Role of different levels of CNS in regulation of motor functions. Role of spinal cord in regulation of motor activity of human.

One of the major functions of CNS is to control motor activity of the organism.

Movement can be divided into:-Voluntary-Un-voluntary-Simple (flexion, extension)-Complex (language, mimics)

Motor functions of organism are maintained at different level of CNS, starting from spinal cord and up to brainstem and cerebral cortex. Spinal cord plays a very significant role in control of body movements.

Spinal cord: cylindrically shaped 43-45cm bundle of nervous tissueSegmental structure:-8 cervical (C1-C8)-12 thoracic (Th1-Th12)-5 lumbar (L1-L5)-5 sacral (S1-S5)-1 coccygeal (Co1)

Segment is a part of spinal cord from which two pairs of roots (ventral and dorsal) on each side come out.

Bell-Magendie lawDorsal (sensory root) contains afferent nerve fibers that carry information from periphery to CNS. Its damage leads to loss of sensitiveness in the innervated are.

Ventral (motor) root contains efferent nerve fibers that carry information from CNS to effectors. Its damage leads to paralysis in the innervated area.

Cross section of spinal cordGray matter is the central butterfly-shaped area and is composed of neurons cell bodies and short neurons externsions (dendrites).

White matter surrounds the gray matter and is composed of myelinated axons of interneurons.

Dorsal (posterior) horn sensory inputVentral (anterior) horn contains motor neuronsLateral horn contains a vegetative neurons (present only from Th1-L5 segments)

Rexed laminae1 to 4 - sensory laminae5-8 interneurons9 motor neurons (alpha and gamma)10 commissural fibers

Functions of gray matter of spinal cord1)to route sensory or motor stimulus to interneurons of the CNS2)to form integrating centres of reflexes

Motor neurons that lie in anterior horns of gray matter r of two types:1) alfa-motor neurons their axons form motor nerves (directly provide contraction)2) gamma-motor neurons (Y) control muscle tone

97% of all neurons in gray matter are interneurons.

White matter of spinal cord is arranged in funiculi: ventral, dorsal, lateral

Axons in white matter connect different areas of gray matter and form nervous pathways (tracts)a)Ascending (sponothalamic, spinocerebellar etc)b)Descening (corticospinalis, vestiblospinalis etc)

Functions of white matter of spinal cord1)to transmit information within parts of CNS2)to connect different levels of spinal cord between themselves and with brain

Functions of spinal cord1.Conductive spreading of impulses with the help of nervous pathways of white matter2.Reflexive important integration centre for reflexesIf we need to examine specific functions of spinal cord we should separate spinal cord from upper levels of CNS (brainstem, forebrain).

Reflexes if spinal cord:1.Motor (somatic)2.Vegetative

Motor reflexes can be:a)Phasic (dynamic) motor reflexes that provide quick responses.b)Tonic (static) motor reflexes that provide prolonged contractions thus creating tonus.

The tonic reflexThis reflex contributes to muscle tone and helps to maintain posture. This is myotatic reflex. Occur during stimulation of muscular receptors. Provided by gamma efferent system (muscle spindle) of the spinal cord. Centers of these reflexes are in a spinal cord. In the anterior horns of the spinal cord are two types of neurons L(alfa)-motor neurons and Y(gamma)-motor neurons.

Muscle spindle is one type of proprioreceptor that provides information about changes in muscle length.

Structure of muscle spindle:Muscle spindle consists of nuclear bag (connective-tissue capsule) with stretch (spiral) receptor (endings of afferent nerve from stretch receptor to alpha motor-neuron) and intrafusal fibers. The skeletal-muscle fibers that form the bulk of the muscle and generate its force and movement are the extrafusal fibers. The stretch (spiral) receptor are parallel to the exrafusal fibers. Intrafusal fibers are innervated by gamma motor-neuron. Extrafusal fibers are innervated by alpha motor-neuron.

Role of muscle spindlesGamma efferent system consists of alpha and gamma motor neurons, muscle spindle and afferent neuron. Gamma efferent system ensures the tensions of muscles. There are three ways of excitation of the spiral receptor:1)relaxation of muscle2)excitation of gamma motor neuron3)passive extension of muscle

Relaxation of muscle extension of spiral receptor action potential transmits to the alpha motor neuron through afferent neuron contraction of extrafusal fibers contraction of muscle

Excitation of gamma motor neuron contraction of intrafusal fibers extension of spiral receptor action potential transmits to the alpha motor neuron through the afferent neuron contraction of extrafusal fibers contraction of muscle

Passive extension of muscle extension of spiral receptor action potential transmits to the alpha motor neuron through the afferent neuron contraction of extrafusal fibers contraction of muscle.

Role of muscle spindles

Classification of spinals reflexesa)Stretch (myotatic) reflex is produced by activatin muscle spindles. Function maintaining postural stability, countering sudden loads. Tonic reflec.

b)Stretch (myotatic) reflex (autogenic inhibition) is produced by activating Golgi tendon organs. Function regulation of muscle tension, prevention of damage to tendon, bone. Monosynaptic reflex. Phasic reflex.

c)Stretch reciprocal reflexes (pain withdrawal, crosses extensor) are produced by activating cutaneous receptors, nociceptors and have protective function. Phasic reflex.

Rhythmical reflexes scratching reflex phasic reflexLocomotor reflexes (running, walking, swimming) - phasic reflexes

Clinically important stretch reflexesKnee jerk reflex (L3-L4)Ankle jerk reflex (Achilles) (S1-S2)Plantar reflex (L5-S1)Elbow jerk reflexes (C4-C5)Abdominal skin reflexes (Th7-Th12)

Golgi tendon organ proprioreceptorReceptors in tendons (Golgi tendons organs) react for the rension of muscles. Impulses from Golgi tendon organ transmit to the alpha motor neuron through the afferent neuron and cause inhibition his. They also stimulate the motor neurons of the antagonistic muscles. This reflex protects the tendon from rupture.

To summarize. The activity of afferent fibers from the Golgi tendon organ supplies the motor control system information about the muscles tension. In contrast, the spindle afferent fibers provide information about the muscles degree of stretch. Role of Golgi tendon organs

Reciprocal reflexes (crossed extensor reflex)

Spinal shockAfter separation of spinal cord from brain spinal shock occurs.Spinal shock is manifested by disappearing of all motor activities and all kinds of sensitiveness (pain, touch, temperature) in the areas of body that are located below the place of injury (!). In human spinal shock lasts for weeks or months.

Main cause of spinal shock blockage of excitatory influences of higher levels of CNS on spinal cords neuron.Using tendon stretch reflexes helps to locate at what level injury of spinal cord is located.

One-sided damage of the spinal cord is named syndrome of Braun-Sekar -it is paralysis on the side of damage, loss of pain and temperature sensitiveness on the lateral side and decrease of touch sensitiveness from both sides.