Behavioral Neuroscience X
How does our Brain make us Move?
Motor System: How we Move
Muscle Vocab
There are three kinds of muscle: cardiac muscle found in the heart, smooth muscle found in the autonomic system, and striated muscle found everywhere else. Since striated muscle is the one we have voluntary control over, we are going to talk about this kind exclusively.
Muscles are made up of thousands of muscle fibers, a long cell with many nuclei. Each muscle fiber is connected to a single motor neuron. Motor neurons release ACH onto muscle fibers causing excitatory contractions. While muscle fibers only connect to one motor neuron, motor neurons can connect to many muscle fibers, between 3 and 30. A motor unit refers to a motor neuron and the muscle fibers it is connected to.
Muscle tension (the strain you feel when you lift something heavy) is increased by recruitment of increasing numbers of motor units. Weak stimulus activates small (low threshold) motor neurons found in slow-twitch muscles. Slow-twitch muscles use oxygen and are good for long distances. Strong stimulation activates large (high threshold) motoneurons found in fast-twitch muscles. Fast-twitch muscles are anaerobic and are good for sprinting but get tired easily.
Motor System
The motor neuron receives input from several places:
- Sensory neurons to make simple reflexes
- Spinal cord neurons that are involved in pattern generators (walking, chewing, etc.)
- Pyramidal System carrying brain signals that initiate activity
- Extrapyramidal System carrying brain signals that modulate activity
The first bullet, sensory neurons, represent an unconscious quick response to stimuli. When a sensory neuron detects something really bad, such as our hand on a hot stove, the signal doesn’t go to the brain for a decision—it would take too long. Instead, when it hits the spinal cord it is immediately sent to the relevant motor neurons. This is why if you touch something hot, your hand jerks away seemingly before you even register it is hot. This reflex system helps you minimize as much avoidable pain/damage as possible.
The second bullet, spinal cord neurons, represent an unconscious activation of motor neurons. This motor system is sometimes called muscle memory. It would be overwhelming to have to remember how to walk, or chew, or run, or ride a bike, every time you needed to (have you ever played QWOP? You have no idea how your to control the muscles in your legs correctly to walk). This system allows your brain to free itself from having to remember everything involved in these activities so it can focus on other, more important things.
The third and fourth bullets are best understood in context of how the brain processes and creates motor stimuli. There are two motor circuit paths: simple & fine movement generation, and complex action generation.
Simple & fine motor movements are generated in the basal ganglia and the primary motor cortex. Much like the primary sensory cortex, the primary motor cortex (M1) contains a motor homunculus: a representation of the body in terms of how much dexterity the body part necessitates. Hands, mouth, and tongue are the three largest areas of M1. M1 sends signals down the pyramidal system of neurons, which have large diameter axons (meaning they are fast conducting) and initiate movements; their name arises from the pyramidal shape of the white matter tracts the neurons travel down. While there isn’t a 1-to-1 representation of M1 neurons and particular muscles/movements, there is quite a strong correlation. The Basal Ganglia is a group of brain structures that each contain a motor homunculus as well. It is mostly responsible for modulating activities initiated by M1.
Complex actions are generated in the cerebellum and supplementary motor area. The Cerebellum is a complex portion of the brain that receives input from both sensory and motor systems, allowing for the feedback control of movement. This feedback is crucial for activities like balance. This modulation is sent down the extrapyramidal system of neurons to the muscles. The supplementary motor area (SMA) is active during mental rehearsal, or preparation of skilled movements or planned actions.
Motor Disorders
As strange as it sounds, oftentimes the biggest breakthroughs in neuroscience and psychology happen because of disease and disorders. The brain is such a black box, that we only really learn what a certain part of the brain does once someone has that part of the brain removed or injured. This is the case in a majority of neuroscience knowledge. Because of this, here are a few examples of disorders and diseases that effect the motor system.
A stroke in M1 causes spasticity in movement, and fine motor control is often lost completely. A stroke in SMA, or areas around M1, cause an inability to plan actions of perform complex acts. Sometimes event simple task performance is inhibited.
Parkinson’s Disease is a degeneration of dopamine cells in the substantia nigra, a part of the Basal Ganglia. This leads to a resting tremor and a difficulty initiating movement due to the lack of basal ganglia modulation to movement signals. Huntington’s Disease, which begins showing signs around 40 years old, is a degeneration of the Basal Ganglia in which inhibition of movement is difficult, leading to excessive movements.
Damage to the cerebellum leads to intention tremors, due to overshooting/overcorrecting movements. Damage can also lead to loss of balance and coordination.
Recap:
- Muscles are made up of motor units, that receive inputs from a variety of places.
- M1, Basal Ganglia, Cerebellum, nearby areas all work together to plan, initiate, and modulate signals to muscles for finer control