Flashcards in W6 L10 211 Deck (22)
For movement there is no single starting point!
It's a brain mechanism
The Motor Homunculus
Stimulation of the Motor Cortex (Graziano and Aflalo 2007)
Prolonged stimulation produces
Organisation of the Motor Cortex
§ The primary motor cortex (M1) is the major input of cortical sensorimotor signals and the major output of sensorimotor signals is to the descending motor pathways that initiate movement.
§ The supplementary motor area (SMA) is located on the medial surface of the brain.
§ The premotor cortex is located primarily on the lateral surface.
§ The SMA and premotor cortex receive sensory information from the parietal and temporal lobes.
§ M1 also receives information from the somatosensory cortex
The Supplementary Motor Cortex (SMA)
§ The SMA is involved in learning sequences of movements.
§ Monkeys with lesions to the SMA are unable to perform a simple two-sequence response: pushing in a lever and then turning it to the left
Neural Activity in Monkey SMA (Shima and Tanji, 2000
Motor neurons Sam response sequence
Firing pattern of single neuron in SMA. This neuron fires only when a push response follows a pull response
§ Monkeys were taught six sequences of three motor responses (e.g. push- pull-turn, or turn-push-pull, etc).
§ Neurons in SMA were recorded. The activity of these neurons encoded certain elements of the sequence.
§ The SMA was then temporarily inactivated with muscimol (a GABA agonist which inhibits neural activity).
§ After inactivation, monkeys could still reach for an object or make a response, but they could not perform the sequence of three responses that they had previously learned.
Studies of SMA Activity in Humans
§ In humans, learned sequences of button presses activate the SMA (Hikosaka et al., 1996)
§ When SMA is disrupted with TMS in people who learned to play 16 finger presses on a piano, the subjects continue to play for about one second after stimulation but then reported that they “did not know which series of keys to press next” (Gerloff et al., 1997)
§ The SMA and pre-SMA, a region just rostral to the SMA is thought to be involved in the desire to move. If we stimulate these regions, it provokes an urge to make a movement or you anticipate that a movement is about to occur.
The Premotor Cortex
§ The premotor cortex is involved in learning and executing complex movements that are guided by sensory information.
§ Movement can be guided by nonarbitrary (reaching for an object in a polar location) or arbitrary information (movement by signals not related to them, touch nose when hear. Tone).
§ Monkeys with premotor inactivation using muscimol are able to move their hand towards a signal light located to the left or right (a nonarbitrary stimulus), but were not able to make previously learnt left and right movements in response to arbitrary red and green stimulus lights (Kurata and Hoffman, 1994).
§ Humans with premotor damage can make several different movements in response to spatial cues but cannot make a specific movement in response to arbitrary visual, auditory or tactile stimuli.
Interesting similarity between monkey and humans
Mirror Neurons (Rizzolatti et al., 2001)
§ The ventral premotor cortex (area F5) in the monkey brain contains mirror neurons. Discovered by accident, while doing a study, response to particular movements by investigator.
§ A mirror neuron fires when a monkey performs an action OR when the monkey observes the same action performed by another monkey or human.
§ Mirror neurons play a role in a monkeys ability to imitate the movements of other monkeys.
§ Mirror neurons are activated by sounds that indicate the occurrence of a familiar action. Like breaking peanuts.
It has been suggested that the mirror neuron system helps us
understand the actions of others and other people's intentions.
Apraxia is caused by damage to
left frontal or the parietal lobe.
There are four types of apraxia:
The Apraxias: Deficits in Skilled Movements coordinated purposeful movement.
Limb apraxia - problems with movements of arms, hands and fingers.
Oral apraxia - problems with muscles used in speech.
Apraxic agraphia - problems with writing
Constructional apraxia - problems with drawing or constructing objects.
§ Limb apraxia is charaterised by movement of the wrong limb, incorrect movement of the correct part of the limb or correct movements in the incorrect sequence.
§ It is assessed by asking people to imitate (pantomime) hand gestures. Air Key, guitar etc.
Limb Apraxia can be caused by three types of lesions:
- Lesions in the anterior corpus callosum (saw video) colossal apraxia.
- Lesions of the motor cortex in the left frontal lobe
- Lesions in and around the intraparietal sulcus of the left parietal
§ Constructional apraxia is caused by lesions to the right parietal lobe
§ People with constructional apraxia are unable to draw objects or pictures or assemble objects from elements.
§ These patients are unable to perceive and imagine geometrical relationships.
§ In addition to apraxia, these patients also have difficulty with other tasks involving spatial perception and navigation (e.g. reading a map).
The Basal Ganglia
The caudate-putamen and globus pallidus constitute the main motor components of the basal ganglia.
The basal ganglia receive most of their input from the entire cerebral cortex, especially primary motor cortex and somatosensory cortex, AND the substantia nigra which houses cell bodies of the dopamine neurons.
The major outputs are the primary motor cortex, SMA and premotor area (via the thalamus) and motor nuclei of the brain stem.
Basal Ganglia and Movement
Cause involuntary movements (hyperkinetic symptoms)
Unwanted choreiform (writhing and twitching).
(e.g. Huntington’s Chorea, Tourettes syndrome
Cause hypokinetic symptoms, a difficulty in making voluntary movements
(e.g. Parkinson’s Disease)
Movement Pathways of the Basal Ganglia (Alexander and Crutcher, 1990)
see diagram slide 17 l10 better than the textbooks!
An inhibitory (gray) and excitatory (blue) pathway converge on the GPi.
GPi è Thalamus è Cortex
The GPi influences the thalamic projection, which modulates the size or force of the movement produced by the cortex.
If activity in the inhibitory pathway is high relative to the excitatory pathway, the GPi will be inhibited and the thalamus is free to excite the cortex.
If activity in the excitatory pathway is high relative to the inhibitory pathway, the GPi will be excited thereby inhibiting the thalamus, and reducing input to cortex.
§ Parkinson’s disease is a neurodegenerative disorder. It is part of the basal ganglia group of disorders.
§ The primary symptom of Parkinson’s disease is muscular rigidity, slowness of movement, a resting tremor and a postural instability.
§ Caused by an imbalance in the direct and indirect pathways; the indirect pathways shows a decrease in inhibitory output due to reduced dopaminergic input to the caudate-putamen.
§ Standard treatment is L-DOPA which increases the amount of dopamine. BUT, too much dopamine stimulation causes involuntary movements and postures (dyskinesia and dystonia).
§ L-DOPA does not work indefinitely; the number of dopamine neurons decline and the symptoms gets worse.
Sim to Lewy body says prof!
§ Huntington’s Chorea is a genetic disorder. It is part of the basal ganglia group of disorders.
§ Huntington’s patients show uncontrollable movements, especially jerky limb movements. Sometimes known as Huntington’s dance.
§ Caused by degeneration of the caudate-putamen which leads to a reduction in the inhibition provided by GABA-secreting neurons of the caudate-putamen. This effects the overall activity of the indirect pathway.
§ There is no treatment for Huntington’s disease.