Motor System Spinal Cord2

Motor system Spinal cord

Components of spinal motor control system • • • •

Spinal neurons Motor unit Muscle spindles Golgi tendon organs

Dorsal root ganglion cell

Upper motor neuron of extrapyramidal tract

Upper motor neuron of corticospinal tract α-motor neuron in the spinal cord Neuro muscular junction

Muscle spindle Golgi Tendon organ

α-mn is directly responsible for generation of force by muscle

muscle

50 muscles of the arm innervated from spinal segments C3-T1

Figure 5.28 Page 173

Cervical cord

Thoracic cord

Lumbar cord Sacral cord

Cervical nerves

Vertebrae

Muscles of the leg innervated from spinal segments L1-S3

Thoracic nerves

Lumbar nerves

Sacral nerves Coccygeal nerve

Cauda equina

Cell body of efferent neuron

White matter

Gray matter Interneuron

Cell body of afferent neuron

Dorsal root Dorsal root ganglion

Efferent fiber From receptors

To effectors

Ventral root

Spinal nerve

Figure 5.29 Page 174

Figure 5.31 Page 176

Dorsal horn (cell bodies of interneurons on which afferent neurons terminate) Central canal

Lateral horn (cell bodies of autonomic efferent nerve fibers) Ventral horn (cell bodies of somatic efferent neurons)

Motor neuron pool of a muscle. • Those motor neurons innervating a single muscle • The motor neuron pools are segregated into longitudinal columns extending through two to four spinal segments. • The longitudinal orientation of motor neurons and their dendrites matches that of primary afferent terminals in that zone. • Thus impulses in a given afferent axon tend to be distributed to motor neurons innervating the same muscle or muscles with similar function.

Figure 8.15 Page 269

Spinal cord

= Motor unit 1 = Motor unit 2 = Motor unit 3

A motor unit is one motor neuron and the muscle fibers it innervates

The size principle: the orderly recruitment of motor units • The first motor units to be activated are those with smallest motor axons; – these motor units generate the smallest contractile forces – and allow the initial contraction to be finely graded.

• As more motor units are recruited, – the alpha motor neurons with progressively larger axons become involved – and generate progressively larger amounts of tension

Motor unit and motor neuron pool

Dorsal root Dorsal root ganglion

Ventral root

Figure 5.29 Page 174

Whole muscle tension depends on  • the size of the muscle,  • the extent of motor unit recruitment, • the size of each motor unit. • The number of muscle fibers varies among  different motor units.

– Muscles performing refined, delicate movements  have few muscle fibers per motor unit.  – Muscles performing coarse, controlled movements  have a large number of fibers per motor unit. – The asynchronous recruitment of motor units  delays or prevents muscle fatigue.

• One group of motor neuron pools is located in the medial part of the ventral horn, and the other much larger group lies more laterally.

Somatotopic organization of spinal cord motor neuron

trunk

α-mn: the final common pathway

The ventral root

extremities flexors

extensor s

Functional rule • The motor neurons located medially project to axial muscles (muscles of the neck and back): those located more laterally project to limb muscles (arms and legs). • Within the lateral group the most medial motor neuron pools tend to innervate the muscles of the shoulder and pelvic girdles, while motor neurons located more laterally project to distal muscles of the extremities and digits. • In addition the motor neurons innervating the extensor muscles tend to lie ventral to those innervating flexors.

Descending tracts Dorsal surface

Lateral corticospinal Gray

matter

Rubrospinal Ventral corticospinal Vestibulospinal

Ventral surface

Figure 5.30 (1) Page 174

Motor neurons • Alpha motor neuron – Thick myelinated fast conducting axons – Motor end plate of extrafusal skeletal muscle fibers

• Gamma motor neuron – Thin myelinated slower conducting axons – Supply the intrafusal fibers of Muscle spindles in skeletal muscles γ-static γ-dynamic

Spinal interneurons • Points of convergence for – most of the input of the brain descending tracts – Sensory afferents & collaterals of LMN axons

• Intersegmental; same side of spinal cord • Commissural: cross midline

Spinal reflexes • • • •

Contribute to Muscle tone Body posture Locomotion

Muscle spindles • Lie parallel to regular muscle fibers • contain nuclear bag and nuclear chain intrafusal muscle fibers.

Capsule

Alpha motor neuron axon

Gamma motor neuron axon

Secondary (flower-spray) endings of afferent fibers

Extrafusal (“ordinary”) muscle fibers

Intrafusal (spindle) muscle fibers

Contractile end portions of intrafusal fiber

Noncontractile central portion of intrafusal fiber

Primary (annulospiral) endings of afferent fibers

Muscle spindles • Can be stimulated by 2 ways • Stretching the entire muscle • Causing contraction of intrafusal fibers while extrafusal fibers remain at the same length.

Muscle spindles • Group Ia afferent fibers form primary endings on nuclear bag and chain fibers, • Group II fibers form secondary endings on nuclear chain fibers. • Dynamic motor axons end on nuclear bag fibers and static motor axons on nuclear chain fibers.

Muscle spindles • Primary endings demonstrate both static and dynamic responses, which signal muscle length and rate of change in muscle length. • Secondary endings demonstrate only static responses and signal only muscle length. • Motor neurons cause muscle spindles to shorten, which prevents the unloading effect of muscle contraction.

Golgi tendon organs • Located in the tendons of muscles and are arranged in series. • They are supplied by group Ib afferent fibers and are excited both by stretch and by contraction of the muscle (very sensitive to changes in muscle tension)

Extrafusal skeletal muscle fiber Spinal cord Intrafusal muscle spindle fiber

Afferent input from sensory endings of muscle spindle fiber Alpha motor neuron output to regular skeletal-muscle fiber Stretch reflex pathway γ motor-neuron output to contractile end portions of spindle fiber Descending pathways coactivating α and γ motor neurons Figure 8.26 (1) Page 287

Relaxed muscle; spindle fiber sensitive to stretch of muscle

Contracted muscle in hypothetical situation of no spindle coactivation; slackened spindle fiber not sensitive to stretch of muscle

Contracted muscle in normal situation of spindle coactivation; contracted spindle fiber sensitive to stretch of muscle

• Nuclear bag fibers • Ia fibers • Show a dynamic response: – Discharge most rapidly while the muscle is being stretched & less rapidly during sustained contraction

• Nuclear chain fibers • Ia fibers • Show a Static response – Discharge at an increased rate throughout the period when a muscle is stretched

• Signal the amount of displacement

Primary endings Signal Velocity and amount of change in muscle length

Alpha-gamma linkage  Enhancement of voluntary muscle contraction by co-activation of gamma and alpha motor neurons

The stretch reflex includes • a monosynaptic excitatory pathway from group Ia (and II) muscle spindle afferent fibers to a motor neurons that supply the same and synergistic muscles and • a disynaptic inhibitory pathway to antagonistic motor neurons.

Myotatic stretch reflex • The simplest reflex • Monosynaptic • Physiological significance: – Resting muscle tone and thus A key reflex in maintenance of posture

The tonic stretch reflex • Physiological significance: Resting muscle tone – Judged by the resistance that a joint offers to bending – Receptors: Ia & II from muscle spindle – Triggered by the static responses of group Ia and II afferents. – Any slight extension or flexion (during standing) will elicit a tonic stretch reflex in muscles required to oppose the movement, thus helping an individual to stand upright.

Phasic stretch reflex • Physiological significance: • Receptors: Ia from muscle spindle • Triggered by the dynamic responses of group Ia fibers • Enhancement of voluntary muscle contraction by co-activation of gamma and alpha motor neurons

Myotatic stretch reflex • Clinical significance in diagnosis of diseases – tendon jerks – muscle tone

Muscle stretch reflex

Extensor muscle of knee (quadriceps femoris)

Patellar tendon

Muscle spindle

Alpha motor neuron

Figure 8.27 Page 288

Inverse stretch reflex • Disynaptic (inhibitory interneuron+ α-mn ) • Inhibition of α-mn of same muscle • Receptor: Golgi tendon organ (in series with muscle fibers) • Stimulus: increase in muscle tension by – excessive stretch – excessive active muscle contraction

• Result: relaxation (sudden stop in contraction) • Safety: – regulates muscle tension – protects the tendon from tearing

Withdrawal reflex • Polysynaptic • Protective • Painful stimulation of skin, subcutaneous tissue or muscle • Stimulation of flexorscontraction • Reciprocal innervation • Simultaneous inhibition of antagonists relaxation

= Inhibitory interneuron = Excitatory interneuron = Synapse = Inhibits = Stimulates

Figure 5.33 Page 178 Thermal pain receptor in finger

Components of a reflex arc Receptor Afferent pathway Integrating center Efferent pathway Effector organs

Ascending pathway to brain Afferent Pathway

Stimulus

Biceps (flexor) contracts Hand withdrawn

Efferent pathway

Triceps (extensor) relaxes Effector organs

Response

Integrating center (spinal cord)

Crossed extensor reflex • Supporting reflex, serves to maintain posture • Polysynaptic • Irradiation of stimulation • Reciprocal innervation • Flexion and withdrawal of the painfully stimulated limb • + extension of the other limb

Figure 5.34 Page 179 Afferent pathway

Efferent pathway Efferent pathway

Integrating center (spinal cord) Flexor muscle relaxes

Flexor muscle contracts

Extensor muscle relaxes

Pain receptor in heel

Injured extremity (effector organ) Response

Extensor muscle contracts

Response

Stimulus Opposite extremity (effector organ)

Dorsal root ganglion cell

Upper motor neuron of corticospinal tract

Interneuron in the spinal cord

S

α-motor neuron in the spinal cord

Y

Effector

W Receptor

X

U

V Z T