Locomotion Flashcards
What is locomotion?
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
Locomotion requires what type of neurones?
Neurones with the ability to generate plateau potentials.
Supraspinal regulation of stepping can be divided into three functional systems:
- Activation and control of locomotion
- Refinement of motor pattern in response to feedback from limbs
- Guided limb movements in response to visual input
How is spinal locomotion activated and controlled?
Descending pathways from brainstem via glutamatergic reticulospinal pathway -> neurones in the medullary reticular formation, whose axons descend in ventrolateral region of SC
How are motor patterns refined in response to feedback from limbs?
Cerebellum fine-tunes the locotmotor pattern by regulating the timing and intensity of descending signals
What do the different parts of the spinocerebellar tract do?
dorsal -> leg proprioceptors - provide cerebellum info. ventral -> CPG - provides cerebellum with info about state of spinal locomotor network
How is limb movement guided by visual input?
- > 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
Definition of locomotor Central Pattern Generator (CPG)
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.
The generation of rhythmic motor activity by CPG’s depends on three factors:
- the cellular properties of individual nerve cells within the network
- the properties of the synaptic junctions between neurones
- the pattern of interconnections between neurones
How CPGs can be modulated
Modulatory substances, usually amine or peptides, can alter cellular and synaptic properties, thereby enabling a CPG to generate a variety of motor patterns.
The simplest CPGs contain neurones that are able to
burst spontaneously
Bursters must be regulated and induced by
Neuromodulators
Why are neuromodulators useful?
Alter cellular properties of neurones -> enable plateau potentials to open via ipsilateral NMDAR opening
What is mutual inhibition?
Interneurones that fire out of phase with each other are usually reciprocally coupled by inhibitory connections.
Why is rate of recovery from inhibition important?
It can influence the relative time of onset of activity in two neurones simultaneously released from inhibition
Why is mutual excitation important?
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.
Outline mechanism for plateau potential generation and termination cycle:
- Inhibition from contralateral I interneurone is terminated
- NMDAR channels in all ipsilateral neurones are opened by a depolarisation resulting from post inhibitory rebound
- Voltage dependency generates plateau potential
- Activation of low-voltage Ca2+ channels further strengthens depolarisation
- Ca2+ influx through these channels and NMDAR channels activates calcium dependent K+ channels
- Resultant K+ increase terminates plateau potential
In what ways are plateau potentials terminated?
- summation of slow after-hyperpolarisations
- delayed excitation of L interneurones which inhibit the I interneurones and thereby remove inhibition from contralateral half of network -> active
Do locomotor CPGs exist in humans?
Human walking may involve spinal pattern generators.
- evidence for existence of spinal rhythm generating networks in humans comes from studies of development.
How are supposed circuits for locomotion in humans brought under supra spinal control?
1/. infant develops ability to control locomotion voluntarily -> development of reticulospinal pathways and regions activating reticulospinal neurones
- 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
How does human locomotion differ from other animals?
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.
How is human locomotion similar to that of other species?
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.
