Neural Control

1. Neural Control Chapter 37

2. Nervous Systems • two kinds of generalized cells • neurons • basic cells of the nervous system; nerve cells • one of the four basic tissue types • receive and transmit information • chemical signals  electrical impulses • detection, integration, response • stimuli The three fundamental phases of neural activity

3. glial cells • neuroglia; several different types • structural support and framework for neurons • produce myelin sheath • oligodendrocytes and Schwann cells • some metabolic functions • help nourish neurons, regulate K • act as selective barrier • microglia and astrocytes Fig. 31.6 Neurons and glial cells

4. neurons • three categories • sensory neurons (afferent neurons) • detection phase • receive information from external and internal environment • interneurons • integration phase • process and route information ("middle men") • motor neurons (efferent neurons) • response phase • transmit information signals to organs, tissues, or muscles • cause motor responses (reactions) • myelin sheath • protective covering and electrical insulator of neurons • nodes (nodes of Ranvier) vs. internodes • structure of neurons • cell body (nucleus and organelles) • produces neurotransmitters • chem. substances necessary for elec. impulses to be transmitted • dendrites • receiving end of neuron (chemical signals) • receptor cells: larger/thicker dendrites of sensory neurons • axons • transmitting end of neuron • chemical signals  electrical impulses • one axon branches (at ends)  axon trees  axon terminal • neuromuscular junctions

5. Fig. 37.4 Neuron anatomy

6. nerves • thousands of neurons surrounded by layers of tough connective tissue Page 692 Nerve anatomy

7. Neural Impulses • overview • electrical impulses are generated through action potentials • change in voltage across a membrane from -65 mV to +40 mV • polarized neuron  depolarized repolarization • specifics • action potentials do not diminish with distance • occur in axons • controlled ion movement, active transport, and diffusion • neurons are polarized in their resting state • interior more negative than exterior (net negative charge) • neg. charged proteins and Cl; pos. charged Na, K ions • inside neuron: neg. proteins, Cl-, K+ • outside neuron: Na+, some K+ • Na/K ion exchange pumps maintain ion distribution • pump Na+ out and most K+ in • exterior positive (Na+), interior negative (proteins, Cl) • K+ acts as a “balancer” • resting potential = -65 mV

8. depolarization • begins as a change in permeability of plasma membrane • dendrites receive information as chemical signals • cell body releases a neurotransmitter • neurotransmitter affects ion gates and channels in axon membrane • separate channels for Na+ and K+ ions • Na+ ion gates open  Na+ ions rush inward • inrush changes balance of charges (-65 mV to +40 mV) • upper limit of change can vary between +30 and +40 mV • proceeds as rapidly moving wave across entire axon • threshold voltage (= about -40 mV) • amount of depolarization needed to start an action potential • action potentials are example of positive feedback • once started, the wave proceeds with constant magnitude and speed • rate depends on strength of stimulus • typical action potential occurs in 2 milliseconds • non-myelinated axons  speed of 1 meter/second • potential travels down axon, one small section at a time • myelinated axons  speeds up to 200 meters/second (450 miles/hour) • saltatory conduction • gated ion channels are concentrated at nodes (nodes of Ranvier) • potential jumps over myelinated areas (internodes)  faster, more efficient

9. Fig. 37.5 Resting potential and depolarization

10. Page 685 Saltatory conduction

11. repolarization • K+ ion gates open  K+ ions move to outside of plasma membrane • causes an undershoot of resting potential (-80 mV) • what else is wrong now? • ion exchange pumps quickly redistribute K+ and Na+ • restores resting potential to -65 mV • refractory period • time between depolarization and repolarizaton • 2nd potential cannot occur unless exceedingly strong Fig. 37.5 Repolarization and an entire action potential

12. An action potential

13. Communication Among Neurons and Between Neurons and Muscles • synapses • axons linking to dendrites or cell body • axons linking to organ, muscle, or tissue • synaptic cleft • tiny gap between two neurons, where they meet at a synapse • nerve impulses cannot cross this gap • transmission across a synapse • accomplished by secretion of more neurotransmitters • e.g., acetylcholine, serotonin, endorphin, dopamine, etc. • can have other physiological effects • transmitting neuron secretes a neurotransmitter across cleft • this starts a new action potential in the receiving neuron • very fast and versatile Fig. 37.7a Synapses

14. neuromodulators • molecules that block release of neurotransmitters • can also modify a neuron’s response to a neurotransmitter • e.g., endorphins block/modify neuron responses  act as natural painkillers Fig. 37.6 Synapse structure and function

15. excitatory vs. inhibitory synapses • excitatory • most synapses in body • cause motor responses and stimulate neural activity • inhibitory • inhibit motor responses and make them less likely to occur • act as a control mechanism in body • involve different neurotransmitters Fig. 37.7b Excitatory and inhibitory synapses

16. reflex arcs • simplest model of neural activitycan involve as few as 2 or 3 neurons • often bypass brain entirely • operate through spinal cord only Fig. 37.13 A reflex arc