7: The Motor System

1. 7: The Motor System Cognitive Neuroscience David Eagleman Jonathan Downar

2. Chapter Outline • Muscles • The Spinal Cord • The Cerebellum • The Motor Cortex • The Prefrontal Cortex • Basal Ganglia • Medial and Lateral Motor Systems • Did I Really Do That?

3. Muscles • Skeletal Muscle: Structure and Function • The Neuromuscular Junction

4. Skeletal Muscle: Structure and Function • Bringing about movement is the ultimate goal of the brain. • Muscles attach to the skeleton at the origin and insertion. • Muscles are collections of many muscle fibers.

5. Skeletal Muscle: Structure and Function Figure 7.2. The major structures of a skeletal muscle.

6. Skeletal Muscle: Structure and Function • Muscle spindles and Golgi tendon organs provide proprioceptive information from the muscles. • Muscles are organized into antagonistic pairs, with extensors extending the joint and flexors contract the joint.

7. The Neuromuscular Junction • Motor neurons release neurotransmitters to cause muscle contraction at the neuromuscular junction. • The neurotransmitter acetylcholine binds to ionotropic receptors, causing depolarization. • If there is enough localized depolarization, voltage-gated ion channels will open.

8. The Neuromuscular Junction • The rapid depolarization caused by the opening of voltage-gated ion channels causes the release of calcium. • Calcium inside the muscle causes actin and myosin proteins to interact, which brings about a muscle contraction. • Acetylcholinesterase removes the neurotransmitter and ends the contraction.

9. The Neuromuscular Junction Figure 7.3. The neuromuscular junction.(a) The microscopic structure of the neuromuscular junction. (b) In myasthenia gravis, antibodies block the receptor for the neurotransmitter acetylcholine, weakening the muscular contractions. The neuromuscular-blocking medication rocuronium also blocks the acetylcholine receptor, and it is sometimes used to relax the body’s musculature during surgery.

10. The Spinal Cord • Lower Motor Neurons • Spinal Motor Circuits: Reflexes • Spinal Motor Circuits: Central Pattern Generators • Descending Pathways of Motor Control

11. Lower Motor Neurons • Lower motor neurons project from the ventral horn of the spinal cord. • Alpha motor neurons cause contraction of the skeletal muscles. • Gamma motor neurons adjust the tension in the muscle spindle fibers so they can accurately detect a stretch. • The motor unit is the alpha motor neuron and all the muscle fibers it innervates.

12. Lower Motor Neurons Figure 7.4. Lower motor neurons of the spinal cord. Alpha and gamma motor neurons are found in the ventral horns of the gray matter of the spinal cord. A motor unit consists of all the muscle fibers innervated by a single alpha motor neuron. A motor pool includes all the motor units of a single muscle. The gamma motor neurons innervate and maintain tension in muscle spindles.

13. Spinal Motor Circuits: Reflexes • Reflexes are simple movements coordinated by the spinal cord. • Proprioceptors detect a stretch and trigger a motor response to counteract the stretch. • The deep tendon reflex, or knee-jerk reflex, is an example of this.

14. Spinal Motor Circuits: Reflexes Figure 7.5. The neural circuitry of a deep tendon reflex. Striking the tendon stretches the muscle spindle in the attached muscle, sending a sensory signal via the dorsal root ganglion to the spinal cord. After crossing an interneuron, the signal returns to the muscle to trigger a counterbalancing contraction. An inhibitory efferent signal also relaxes the opposing muscle to prevent another stretch reflex from occurring.

15. Spinal Motor Circuits: Central Pattern Generators • Neurons within the spinal cord influence rhythmic behaviors, such as walking. • Excitatory interneurons stimulate alpha motor neurons to cause a muscle contraction. • Inhibitory interneurons are also stimulated, eventually overwhelming the excitation. • After a period of inactivity, excitation resumes. • Inhibitory interneurons cross the midline, causing alternating contraction and relaxation.

16. Spinal Motor Circuits: Central Pattern Generators Figure 7.6. Spinal motor pattern generator circuits. Central pattern generator circuits in each segment of the spinal cord create naturally oscillating, alternating rhythms of activity that can be used to drive alternating movements of each side of the body. Each segment fires just a little after the one anterior to it, so that waves of alternating activity pass down along the body to drive locomotion.

17. Descending Pathways of Motor Control • Upper motor neurons from the primary motor cortex project to the spinal cord. • About 80% of the axons of the upper motor neurons decussate at the medulla, forming the lateral corticospinal tract. • About 10% decussate at the point where they exit the spinal cord. • The remainder remain ipsilateral.

18. Descending Pathways of Motor Control Figure 7.7 The corticospinal tract. The corticospinal tract travels from the upper motor neurons of the primary motor cortex down through the corona radiata and the internal capsule, through the cerebral peduncles, crossing over the opposite side of the body at the pyramidal decussastions, and then continuing down the spinal cord to the lower motor neurons.

19. Descending Pathways of Motor Control • Other descending pathways also influence movement. • The rubrospinal tract influences the limbs. • The vestibulospinal tract influences balance of the trunk. • The tectospinal tract coordinates movements to capture or avoid targets. • The reticulospinal tract coordinates startle and escape reflexes.

20. Descending Pathways of Motor Control Figure 7.8. Non-corticospinal motor control pathways.(a) The rubrospinal tract. (b) The vestibulospinal tract. (c) The tectospinal tract. (d) The reticulospinal tract.

21. The Cerebellum • The Circuitry of the Cerebellum • Motor Functions of the Cerebellum • Nonmotor Functions of the Cerebellum

22. The Circuitry of the Cerebellum • The cerebellum is important for motor coordination. • Injury to the cerebellum results in impairments to the coordination, accuracy, and timing of movements.

23. The Circuitry of the Cerebellum • There are three cellular layers of the cerebellum • Granule cell layer • Purkinje cell layer • Molecular cell layer

24. The Circuitry of the Cerebellum Figure 7.10. The microscopic circuitry of the cerebellum. The connection patterns of the neurons in this cerebellar circuit are repeated over and over again, like a motif, in a “crystalline” fashion throughout the cerebellar cortex. Figure 7.9 Macroanatomy of the cerebellum. The cerebellum contains as many neurons as the entire cortex. Its gray matter is densely packed into leaflike folia, which themselves are grouped into larger lobules and lobes. It is responsible for smoothness and accuracy of movements. Lesions of the cerebellum cause overshooting and overcorrection of movements, resulting in wobbly movement trajectories.

25. The Circuitry of the Cerebellum • Purkinje cells generate the output of the cerebellum via inhibitory projections to deep cerebellar nuclei. • These nuclei send excitatory connections to the brain and spinal cord.

26. The Circuitry of the Cerebellum • Mossy fibers send excitatory input to the granule cells, which excite the molecular cell layer. • Climbing fibers project from the olivary nuclei to provide excitatory input to the Purkinje cell bodies. • Basket cells and stellate cells provide lateral inhibitory connections.

27. Motor Functions of the Cerebellum • Cerebellum may provide forward modeling to fine-tune motor control. • It combines sensory and motor information to predict where an object will be at some future point in time.

28. Motor Functions of the Cerebellum Figure 7.11 Forward modeling. (a) When trying to touch a moving object, you will miss the object if you steer your hand toward its current location, which will be out of date by the time your hand arrives there. (b) If you can forward-model the expected location of the ball by the time your hand arrives, you will be closer. However, you may overshoot, since your information about your arm position is also slightly out of date. (c) If you forward-model the position of both your arm and the ball, then you can reach for it accurately.

29. Nonmotor Functions of the Cerebellum • The cerebellum sends projections to the frontal lobe and influences cognition, emotion, motivation and judgement. • Damage to the cerebellum impairs cognition, language perception, and grammar.

30. The Motor Cortex • Motor Cortex: Neural Coding of Movements • Motor Cortex: Recent Controversies

31. Motor Cortex: Neural Coding of Movements • The primary motor cortex (M1) is in the frontal lobe, immediately anterior to the central sulcus. • There is a motor homunculus in M1, similar to the somatosensory homunculus found in S1. • Areas with more motor control or sensory input are larger in the homunculus.

32. Motor Cortex: Neural Coding of Movements Figure 7.12 The primary motor cortex. (a) The primary motor cortex, located along the precentral gyrus, occupies less than 10% of the entire cortical surface. (b) Stimulation at different points along the precentral gyrus elicits movements of different body parts, from the posterior to the anterior end of the organism. (c) The “motor homunculus” illustrates that the size of each body part’s representation in the cortex is proportional to its dexterity, not its physical size.

33. Motor Cortex: Neural Coding of Movements Figure 7.13 The primary somatosensory cortex. (a) The primary somatosensory cortex is located along the postcentral gyrus, in close physical proximity to its motor counterpart. (b) Stimulation at different points along the precentral gyrus elicits sensations in different body parts, from the posterior to the anterior end of the organism. (c) The “somatosensory homunculus” illustrates that the size of each body part’s representation in the cortex is proportional to its sensitivity, not its physical size.

34. Motor Cortex: Neural Coding of Movements • The lateral premotor area, supplementary motor area, and pre-supplementary motor area are anterior to M1. • These are motor planning areas and each have their own somatotopic map.

35. Motor Cortex: Neural Coding of Movements Figure 7.14 Premotor regions. (a) Lateral premotor regions include the premotor cortex, just anterior to the primary motor cortex, and the frontal eye field, just anterior and dorsal to the premotor cortex. (b) Just anterior to the medial primary motor cortex are several medial premotor regions including the supplementary motor area, supplementary eye field, and presupplementary motor area.

36. Motor Cortex: Neural Coding of Movements • The upper motor neurons of M1 project to the lower motor neurons via the corticospinal tracts. • They also connect with the interneurons of the spinal cord to influence reflexes and central pattern generators. • M1 seems to use population coding to encode direction of movement.

37. Motor Cortex: Neural Coding of Movements Figure 7.15 Population coding. (a) An individual neuron in the primary motor cortex fires more rapidly during movements of a joystick in certain directions—in this case, forward and to the left. However, the preferred direction is “fuzzy” at the individual-neuron level. (b) During movements in a given direction, neurons fire at higher rates when the direction is closer to their preferred direction. Summing together the firing rates across the entire population gives a much less fuzzy, precise prediction to the movement direction. This finding has been used to suggest “population encoding” of movement directions in the primary motor cortex.

38. Motor Cortex: Recent Controversies • Newer research with longer stimulation of M1 suggests the map may be more complex than the homunculus. • Longer stimulation evokes complete movements, like moving the hand to the mouth and opening the mouth. • There is no obvious population coding of direction with longer stimulation.

39. Motor Cortex: Recent Controversies Figure 7.16 Feedback mapping. (a) Longer trains of stimulation in the primary motor cortex tend to drive the limb toward a specific final position (blue) and posture, regardless of initial position (red). (b) Classically, each neuron in the primary motor cortex relays signals through the spinal cord to drive a specific muscle in a one-to-one mapping. In the more complex view of many-to-many mapping, each neuron can drive many muscles and each muscle can be driven by many neurons. In the still more complex view of feedback mapping, incoming sensory feedback signals from the muscles and joints can resculpt the many-to-many mapping patterns between upper motor neurons and muscles.

40. The Prefrontal Cortex: Goals to Strategies to Tactics to Actions • The Functional Organization of the Prefrontal Cortex in Motor Control • Sensory Feedback • Mirror Neurons in Premotor Cortex • Control Stages of the Motor Hierarchy

41. The Functional Organization of the Prefrontal Cortex • Actions are the body’s way of transforming needs into goals and then into behaviors. • Primary motor cortex and premotor cortex have direct connections to spinal cord to influence movement. • Prefrontal cortical areas influence M1 and the premotor cortex, not the spinal cord directly.

42. The Functional Organization of the Prefrontal Cortex Figure 7.18 The hierarchy of behavioral control in the frontal lobes. From posterior to anterior, the primary motor cortex directs simple movements, the premotor cortex directs more complex actions, the lateral prefrontal cortex directs complex cognition and planning, and the frontopolar cortex represents long-term goals. The detailed somatotopic organization of the primary motor cortex is gradually lost in the more anterior areas, higher in the hierarchy. The frontopolar cortex has no direct sensory input at all.

43. The Functional Organization of the Prefrontal Cortex • Most motor areas receive extensive input from somatosensory areas. • The frontopolar cortex receives no sensory input and connects with other prefrontal areas. • This helps set and maintain long-term goals.

44. Sensory Feedback • Tactile, proprioceptive, and nociceptive somatosensory feedback helps guide movements. • The intraparietal sulcus contains several areas that represent the location of objects in space in relation to different parts of the body.

45. Sensory Feedback Figure 7.19. Spatial maps of the parietal lobe. Parietal lobe regions provide guidance to lateral prefrontal regions. To do this, they use input from the external senses: vision, hearing, and touch. The intraparietal sulcus contains multiple maps of the shapes and locations of objects in our surroundings. Each map is centered on a different body region and connects to the appropriate region of the prefrontal cortex to guide that body part.

46. Mirror Neurons in Premotor Cortex • Mirror neurons are active when performing an action or when observing another individual perform a similar action. • Mirror neurons are found in the ventral premotor cortex.

47. Mirror Neurons in Premotor Cortex • The action must be goal-directed to cause motor neurons to fire. • These neurons may be important for our ability to understand the thoughts and feelings of others.

48. Mirror Neurons in Premotor Cortex Figure 7.20 Mirror neurons. (a) In both humans and nonhuman primates, mirror neurons are found in the premotor cortex and anatomically connected regions of the superior temporal sulcus. (b) Mirror neurons are active both when individuals perform an action and when they observe another individual performing the same action.

49. Control Stages of the Motor Hierarchy • Posterior lateral premotor areas select actions based on sensory input. • Intermediate lateral premotor areas choose which sensory rules to use in the current context. • Anterior lateral premotor areas select the appropriate context of choosing an action. • Most anterior areas keep track of overall goals.

50. Control Stages of the Motor Hierarchy Figure 7.21 Control of complex behavior in the prefrontal cortex. More anterior areas of the prefrontal cortex guide progressively more complex forms of conditional behavior. The premotor cortex controls actions regulated by sensory cues using simple if–then rules (if go, then press button). More anterior areas can override these simple if–then rules based on context (if go, then press button unless uppercase). Still more anterior areas can switch contexts episodically (if lowercase is go last time, then uppercase is go this time). Still more anterior areas can switch contexts conditionally (if a green go appears, then its case becomes go from now on). With these capabilities, bewilderingly complex behaviors become possible.