**Why Is the Motor System Important** The fact that we have a brain has to do with the fact that we move (counterexample, some animal in the ocean is capable of digesting its own brain as soon as he finds a stable spot to live). - All observable behavior is directly related to activity in the motor system. - Without the motor system we could experience sensation, think, reason, problem solve, read, write and do mental math, but we would not be able to communicate our thoughts and abilities to anyone. **Overview of Motor System** - Spinal Reflexes - Corticospinal and Corticobulbar Tracts - Cortical-subcortical-thalamo-cortical systems - Involving Basal Ganglia - Involving Pons and Cerebellum ### Muscles **Skeletal Muscles (vs. Smooth Muscles)** ![[ETH/ETH - Systems Neuroscience/Images - ETH Systems Neuroscience/image84.png]] Striated vs Smooth because the smooth muscles are the ones that control our interal organs. - Striated (striped) appearance because they are comprised of muscle fibers. - Move through a pull action (contraction) - Work in pairs with a reciprocal muscles (bicep contracts & triceps relaxes) - Stimulated by a Motor Neuron (neurons that bring the electrical pulses to muscles). **Anatomy of the Muscle** Striated muscles are made of muscle fibers that have two parts, **Outer** and **Inner**: The following image represents only one muscle fiber - a muscle has many fibers ![[ETH/ETH - Systems Neuroscience/Images - ETH Systems Neuroscience/image85.png]] Wrapped around the intrafusal fiber, there is a sensory nerve that picks up the sensation of stretch. Good part of the innervation in the muscles senses the stretch of the muscles rather than controlling the muscle movements. Each muscle fiber has a **Gamma Motor Neuron** that synapses on the intrafusal fiber. The **Alpha Motor Neuron** synapses on the extrafusal fibers. One alpha motor neuron can stimulate numerous fibers. This is called **Motor Unit**. The neural link between the alpha motor neuron and the muscle fiber is called the **Neuromuscular Junction**. **The Muscle -- Spindle Feedback Circuit** This loop is the basis of reflexes. ![[ETH/ETH - Systems Neuroscience/Images - ETH Systems Neuroscience/image86.png]] The ratio between the alpha motor neuron and the number of muscle fibers it innervates is associated with the degree of dexterity needed in the movement: - **High Ratio** (1:150) = contraction of large muscles - **Low Ratio** (1:10) = contraction of small muscles needed for fine movements (Finger, Lips) Motor Homunculus is related to the number of alpha motor neurons needed to innervate muscles of various regions of our body. **Comparing the Anatomy of the CNS with the Anatomy of the Neuromuscular Junction** ![[ETH/ETH - Systems Neuroscience/Images - ETH Systems Neuroscience/image87.png]] ### Limb Position **How is Limb Position Maintained?** - **Involuntary Movement** (i.e., posture): continual contraction and relaxation of the muscles in our feet and calves. - **Voluntary Movement**: stretch of the intrafusal fiber causes contraction of the extrafusal fiber via alpha motor neuron. Keeping the movement at this position requires a direct signal from the brain. **Automatic Maintenance of Limb Position** | ![[ETH/ETH - Systems Neuroscience/Images - ETH Systems Neuroscience/image89.png]] | ![[ETH/ETH - Systems Neuroscience/Images - ETH Systems Neuroscience/image90.png]] | |---|---| How is the position maintained with an external force? The intrafusal motor neuron (Gamma) signals the change, which is accounted by the Extrafusal Motor Neuron (Alpha) by increased firing. ### Gamma Motor Neurons **Intrafusal Motor Neurons** ![[ETH/ETH - Systems Neuroscience/Images - ETH Systems Neuroscience/image88.png]] The Gamma Motor Neuron makes sure that the intrafusal muscles show always a state of tension to exert a reaction when needed (e.g., the previous cat). **Remember that muscles work in pairs,** so if one contracts the other relaxes. This is referred to as **Reciprocal Innervation**. What if both muscles contracted at the same time? **Reciprocal Innervation of Antagonistic Muscles & The Elicitation of a Stretch Reflex** | ![[ETH/ETH - Systems Neuroscience/Images - ETH Systems Neuroscience/image91.png]] | ![[ETH/ETH - Systems Neuroscience/Images - ETH Systems Neuroscience/image92.png]] | |---|---| Neurologist often measure this reflex because the presence of such reflex indicates a functioning spinal cord. ### Alpha Motor Neurons Alpha Motor Neuron is the **Final Common Path** for all movements. Movement can be generated from: - Sensory signals in the muscle spindle like the stretch reflex. - Sensory signals from skin as in the pain withdrawal response. - Involuntary signals from the brainstem for posture, keeping us upright without conscious attention. - Signals from the brain for voluntary movement. But, regardless of where the signal originates, all movement is the result of activity in the Alpha Motor Neuron - making this the **Final Common Path**. ### The Two Divisions of the Dorsolateral Motor Pathway | ![[ETH/ETH - Systems Neuroscience/Images - ETH Systems Neuroscience/image93.png]] | ![[ETH/ETH - Systems Neuroscience/Images - ETH Systems Neuroscience/image94.png]] | |---|---| **Corticospinal Tract** - Origins: Primary Motor Cortex (MI), Premotor Cortex, Supplemental Motor Cortex, Anterior Paracentral Gyrus, Parietal Lobe (including SI) and Cingulate Gyrus. - Collaterals: Small percentage of Corticospinal Neurons 1. Midbrain (primarily red nucleus) 2. Trigeminal Nuclei 3. Pontine Nuclei - Termination is Spinal Cord: mostly laminae 3-7, few in ventral horn and laminae 1-2; mostly innervating interneurons, although some innervation of alpha motor neurons. - Neurotransmitter: Glutamate and/or Aspartate. **Corticobulbar Tracts** - Control over facial muscles; bilateral input to motor neurons controlling muscles in upper face, but contralateral input to motor neurons controlling lower face (in humans, not sure about rodents). - Control over muscles of mastication: motor trigeminal and RF. (Reticular Formation) - Control over external eye muscles: input comes from frontal and parietal eye fields, rather than from MI; projection to midbrain and paramedian pontine RF - Control over tongue: hypoglossal and RF - Control over swallowing reflexes: nucleus ambiguous and RF. ### Motor & PreMotor Cortex **Voluntary Movement: Instructions from Cerebral Cortex** - Dorsolateral Prefrontal Cortex: directs movement of our limbs (as in reaching) and movements of our fingers. - Actual signal for movement must go through pre-motor cortex, then motor cortex. - From motor cortex, signal travels down spinal cord eventually reaching the alpha motor neuron. - But, the instructions for this movement ultimately comes from our Parietal lobe, which receives sensory input. ![[ETH/ETH - Systems Neuroscience/Images - ETH Systems Neuroscience/image95.png]] It is important to note in the picture that the **Posterior Parietal Association Cortex** is the location where all these sensory information are integrated, as well as the point of origin of the decision to perform a particular movement. | ![[ETH/ETH - Systems Neuroscience/Images - ETH Systems Neuroscience/image96.png]] | ![[ETH/ETH - Systems Neuroscience/Images - ETH Systems Neuroscience/image97.png]] | |---|---| In the **Premotor Cortex** the sequence of muscles to be activated is planned. Then, in the **Primary Motor Cortex** all the muscles needed to perform a particular movement are "read" and the exact contractions and relaxation information are calculated. ![[ETH/ETH - Systems Neuroscience/Images - ETH Systems Neuroscience/image98.png]] Where is the best place to implant electrodes to read impulses and try to decode the intentions of an individual, which can be used to actuate prosthesis? There are **two school**: - **Primary Motor Cortex** to read the output signals of the brain, but that's already very difficult as the inputs are already decomposed and spread around the area. - **Parietal Cortex** in order to read the high-level commands of the movements that the individual would like to perform. **Control of Movement by Motor Cortex** - **Micro-stimulation Studies**: in MI movements of particular contralateral joints (e.g., distal finger) can be elicited by micro-stimulation; in MII contractions of groups of muscles sequentially to produce overall movements of limbs, often bilaterally. - **Electrical Activity During Movement**: corticospinal neurons active just before initiation of a movement; activity related to amount of force necessary to produce movement; directionally-sensitive corticospinal neurons; higher-order motor cortex involved in calculating trajectories in space (probably in close communication with cerebellum) and in planning larger-scale movements (probably in close communication with the basal ganglia). - **Imaging Studies in Humans**: random movements of digits activates MI (precentral gyrus); planned movements activate MI and supplemental motor cortex; thinking about planned movements activates supplemental motor cortex, but not MI. There has been a lot of work done about thinking of movements, which leads to an activation of this area that are usually involved in planning. It involves mirror neurons, which is a population of neurons shown to be active in humans/animals when they observe someone else performing a particular movement, which are the same neurons that activate when someone plans the same movement. Hypothesis consider them involved in the process of learning by imitation and they are considered to be very important in the process of learning language. Activation of different cortical areas involved in performing a sequence of actions that has been well practiced. Compared to performing a sequence of actions that has not been extensively practiced | ![[ETH/ETH - Systems Neuroscience/Images - ETH Systems Neuroscience/image99.png]] | ![[ETH/ETH - Systems Neuroscience/Images - ETH Systems Neuroscience/image100.png]] | |---|---| **Of course, this is really too simple...** Other brain areas are involved in movement: - **Ventromedial Frontal Cortex** - involved in body control, posture and whole body movements. - **Cerebellum** - **Basal Ganglia** - **Brainstem** - **Frontal Eye Fields** are involved in control of saccadic movements, and they can be found in the frontal part of the brain In the end, all movement funnels through the alpha motor neuron (final common path). **Motor Hierarchy and Loops** ![[ETH/ETH - Systems Neuroscience/Images - ETH Systems Neuroscience/image101.png]] **Disorders of the Motor System** - **Amyotrophic Lateral Sclerosis** - motor neurons of the brainstem & spinal cord are destroyed. - **Huntington's Disease** - progressive destruction of the basal ganglia (GABA). - **Muscular Dystrophy** - biochemical abnormality affecting the utilization of Ca++ causing wasting away of muscles. - **Myasthenia Gravis** - autoimmune disorder that destroys Ach receptors (starts with head as in drooping eyelids then progresses to swallowing & respiration). - **Parkinson's Disease** - degeneration of neurons in the striatum due to loss of cells in the substantia nigra that synthesis/release dopamine.