C H A P T E R 2

1. C H A P T E R 2 NEUROLOGICAL CONTROL OF MOVEMENT

2. w Discover how neurons communicate with one another and learn the role of neurotransmitters in this communication. (continued) Learning Objectives w Learn the basic structures of the nervous system. w Follow the pathways of nerve impulses from initiation to muscle action.

3. Learning Objectives w Understand the functional organization of the central nervous system. w Become familiar with the roles of the sensory and motor divisions of the peripheral nervous system. w Learn how a sensory stimulus gives rise to a motor response. w Consider how individual motor units respond and how they are recruited in an orderly manner depending on the required force.

4. ORGANIZATION OF THE NERVOUS SYSTEM Skeletal Muscles

5. STRUCTURE OF A NEURON (NERVE CELL)

6. Alpha-Motoneuron/Motor Unit Brooks G, et al., Exercise Physiology, Mayfield, 2000.

7. Action Potential (Impulse) An electrical event that passes down an excitable cell membrane by ionic fluxes and from one neuron to the next via chemical transmission and finally to an end organ via chemical transmission, such as a group of muscle fibers.

8. Resting Membrane Potential (RMP) w Difference between the electrical charges (ions) inside and outside a cell, caused by separation of charges across a membrane w High concentration of potassium ions (K+) inside the neuron and sodium ions (Na+) outside the neuron w K+ ions can move more freely through the membrane, i.e., the membrane is more permeable to this ion; thus some K+ ions “slip” out through the membrane, making the outside more positive wSodium-potassium pump actively transports K+ in and Na+ out through the membrane to maintain imbalance w The constant imbalance keeps the RMP at –70 mV

9. Changes in Membrane Potential Depolarization—inside of cell becomes less negative relative to outside (> –70 mV) Hyperpolarization—inside of cell becomes more negative relative to outside (< –70 mV) Graded potentials—localized changes in membrane potential (either depolarization or hyperpolarization) Actionpotentials—rapid, substantial depolarization of the membrane (–70 mV to +30 mV to –70 mV all in 1 ms)

10. What Is an Action Potential? w Starts as a graded potential w Requires depolarization greater than the threshold value (e.g., –50 to –55 mV) w Once threshold is met or exceeded, the all-or-none principle applies

11. Action Potentials w Once threshold is met or exceeded, the all-or-none principle applies; the action potential is propagated down the neuron or muscle fiber membrane Threshold

12. Events During an Action Potential 1. The resting state 2. Depolarization 3. Propagation of an action potential 4. Repolarization 5. Return to the resting state via the sodium-potassium pump

13. RESTING STATE

14. ACTION POTENTIAL

15. Myelinated fibers wSaltatory conduction — action potential “jumps” from one Node of Ranvier to the next. w Action potential is 5 to 150 times faster in myelinated compared to unmyelinated axons. Diameter of the neuron w Larger diameter neurons conduct nerve impulses faster. w Larger diameter neurons present less resistance to current flow. Velocity of an Action Potential Alpha-motoneurons and the sensory neurons from muscle spindles and Golgi tendon organs are the largest, fastest neurons in the PNS (~100 m/s); an example of small, relatively slow unmyelinated neurons are the sensory neurons from some nociceptors (dull, diffuse aching sensation) (~15 m/s)

16. Synapse w A synapse is the site of impulse transmission between two neurons. w An action potential travels to a presynaptic axon terminal where it causes synaptic vesicles on the terminal to release chemicals (neurotransmitters) into the synaptic cleft (gap). w The neurotransmitters bind to postsynaptic receptors on an adjacent neuron, usually on the dendrites (80-95%), which results in increased permeability to sodium. w Neural impulses can only be transmitted from the dendrite or cell body through the axon to the adjacent neuron since the neurotransmitters are released only from the terminal end of the axon.

17. CHEMICAL SYNAPSE

18. Neuromuscular Junction w The junction is a site where a motor neuron communicates with a muscle fiber. w Motor axon terminal releases acetylcholine (ACh) which diffuses across the cleft and binds to receptors on the muscle fiber membrane, increasing permeability to sodium ions. w This causes depolarization, leading to development of an action potential on the muscle fiber. w The action potential spreads across the sarcolemma into the t-tubules causing the muscle fiber to contract.

19. NEUROMUSCULAR JUNCTION

20. Refractory Period w Period of repolarization of a neuron or muscle fiber. w The cell is unable to respond to any further stimulation during the refractory period. w The refractory period limits a motor unit's firing frequency. Nonetheless, fast-twitch motor units may fire at frequencies as high as 100 Hz (Hertz), or 100 action potentials per second.

21. Thought Question Where in the body are the longest alpha-motoneurons located? At 100 m/s, about how long would it take an action potential to get from the CNS to a motor unit in this location?

22. Key Points Neural Synapses w Neurotransmitters released by neurons bind to the post-synaptic receptors and cause depolarization (excitation) or hyperpolarization (inhibition) depending on the specific neurotransmitter and the specific receptor to which it binds.

23. Key Points Postsynaptic Response w An excitatory presynaptic impulse causes depolarization. w An inhibitory presynaptic impulse causes hyperpolarization. w Whether or not the membrane reaches threshold and generates an action potential depends on the balance of the hundreds of excitatory and inhibitory inputs.

24. Brain w Cerebrum: Site of intellect; primary motor and sensory areas. w Diencephalon: Location of the hypothalamus, which is central in the regulation of homeostasis. w Cerebellum—Plays crucial role in coordinating movement. w Brain stem—Connects brain to spinal cord; coordinates skeletal muscle function and maintains muscle tone; contains regulators of respiratory and cardiovascular systems. Central Nervous System Spinal cord

25. REGIONS OF THE BRAIN

26. Peripheral Nervous System w 12 pairs of cranial nerves connected with the brain. w 31 pairs of spinal nerves connected with the spinal cord. wSensory division — carries sensory information from sensory receptors in the body by afferent fibers to the CNS. wMotor division — transmits information from CNS via efferent fibers to target organs (e.g., muscles). wAutonomic nervous system — controls involuntary internal functions – sympathetic and parasympathetic components.

27. w Heart rate and strength of heart contraction w Blood pressure w Blood supply to the heart and active muscles w Metabolic rate and release of glucose by the liver w Rate of gas exchange between lungs and blood w Mental activity and quickness of response Sympathetic Nervous System Fight-or-flight—prepares you for acute stress or physical activity; norepinephrine is the neurotransmitter; also stimulates release of catecholamines (norepinephrine and epinephrine) from the adrenal medulla glands Facilitates the motor response with increases in:

28. w Decreases heart rate w Constricts coronary vessels w Constricts tissues in the lungs Parasympathetic Nervous System Internal “Housekeeping”— controls digestion, urination, glandular secretion, and energy conservation; ACh is the primary neurotransmitter Actions oppose those of the sympathetic system

29. SENSORY RECEPTORS AND PATHWAYS

30. Types of Sensory Receptors Mechanoreceptors—respond to mechanical forces such as pressure, touch, vibration, or stretch. Thermoreceptors—respond to changes in temperature. Nociceptors—respond to painful stimuli. Photoreceptors—respond to light to allow vision. Chemoreceptors—respond to chemical stimuli from foods, odors, and changes in blood concentrations of gases and substances. Proprioceptors—sensitive to bodily movement and position

31. Muscle and Joint Proprioceptors w Kinesthetic receptors in joint capsules sense the position and movement of joints. w Muscle spindles sense muscle length and rate of length change. w Golgi tendon organs (GTOs) detect the tension of a muscle on its tendon, providing information about the strength of muscle contraction.

32. Muscle Spindles w A group of 4 to 20 small muscle fibers (intrafusal) with sensory and motor nerve endings, covered by a connective tissue sheath, and connected in parallel with extrafusal (or regular) muscle fibers. w The middle of the spindle can stretch, but cannot contract as it contains little or no actin and myosin. w When extrafusal fibers attached to the spindle are stretched, sensory neurons on the spindle transmit information to the CNS about the muscle’s length. w Reflexive muscle contraction is triggered through the alpha motoneuron to resist further stretching. w Gamma motoneurons activate intrafusal fibers, causing the middle of the spindle to stretch, making the spindle sensitive to small degrees of stretch.

33. Golgi Tendon Organs (GTOs) w Encapsulated sensory organs through which muscle tendon fibers pass w Located close to the tendon's attachment to the muscle w Sense small changes in tension w Inhibit contracting (agonist) muscles and excite antagonist muscles to prevent injury

34. MUSCLE BODY, MUSCLE SPINDLE, AND GTO

35. MUSCLE SPINDLE Eccles, JC. The Understanding of the Brain, McGraw-Hill, 1977.

36. SENSORY-MOTOR INTEGRATION

37. Integration Centers Spinal cord—simple motor reflexes such as pulling your hand away after touching something hot. Lower brain stem—more complex subconscious motor reactions such as postural control. Cerebellum—subconscious control of movement such as that needed to coordinate multiple movements. Thalamus—conscious distinction among sensations such as feeling hot or cold. Cerebral cortex—conscious awareness of a sensation and the location within body where the sensation originates.

38. Motor Control w Sensory impulses evoke a response through a motoneuron. w The closer to the brain the impulse stops, the more complex the motor reaction. w A motor reflex is a preprogrammed response that is integrated by the spinal cord or brain without conscious thought.

39. MOTOR PATHWAYS

40. Conscious Control of Movement w Neurons originating in the primary motor cortex control voluntary muscle movement (cerebro-spinal motoneurons, or “upper motoneurons”). w Clusters of nerve cells in the basal ganglia initiate sustained and repetitive movements—walking, running, maintaining posture, and muscle tone. w The cerebellum compares the actual movement with the intended movement. For this reason it is called a “comparator.” Engrams are learned motor patterns stored in the cortex.

41. Thought Question How might the cerebellum (comparator) use information from the muscle spindles to monitor the muscle actions? If the movement is not going as intended, what would the cerebellum do to correct the movement?

42. Motor unit size/function Muscles controlling fine movements, such as those controlling the eyes, have a small number of muscle fibers per motor neuron (about 1 neuron for every 15 muscle fibers). Muscles with more general function, such as those controlling the calf muscle in the leg, have many fibers per motor neuron (about 1 neuron for every 2,000 muscle fibers).