muscle action and reflexes: motor unit and control, reflex
TRANSCRIPT
1. Motor unit and muscle action
- motor unit
- organization of muscles
- regulation of muscle contraction
- motor end plate
- muscle spindle
2. Basic motoric pathways
- stretch reflex
- flexion-withdrawal reflex
- modification of reflexes
Muscle action and reflexes: Motor unit and
control, reflex pathways
Motor unit: single motoneuron (a-motoneuron) + all muscle fibers that are
innervated by this neuron
Number of muscle fibers innervated by a single motor
neuron differ: 2-3 (finger), 50-60 (larger muscles)
Motor unit and muscle action
Somatotopic organization of the motor nuclei
Motor neurons for one muscle are clustered into an
elongated motor nucleus in the ventral spinal
cord (extends 1-4 segments)
Distal limbs:
more lateral
Proximal limbs:
more medial
Motor unit and muscle action
One example for one of the largest nerve tract: Nervus ischiadicus
(sciatic nerve)
Controls many of the
muscles of the leg and foot
Practical relevance:
Sciatic pain syndrome
(sciatica, sciatic neuritis)
(“Ischiassyndrom”)
Motor unit and muscle action
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
Contains thick and thin filaments (myosin,
F-actin)
Z-disk
Motor unit and muscle action
(Huxley and Niedergerke (1954)
Structural changes in muscle during
contraction. Nature)
Interference microscopy
Organization of the sarcomere as the functional unit of a striated muscle
„sliding filament hypothesis“
(A.F. Huxley et al., 1950s)
Motor unit and muscle action
2 types of muscles:
- Striated muscle
(voluntary except cardiac muscle)
- Smooth (non-striated) muscle
(involuntary)
„sliding filament hypothesis“
(A.F. Huxley et al., 1950s)
Organization of the sarcomere as the functional unit of a striated muscle
Motor unit and muscle action
Actin_Myosin Movie.avi
F-actin
Troponin-
Tropomyosin
Ca2+ATP/
ADP
F-actin
Troponin-
Tropomyosin
Ca2+
ATP/
ADP
Myosin
Myosin
Required: Ca2+-Ions, ATP
Troponin-tropomyosin complex
Regulation of muscle contraction: Ca2+
Rigor mortis
(„Leichenstarre“)
Motor unit and muscle action
striated but mononuclear
functional syncytium: coupled by gap junctions („Glanzstreifen“)
pacemaker potential: sinus node
Cardiac muscle (striated muscle)
Smooth muscle (nonstriated muscle)
Not striated, mononuclear
dense bodies; Ca2+-
dependent phosphorylation
of myosin head
Motor unit and muscle action
Development of the motor end plate
3 interacting cell types: motor
neuron, skeletal muscle,
Schwann cell
Structures:
- presynaptic specialization
- postsynaptic specialization
- basal lamina
„protoplasmic kiss“ (Ramon y Cajal)
Motor unit and muscle action
romantic
phasemarriage marital phase
(adjustment and adaptation)(initial recognition)
divorce?
• Fusion of myoblasts
• Motor neurons secrete acetylcholine
• Patches of basal lamina form
• Clustering of acetylcholine receptor
• Basal lamina becomes continuous
• Accumulation of acetylcholine esterase in cleft
• Extrasynaptic acetylcholine receptor decreases
• Fold forms
• Subunit structure of the acetylcholine receptor changes
Motor unit and muscle action
Function of the end plate:
Release of acetylcholin → depolarization of the postsynaptic
membrane (via nicotinic cholinergic receptors)
Motor unit and muscle action
Triggering of an action potential (skeletal muscle) that propagates through
T-tubules
Voltage-gated Ca2+ channels
SERCA
(Sarcoplasmic/endoplasmic
reticulum Ca2+ ATPase)
Motor unit and muscle action
Action potential along an axon and along a T-tubule are in principle the same
Resting potential in each case about –80 mV, threshold about –45 mV
Length of the action potential: about 1ms
Speed: about 6 m/s (Skeletal muscle), C-fiber: 0.5-2 m/s, A-fiber: 12-120 m/s
Different shape: Axon: hyperpolarization
Myofibril: depolarization
Action potentials in myofibrils
Motor unit and muscle action
Sensing muscle tension
Muscle spindles: encapsulated sensory receptors
located within muscles
signal changes in the length of the muscle (stretch sensors)
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)
Motor unit and muscle action
Coordinated work of muscles
Muscles pull but cannot push →
hinge requires at least two antagonist muscles
Basic motoric pathways
Basic motor unit: a-motoneuron + 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 alpha-motoneurons
Basic motoric pathways
Recruitment of motoneurons
(Campbell, Biologie)
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)
Basic motor unit: a-motoneuron + innervated muscle fibers
Basic motoric pathways
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
Basic motoric pathways
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
Basic motoric pathways
Flexion-withdrawal reflex
Cutaneous receptors
Polysynaptic pathway
Reciprocal innervation
Crossed-extension reflex
Basic motoric pathways
Modification of reflexes
Supraspinal contacts to alpha or
gamma motoneurons
„Spinal shock“
Basic motoric pathways