Locomotion

Question 1

What is locomotion?

Answer 1

a purposeful, goal-directed behaviour initiated by signals arising from either volitional processing in the cerebral cortex or emotional processing in the limbic system accompanied by automatic controlled movement processes

Question 2

Locomotion requires what type of neurones?

Answer 2

Neurones with the ability to generate plateau potentials.

Question 3

Supraspinal regulation of stepping can be divided into three functional systems:

Answer 3

1. Activation and control of locomotion
2. Refinement of motor pattern in response to feedback from limbs
3. Guided limb movements in response to visual input

Question 4

How is spinal locomotion activated and controlled?

Answer 4

Descending pathways from brainstem via glutamatergic reticulospinal pathway -> neurones in the medullary reticular formation, whose axons descend in ventrolateral region of SC

Question 5

How are motor patterns refined in response to feedback from limbs?

Answer 5

Cerebellum fine-tunes the locotmotor pattern by regulating the timing and intensity of descending signals

Question 6

What do the different parts of the spinocerebellar tract do?

Answer 6

dorsal -> leg proprioceptors - provide cerebellum info. ventral -> CPG - provides cerebellum with info about state of spinal locomotor network

Question 7

How is limb movement guided by visual input?

Answer 7

• > the motor cortex is involved in the control of precise stepping movements in visually guided walking
• > neurones in motor cortex may regulate the activity of interneurones that form part of the CPG for locomotion

Question 8

Definition of locomotor Central Pattern Generator (CPG)

Answer 8

A neuronal network capable of generating a rhythmic pattern of motor activity in the absence of phasic sensory input from peripheral receptors. The basic pattern produced by a CPG is usually modified by sensory information from peripheral receptors and signals from other regions of the CNS.

Question 9

The generation of rhythmic motor activity by CPG’s depends on three factors:

Answer 9

1. the cellular properties of individual nerve cells within the network
2. the properties of the synaptic junctions between neurones
3. the pattern of interconnections between neurones

Question 10

How CPGs can be modulated

Answer 10

Modulatory substances, usually amine or peptides, can alter cellular and synaptic properties, thereby enabling a CPG to generate a variety of motor patterns.

Question 11

The simplest CPGs contain neurones that are able to

Answer 11

burst spontaneously

Question 12

Bursters must be regulated and induced by

Answer 12

Neuromodulators

Question 13

Why are neuromodulators useful?

Answer 13

Alter cellular properties of neurones -> enable plateau potentials to open via ipsilateral NMDAR opening

Question 14

What is mutual inhibition?

Answer 14

Interneurones that fire out of phase with each other are usually reciprocally coupled by inhibitory connections.

Question 15

Why is rate of recovery from inhibition important?

Answer 15

It can influence the relative time of onset of activity in two neurones simultaneously released from inhibition

Question 16

Why is mutual excitation important?

Answer 16

Mutual excitation is important for establishing the synchrounous firing of a group of neurones, Electrical junctions often mediate mutual excitation, particularly when it’s important to rapidly generate a high intensity burst within a group of neurones.

Question 17

Outline mechanism for plateau potential generation and termination cycle:

Answer 17

1. Inhibition from contralateral I interneurone is terminated
2. NMDAR channels in all ipsilateral neurones are opened by a depolarisation resulting from post inhibitory rebound
3. Voltage dependency generates plateau potential
4. Activation of low-voltage Ca2+ channels further strengthens depolarisation
5. Ca2+ influx through these channels and NMDAR channels activates calcium dependent K+ channels
6. Resultant K+ increase terminates plateau potential

Question 18

In what ways are plateau potentials terminated?

Answer 18

1. summation of slow after-hyperpolarisations
2. delayed excitation of L interneurones which inhibit the I interneurones and thereby remove inhibition from contralateral half of network -> active

Question 19

Do locomotor CPGs exist in humans?

Answer 19

Human walking may involve spinal pattern generators.

- evidence for existence of spinal rhythm generating networks in humans comes from studies of development.

Question 20

How are supposed circuits for locomotion in humans brought under supra spinal control?

Answer 20

1/. infant develops ability to control locomotion voluntarily -> development of reticulospinal pathways and regions activating reticulospinal neurones

2. Stepping pattern gradually develops from a primitive flexion extension pattern that generates little effective propulsion to complex mature pattern -> maturation of descending systems originating from motor cortex and brain stem nuclei modulated by cerebellum

Question 21

How does human locomotion differ from other animals?

Answer 21

Human locomotion differs from most animal locomotion in that it is bipedal, placing significantly greater demands on descending systems that control balance during walking. Likely that spinal networks that contribute to human locomotion are more dependent on supra spinal centres than those in quadrupedal animals.

Question 22

How is human locomotion similar to that of other species?

Answer 22

Human waling relies on same general principles of neuronal organisation as walking in other mammals: intrinsic oscillatory networks are activated and modulated by other brain structures and by afferent input.