Ventral horn parts
Dorsolateral part
- Distal muscles
- Fine movements
Ventromedial part
- Proximal muscles
- Posture
Alpha motor neurons
The neurons that attach to the muscles that are involved in voluntary movement
Diseases that affect Alpha Motor Neurons
- ALS
- Poliomyelitis: viral infection that selectively attacks AMN’s
The stretch reflex
This is to keep posture.
The muscle spindle senses stretching. Activates AMN and the muscle contracts.
Gamma motor neuron contracts muscle spindle during voluntary movements so that they stay short enough to sense stretching when muscles are short.
Crossed extensor reflex
When one limb flexes, the other extends
Fully automatic and highly coordinated movement patterns
- Walking
- Swallowing
- Breathing
- Orienting
Withdrawal reflex
When you sense pain in your hand in pull it away quickly.
Golgi tendon reflex
Protects the muscle from excessively heavy loads by causing the muscle to relax and drop the load. The golgi tendon causes relaxation of the muscle.
Extrapyramidal systems (involuntary)
- Rubrospinal tract: distal limb muscles, precise movement
- Tectospinal tract: receives visual and auditory info. Reflex orienting response.
- Vestibulospinal tract: head position to maintain balance and posture.
- Reticulospinal tract: reflexes, state of arousal
Pyramidal systems (voluntary)
- Corticobulbar tract: move facial muscles and throat
- Corticospinal tract: move all non facial muscles
Damage to corticospinal tract
- Paralysis/paresis
- Spasticity/flaccidity
- Change in reflexes
Cerebellum role in motor system
- Fine tuning of movements
- Timing of automated movement sequences, motor memory
- Maybe also timing in general
Cerebellum parts
- Spinocerebellum: balance, walking, affected by alcohol
- Neocerebellum: control of fine movements
- Vestibulocerebellum: coordination of eye movements with body movements
Cerebellar ataxia
Endpoint tremor, slurred speech
Basal Ganglia anatomy
- Striatum (Caudate nucleus and Putamen)
- Globus Pallidus (Externa and Interna)
- Substantia Nigra
- Subthalamic nucleus
Role of the Basal Ganglia in the motor system
The basal ganglia inhibits most of the movements sent out by the motor area. Only the most active response gets let through.
Parkinson’s Disease
Tremor of hands, Rigidity, Bradykinesia
Medication: L-dopa, Stem cells, DBS
Cause: no dopamine production in the substantia nigra
Motor cortex parts
- M1: direct motor control
- PMC (and PPC): externally guided, stimulus driven action
- SMA (and PFC): internally guided action
Hemiplegia
Half sided paralysis due to lesions of upper motor neurons coming from M1
Apraxia
- Loss of motor skill
- Lesions in SMA, PMC, PPC
- Ideomotor apraxia: rough idea of movements can be executed
- Ideational apraxia: no idea what to do, uses wrong tools
Motor tuning in M1 and PMC
Individual motor neurons encode vector of movement, they are tuned for the direction of limb movement.
Tuning is fairly broad.
Actual movement is the vector sum of population of M1 cells.
Affordance competition hypothesis
Sensory inputs create many potential motor responses (affordances). Depending on needs and potential payoffs, one of these has to be selected.
Premotor cortex
Encodes population vectors of multiple potential actions, until colour cue is given to perform one action and not the other.
Posterior Parietal Cortex (PPC)
Translating movement from retinal to hand, head or body centered reference frames.
