14. Proprioception Flashcards
What determines what muscle is moved and where?
The amount and area of the motor cortex activated
What is Henneman’s size principle?
Smaller motor units are recruited first to minimise fatigue
What is proprioception?
The sensing of the bodies position and motion
What types of proprioceptors are located in the limbs?
Muscle spindles - give info about changes in muscle length
Golgi tendon organ - provides info about changes in muscle tension
Also joint angle info
Where are muscle spindles found?
Throughout the body of the muscle, in parallel with extrafusal fibres (typical muscle fibres)
What are intrafusal fibres?
Small, specialised muscle fibres found in the muscle spindle
What is the structure of an intrafusal fibre?
Contractile proteins at the ends, with a central region devoid of contractile proteins. Central region wrapped by sensory dendrites of the muscle spindle afferent
How are APs triggered in the spindle afferents?
The muscle lengthening opens ion channels on the stretched spindle. AP begins
What type of efferent neurons innervate intrafusal fibres?
What type of efferent neurons innervate extrafusal fibres?
Gamma Motor neuron
Alpha motor neuron
How are muscle spindle afferents keep responsive regardless of muscle length?
The gamma motor neuron stimulates contraction at the two ends of the intrafusal fibre to keep the central region taut.
What is meant by alpha-gamma coactivation?
When extrafusal fibres are stimulated to contract by alpha motor neurons, the gamma motor neuron is simultaneously excited
Where are Golgi tendon organs found?
What is their structure?
Located in tendons, in series with the muscle fibres.
Sensory dendrites interweave with collagen fibrils in tendon
What activates the Golgi tendon organ afferent?
The contraction of the muscle resulting in collagen fibrils being pulled tight
Why does the Golgi tendon organ not detect muscle stretch, only contraction?
Most of the stretch of the muscle is absorbed by the muscle itself, so the Golgi tendon organ is not stimulated
What are the four types of mechanoreceptors?
Merkel
Meissen
Ruffini
Pacinian
What are slowly adapting fibres?
Respond while the stimulus is present.
Merkel discs and Ruffini cylinders
What are rapidly adapting receptors?
Meissner corpuscles and Pacinian corpuscles.
Respond to stimulation with a burst of firing at the beginning and end of stimulation
What does the size of the receptive field of a receptor determine?
Its position in the skin. Small receptor fields are located closer to the surface of the skin, eg Merkel in epidermis, Meissner in top of dermis
What type of stimulation do each of the mechanoreceptor types respond to?
Merkel - pressure
Meissner corpuscle - flutter
Ruffini cylinder - stretching
Pacinian corpuscle - vibration
What is the structure of complex reflexes?
First order neuron - delivers sensations to CNS, ie brainstem
Second order neuron - delivers to higher level CNS, ie thalamus
Third order neuron - carries sensory info to appropriate area of the cerebral cortex
Why does dizziness occur?
Vestibulo-ocular reflex gone wrong. Sensory cells fire after stimulus stops and cannot tell which way is up
