Neural Control and the Senses

1. Neural Control and the Senses Chapter 25

2. Neurons • Communication units of nervous systems • Detect information about internal and external conditions • Issue commands for responsive actions

3. stimulus (output) Types of Neurons receptors sensory neurons • Sensory neurons • Detect and relay information • Interneurons • Receive and process information • Motor neurons • Transmit signals from interneurons to effectors integrators interneurons of brain, spinal cord motor neurons effectors muscles, glands response (output)

4. Structure of a Neuron dendrites INPUT ZONE cell body axon OUPUT ZONE TRIGGER ZONE CONDUCTING ZONE axon endings

5. Fig. 25-1b, p.423

6. Neuroglia • Cells that metabolically assist, structurally support, and protect neurons • Make up more than half the volume of the vertebrate nervous system

7. Resting Membrane Potential • Electrical gradient across membrane • About -70 mV • Maintained by sodium-potassium pump • Potassium (K+) higher inside • Sodium (Na+) higher outside more Na+ flows into the neuron neuron becomes more positive inside more gated channels for Na+ open

8. Na+ K+ outside plasma membrane inside Na+ K+ p.424a

9. How Ions Move across Membrane interstitial fluid cytoplasm Na+/K+ pump passive transporters with open channels passive transporters with voltage-sensitive gated channels active transporters lipid bilayer of neuron membrane

10. Action Potential • Brief reversal in membrane potential • Voltage change causes voltage-gated channels in membrane to open • Inside of neuron briefly becomes more positive than outside

11. Action Potential 1 2 Na+ Na+ Na+ K+ K+ K+ K+ K+ K+ K+ Na+ Na+ Na+ Na+ 3 4 Na+ Na+

12. interstitial fluid cytoplasm Fig. 25-4a, p.425

13. Na+ Na+ Na+ Fig. 25-4b, p.425

14. K+ K+ K+ Na+ Na+ Na+ Fig. 25-4c, p.425

15. Na+/K+ pump K+ K+ K+ Na+ Na+ Na+ K+ Fig. 25-4d, p.425

16. Positive Feedback more Na+ ions flow into the neuron more gated channels for Na+ open neuron becomes more positive inside

17. All or Nothing • All action potentials are the same size • If stimulation is below threshold level, no action potential occurs • If stimulation is above threshold level, cell always depolarizes to same level

18. Repolarization • Once action potential peak is reached, Na+ gates close and K+ gates open • Movement of K+ out of cell • The inside of the cell once again becomes more negative than the outside

19. Recording of Action Potential action potential +20 0 -20 Membrane potential (millivolts) threshold -40 resting membrane potential -70 5 0 2 3 1 4 Time (milliseconds)

20. Propagation of Action Potentials • Action potential in one part of an axon brings neighboring region to threshold • Action potential moves from one patch of membrane to another • Can only move one direction

21. Chemical Synapses • Action potentials cannot jump from cell to cell • Signal is transmitted from axon end, across a synaptic cleft, by chemical signals called neurotransmitters

22. Chemical Synapse • Gap between the terminal ending of an axon and the input zone of another cell plasma membrane of axon ending of presynaptic cell plasma membrane of postsynaptic cell synaptic vesicle synaptic cleft membrane receptor

23. Synaptic Transmission • Action potential in axon ending triggers release of neurotransmitter from presynaptic cell into synaptic cleft vesicle inside presynaptic cell synaptic cleft postsynaptic cell

24. Synaptic Transmission • Neurotransmitter diffuses across cleft and binds to receptors on membrane of postsynaptic cell • Binding of neurotransmitter to receptors opens ion gates in membrane of postsynaptic cell

25. Ion Gates Open neurotransmitter ions receptor for neurotransmitter gated channel protein

26. Synaptic Integration • Many signals reach a neuron at the same time • Signals may suppress or reinforce one another • Whether or not an action potential occurs depends on the sum of the signals the neuron receives

27. Neuromuscular Junction • Synapse between motor neuron and skeletal muscle fiber • Neuron releases chemical neurotransmitter acetylcholine (ACh)

28. A Neuromuscular Junction neuromuscular junction motor neuron axons from spinal cord to skeletal muscle fibers transverse slice of spinal cord part of a skeletal muscle Fig. 25-6a, p.427

29. A Neuromuscular Junction muscle fiber axon ending Fig. 25-6b, p.427

30. Neurotransmitters • Acetylcholine (ACh) • Norepinephrine • Epinephrine • Dopamine • Serotonin • GABA

31. Cleaning Up • After neurotransmitter has acted, it is quickly removed from synaptic cleft • Molecules diffuse away, are pumped out, or broken down

32. interneuron motor neuron sensory neuron Information Flow

33. Organization • Neurons are bundled in nerves • Nerves are organized in circuits and reflex pathways • Information from sensory neurons is relayed to interneurons in spinal cord and brain • Motor neurons carry signals to body

34. Nerve axon myelin sheath • A bundle of axons enclosed within a connective tissue sheath many neurons inside a connective tissue sheath

35. Myelin Sheath • Sheath blocks ion movements • Action potential must “jump” from node to node • Greatly enhances speed of transmission

36. Multiple Sclerosis • A condition in which nerve fibers lose their myelin • Slows conduction • Symptoms include visual problems, numbness, muscle weakness, and fatigue

37. Reflexes • Automatic movements in response to stimuli • In simplest reflex arcs, sensory neurons synapse directly on motor neurons • Most reflexes involve an interneuron

38. Stretch Reflex STIMULUS Biceps stretches. sensoryneuron motorneuron RESPONSE Biceps contracts.

39. Invertebrate Nervous Systems • All animals except sponges have some sort of nervous system • Nerve cells interact with one another in signal-conducting and information-processing highways

40. Bilateral Nervous System rudimentary brain branching nerve nerve cord ganglion (one in most body segments)

41. Vertebrate Development • Earliest fishlike vertebrates had a hollow, tubular nerve cord • Modification and expansion of nerve cord produced spinal cord and brain • Nerve cord persists in vertebrate embryos as a neural tube

42. Central and Peripheral Nervous Systems • Central nervous system (CNS) • Brain • Spinal cord • Peripheral nervous system • Nerves that thread through the body

43. Vertebrate Nervous Systems

44. Major Nerves Brain cervical nerves (eight pairs) cranial nerves (twelve pairs) Spinal Cord thoracic nerves (twelve pairs) ulnar nerve (one in each arm) lumbar nerves (five pairs) sacral nerves (five pairs) sciatic nerve (one in each leg) coccygeal nerves (one pair) Fig. 25-12, p.431

45. Peripheral Nervous System • Somatic nerves • Motor functions • (Shown in green) • Autonomic nerves • Visceral functions • (Shown in red)

46. Two Types of Autonomic Nerves Sympathetic Parasympathetic • Most organs receive input from both • Usually have opposite effects on organ

47. optic nerve midbrain eggs medulla oblongata salivary glands heart vagus nerve cervical nerves (8pairs) larynx bronchi lungs stomach thoracic nerves (12 pairs) liver spleen pancreas kidneys adrenal glands small intestine upper colon lower colon rectum lumbar nerves (five pairs) (all ganglia in walls of organs) (most ganglia near spinal cord) sacral nerves (five pairs) bladder uterus pelvic nerve genitals Autonomic Nervous System Fig. 25-13, p.432

48. Sympathetic Nerves • Originate in thoracic and lumbar regions of spinal cord • Ganglia are near the spinal cord • Respond to stress or physical activity (fight-or-flight response)

49. Parasympathetic Nerves • Originate in brain and sacral region of spinal cord • Ganglia are in walls of organs • Promote housekeeping responses such as digestion

50. Opposing Systems • Most organs receive both sympathetic and parasympathetic signals • Example: Sympathetic nerves signal heart to speed up; parasympathetic stimulate it to slow down • Synaptic integration determines response