Lecture 8: Proprioceptive system Flashcards
What are muscle spindles
Proprioception receptors that detect muscle stretch. They lie parallel to (and are attached at either end to) extrafusal muscle fibres , and consist of a connective tissue capsule with specialised intrafusal fibres. Because the intrafusal fibres are attached at either end of extrafusal fibres, the muscle spindle stretches when the muscle stretches (active or passive stretch)
There are two types of intrafusal fibres in a muscle spindle:
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Nuclear chain fibres
Which are narrow and have a single row of nuclei -
Nuclear bag fibres
Which are wider and have a cluster of nuclei in the middle
Two types of afferents arise from intrafusal fibres
- Annulospinal endings
- Innervate the central part of both chain and bag fibres (wrapped around like a spiral).
- are primary afferents (type 1a axons) which are activated by brief stretch (tendon tap) or vibration of the muscle as well as by a sustained stretch.
- Tonically active, ensuring optimal muscle length
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Flowerspray endings
innervate the ends/poles of both chain and bag fibres at either side of the annulospinal endings
- are secondary afferents (type II axons) which are only activated during sustained stretch on the muscle
What is the function of proprioception
For skeletal muscles to perform effectively, the brain must be continually informed of their state, and they must have healthy tone to be in an optimal position and length to either contract or relax/
Describe what happens when you initially pick up something heavy (Eg. a bowling ball)
- When pick up weight (bowling ball) extrafusal and intrafusal muscle fibres are pulled down by the weight / stretched initially.
- Type Ia axons wrapped around intrafusal fibres in muscle spindle are activated by stretch and send action potentials to spinal cord.
- Alpha motor neurons in spinal cord are excited by type Ia axons and trigger extrafusal muscle fibre contraction.
How does a muscle maintain optimal muscle length for responsivness?
Note: When a muscle is contracted, it shortens and this effectively reduces the length of the muscle spindle as well. A shortened muscle spindle contains shortened intrafusal fibres resulting in ‘slack’ and less responsive type Ia axons, … so there is an inbuilt mechanism to keep the muscle spindle at optimal length or for optimal type Ia axon ‘responsiveness’ at all times.
- Intrafusal muscle fibres also receive motor input from spinal gamma motor neurons originating in the spinal cord.
- Gamma motor neurons regulate the intrafusal fibres sensitivity to stretch by triggering the two poles of the muscle spindle to contract to effectively tighten the centre portion.
- This intrafusal fibre contraction keeps type Ia axons ‘tight’ and responsive even when extrafusal fibres are contracted.
Describe the myotactic reflex
- When patellar tendon is tapped it pulls on quadriceps (extensor) muscle, stretching both extrafusal muscle fibres and intrafusal muscle fibres inside the muscle spindles.
- Type Ia axons wrapped around intrafusal muscle fibres in muscle spindle are activated by stretch and send action potentials to spinal cord.
- Alpha motor neurons in spinal cord are excited directly (monosynaptic) by type Ia axons and trigger extrafusal muscle fibre contraction in the stretched quadriceps.
- This, contraction of the quadriceps (in concert with relaxation of the hamstring (flexor) muscles due to inhibition of extrafusal muscle fibres of the hamstring (due to inhibitory interneuron involvement)) results in a forward kick of the lower leg and signifies that proprioceptive connections in the spinal cord are intact and functioning.
What are golgi tendon organs?
Golgi Tendon Organs detect, and respond to, active changes in muscle tension ultimately to protect muscles from excessive contraction.
- are located at the junction between muscle and tendon.
- consist of a connective tissue capsule ensheathing a meshwork of collagen fibres, intermingled with and encircled by Type Ib primary afferent axons.
active contraction of a muscle changes the tension of the collagen bundles in its tendon, squashing the intermingling type Ib axons of the Golgi Tendon Organ
What is the inverse myotactic reflex
The inverse myotactic reflex occurs when excessive load is placed on a muscle. Golgi tendon reflex causes relaxation, therefore protecting the muscle. But primarily important for regulating muscle tension within an optimal range: particularly important for fine motor acts such as manipulation of fragile objects with fingers … allows for steady but not crushing grip.
- Muscle contraction shortens muscle belly, increasing tension in muscle tendons.
- The increased tension in collagen bundles in the muscle tendon Golgi Tendon Organ squashes type Ib axons stimulating them to send action potentials to the spinal cord.
- In the spinal cord type Ib axons make an excitatory synapse with inhibitory interneurons which release inhibitory neurotransmitter in their synapse with alpha motor neurons (poly-synaptic).
- Inhibition of alpha motor neurons causes a dampening, or cessation all together, of extrafusal muscle fibre contraction in the muscle linked to the tensed tendon.
- As muscle tension increases, firing of type Ib axons in the Golgi Tendon Organ increases and inhibition of the alpha motor neuron increases to slow, and even stop (drop load), muscle contraction.
- As muscle tension decreases, firing of type Ib axons in the Golgi Tendon Organ decreases and inhibition of the alpha motor neuron also decreases, increasing muscle contraction.
What pathway is involved in conscious proprioception?
What else is conveyed by this pathway?
Describe the pathway.
Dorsal column medial lemniscus pathway (also involved in touch, pressure and vibration)
- Information conveyed from type Ia and Ib axons of primary afferent proprioceptors in periphery to the spinal cord.
- Primary afferents ascend spinal cord in the dorsal column tracts to synapse with second order neurons in the dorsal column nuclei in the medulla.
- Second order neurons cross to the contralateral side of the medulla in the medial lemniscus and project to synapse with third order neurons in the thalamus.
- Third order neurons project to somatosensory cortex of the cerebrum contralateral to proprioceptive input.
How does information get from the muscle spindles and golgi tendon organs to the spinal cord?
Via 1a (muscle spindles and 1b (golgi tendon organs) axons in a mixed spinal nerve. Cell bodies of pseudo unipolar primary afferents are located in the dorsal root ganglion.
The myelinated axons project into the spinal cord where they either synapse in various ‘Laminae’ (I-VII) of the dorsal horn or project to higher levels of the neuraxis without synapsing.
Which two pathways are involved in unconscious proprioception of the upper limbs?
Cuneocerebellar and rostral spinocerebellar
Which two pathways are involved in carrying proprioception from the lower limbs?
Dorsal spinocerebellar tract and ventral spinocerebellar tract
What is the function and pathway of the dorsal spinocerebellar tract?
The dorsal spinocereballar tract is responsible for conveying information about muscle tone AND muscle tension from lower limb muscles to allow adjustment of posture.
From muscle spindles (1a axons) and golgi tendon organs (1b axons), primary afferent neurons synapse in clarke’s nucleus in laminae 7 of the dorsal horn in C8-L2 spinal cord. Second order neurons continue ipsilaterally from clarke’s nucleus up to the inferior cerebellar peduncles via the dorsal spinocerebellar tract.
What is the function and pathway of the ventral spinocerebellar tract?
To convey muscle tension from lower limb, but also related to attempted movement as well.
From golgi tendon organs (1b axons), primary afferent neurons enter the ipsilateral dorsal hron to synapse in lamina V-VII. Second order neurons arise from lamina V-VII and immediately cross the midline via the anteior white commissure and ascend via the ventral spinocerebellar tract. Second order neurons then enter the contralateral superior cerebellar peduncle before recrossing to terminate in the ipsilateral cerebellar cortex.
This is why it may be called “the double crosser”
What is the function and pathway of the cuneocerebellar tract?
Responsible for muscle stretch and tension in the upper limbs.
From golgi tendon organs (type 1b) and muscle spindles (type 1a) primary afferent neurons project to the ipsilateral dorsal horn and enter the cuneate fasiculus, projecting up to the brain stem to synaps of the lateral accessory cuneate nucleus. Secondary sensory afferents arise from the lateral accessory cuneate nucleus and enter the inferior cerebellar peduncle to synapse on the cerebellar cortex.
What is the function and pathway of the rostral spinocerebellar tract?
Responsible for conveying muscle tension from upper limb.
From golgi tendon organs in upper limb muscles, primary sensory afferents (type 1b) synapse in layer VII of the dorsal horn. Second order neurons ascend in the rostral spinocerebellar tract and project to ipsilateral cerebellum via the inferior and superior peduncles.
