physio psych: ch 7 lecture (Movement)

Three categories of vertebrate muscles

Smooth muscle: Long, thin cells found in the intestines and other organs. Skeletal muscle: Long, cylindrical fibers with stripes (striated). Cardiac muscle: Heart muscle consisting of fibers that fuse together at various points and contract together, not independently.

Muscle fiber innervation

Each muscle fiber receives innervation from one axon. Movements are more precise when each axon innervates only a few muscle fibers (eye muscles) compared to many fibers (biceps).

Neuromuscular junction

The connection between a motor neuron and a muscle fiber where acetylcholine (ACh) is released to stimulate muscle contraction.

Acetylcholine (ACh)

A neurotransmitter released at the neuromuscular junction that activates muscle contractions.

Myasthenia gravis

An autoimmune disorder affecting acetylcholine receptors at the neuromuscular junction, causing muscle weakness.

Myopathic movement disorders

Disorders involving problems with the muscles themselves, such as muscular dystrophy.

Flexor and extensor muscles

Muscles that work in antagonistic pairs. Flexors bend a joint (biceps), while extensors straighten a joint (triceps).

Antagonistic muscles

Pairs of muscles that produce opposite movements. Example: The biceps is a flexor, while the triceps is an extensor.

Proprioceptors

Sensory receptors that provide information about the position, movement, and tension of muscles. Two types are muscle spindles and Golgi tendon organs.

Muscle spindles

When a muscle is stretched, nerves from the muscle spindles transmit impulses that lead to contraction of the muscle.

Golgi tendon organs

Receptors activated by muscle tension during contraction. They act as a brake or shock absorber to prevent contractions that are too forceful or extreme.

Primary motor cortex (M1)

Responsible for executing movements by sending direct muscle commands. Controls specific areas of the opposite side of the body (contralateral).

When is the primary motor cortex active?

When people intend to make a movement. It sends commands to produce the intended movement.

Contralateral control

The primary motor cortex controls movements on the opposite side of the body.

Primary somatosensory cortex and primary motor cortex

The motor area responsible for moving a body part is aligned with the somatosensory area responsible for feeling that body part. Communication between sensing and moving is essential.

Motor homunculus

A somatotopically organized representation of body parts in the primary motor cortex, with different areas controlling different body parts.

Posterior parietal cortex

Keeps track of the body's position relative to the world and transforms visual information into motor commands to guide movements.

What happens when the posterior parietal cortex is damaged?

Difficulty coordinating visual stimuli with movements, such as reaching toward an object.

Posterior parietal cortex and movement planning

Important for planning movements. Active during the delay between detecting a signal to move and receiving the second signal indicating how or where to move.

Premotor cortex

Responsible for preparing movements using external sensory cues. Receives information about a target and organizes movement direction in space.

Premotor cortex and body position

Integrates information about body position and posture and controls proximal muscles and trunk muscles. Helps orient the body before reaching for an object.

Supplementary motor area (SMA)

Responsible for sequencing movements based on internal intent or memory. Organizes rapid movements in a specific order and can inhibit movements when necessary.

Supplementary motor area and complex movements

Helps plan complex movements and coordinate two-handed movements. Becomes active seconds before movement.

Prefrontal cortex

Responsible for decision-making and goal-oriented planning. Calculates predictable outcomes of actions and plans movements accordingly.

Prefrontal cortex and sensory signals

Responds to lights, noises, and other sensory signals that lead to movement.

What happens when the prefrontal cortex is damaged?

Movements may become illogical or disorganized because the person has difficulty considering the probable outcomes of actions. Example: Difficulty choosing appropriate clothes while showering.

Corticospinal tracts

The main pathway for voluntary movement in humans, carrying motor commands from the cerebral cortex to the spinal cord.

Lateral corticospinal tract

Crosses from one side of the brain to the opposite side of the spinal cord. Controls precise, discrete movements of the extremities, including the hands, fingers, and feet.

Medial corticospinal tract

Controls trunk muscles for postural adjustments and bilateral movements, such as standing, bending, turning, and walking.

Difference between lateral and medial corticospinal tracts

Lateral: Precise movements of the extremities. Medial: Trunk muscles, posture, and bilateral movements.

Basal ganglia

Subcortical structures that enable practiced motor programs and help with learning new motor skills by organizing motor programs for complex movements.

Caudate nucleus and putamen (striatum)

Receive input from the cerebral cortex and send output to other basal ganglia structures, including the globus pallidus.

Globus pallidus

Sends inhibitory signals to the thalamus, which relays information to motor areas and the prefrontal cortex.

Thalamus in motor control

Relays information from the basal ganglia to the motor cortex, helping regulate movement.

How do the basal ganglia select movements?

By ceasing to inhibit a movement, allowing it to occur rather than requiring an external stimulus to guide the action.

Basal ganglia and learned movements

Help organize and execute practiced motor programs and contribute to learning new motor skills.

Parkinson's disease

A movement disorder characterized by muscle tremors, rigidity, akinesia, slow movements, and postural instability.

TRAP symptoms of Parkinson's disease

T: Tremor. R: Rigidity. A: Akinesia/bradykinesia (difficulty initiating movement or slow movement). P: Postural instability.

Akinesia

Difficulty initiating voluntary movements, particularly spontaneous movements without external stimuli to guide the action.

Bradykinesia

Abnormally slow movements, commonly seen in Parkinson's disease.

Cognitive and nonmotor symptoms of Parkinson's disease

Depression, memory and reasoning deficits, loss of olfaction (smell), and other cognitive difficulties.

Immediate cause of Parkinson's disease

Gradual and progressive death of neurons, especially dopamine-producing neurons in the substantia nigra pars compacta.

Substantia nigra pars compacta

A brain region that sends dopamine-releasing axons to the caudate nucleus and putamen. Degeneration of these neurons contributes to Parkinson's disease.

Dopamine and Parkinson's disease

Loss of dopamine disrupts basal ganglia activity, reducing stimulation of the motor cortex and slowing the initiation of movements.

Excitatory vs. inhibitory pathways

Excitatory pathways: Increase activity in target neurons. Inhibitory pathways: Decrease activity in target neurons. In the lecture diagram, excitatory pathways are green and inhibitory pathways are red.

What happens when excitation from the substantia nigra decreases?

Decreased excitation from the substantia nigra reduces inhibition from the striatum, leading to increased inhibition from the globus pallidus.

How does Parkinson's disease affect the thalamus?

Increased inhibition from the globus pallidus reduces excitatory output from the thalamus to the motor cortex.

Why does Parkinson's disease cause slower movements?

Dopamine loss leads to increased inhibition of the thalamus, resulting in less stimulation of the motor cortex and difficulty initiating movements.

Parkinson's disease and voluntary movement

The conscious decision to move and the actual movement occur at different times because initiating movement becomes difficult.

Primary motor cortex (M1)

Primary role: Execution. Activation trigger: Direct muscle command.

Premotor cortex

Primary role: Preparation. Activation trigger: External sensory cues.

Supplementary motor area (SMA)

Primary role: Sequencing. Activation trigger: Internal intent or memory.

Posterior parietal cortex

Primary role: Guidance. Activation trigger: Spatial orientation.

Prefrontal cortex

Primary role: Decision. Activation trigger: Goal-oriented planning.