Muscle action and reflexes: Motor unit and control, reflex ...

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


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”)


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


(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)


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


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“)


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


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)


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


Function of the end plate:

Release of acetylcholin → depolarization of the postsynaptic

membrane (via nicotinic cholinergic receptors)


Triggering of an action potential (skeletal muscle) that propagates through

T-tubules

Voltage-gated Ca2+ channels

SERCA

(Sarcoplasmic/endoplasmic

reticulum Ca2+ ATPase)


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


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)


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


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


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


Flexion-withdrawal reflex

Cutaneous receptors

Polysynaptic pathway

Reciprocal innervation

Crossed-extension reflex


Modification of reflexes

Supraspinal contacts to alpha or

gamma motoneurons

„Spinal shock“


Muscle action and reflexes: Motor unit and control, reflex ...

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