Motor function: brainstem and spinal cord Flashcards
1
Q
What is a neuromuscular junction?
A
A synapse between a muscle fiber and a branch of motor neuron axon. The axon of a motor neuron divides into a number of branches at the surface of the muscle fiber. Each branch ends in a bouton.
2
Q
How wide is the cleft of a NMJ?
A
50nM across
3
Q
The postjunctional membrane, the endplate, which is thrown into folds, has a high density of what receptors?
A
nicotinic acetylcholine receptors under the active zones where acetylcholine (ACh) is released.
4
Q
What is the structure of the basal lamina of the cleft?
A
It is a collagenous basement membrane to which is bound acetylcholinesterase (AChE). Soluble forms of the same enzyme are also secreted into the cleft.
5
Q
What happens when ACH binds to Nicotinic acetylcholine receptors (nAChR)?
A
These are ligand-gated ion channels that mediate fast ACh transmission. Binding of ACh opens the channel, allowing Na+influx and K+ efflux. The reversal potential for this current is close to 0 mV, so activating nAChR causes depolarization.
6
Q
What occurs after a spontaneous release of a single quantum of ACh at the nmj?
A
This causes a 0.4 mV depolarization at the endplate called a miniature endplate potential (mepp).
7
Q
The arrival of an action potential at the motor nerve terminal triggers what?
A
Triggers the release of 200–300 quanta producing numerous mepps which sum to produce a depolarization to about -20 mV, an endplate potential (epp). This greatly exceeds the threshold for activating voltage-dependent sodium channels in the muscle membrane, so an action potential which is propagated over the muscle fiber membrane.
8
Q
How long does it take for the concentration of AcH in the NMJ to reach 1mM, and how long does it take to fall back to baseline?
A
within about 200 μs of the arrival of an action potential at the motor nerve terminal but within a millisecond or so the Ach concentration has fallen back to baseline levels because of the high activity of AChE in the cleft.
9
Q
How is AcH removed from the cleft?
A
AChE hydrolyzes ACh to choline and acetate. Choline is taken back into the nerve terminal via a Na+ -dependent transporter.
10
Q
What is the mechanism of action of Nondepolarizing muscle relaxants, such as the tetrahydroisoquinoline derivatives (e.g., the prototypical d-tubocurarine, atracurium) and the aminosteroids (e.g., pancuronium)?
A
They are competitive antagonists of nAChR. The duration of action of these drugs ranges from 15 to 180 minutes.
11
Q
What is the mechanism of action of neostigmine to reverse action of non depolarizing muscle relaxants?
A
It is an AChE inhibitor that causes a rise in the concentration of ACh which then out-competes the drug for the nicotinic receptor.
12
Q
Which is the only depolarizing muscle relaxant of clinical importance?
A
Succinylcholine
13
Q
What is the mechanism of action of succinylcholine?
A
It is a nAChR agonist
14
Q
What is the phase 1 block by succinylcholine?
A
Initially, agonist binding opens the nicotinic receptor channel causing persistent depolarization of the endplate. This first causes generalized disorganized contractions of muscles called fasciculations, and is followed by flaccid paralysis as muscle Ca2+ channels inactivate and the contraction mechanism fails as a result. This arises as a result of an ACh-like depolarization and so is augmented rather than reversed by AChE inhibitors.
15
Q
What is a phase 2 block by succinylcholine?
A
With continuing exposure phase II block occurs in which the nAChR either desensitizes, or suffers open channel blockade by the drug. Phase II block can be reversed by AChE inhibitors.
16
Q
Why is the duration of action of succinylcholine only 5 minutes?
A
Succinylcholine is rapidly hydrolyzed by circulating esterases.
17
Q
What is a motor unit?
A
Consists of a motor neuron and the muscle fibers it innervates. In mammals each muscle fiber is supplied by only one motor neuron, however, each motor neuron synapses with anything from 6 to a few thousand muscle fibers within a single muscle. The fibers of a single unit are scattered widely throughout a muscle so no part of a muscle is controlled by just one motor unit.
18
Q
What is the size of a motor unit related to?
A
To the precision of motor control required of a given muscle. Finely regulated muscles consist of small motor units, less finely regulated muscles have larger ones
19
Q
What does a single action potential in the motor neuron cause?
A
A twitch (single contraction) in all of the muscle fibers to which it is attached.
20
Q
What is unfused tetanus?
A
The contraction and relaxation of muscle fibers is loner than the muscle AP (3ms). If a volley of APs is fired (at 12HZ) and there is insufficient time for the muscle to relax between successive impulses the twitches summate to increase the force which oscillates about a plateau value.
21
Q
What is fused tetanus?
A
As firing frequency increases (30HZ), the oscillations smooth out and the plateau reaches maximum force.
22
Q
Which type of motor units are the most common?
A
Slow twitch (S) motor units
23
Q
What are the properties of the force generated by the fibers of S motor units?
A
they take 50 ms to reach peak force and show little decline, even after an hour of repetitive stimulation. They are capable of exerting low forces for long periods. They form the bulk of the antigravity or postural muscles of the trunk and legs.
24
Q
What are the properties of the motor neurons of the S motor units?
A
They are small, have low conduction velocity and long refractory periods because they contain a high density of calcium activated K channels which cause a long afterhyperpolarization.
25
What is the significance of the long refractory periods of S motor units?
This limits maximum firing frequencies to low rates, but fused tetanus is achieved at low frequencies (15-20 Hz)
26
What are the properties of the Type 1 muscle fibers of the S motor units?
They are rich in mitochondria, have high activities of Krebs cycle enzymes which fits them for high rates of aerobic metabolism and form red muscle because of their high myoglobin content.
27
What are the properties of the forces generated by fast twitch motor units?
They contract maximally in 5-10 ms but cannot sustain the contraction for very long. With repetitive stimuli fatigue resistant units can sustain moderate force for 5 minutes or so before a steady decline sets in that takes many minutes. Fast fatigue motor units achieve the greatest force, but with repetitive stimuli the force falls precipitously after about 30 seconds. Both FF and FR are adapted for producing rapid, large forces and so are found in muscles involved in executing fast movements.
28
What are the properties of the motor neurons of fast twitch motor units?
They are large with high conduction velocities. For brief periods they fire at high rates but action potential volleys are of short duration, particularly for FF units
29
Fast twitch units contain type 2 muscle fibers, what are the properties of these?
They form white muscle because of their low myoglobin content and require firing frequencies of 40 -60 Hz to produce fused tetanus. Type 2b (FF) are anaerobic, Type 2a (FR) are intermediate between types 1 and 2b in terms of metabolism.
30
How are the properties of muscle fibers and motor neurons matched for optimal performance?
Muscle fiber properties are determined by the motor neurons that innervate them. If type 1 muscle fibers are denervated and the axon of a FF unit sprouts to establish new connections with the denervated fibers, they acquite the characteristics of type 2b muscle fibers
31
What is a motor pool?
Formed by motor neurons that innervate the same muscle. They are topographically localised in motor nuclei of the brainstem and spinal cord
32
What are the properties of spinal motor nuclei?
They extend over several spinal segments. Axons of motor neurons leave the ventral horn of the spinal cord to run in the spinal nerve of the same spinal segment.
33
Where does the sorting of fibers destined for the same muscle but originating from different spinal segments occur?
The nerve plexuses. Axon collaterals of motor neurons ascend and descend a few segments to influence the behaviour of other motor neurons in the same pool
34
How is the force of a muscle contraction determined by the motor pool?
The rate at which individual motor neurons fire and the number of motor neurons in the pool that are firing.
35
How is the force of a contraction increased?
Small increases in force are met mostly by increased firing rate, but larger contractions involve increasing the number of active motor units, a process called recruitment.
36
How is the process of recruitment carried out?
It is done in an orderly manner. The earliest units to be recruited are S, followed by FR and finally FF, and order determined by the size principle. 2 effects are at work – the size of the motor neurons, and how synapses onto them are organised.
37
How does size of a motor neuron determine the size principle?
Small cells offer a bigger resistance to the flow of current than large ones. Ohms law states that membrane voltage = Current x resistance. Neurons in a motor pool are excited by common inputs. Therefore, for a given synaptic current input, the small cell body of an S motor neuron will have a bigger excitatory postsynaptic potential than the larger cell body of a fast twitch unit. This means that the weakest inputs recruit the S units as they have the lowest threshold for synaptic activation.
38
What is the second effect determining size principle?
The synaptic inputs to the three classes of motor unit are weighted in such a way that as input strength increases so motor units are recruited in the sequence S-FR-FF.
39
Does recruitment always obey the size principle?
No, in some instances synapses are arranged so that large motor neurons get more excitation than small ones. For eg, in humans, cutaneous afferents preferentially excite fast twitch motor units.
40
How are motor neurons modulated?
They are modulated by monoaminergic neurons (NA, 5-HT) that projects in the reticulospinal tracts. This enhances the response of the motor neuros to excitatory input, eg, S motor units fire tonically at 20 -30 Hz but with elevated monoamine input, Ia afferents will produce firing rates in excess of 50 Hz.
41
When do the firing rates of monoaminergic neurons increase?
With arousal. Modulation by NA allows the generation of higher forces from the same inout when arousal is high. Serotonin neurons fire during locomotion and their firing rate increases with speed, driving motor neurons to fire at higher frequency for a given input.
42
How do monoamines bring about motor neuron hyperexcitability?
By the enhancement of a depolarizing current through L-type calcium channels in their dendrites
43
What is a reflex?
It is a stereotyped response to a particular stimulus. When it involves the autonomic nervous system it is an autonomic reflex and the effector is typically cardiac or smooth muscle or a gland. When it occurs in the somatic nervous system it is a motor reflex and the effector is skeletal muscle
44
Reflexes are mediated by specialized neural circuitry called a reflex arc, what does this consist of?
A sensory neuron, a motor neuron and usually interneurons interposed between the two, which may be excitatory or inhibitory.
45
What are polysynaptic reflex arcs?
Reflex arcs with two or more interneurons that have three or more central synapses
46
What is a disynaptic reflex?
A reflex arc with only one interneuron
47
What is the only example of a monosynaptic reflex, where there is no interneuron?
The stretch reflex
48
What is usually the effect of afferents firing in a reflex arc?
A sensory neuron will form synapses with several interneurons (or motor neurons in the case of a monosynaptic reflex). Usually the effect of afferent firing is to produce quite large epsps on a few neurons and more modest epsps in a bigger group, depending on the number of synapses.
49
What provides for the possibility of integration in reflexes ?
The connections made by several sensory neurons on interneurons overlap
50
Integration at reflexes is often nonlinear, what does this mean ?
the excitation of the motor neuron can be bigger than the sum of the individual inputs, facilitation, or dominated by one input and hence little affected by additional ones, occlusion.
51
What is reflex latency or reflex time and what does it result from?
The time between the stimulus and response. It results from the conduction time along afferent and efferent fibers, but also includes the time taken for sensory transduction and for activation of the effector (muscle or gland). A small interval is taken up by the synaptic delay, usually between 0.5 and 1 ms.
52
What do reflex latencies reflect?
When conduction time is taken into account reflex latencies reflect the number of central synapses
53
What effect does increasing the stimulus intensity have on a reflex?
It will change the amplitude of the reflex (e.g., the amount by which a limb moves) but may also alter the form of the response by recruiting addi- tional muscles, this is called irradiation
54
What is the local sign?
The exact form of the reflex response depends on precisely where the stimulus is applied and so which afferents are excited
55
What is habituation ?
The attenuation of a reflex by the repeated application of a constant innocuous stimulus. It is caused by synaptic depression
56
What is dishabituation?
The return of the reflex to its baseline state. It is caused by any change to the stimulus (e.g., in its intensity
57
What is sensitization?
The enhancement of a reflex by a decrease in latency, increased amplitude, or irradiation caused by repeated application of a noxious stimulus. It results from increased transmitter release
58
What are habituation and sensitization of the reflexes examples of?
Nonassociative learning because only one stimulus is involved
59
What are conditioned reflexes?
reflexes that are capable of the more complicated associative learning, in which a response occurs if two stimuli are paired in time.
60
What is the most elementary modulation of motor unit output?
It is made by sensory input from the muscle spindles which measure the length and rate of change of length (velocity) of the muscle.
61
What is the muscle spindle reflex/ stretch reflex/ myotatic reflex?
Any attempt to stretch the muscle rapidly, for example by suddenly loading it, is met by contraction. It is a negative feedback mechanism which defends a constant muscle length in the face of external forces which act to perturb it.
62
How is a stretch reflex elicited?
by sharply tapping the tendon of any muscle. The resultant stretch causes the muscle to contract. The stretch reflex is most easily demonstrated by tapping the patellar ligament between its insertion into the tibia and the kneecap, causing the contraction of the quadriceps femoris, the powerful group of extensor muscles on the front of the thigh
63
The sensory side of the stretch reflex consists of the muscle spindle and its afferents. What is the structure of the spindle?
Muscle spindles lie in parallel with the standard extrafusal fibers. Each muscle spindle is a fluid- filled capsule of connective tissue, 4–10 mm long and 100 μm in diameter, containing about seven modified muscle fibers called intrafusal fibers. Intrafusal fibers have contractile ends but their central regions are noncontractile. There are two types of intrafusal fiber, nuclear bag and nuclear chain
64
What is the general structure of nuclear bag fibers?
They are swollen at their center, where their nuclei are clustered, and are innervated by large diameter myelinated (Ia) primary afferents, the ends of which spiral around the central region of the fiber.
65
What are the two types of nuclear bag fibers and how are they differentiated?
They can be recognized by whether in addition to primary afferent innervation they also receive secondary, group II, myelinated afferents. Those that do not are dynamic (b1), those that do are static (b2).
66
Why do the primary afferents of nuclear bag fibers show dynamic responses, responding to the rate of change of the length (velocity)?
This is because of the properties of the dynamic nuclear bag fiber. When
stretched the central region elongates causing the Ia afferent to fire a volley of action potentials. Subsequently, however, the poles of the fiber elongate slowly permitting the central region to creep back to a shorter length so that the firing rate of the Ia afferent
drops off.
67
How do the primary afferents of nuclear bag fibers show static responses, signaling muscle length?
Due to the innervation of static (b2) nuclear bag fibers which are stiffer than the dynamic fibers and hence elongate in proportion to muscle stretch.
68
What is the structure of nuclear chain fibers?
they are of uniform diameter, are about half the size of b fibers and their central region contains a line (chain) of nuclei. They are innervated by primary and
secondary afferents
69
What do the afferents of nuclear chain fibers respond to?
being stiff (like the b2
fibers) these afferents respond to muscle length.
70
Which intrafusal fibers typically are found in muscle spindles ?
a spindle will contain one b1, one b2, and three to five c intrafusal fibers.
71
Where do spinal Ia afferents make their synapses?
The majority of Ia spinal afferents form synapses on homonymous motor neurons, that is, motor neurons going to the same muscle. However, about 40% make synapses with
motor neurons which go to synergistic muscles. For example, the quadriceps femoris consists of four muscles that act synergistically (they are all leg extensors). Afferents from spindles in any one of them will establish connections with its own motor pool and the pools of the other three muscles.
72
What is the phasic component of the stretch reflex?
It is seen by tapping the tendon of a muscle. It occurs rapidly, is brief, and occurs because of the dynamic activity of the Ia afferents.
73
What is the tonic component of the stretch reflex?
component is the much more sustained contraction brought about by the static activity of the Ia afferents and the secondary, group II afferents. This component is particularly important in maintaining posture.
74
Which neurons which drive the extrafusal fibers to produce muscle contraction?
Aa class with a cell body diameter averaging 80
μm, usually referred to as a motor neurons.
75
Where do y motor neurons send axons to?
They are fusimotor fibers going to muscle spindles All intrafusal fibers have their contractile ends innervated by these y motor neurons.
76
What is the purpose of y efferent discharge?
Contraction of the ends of the intrafusal fibers keeps the central region taut so that it can respond to muscle stretch. Y efferent discharge maintains the sensitivity of the muscle spindle to changes in length over a wide range of lengths. Without it, muscle contraction would cause the intrafusal fibers to slacken and fail to respond to stretch
77
What are the two categories of y motor neurons that can be independently activated by the cns?
Y1 (dynamic) innervate b1 while y2 (static) innervate b2 and c fibers.
78
How do y efferents increase the gain/ sensitivity of the muscle spindle?
Stimulation of g1 fibers increases the sensitivity of the b1 fibers, so that the primary afferent firing rate is higher in rapid than slow stretch. Stimulation of g2 fibers enhances firing of secondary afferents in response to constant stretch
79
When are firing rates of y efferents increased?
When performing movements that are particularly complex
80
Stretch reflexes must be overridden to allow the execution of a movement since the muscle must contract isotonically and shorten. How is this achieved?
by descending motor pathways exciting both a and g motor neurons at the same time. This is called coactivation. It makes the extrafusal and intrafusal fibers shorten together in such a way that the intra- fusal fibers are always sufficiently taut to respond to stretch
81
Where are golgi tendon organs located?
In the tendons in series with muscle fibers
82
Golgi tendon organs measure muscle tension what does an increase in muscle tension cause?
It activates a negative feedback reflex, the inverse myotatic (Golgi tendon) reflex, which opposes the increases in tension. It is brought about by GTO input activating inhibitory interneurons that synapse with a motor neurons supplying the muscle
83
What is the structure of a golgi tendon organ?
It is composed of the collagen fibers which join muscle fibers to tendons, inter- woven through which are axon branches of a group Ib afferent neuron
84
What is the mechanism of Golgi tendon organs measuring muscle tension?
The increased tension that occurs with muscle contraction stretches the collagen fibers, distorting the terminals of the Ib afferent which fires. Individual Ib afferents respond statically, reflecting the level of tension in response to the activation of a single motor unit.
85
Why don’t GTOs measure the average tension of the muscle?
because less than one percent of fibers in motor units are coupled to GTOs.
86
What is the pathway of the 1b afferents of GTOs?
The Ib afferents enter the spinal cord to synapse in the intermediate zone (Rexed laminae VI–VIII) on inhibitory neurons, which then synapses with motor neurons of the homony- mous and synergistic muscles. These inhibitory neurons are all specific to this disynaptic reflex pathway and so are designated Ib inhibitory neurons (IbINs)
87
How is Inhibition of homonymous and synergistic muscle by GTOs augmented?
by inputs from Ia spindle afferents, joint afferents, and cutaneous mechanoreceptor afferents onto IbINs. The functional importance of these connections is not clear. Descending motor systems may either excite or inhibit IbINs.
88
How do the muscle spindle reflex for maintaining muscle length and the GTO reflex for keeping constant tension work in opposition?
When the loading on a muscle is altered either the muscle is stretched, in which case its passive tension increases or it must contract isometrically to maintain a constant length, in which case the active tension rises due to contraction. It is impossible for both length and tension to be held constant at the same time.
89
What is muscle stiffness?
k, a constant that describes how much the length of a muscle is altered, DL, by a change in load, DF. k = DF/DL
90
How do the spindle and GTO reflexes relate to muscle stiffness?
muscle spindle and GTO reflexes between them compensate for the complicated way in which muscle stiffness changes over the normal working range of muscle. This allows the organization of supraspinal motor systems to be simpler.
91
What proportion of the neurons in the spinal cord are motor neurons?
they make up less than 2% of the total number of neurons in the spinal cord, most
of which are interneurons.
92
What does organisation of movement by the spinal cord depend on?
depends on reflex circuits and central pattern generators (CPGs).
93
Which motor reflexes are seen in isolation in physiological conditions?
Very few protective trelfexes such as the flexion withdrawal reflex.
94
Motor reflexes are usually never seen in isolation, they are usually elements of circuits which do what?
They are elements of
circuits which operate coherently to allow descending motor control of muscles to be continually modified on the basis of proprioceptor input from muscles and joints, and input from the skin, enabling the smooth execution of the appropriate movement.
95
What does locomotion involve?
It involves cycles of activity in which muscle groups are made to contract in a
precisely timed sequence. This requires neural networks that can generate the required rhythmic output.
96
What are central pattern generators?
They are neural networks which provide the required rhythmic output for locomotion. They were thought to be autonomous, though modifiable by reflexes and activated by supraspinal influences.
96
What are central pattern generators?
They are neural networks which provide the required rhythmic output for locomotion. They were thought to be autonomous, though modifiable by reflexes and activated by supraspinal influences.
97
What are central pattern generators?
They are neural networks which provide the required rhythmic output for locomotion. They are networks of interneurons in the spinal cord that generate the precisely timed
sequences of a motor neuron activation without the need for sensory input. They were thought to be autonomous, though they are modifiable by reflexes and activated by supraspinal influences.
98
What are the three types of inhibition that contribute to spinal cord function?
reciprocal, presynaptic, and
recurrent.
99
What is the definition of reciprocal inhibition?
The inhibition of mutually antagonistic muscle motor neurons
100
What is the mechanism of reciprocal inhibition?
Axon collaterals of Ia afferents from muscle spindles synapse with glycinergic Ia inhibitory interneurons in lamina VII (IaINs) that project to motor neurons of antagonist muscles. This disynaptic circuit allows antagonist muscles to be relaxed during agonist contraction.
101
What is IaIN activity mediating reciprocal inhibition modified by?
by descending motor pathways (corticospinal, rubrospinal, and vestibulospinal) and locomotor networks in the cord.
102
Why do muscles only follow slowly changing neural output ?
Because muscle contractions are so long lasting
103
How does the motor system produce fast fluctuations in muscle tension?
Rapid changes in motor
neuron firing cannot be translated into corresponding alterations in muscle tension. Hence to facilitate rapid movements, the motor system alternates contraction in agonist and antagonist muscles. This is helped by reciprocal inhibition.
103
How does the motor system produce fast fluctuations in muscle tension?
Rapid changes in motor
neuron firing cannot be translated into corresponding alterations in muscle tension. Hence to facilitate rapid movements, the motor system alternates contraction in agonist and antagonist muscles. This is helped by reciprocal inhibition.
104
How is the stiffness of a joint increased?
By co-contraction of muscles with opposing actions at the joint. It requires suppression of reciprocal inhibition.
105
What is the purpose of co-contraction?
stabilizes a joint, it provides better control when loads change unexpectedly because a given difference between expected and actual load will have a smaller effect on limb trajectory if a joint is stiffer.
106
What is the mechanism of presynaptic inhibition?
GABAergic interneurons in the spinal cord make axoaxonal synapses on the afferent terminals. GABA secreted at these synapses acts on GABAA receptors to bring about depolarization because in these sensory neurons the membrane potential is more negative than the reversal potential of the chloride current through the GABAA receptor channels. The effect of this primary afferent depolarization (PAD) is inhibitory in that when an action potential sweeps into the terminal its amplitude gets smaller, less Ca2+ influx occurs, so transmitter release from the terminal is reduced.
107
How is presynaptic inhibition of Ia afferents organized?
on a reciprocal basis in which flexor afferents inhibit extensor afferents and vice versa. At the start of a movement presynaptic inhibition to Ia terminals going to agonist motor neurons is reduced whereas inhibition of antagonist muscle Ia terminals is increased. This means that spindle activity in the agonist is reinforced whereas that in the antagonist muscle is dampened.
108
Why is it important that presynaptic inhibition dampens spindle activity in the antagonist muscle during any movement?
agonist action lengthens the antagonist muscle and this would provoke it to contract were its stretch reflex not suppressed
109
What is the purpose of presynaptic inhibition of Ib and II afferents?
It allows control of tension and tonic length signals independently of phasic length signals.
110
What is the mechanism of recurrent inhibition?
A population of interneurons called Renshaw cells in the ventral horn are activated by axon collaterals of a motor neurons and project to neighboring, homonymous, and synergistic a motor neurons. These cells fire high frequency bursts of action potentials that produce fast large ipsps
111
What is the effect of recurrent inhibition?
It is to silence motor neurons excited weakly and firing at low rates, and to dampen the firing frequency of strongly excited motor neurons. It enhances contrast, and so enables economical movements.
112
How does strychnine and Tetanus, caused by infection with the organism Clostridium tetani cause convulsions?
Renshaw cells are glycinergic and blockade of glycine receptors with strychnine results in convulsions due to the failure of recurrent inhibition. Tetanus produces the same effect because its toxin blocks glycine release from Renshaw cell terminals.
113
What are flexor reflex afferents?
They are a variety of afferents, including group II and III muscle afferents, joint afferents, and skin mechanoreceptor and nociceptor afferents that elicit flexor reflexes in the ipsilateral limb
114
What is the crossed extensor reflex?
Flexor reflex afferents trigger an extension of the contralateral limb.
115
How do the flexor reflex afferents differ?
Different types of FRAs are connected to specific subsets of interneurons so the reflexes they excite differ in form and timing.
116
What are half centers?
the sets which interneurons targeted by the flexor reflex afferents are organised into, between which reciprocal inhibitory connections exist. They are also components of central pattern generators
117
Where do half centers get there input from?
A flexor half center gets inputs from ipsilateral FRAs and excites flexor motor neurons, while an extensor half center is activated by contralateral FRAs driving extensor motor neurons
118
How are flexor reflex afferents involved in the execution of a particular movement?
For a movement, descending motor axons are activated which project to the specific set of FRA interneurons that bring about the movement. The reciprocal connections ensure that the alternative set is inhibited. As the movement proceeds, FRA input from muscles, joints, and skin reinforces the movement and allows it to be
fine-tuned.
119
What is the flexor withdrawal reflex triggered by?
Ad or C fiber (group IV) nociceptors
120
How is the withdrawal flexor reflex distinct from other flexor reflexes that are recruited as elements of normal movement?
It overrides ongoing movement, involves flexor muscles throughout the limb
so the response is dramatic, and it is long lasting. It is protective.
121
What does the gait or manner of locomotion depend on?
the speed. The gait adopted is the one which minimizes the energy expenditure for that speed.
122
In humans, what are the phases of stepping?
each leg has a stance phase, when extensors are most active, and a swing phase, when the flexors are the most active. The same sequence in the opposite leg
is out of phase
123
How do the phases of stepping change as the speed of locomotion changes?
during walking there is a brief overlap of the stance phase in both legs. As the speed increases the stance phase shortens until there is no overlap and the switch to running occurs.
124
What is the structure of central pattern generators?
Each limb has an array of CPGs. Each CPG is an oscillator with two half centers, one driving flexors,
the other driving extensors, with reciprocal connections between them.
125
In a CPG, how does firing of the half centers occur?
Each half center
produces rhythmic bursts of action potentials that are terminated in a time and manner determined by the intrinsic excitable properties of its constituent neurons. The cessation of firing of one half center releases its opposite number from reciprocal inhibition, allowing it to fire a burst. In this way burst firing alternates between the two half centers.
126
How do half centers stimulate motor neurons?
Depolarization of excitatory (E) cells in one half center activates their NMDA receptors. The resulting calcium influx prolongs
the depolarization so that the E cell fires a burst of action potentials. The burst firing
of the E cells stimulates motor neurons.
127
What features of a CPG allow it to flip-flop
between bursting of first one half cell then the other?
Firstly, the E cells stimulate L cells which inhibit the I neurons responsible for the reciprocal inhibition. This disinhibits the opposite half center. Secondly, the E cell burst ends because the calcium activates KCa
channels, allowing K+ efflux and hyperpolarization.
128
What is locomotor activity initiated by?
by activity in the mesencephalic locomotor region (MLR) which projects to reticular nuclei in the medulla. Axons from here run in the reticulospinal tracts to the spinal cord. These
129
What do the axons of reticular nuclei do in the spinal cord?
They are excitatory, releasing
glutamate to produce a large depolarization of the CPG neurons which then produce
oscillating output for as long as the MLR input continues.
130
Why are CPGs interconnected?
so that timing of events in all limbs is coordinated.
131
What is the function of postural mechanisms?
They prevent the body from being destabilized by forces (including gravity and those produced by limb movements) acting to shift the center of mass.
132
What are antigravity muscles?
Muscles either oppose or assist gravity when contracting; those that oppose are described as antigravity muscles. Many antigravity muscles, such as the leg extensors and the short deep extensor muscles of the back (axial muscles), are involved in maintaining posture. In humans, the flexor muscles of the arms are also antigravity muscles.
133
Which are the strongest muscles in the human limbs?
Since antigravity muscles are generally more powerful than muscles assisted by gravity, in human limbs the strongest muscles are the leg extensors and arm flexors.
134
Where do commands for postural adjustments necessary during the execution of a movement come from?
Forebrain and cerebellum. These are feed forward postural adjustments which anticipate the unbalancing forces that act during the movement
135
What do postural adjustments/ set depend on?
precisely on the initial position and nature of the intended movement and they must be learnt.
136
Postural adjustments often have to be made to compensate for unpredicted disturbances in body position and movement. This cannot be done by feedforward, how is it done?
these adjustments are made by postural reflexes, organized by the brainstem, which are negative feedback mechanisms
137
Where is the sensory input for the postural reflexes from?
-Vestibular, from the otolith organs
-Proprioceptor, from muscle spindles, Golgi tendon organs, and joint receptors
-Visual, from the superior colliculus
These inputs are highly integrated to recruit the required sequence of corrective muscle activity.
138
What does the exact nature of postural reflex adjustments made by humans depend on?
context, that is, the initial position of the body and the size and direction of the destabilizing force.
139
What muscles would swaying, produced by sudden displacement of the surface on which a person is standing, activate?
It will activate quite different sets of muscles depending on the direction of sway, but in general distal muscles are excited before proximal ones, with most movement occurring at the ankle joint.
140
What muscles does rotation or tilt of the surface activate?
results in bending at the hips.
141
What is the aim/ goal of postural reflexes?
In extremis postural reflexes try to maintain the center of gravity to prevent falling, or to put the limbs in a position to brace against falling.
142
Why can’t postural reflexes be easily isolated in healthy humans?
because motor functions are normally so highly integrated. They may be seen in newborn infants, in whom motor systems are immature.
143
What is the function of sensory input from otolith organs and semicircular ducts in terms of postural reflexes ?
It is used to stabilize the orientation of the head in space. Any tilting or rotation of the head and body as a unit activates motor neurons to muscles that maintain the head vertical with respect to gravity. These (mainly tonic) reflexes have a latency of about 40–200 ms.
144
What are vestibulocollic reflexes?
Those which activate motor
neurons to neck muscles. They act on the neck muscles to keep the head upright. If the body sways forward the neck extensors contract bringing the head up. If the body sways backwards, neck flexors are activated.
145
What are vestibulospinal reflexes?
act on limb muscles. They trigger contraction of arm extensor muscles and leg flexor muscles when falling, to reduce the impact of landing. Swaying sideways triggers extension of the ipsilateral limbs to brace against further tilt in that direction, and contralateral flexion.
146
What does turning the neck relative to the body result in?
This excites spindles in neck muscles and afferents from the cervical vertebral joints. This evokes reflex contractions of neck muscles (cervicocollic reflexes) and limb muscles (cervicospinal reflexes) with both phasic and tonic components.
147
What are cervicocollic reflexes?
These contract neck muscles that are stretched and so act to reorientate the head on the body. Cervicocollic and vestibulocollic reflexes are synergistic.
148
What are cervicospinal reflexes?
They cause contraction of limb muscles in response to rapid head movement. In standing humans a force which throws the head backwards on the trunk
activates all the limb extensors, whereas a force throwing the head forwards activates all
limb flexors.
149
How are vestibulospinal and cervicospinal reflexes related?
They are antagonistic. If the head and trunk are tilted as one to the left, the vestibulospinal reflex causes left arm extension. But
if the trunk alone is passively tilted to the left (with the head remaining fixed in relation to space) the cervicospinal reflex will flex the left arm. In the more usual situation that
the head is tilted while the trunk remains stationary, these opposing reflexes cancel out.
150
What are the two sets of pathways used for supraspinal descending control of movement?
Those mediating postural reflexes, which go from brainstem to spinal cord, are collectively called the medial motor pathways to distinguish them from the lateral motor pathways for making voluntary movements
151
What does the medial motor system comprise of?
the vestibulospinal and reticulospinal tracts which terminate in the intermediate and ventral gray matter of the spinal cord.
152
What is the pathway of input from the vestibular labyrinth (mediates vestibulocollic reflexes)?
This input goes to medial and inferior vestibular nuclei, which give rise to the bilateral medial vestibulospinal tract. This makes connections with neck muscle motor neurons. In general, ipsilateral motor neurons are excited whilst contralateral ones are inhibited
153
What is the pathways of vestibular afferents to lateral vestibular nucleus implicated in control of limb muscles?
The lateral vestibular nucleus projects via the uncrossed lateral vestibulospinal tract to all segments of the spinal cord. Neurons in this pathway facilitate extensor motor neurons and inhibit flexor motor neurons. This pathway is responsible for some vestibulospinal reflexes.
154
There are two reticulospinal tracts. What is the pathway of the medial reticulospinal tract?
It originates from the pontine reticular nuclei and is ipsilateral. Its neurons excite axial and limb extensor motor neurons, but are inhibitory via polysynaptic pathways to limb flexors.
155
What is the pathway of the lateral reticulospinal tract?
The medullary reticular nuclei give rise to the lateral reticulospinal tract which is bilateral and produces monosynaptic inhibition of neck and axial motor neurons and polysynaptic inhibition (excitation) of proximal limb extensors (flexors). The medullary reticular nuclei get input from the mesencephalic locomotor region and project to the central pattern generators.
156
Why do the reticulospinal tracts service both neck and vestibular reflexes?
They receive proprioceptor input from muscle spindles and vertebral joint receptors, and vestibular input. otolith signals for head forward and back pitching components of the vestibulocollic reflexes are not transmitted through vestibulospinal tracts but probably involve the reticulospinal tracts.
157
What allows postural reflexes to be modified by feedforward during intentional movement?
The reticular nuclei receive input from the premotor cortex
