Nervous system II

reflex centres loctation

spinal cord, or in the subcortical and cortical areas

Motor function

Impulses are conducted from the cortex → motor nuclei of the cranial nerves located in the brainstem + motor neurons located in the ventral horn of the spinal cord.

Motor neurons

Aα (innervates muscle fibers) and Aγ motor neurons (innervates intrafusal fibers)

Muscle tone

active stretch of the muscle, their partially contracted state.

Pyramidal system

voluntary control, the implementation of the fine, highly resolved movements, learn the new movement elements and influences the muscle tone (in cooperation with the extrapyramidal system).

Extrapyramidal system

all of the descending pathways except pyramidal.

Regulation of automatic movements (programmed movements), regulation of muscle tone, regulation of rough movements

Somatic reflexes:

proprioceptive ("sense of self")
reflexes myotatic reflex (stretch reflex)
inverse myotatic reflex

exteroceptive reflexes

flexor reflexes
crossed flexor-extensor reflex

Myotatic (stretch) reflex

a muscle contraction in response to stretching of the muscle

Receptor: muscle spindle

Muscle stretch → deformation of the annulospiral endings → activation of the mechanoreceptors → receptor potential→ action potential

Adequate stimulus of sensory fibers (coupled to the intrafusal fibers): passive tension of skeletal muscle fibers or contraction of intrafusal fibers.

Its role is the keeping the normal posture in the gravitational space.

Inverse myotatic reflex

Receptor: Golgi tendon organ
Stimulus: intense stretch of the muscle Reflex arch with 3 neurons, bisynaptic reflex (inhibitory interneuron)
Latency period is two times longer, than in the monosynaptic reflexes.
Passive stretch induces prompt reflex relaxation.
Localized, only the muscle under passive stretch respond to the stimulus.

Biological role: protection against overstretch

Exteroceptive reflexes

• Polysynaptic, longer latency period
• Response is graded: reflex irradiation
• Responses depend on the stimulus intensity (reactions with increasing stimulus isntensity): ipsilateral flexion, ipsilateral flexion – contralateral extension, bilateral flexion

Biological function: protection (to take away the source of stimulus, to avoid the source of stimulus)
Reciprocal innervation: inhibition of antagonists is characteristic for the reflex action

Basal ganglia

caudate nucleus, putamen, globus pallidus (putamen + globus pallidus: lentiform nucleus), subthalamic nucleus and substantia nigra.

Movement regulation

• Regulation of the muscle tone distribution

• Movement patterns, automatisms

• Organisation of the voluntary movements

• Organisation of the emotional motor reactions

Role of primary motor cortex

initiation of the voluntary movement, learning of the new motor elements, controlling the implementation of the voluntary movement actions.

Myasthenia gravis

• Decrease in the number of functioning acetylcholine receptors due to autoimmune process.
• Symptomes: fatigability of muscles, eye muscle weakness, weakness of any other muscles, ptosis

Motor unit

A motor unit consists of a single α-motor neuron and the population of muscle fibers innervated by the same motor neuron.

Levels in the hierarchy

Brain cortex → limbic system → hypothalamus → midbrain → medulla oblongata → spinal cord