al. (2003) What
the cerebellum computes. Trends
Neurosci. 26: 222-227)
Learning depends on the interstimulus interval (ISI):
Extrapyramidal system of movement
The cerebellum in motor learning
Simple model system: Rabbit eyelid conditioning
tonePuff (CS)(US)
Mossy
fiberClimbing
fiber
Cerebellar cortex
Deep nuclei of the
cerebellum
(Interpositus nuclei)
-
++
++
Extrapyramidal system of movement