L9. Functional Organisation of the Spinal Cord Flashcards
What is the difference between a segmental reflex and an inter-segmental reflex?
A segmental reflex is when information enters the spinal cord, is processed and exits the spinal cord all that the same level.
A inter-segmental reflex involves a few regions of the spinal cord. These are usually for complex movements.
Where does the spinal cord usually end for adults?
Why is this so?
L1/2
Because of the differential rates of growth of the spinal cord and the spinal column
At what level are lumbar punctures usually performed and why?
At L3-4
Because it is after the termination of the spinal cord and thus without the danger of injurying neural tissue.
How does white and grey matter appear histologically?
White matter stains dark while grey matter appears light
Describe the distribution of white matter as you descend down the spinal cord
Progressing down to lower levels of the spinal cord there is less white matter as they all progressively terminate where they need to leaving only the lower distal limbs at the caudal end of the spinal cord.
by the end there is only a thin rim surrounding the grey matter.
What are the main ways that white matter fibres travel through the spinal cord? [2]
What is the most common of them?
- Running up and down the spinal cord in the same axis of it (most common)
- Running across the spinal cord, transverse to the axis of the spinal cord
What are the white matter tracts linking sections purely within the spinal cord (ie. not to/from the CNS) called?
Propriospinal cords
How do motor neurons tend to lie in the spinal cord segments?
They tend to lie in clusters/pools in the anterior ventral horn projecting out through the ventral root to the periphery
Other than the (ascension to the CNS) and the dorsal synaptic connections what other local synapse do primary sensory afferents make and why?
They make local synaptic connections with motor neurons in the ventral horn
This is to coordinate muscle movement and respond to the stimuli
What is the definition final common pathway for movement?
Whay is it named this?
The common pathway that is shared by any kind of movement.
It is called this because all movements must occur via activity of this motor neuron pathway.
What are the major descending/controlling inputs of the final common pathway for movement?
Which of the 2 is a major component of control?
- Descending control from cortical areas for voluntary movement
- Indirect interneuron control for reflex movement (most of the control)
Describe the topographical mapping of the ventral horn of the spinal cord
Medially send axons to proximal muscles
Laterally have motor neurons that project out to control distal muscles.
(The ventral surface also tends to spread a superior and inferior mapping as well). However, they overlap significantly.
What is a motor pool?
The collection of all motor neurons in the spinal cord that innervate one particular muscle.
Eg. the biceps muscle has a motor pool (collection) of thousands of motor neurons that contribute to its innervation.
What is meant by the functional grouping/antagonism of muscles?
Because innervation of muscles is only excitatory (to contract) muscles and there are no neurons to inhibit and relax muscles, the limbs and neural supply to the limbs are organised in such a way that around a joint there is an organisation into push/pull or flexor/extensor pairs
Contraction of one muscle of the pair antagonises contraction of the other muscle
Muscles have “muscle sense” or mechanoreception. What does this men?
There are specialised mechanoreceptors in the muscles (and tendons) that are important in providing sensory information regarding state of contraction, force and position to enable coordination and balance.
What is a motor unit?
One neuron innervates a number of muscle fibres.
A motor unit is made up of a
- motor neuron
- the skeletal muscle fibers innervated by that motor neuron’s axonal terminals.
The more motor units active, the larger the force of contraction
Describe the relationship between a motor unit and motor neuron pool
Groups of motor units often work together to coordinate the contractions of a single muscle
All of the motor units within a muscle are considered a motor pool.
What is a muscle spindle?
A major component to motor control, present in all muscles of the body (except in the eye).
They are sensory receptors within the belly of a muscle that primarily detect changes in the length of this muscle.
What is the main function of the muscle spindle?
To convey length information to the cCNS via sensory neurons. This information can be processed by the brain to determine the position of body parts.
Describe the development (differentation) of the muscle spindles
During development nerves growing out of DRG grow into the muscle
The nerves induce normal muscle fibres to differentate: forming a CT sheath around it (intrafusal muscle fibres) so it becomes a sensory structure
They can still contract but its main job is to report back to the sensory nerve how much stretch is occuring.
What is the Golgi Tendon Organ?
A proprioceptive sensory receptor organ that is at the origins and insertion of skeletal muscle fibers into the tendons of skeletal muscle
The neuron interdigitates within collagen and elastin matrix of the tendon (force transducer meshed in the structure)
Compare the information gained from a muscle spindle with the information from a golgi tendon organ.
The muscle spindles are parallel with the muscle fibres so are able to give information about the length of the muscle.
The golgi tendon organ is part of the chain (series) (thus gives information about force - no knowledge of the length of the muscle)
What is the main function of the motor reflexes (also known as the deep tendon reflexes)
They are the foundation for motor control (all behaviour relies on reflex control mechanisms to ensure the body is aware of limb position and movement at all times)
Why is the Deep Tendon Reflex also known as the monosynaptic reflex?
Because only one synapse is involved in the entire reflex pathway.
(no interneurons)
Describe how the deep tendon reflex elicits the response that it does
Tapping on a tendon causes a slight pulls in the muscle that it is attached to. Stretch receptors (highly sensitive) embedded in muscles are activated as a result.
These stretch receptors release a burst of neural activity via the muscle spindles at about 12om/s (fastest in the PNS).
This leads to a relay of information through the afferent neuron to the spinal cord where there is an excitatory synapse with the motor neuron to cause the extensor muscle to contract further.
Describe the monosynaptic stretch reflex
- Stretch receptors activated in the muscle by the tendon tap
- Action potentials rapidly propagate through the primary afferent neuron to the level of the spinal cord associated with innervation
- One excitatory synapse is made with a motor neuron in the ventral horn
- This excites an action potential in the motor neuron to cause muscle contraction of the muscle.
- The same afferent fibre that excited the motor neuron also inhibits the activity of the anagonistic neuron through an inhibitory interneuron to reduce activity of the flexor muscle
Compare the threshold for activation of the muscle spindles and the golgi tendon organs
The muscle spindles (the ones involved in the monosynaptic stretch reflex) are a low threshold receptors.
Golgi tendon organs are high threshold
Imagine a hypothetical where a limb (eg. arm) is in a static situation of supporting an unchanging load. Draw a pathway of the negative feedback pathway to maintain this situation
The reflex acts to sense the length of the muscle in this static position. If it feels the muscle lengthening then it will send signals through the pathway to signal contraction. If it senses shortening then it will do the opposite
The point is to know where in space, the arm is at all times without having to know about it consciouslly
Now imagine that the load is increasing in weight. This is now a dynamic situation. Now draw what changes would occur through the reflex pathway
Adding load will force the flexor muscle to lengthen under the weight. This causes a large activation of the stretch receptors sending a larger action potential through the afferent neuron meaning an even larger stimulation to flex the muscle through the excitatory motor neuron and an even larger inhibition of the extensor muscle
What are the main neurotransmitters used by the inhibitor interneurons in motor pathways?
GABA and glycine
What about the muscle spindles in the extensor muscles? How do they contribute to the pathway?
They work in the exact same mechanism.
Together the pathways work to maintain position
Describe the golgi tendon organ receptors involvement in the pathway
Golgi Tendon organs detect the amount of force generated by the muscle and feedback to the spinal cord.
The more active they are, the more force is being applied by the muscle and this causes a burst of activity that leads to an inhibitory interneuron that inhibits the same neuron.
It also excites the antagonist muscle by an excitatory interneuron.
This point is to control force be reducing it when force increases.
Imagine the situations where the flexor muscle is holding a load but is fatiguing (ie. the force is reducing in the muscle).
What receptor pathway is involved?
Draw the pathway
The golgi tendon organs detect force
The golgi tendon organs would detect a reduction in force as a result of muscle fatigue. This would reduce the action potential firing in the golgi afferent.
Reducing the golgi afferent reduces the firing in the inhibitory interneuron that it innervates. This means that it inhibits the inhibition and reduces inhibition on the contraction of the flexor muscle (and reduces excitation of the extensor)
What is an example of an inter-segmental reflex?
Withdrawal reflexes from noxious stimuli. Muscles involved in withdrawing a limb from the stimuli involves different levels of the spinal cord.
This is because withdrawal of the limb requires support from other limbs as well (bilateral and multisegmental)
Describe the pathway in removing a limb from a noxious stimuli
Flexion (withdrawl) reflex due to a nociceptor fibres. Nociceptors terminate as soon as they enter the dorsal limb. They activate flexors to remove the limb, at the same time extensors are inhibited.
At the same time, the contralateral leg has to increase activity and support body weight thus a pathway excites extensors and inhibits the flexors.
Does the removal of the limb from a noxious stimuli require CNS input?
No, it is a reflex: it is completely through the spinal cord
(eg. a frog without a brain exhibited the ability to perform this reflex correctly)
What is the function/reason why we have propriospinal fibres?
Propriospinal fibres (interneurons) connect different segments of the spinal cord for coordination across large regions (eg. walking pattern distribution to allow for arms to swing)
Describe the distribution of the propriospinal fibres (short vs. long) through the cross section of the spinal cord
Is there a reason for this distribution?
Short are located laterally through the spinal cord cross section.
Long are located medially.
Because lateral motor neurons are controlling the distal muscles (only interested in things close together). Digits are close together. Long fibres medially (muscles are long way away eg. postural control)
