Movement, Muscles and the Brain

• A significant portion of the brain is dedicated to planning and executing movements, not just higher cognitive functions.
• Moving requires precise control of muscles, like mouth and throat muscles for singing or coordinating a ball throw.
• This precise control demands significant computational power.

Types of Muscles

• Skeletal muscles facilitate body and limb movement.
• Smooth muscles control contractions in internal organs, aiding processes like digestion.
• Cardiac muscles drive heartbeats.
• Despite variations in appearance, all muscle types operate similarly.

The Neuromuscular Junction

• Motor neurons connect with muscle fibers at the neuromuscular junction (NMJ). These signal muscles to contract.
• The NMJ is a specialized synapse where motor neurons release acetylcholine (ACh) to trigger muscle contraction.
• Innervation refers to neurons forming synapses with other neurons or muscle fibers.
• Precision in muscle movement is determined by the degree of innervation a muscle receives.
• Muscles requiring fine motor control, such as those in the eye, receive more innervation than larger muscles like the biceps

Spinal Reflexes

• The spinal cord, roughly the width of a finger, transmits information to and from the brain and controls rapid spinal reflexes.
• Like the brain, the spinal cord consists of white and gray matter.
• White matter contains ascending axons carrying sensory information to the brain and descending axons carrying commands to muscles and organs.
• Gray matter, primarily composed of cell bodies, is located in the interior of the spinal cord.
• Reflexes are behaviors controlled through direct sensory-motor neuron connections, bypassing conscious thought.
• Reflexes occur more rapidly than voluntary actions.

Reflex Definition

• Most motor neurons receive input from the brain; however, sensory neurons can connect directly or indirectly with motor neurons through interneurons.
• Interneurons connect two neurons, and are particularly active in reflexes.
• Reflexes are direct or indirect pathways between sensory and motor neurons.
• Reflexes allow for swift adjustments without the delay of conscious processing.

Testing Reflexes

• Tapping the patellar tendon stretches the quadriceps muscle, which is detected by muscle stretch receptors that relay information to the spinal cord.
• In the spinal cord, sensory neurons synapse directly onto motor neurons, causing the quads to contract and extend the lower leg.
• This circuit makes the reflex seem automatic, and this stretch reflex enables muscles to respond quickly to maintain posture.

Brains Plan and Orchestrate Movement

• The motor cortex has a hierarchical organization, starting from the forebrain down through the brain stem and spinal cord.
• The cortex plans, organizes, and executes movements, while modulating activity in the brain stem and spinal cord.

Motor Cortex

• The primary motor cortex has two secondary areas: the supplementary motor area and the premotor cortex.
• These areas contain a somatotopic map, with greater cortical area devoted to body parts requiring finer movements.

Relaying Information About Movement

• Planning begins in the association areas, relays to the primary motor cortex, and then descends to the brain stem.
• Intended movement is modified by inputs from the somatosensory cortex, posterior parietal cortex, basal ganglia, and cerebellum.
• The prefrontal cortex has an executive role, which is well-suited for planning movement.
• The prefrontal cortex plans actions based on their consequences and integrates auditory and visual information with body position.
• It holds information in memory while deciding on the next action, so it plans FOR movements.
• The premotor cortex programs movements by combining prefrontal and posterior parietal cortex information and sending it to the supplementary motor area.
• It assembles movement sequences and coordinates both sides of the body.
• The primary motor cortex organizes and executes voluntary movements, adding information about force and direction.
• It is activated DURING a movement, not prior.
• The basal ganglia modulate cortical and brain stem motor systems, smoothing movement using information from various cortical areas.
• It is active during complex movement sequences and is important for learning habits.
• The cerebellum receives motor cortex information about intended movement.
• It determines muscle contraction order and timing, maintaining posture and balance, refining movements, and controlling compensatory eye movements.
• The supplementary motor area receives information from the prefrontal cortex, assembling movement sequences and coordinating both sides of the body.
• The primary somatosensory cortex receives information from sensory neurons about the body position in space.
• It connects with motor areas and the posterior parietal cortex, informing motor areas about the body’s position.
• The posterior parietal cortex integrates senses of touch, vision, and sound, integrating body position with limb location and object identity.
• It integrates the body with the world, sending this information to motor areas.

Parkinson's Disease

• This is a movement disorder characterized by motor tremors, rigidity, loss of balance and coordination, and difficulty in initiating movements.
• It is caused by the deterioration of dopamine neurons in the substantia nigra.
• Symptoms stem from the degeneration of dopamine neurons in the substantia nigra, which project to the striatum (caudate and putamen).
• A loss of dopamine neurons projecting to these areas results in Parkinson's disease.
• L-dopa is a precursor for dopamine.
• Dopamine itself cannot cross the blood-brain barrier, whereas L-dopa can.
• Deep brain stimulation involves implanting an electrode to stimulate dopamine neurons in the substantia nigra.

Multiple Sclerosis (MS)

• MS is a motor disorder caused by the deterioration of myelin and neuron loss in the central nervous system.
• Early symptoms include impaired tendon reflexes and inability to sense vibrations; later symptoms include muscle weakness, tremor, and impaired coordination.
• MS is an autoimmune disorder where the body attacks myelin.
• Damaged myelin leads to inflamed neurons unable to synchronize neuronal impulses.
• Sclerosis: Areas of hardened scar tissue within the central nervous system (CNS) as neurons die.
• Treatments: Modifying the immune system to reduce inflammation: Drugs may slow the progress of the disease, but they do not repair the damage that has already been done (no cure)