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Biology for Engineers: Cellular and Systems Neurophysiology Christopher Fiorillo BiS 521, Fall 2009 042 350 4326, [email protected] Part 13: The Motor System Reading: Bear, Connors, and Paradiso Chapters 13 and 14 Anatomical Terms •These terms are explained in chapter 7. Memorize them. •Rostral-Caudal (anterior-posterior) •Dorsal-Ventral •Medial-Lateral •Coronal plane •Horizontal plane •Sagittal plane Cortical Anatomy •The frontal cortex (lobe) is greatly expanded in humans relative to other animals. •Memorize the relative locations of all of these structures. I will ask questions about this on the exam. These questions will be very easy if you can simply memorize. •You should also know the location of the cerebellum. It is very important for motor function (and very easy to find). Cortical Anatomy •Primary motor cortex (area 4) is just rostral (anterior) of the central sulcus •It receives many inputs from primary somatosensory cortex and from prefrontal cortex •Somatosensory information is highly relevant to motor output, even without much processing •Prefrontal cortex contains highly processed information from all sensory modalities, and its information is highly relevant to motor output •Thus it may be efficient to have motor cortex located between these two areas, and further from primary auditory and visual cortices, in order to keep “wires” (axons) short Basal Ganglia •The basal ganglia are an interconnected set of motor-related subcortical nuclei •Basal ganglia receive input from the cortex •They send their output to the thalamus, which then sends the information back to the cortex •The basal ganglia receive a lot of dopamine from substantia nigra in the midbrain •Loss of dopamine in the basal ganglia causes Parkinson’s disease •Dopamine in the basal ganglia is probably important for learning “action values” •The striatum is the largest nucleus within the basal ganglia The Spinal Cord •The spinal cord is part of the Central Nervous System •It contains both somatosensory and motor neurons, and many interneurons Anatomy of the Spinal Cord • The Dorsal Root Ganglion contains primary somatosensory neurons •The Dorsal Horn is somatosensory •The Ventral Horn is motor, and contains motor neurons •The Ventral Root contains the axons of motor neurons Inputs to Motor Neurons • Excitatory and inhibitory interneurons of the spinal cord • Primary proprioceptive neurons (excitatory) – Cell bodies are in dorsal root ganglion • Descending inputs (excitatory) Inputs to Motor Neurons • Descending inputs (excitatory) to motor neurons – Corticospinal • – – – – From layer 5 of motor cortex Rubrospinal Tectospinal Vestibulospinal Reticulospinal Motor Neuron Output to Muscle The Alpha Motor Neurons These neurons directly control body movements The Gamma motor neurons assist in proprioception One muscle fiber gets input from one motor neuron One motor neuron may innervate multiple muscle fibers. One motor neuron plus its muscle fibers is called a “motor unit.” This is the smallest unit of motor output All the motor neurons that innervate the same muscle are called a “motor neuron pool” Pathways from Sensory Input to Motor Output A single motor neuron gets inputs, indirectly, from a very large part of the nervous system There are long paths and short paths that connect sensory inputs to motor outputs A long path is needed when sensory inputs, such as light intensity, carry very little information about future reward A motor neuron must “decide” what firing rate (and contraction of muscle) will maximize reward. What unprocessed sensory information should be most relevant in “deciding” upon a motor neuron’s optimal output (and thus have a short path)? The answer: Proprioceptive information indicating the position of the relevant limb, and muscle tension Vestibular information is also important, indicating head position, velocity, and acceleration Sudden pain in the same region of the body Each of these types of sensory information takes a short path to the motor neuron The Myotatic (Stretch) Reflex A 1a proprioceptive neuron is activated by stretch of a muscle Stretch of a muscle provides information about the position of the limb that the muscle controls The 1a axon directly excites motor neurons that innervates the same muscle This is the shortest sensory-motor path in the central nervous system (at least for control of muscles, maybe not for hormone outputs from the hypothalamus) One effect of this would be to stabilize the limb position when the external force on the limb changes. This would usually be beneficial. Inhibition tends to occur at the same time as excitation in motor neurons • The data below are from motor neurons that are mediating repetitive scratching behavior in a turtle • It seemed strange to most people that inhibition and excitation should occur at the same time • That is why this was published in Science • This observation fits well with the theory • • Inhibitory inputs predict the excitatory inputs This could be accounted for by anti-Hebbian plasticity of inhibitory inputs • The same phenomena has been observed in other types of neurons, including retina, visual cortex, and auditory cortex Berg et al., Science, 2007 Motor Patterns • Not all reward information needs to come from the senses • Patterns could be internally generated, and then tested to see if they predict reward • Evidence that patterns (information) may be internally generated – Motor systems generate patterns (and coordinated behaviors) prior to development of sensory systems – Central Pattern Generators Central Pattern Generators A “CPG” generates a pattern even without receiving any patterned input Thus a pattern can be generated in the complete absence of sensory inputs, or when sensory input is constant For example, a simple pattern could be a sinusoidal oscillation If the pattern is very regular, like a heartbeat, then it is called a “pacemaker” If there are several action potential at high frequency, followed by a pause, that is called “bursting” Simple CPG patterns are useful for repetitive events. The are involved in: Walking and swimming Respiration (breathing) Gastrointestinal movements CPGs have been studied extensively in invertebrates, such as the stomatogastric ganglion of the lobster shown at right Marder and Bucher, 2007 Cellular Mechanism of a CPG As described in the textbook, this example is from a type of neuron that is important for swimming in the lamprey Glutamate is necessary to generate this oscillatory pattern However, glutamate concentration can remain constant, with no pattern Other types of neurons oscillate with no external (synaptic) input at all In the constant presence of glutamate, NMDA receptors function as depolarization-activated cation channels. They depolarize the membrane, eliciting multiple action potentials. Calcium concentration increases, via NMDA receptors and other calcium channels that open during each action potential. Calcium then activates calcium-gated K+ channels, which hyperpolarize the membrane. Once it is hyperpolarized, NMDA receptors and other calcium channels close, calcium concentration decreases, K+ channels close, and the membrane depolarizes again to start a new cycle. Bistability in Striatal Neurons • The membrane voltage is stable at an “upstate” and a “downstate,” but not inbetween • A large EPSP will cause the neuron to enter the up-state. The up-state will last for a few hundred ms after the EPSC has finished