Comparative Neurobiology Lecture 4-5

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Reflex Gain is controlled by:

Activation of the fusimotor system

Alteration of alpha motor neuron excitability

Pre-synaptic inhibition of 1a&II afferents.

Reciprocal Inhibition

The stretch reflex works alongside reciprocal inhibition. The muscle spindle sensory afferents reach the spinal chord and synapses in the ventral horn with the motor neurons causing the stretch reflex. Another set of branches at the same time synapse with interneurons in the spinal chord, it is inhibitory. It inhbits the alpha motor neuron supplying the antagonistic muscle. Whilst stimulating the extensor the antagonistic muscle is inhibited. If the antagonistic muscle was contracting hard at the same time. The reflex could not occur. There is a relationship that facilitates alternating action of the muscles.

2nd Image

1a afferent activates the inhibitory neuron to relax antagonistic muscle and also inhibits the other alpha motor neuron.There is dual action, the muscle is caused to interact but the inhibitory interneuron stops antagonistic muscles and prevent the antagonistic circuit from feeding back and inhbiting the alpha motor neuron. This alternating pattern is a short form of central pattern generation for things such as walking.

Autogenic inhibition is provided by feedback from the tendon organ. The tendon organ, the axon ascends into the dosarl horn of the spinal chord and synapses. This makes connections with inhibitory motor neuron and inhibits the alpha motor neuron going back to the same muscle. It inhibits itself. The tendon organ samples the force used by a group of motor units. This process enables balancing of force production along the muscle as a whole. It ensures and even production of force along the whole muscle. They know its a tendon organ as Vincent et al found that the 1b afferent fired in response to muscle contraction. A ramp stretch has very little response, and no response in vibration. When stained it is evident that all the terminals were in the dorsal horn terminals. This shows that its connections are through interneurons.

Recurrent Inhibition

Axon branches recur into the ventral horn and synapse with interneurons renshaw cells which provide an inhibitory input back on the the alpha motor neurons by direct feedback. Renshaw cells have purely inhbitory outputs through the GABA . Inhibiting alpha motor neuron , there is also gamma inhibition and there is also a reciprocal arrangement with the two renshaws with one acting on the flexor and the r on the extensor.When a contraction in the alpha motor neuron feeeds back to the same muscle switching off inhibitory interneuron releasing inhibition which prevents alpha motor neuron action and tehn inhibits the interneuron. This supports renshaw cell action and prevents self inhibition whilst inhibitting the 1a interneuron. This is also part of alternating action of activation and relaxation which occurs in process such as stereotypical walking.

Role of Recurrent Inhibition are:

The cns can regulate both muscle force and muscle stiffness.

This relates to activity-cntrast postural (high stiffness) and ballistic (high force, low stiffness)

For a given force muscle stiffness correlates to the number of active motor units

For a set force recurrent inhibition facillitates different varieties by changing firing rate in turn affecting recruitment and stiffness of motor neurons

Concentric contraction is muscle shortenin when lifting weight. Extentric contraction is when muscle contraction causes legnthening.

When running up hill it is concentric contraction whereas when running down hill it is extentic contraction.

The Withdraw Reflex

All other reflexes act on a single segment of the spinal chord. Some reflexes are poly segmental such as the Withdrawal Reflex, this ascends and descends several different levels. It is pain elicited and activates flexor muscles. At each segment level it synapses with excitatory interneurons which activate flexor muscles especially the proximal muscles.

There are reflexes mediated by sensory feedback from muscle and activation of motor neuron which pplay a critical role in posture and control of movement. The most powerful is the monosynaptic reflex which is the only one with dict connection of sensory neurons and motor neurons. We have a set of sense irgans that provide us with information on position velocity accelaration and force. Muscle spindle provides information on position velocity accelaration with the tendon organ giving info on force. These then feedback into reflexes which control movement.

END OF MAMMALS. MOVES ON TO INVERTEBRATES AT MINUTE 32.

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Invertebrate Proprioceptors

The Locust metathoracic femoral chordotonal organ.

Chordotonal organs are a group of specialized sensory organs whose mechanosensory functions from both extero and proprioeveptive sensing insects.

The insect femoral chordotonal organs are internal proprioceptors which monitor the position and movements of the femur tibia joint of the leg. The locust metathoracic femoral chordotonal organ is composed of rougly 100 neurons with a variety of response properties. Matheson used intracellular recordings to examine the range of fractionation of the phasic and tonic responses to tibial movement and found some neurones responding across the full range of leg angles. Other neurons were restricted to more specific response ranges allowing them to act as labeled line. This paper in 1992 described neurones with maximal firing and mid range angles for the first time in the locust chordotonal organ.

Invertebrates are used a model to compare to mammals. Invertebrates have a whole different range of proprioceptors.

Chordotonal Organ- The Femoro-tibial co of the locust jumping leg.

Image from Slides

The Extensor tibiae is the main muscle that extends the tibia. The flexor tibia tendon is the flexor for the tibiae. Tendons and apodeme are the same thing. The chordotonal organ is in the distal part of the femur and is anchored to the wall of the femur. The body of the organ is like a distorted trampoline, a sheet of connective tisue which can be flexed and distorted in different directions. The organ is attached to the joint pivot and indirectly via the flexo ligament which attaches to the flexor tibiae tendon. As the joint is rotated in different directions the bdy of the organ will be distorted. The flexing and extending of the tibia affects the chordotonal organ. As the locust walks , the movement is detected by sensory nerve 5b1 hich feeds to the ganglion in the ventricle which controls that leg. Within the body of the organ there are individual receptor neurons. Their processes point in different direction iin the body of the organ. As differantial stretch and movement occurs causes stretching and relaxing, the sensory endings are distoted in different ways. The group of sensory endings together is called the scoloperium , each of the individual ones are called the scolopael. As matheson stated there are about a 100 individual nerve cells with long processes going in different directions. Some of them extend out into the ligament itself. If we pull on main or flexor ligament, processes are bent. Depending on where the neurons are they se different parts of the movement which allows feedback on all ranges of the movement between them.

Lecture 5.

FTCO Ligaments (Shelton et al 1992). (Walker, M. 1998)

The main ligaments divided into 3 different segments.

At the end which attaches to the tibia, there is a rod of chitin.

There are gyropes which help hold the organ in position by attaching to the walls of the themur.

The chitin is the tough connective tissue which forms the exoskeleton. It transfers movement directly across the legnth of the chitin rod with no changes of mechanical properties. There is also flexible connective tissue attached to the mainbody forming the first part of the ligament. It is also viscoelastic, the more rapid the stretch the stiffer that part of the organ will be. This is similar to the muscle spindle.

There is a fina region which merges with the stiff rod, the viscoelastic structure and within the body of main ligament it forms up into individual strands. Those strands attach progressively to the chitin rod. The strands at the proximal ends are longer than at the distal end, this region is called the apodeme complex.

The arrangement of strands means if you pull when stretching due to leg flexing each of the strands which are most distal become to pull tight and progressively the overall attachment region pulls tight. Tension is applied first on the ventral surface and progressively upwards across the strand causing a shearing force translated causing a distortion on to the body of the organ stretching the organ resulting in the nerve cells seeing different parts of the stretch as distortion occurs..

Flexor and Extensor image Walker 1998)

In the Leg Flexed when the tibia is pulled the apodeme complex becomes completely tight and flexor ligament wont have any tension on it.

When the leg is extended and the tibia is extending in a big arch , at maximal extension at about 120o angle we see the tightly pulled flexor ligament causing a central rotation resulting in buckling of the apodeme complex as there is no flexibility in the rod.

There are 2 sets of mechanical inputs coming into the body of the organ. The largescale is the movement associated with the flexor or extensor. Secondly the differential tension across the strand as a result of each of these elements pulling tight in turn.

Responses of FTCO (1968

He recorded the global output of the organ and recorded the max action potential activity. He was only able to do this in a maintained position hold

He found the lowest range of firing at mid range of movement

You get full rate of firing at full extension and flexion

You also see that the direction of leg movement affects firing rate

If you are moving away from the centre , e.g from centre to flexion, when moving back firing rate drops. When taking tension off the organ moving into the mid range the firing rate is significantly lower. The firing rate depends on the direction of movement this property is called Histarysis. Normally the graph is expected to be a single curve by histarysis where the graph follows a different line depedant on which way youre moving which is potentially reflecting the pattern of creep that you see as a result of viscoelasticity.

In bag 1 fibres of the spindle you also had a creep back so when the sensory ending location was stretched it creeped back due to the viscoelasticity. This is a similar property as directional stretch shows a higher firing rate when relaxing. Both the muscle spindle and FTCO show histarysis.

Bottom graph shows when either ligament is cut. If the main ligament was cut there was no change in the firing rate during flexion because it is the main ligament that is cut as we go into flexion. The firing rate is at baseline level but when you pull on the flexor ligament for extension the firing rate increases. It is similar when cutting flexor ligament where disortion of the flexor ligament does not cause extension but distortion of the main ligament causes flexion as predicted.

Zill 1985 repeated experiments during movements applied sinosodial movements at stationary, 0.2Hz, 0.5 Hz and 5 Hz.