Motor systems: Reflexes, pyramidal and extrapyramidal ... · 1. Muscle, muscle spindle, motor unit and reflexes 2. Voluntary movement: Organization and major pathways 3. Extrapyramidal

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)


(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


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


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


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


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


(Kandel, Principles of Neural Science)

Flexion-withdrawal reflex

Cutaneous receptors

Polysynaptic pathway

Reciprocal innervation

Crossed-extension reflex


Modification of reflexes

Supraspinal contacts to alpha or

gamma motoneurons

„Spinal shock“


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


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


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


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)


Connections of the basal ganglia (schematic representation)

Reciprocal coupling between - Striatum and S. Nigra

- Pallidum and N. subthalamicus

Nigrostriatal pathway


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“)


Functional structure:


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


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


Input connections of the cerebellar cortex

Two main types of inputs:

- input from mossy fibers - input from climbing fibers


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


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)


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


Inhibitory interneurons

Climbing fiber

Mossy fiber

Neurons of

subcortical nuclei

Principles of function of the cerebellar cortex II


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


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


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



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):



Simple model system: Rabbit eyelid conditioning

tonePuff (CS)(US)

Mossy

fiberClimbing

fiber

Cerebellar cortex

Deep nuclei of the

cerebellum

(Interpositus nuclei)

-

++

++


Documents

Motor systems: Reflexes, pyramidal and extrapyramidal ... · 1. Muscle, muscle spindle, motor unit and reflexes 2. Voluntary movement: Organization and major pathways 3. Extrapyramidal