2Many ways to study motor systems Behavioral perspective - holistic approach to studying motor activity. Investigates the behavioral pattern of an animal.At the turn of the last century with the influence of Darwinian philosophy, naturalists were interested in studying behavior patterns of many species in the wild. The beginnings of Ethology.Why are behaviors elicited? Is there an evolutionary advantage to the modal action pattern (formally referred to as ‘Fixed Action Pattern’)-Species specific behaviors related to specific environmental situations.
3Behavioral perspective (cont.) 19th Century was very big in Reflexology..the study of reflexes.Reflex - a highly stereotyped, unlearned response to a particular stimulus.
4DR. CHARLES SCOTT SHERRINGTON (1852-1957) Nobel Prize in Medicine (1932)..for his discoveries regarding the functions of neurons.Sherrington was born in London, studied in England and gravitated into physiology where he was strongly influenced by Michael Foster - "father of British Physiology”. His interest was in reflexes and he is best known for his lengthy studies in spinal reflexes. Also he will be remembered for his contributions to the physiology of perception, reaction and behavior.
5Sherrington understood that the brain made all necessary computations, but ultimately it was the reflexes that make up the spinal cord system that will be called upon to activate target muscles. He believed no matter how complex the motoric behavior, it was really nothing more than a series of unit reflexes chained together. CNS modulates the series.SRSRSRSRNote: Sherrington was Ivan Sechenov’s contemporary.
6Ivan Sechenov ( )Ivan Petrovich Pavlov ( )Nobel Prize (1904)Sechenov, born in Russia, was known as the Father of Russian physiology. He introduced electrophysiology into laboratories and was also a notable teacher. His life work concentrated on neurophysiology. He wrote a major classic treatise "The Reflexes of the Brain."
7Salivation is supposed to be an unlearned reflex governed by preprogrammed stimuli like food, not something that can be learned to a novel stimulus!
8Ways to study motor systems Behavioral perspective - holistic approach to studying motor activity. Investigates the overt behavioral patterns of an animal.Control systems perspective - an engineering view of systems (systems analysis).commandMotor SystemdetectorController (CNS)inputoutputErrorTrans-ducerClosed-Loop System
9Ways to study motor systems Behavioral perspective - holistic approach to studying motor activity. Investigates the behavioral pattern of an animal.Control systems perspective - an engineering view of systems (systems analysis).outputController (CNS)commandInputMotor SystemOpen-Loop System - faster. Not relying on feedback signalsLearned vs. innate behavior. Closed-loop while learning, then with experience develops into an open-loop efficient expression.
10Ways to study motor systems Behavioral perspective - holistic approach to studying motor activity. Investigates the behavioral pattern of an animal.Control systems perspective - an engineering view of systems (systems analysis).Neurobiological view - interested in investigating the mechanical properties (i.e., the substrate) of motoric behavior.*
11The motor systems of the brain, spinal cord and peripheral nerves provide the means for us to move and thereby act upon our ever-changing environment.Afferent systems - transduce physical energies into neurological signals.Efferent systems - concerned with reverse process.. The transduction of neural signals into mechanical energy (or force).
14Descending tracts: Reticulospinal: Automatic Antigravity & gravity reflexes.Uses sensory information about balance, body position & the visual environment to reflexively maintain posture and balance with movement (proximal / axial muscles).
16Somatic (striate) muscle Fibers control flexors-striate muscles that pull limbs around a joint together (closing the knife).proximalFibers control extensors-striate muscles that pull limbs around a joint apart (opening the knife).
20Cholinergic neuromuscular endplate. Amount of pool divergence or convergence defines motor resolution.More macroscopic..
21Older, more radical treatments: plasmaphoresis; remove thymus membrane immersed in liquid Nitrogen fractures the weak bonds along the hydrophobic layersFreeze fracture of aNeuromuscular JunctionMyasthenia Gravis (grk. “severe muscle weakness”) - body produces antibodies that destroy nicotinic, cholinergic receptors. One remedy: Inhibit ACHE to prolong the presence of ACH in the cleft.Older, more radical treatments: plasmaphoresis; remove thymus
22Multinucleated muscle fiber cell (cytoskeleton is primarily myofibrils). - channels that allow connection to extracellular space. Shaped like a “T”. Contain extracell. fluid.I- high Ca++ like ER plasma in neurons.A- Openings allow membrane voltage change to travel into “deeper” intracellular myofibrils.- analogous to axolemma.
28Transduction process: How does the neural signal translate into contraction?
29The release of Ca++ from the sarcoplasmic reticulum The release of Ca++ from the sarcoplasmic reticulum. Depolarization of the T-tubule membrane causes conformational changes in proteins that are linked to calcium channels in the sarcoplasmic reticulum, releasing stored Ca++ into the cytosol of the muscle fiber.
30The molecular basis of muscle contraction The molecular basis of muscle contraction. The binding of Ca++ to troponin allows the myosin heads to bind to the actin filament. Then the myosin heads rotate, causing the filaments to slide with respect to one another.
31Excitation:AP arrives at a-motor neuron endplate.ACh release into cleft.ACh binds to nicotinic ACH receptors: EPSP.Na+ channels open (inc. gNa+) at sarcolemma.T-tubules continue to convey the membrane depolarization toward the myofibril membranes.Depolarization of the T-tubules causes conformational change of Ca++ SR channels yielding cytosolic Ca++ release.Contraction:Ca++ binds to troponin protein.Myosin binding sites exposed.Myosin heads bind to actin & myosin heads rotate (ratchet).Myosin heads disengage (ATP required)Cycle continues until ATP and Ca++ levels are depleted.
32Relaxation:Ca++ is sequestered by the SR by an ATP-driven pump.Myosin binding sites on actin are once again covered by troponin.A lot of ATP required to run muscles. Here and during contraction, not to mention the usual ATP demands associated with synaptic transmission. Muscles are very metabolically demanding.
33Proprioception is an integral part of kinesthesis-the perception or knowing of the position of the body as muscles, tendons and joints move, in addition to the visual, auditory and vestibular systems responsible for our perception of body position. Proprioception is usually a reference to unconscious perception of body position.
34Proprioception - Motor sensory system designed to signal changes in muscle length. Two main types of proprioceptors:Muscle Spindles - found within the muscle, parallel to the muscle fibers (stretch or length).Golgi tendon apparatus - found in the tendons, in series to the muscle fibers (contraction-’pulls’ at the tendons, tension).
40Resets the spindle stretch receptors so that 1a can pick up subsequent stretch responses. a motor neuron -- myotatic correction and subsequent g-induced reset.start with a passive stretch
41Muscle Spindles - found within the muscle, parallel to the muscle fibers (stretch or length). Golgi tendon apparatus - found in the tendons, in series to the muscle fibers (contraction-’pulls’ at the tendons, tension).
44Knee Jerk reflex is also monosynaptic (intrasegmental). Tap “pulls” on the tendon.This ‘interprets’ as a stretch of the quadricepts which in turn activates the muscle spindles (& thus 1a sensory neuron).Neural signals carry through dorsal root ganglion and excites synapsing a motor neuron.This results in contraction of extrafusal fibers of the quadriceps (extension)....& corresponding inhibition of antagonistic hamstrings.Patellar
50Descending tracts: Voluntary movements Pyramidal tracts A more primitive tract originating from the red nuclei. Function subsumed by corticospinal projections.Pyramidal tracts - Higher end motor activity. Decussate at the enlarged ventral region of the medulla (cross-section looks triangular; thus the name “pyramidal”).
51“rt. hand”M1, Area #4..in the tegmentumTerminates in the dorsolateral portion of the ventral horn.
52Rubrospinal tract is an alternative route for the mediation of voluntary movement. It is responsible for LARGE muscle movement such as the arms and the legs as well as for fine motor control in non-primate species. It facilitates the flexion and inhibits the extension in the upper extremities. It is small and rudimentary in humans.NOTE: In some other primates, however, experiments have shown that over time, the rubrospinal tract can assume almost all the duties of the corticospinal when the corticospinal tract is lesioned.
53Descending tracts:Originates from colliculus..Coordination of eye and head position(e.g., vestibular ocular reflex [VOR] used for perceived image stabilization.)Originates from vestibular nuclei, near the cochlear nuclei (part of VIII). Movement of the head activates vestibular system. Reflex circuitry controls neck and back muscles to ensure head stability (proprioceptive control as our body moves).Balance and body posture. Makes use of spinal reflexes, and sensory information from proprioceptors, visual system & vestibular system.
64Premotor area - involves reticulospinal tracts (proximal) muscle areas. Supplementary motor area - involves distal muscle areas.
65Psychomotor CortexA lot of this through subcortical basal ganglia (i.e., lenticular to thalamus to #6)
66From “What” the actions should be to “How” they will be carried out.. “Get Ready”Posterior Parietal (#5,7)Prefrontal Cortex (#8)“Set”PMA & SMA (#6) - lesions here produce apraxia = lack of complex motor skills.“Go”M1 (#4) - due to release of subcortical inhibition on SMA (#6).Spinal Cord = sends signals to motor pools.
67Edward Evarts (NIH)“Behavioral Neurophysiology”Motoric activity in awake animals and measuring single cell activity--reminiscent of vision science research with RFs decades earlier.
68Basal Ganglia Motor Loop: Part of the extrapyramidal system and key to setting up motor commands for execution through M1 to spinal cord..
75Basal gangliaVLoDisinhibition- globus pallidus is “turned off”, allowing VLo to activate SMA (distal motor pools).
76Cortical cells excite putamen neurons.. 2. Activated putamen cells are inhibitory. They inhibit globus pallidus (GPe) cells.5. Result: Excitation of SMA (a release from active supression).3. GPe cells are inhibitory. They inhibit VL cells..4. but when they are inhibited by putamen cells, they in turn “turn off” their inhibition on VLo cells (disinhibition).
77Basal Ganglia facilitates or inhibits the initiation of volitional movement.. 1. At rest, Globus pallidus always inhibits VLo.2. Striatum inhibits globus pallidus.3. This, in turn, activates VLo (disinhibition).4. This activates SMA..5. which in turn activates M1..
78An imbalance of the motor loop can lead to dysfunction: Hypokinesia: Paucity of movement.e.g., Parkinson’s Disease (s. nigra defect) No striatal excitation. (bradykinesia, akinesia)Hyperkinesia: Excess of movement.e.g., Huntington’s Disease (subthalamic defect) No controlled inhibition of VLo. (chorea, dementia).No disinhibition
79Corticospinal fibers are primarily emerging from layer V and VI of the cortex (M1 is agranular). Largest descending cells are Betz cells in the corticospinal tract. Majority are the pyramidal cells.
80A comparison being made between “What was intended” to “What has happened?” An important loop for motor skills learning.The VLc loop
81Corticospinal cells in M1 code for movement “force” and direction (Georgopolus study suggests a motoric receptive field?)..
82“Motor cell tuning”. This particular cell really “likes” leftward arm movement (180°):
83Population vectors. Combining gaussian- distributed response properties for multiples of M1 cells can lead to many different directional characteristics. (i.e., the ratio of M1-cell broadband outputs can provide a large gamut of directional movements.ANALOGY: Color vision-the ratio of 3 cone outputs in the retina can provide a large gamut of perceived colors.45°