The Nervous System

1. The Nervous System

2. Functions of the Nervous System • Sensory input – gathering information • To monitor changes occurring inside and outside the body • Changes = stimuli = sensory input • Integration • To process and interpret sensory input and decide if action is needed

3. Functions of the Nervous System • Motor output • A response to integrated stimuli • The response activates muscles or glands

4. Structural Classification of the Nervous System • Central nervous system (CNS) • Brain • Spinal cord • Act as integrating & command centers (interpret sensory input & issue commands) • Peripheral nervous system (PNS) • Nerve outside the brain and spinal cord • Spinal nerves carry impulses to & from spinal cord • Cranial nerves carry impulses to & from brain

5. Functional Classification of the Peripheral Nervous System • Sensory (afferent) division • Nerve fibers that carry information to the central nervous system Figure 7.1

6. Functional Classification of the Peripheral Nervous System • Sensory fibers that carry impulses from skin, skeletal muscle & joints = somatic sensory fibers. • Those that carry impulses from the visceral organs = visceral sensory fibers

7. Functional Classification of the Peripheral Nervous System • Motor (efferent) division • Nerve fibers that carry impulses away from the central nervous system to effector organs, the muscles & glands Figure 7.1

8. Functional Classification of the Peripheral Nervous System • Motor (efferent) division • Two subdivisions • Somatic nervous system = voluntary • Autonomic nervous system = involuntary Figure 7.1

9. Functional Classification of the Peripheral Nervous System • Autonomic nervous system = involuntary • 2 parts • Sympathetic – mobilizes the body during extreme situations – called the “fight or flight” division • Parasympathetic – allows us to “unwind” & conserve energy – called the craniosacral division

10. Organization of the Nervous System Figure 7.2

11. Nervous Tissue: Support Cells (Neuroglia = literally “nerve glue”) • Astrocytes • Abundant, star-shaped cells • Brace neurons; anchor to capillaries • Form barrier between capillaries and neurons • Control the chemical environment of the brain Figure 7.3a

12. Nervous Tissue: Support Cells • Microglia • Spider-like phagocytes • Dispose of debris(dead brain cells, bacteria) • Ependymal cells • Line cavities of the brain and spinal cord • Beating cilia circulate cerebrospinal fluid Figure 7.3b–c

13. Nervous Tissue: Support Cells • Oligodendrocytes • Produce myelin sheath around nerve fibers in the central nervous system • Unable to transmit nerve impulses • Never lose their ability to divide • Most brain tumorsare gliomas -tumors formed byneuroglia Figure 7.3d

14. Nervous Tissue: Support Cells • Satellite cells • Protect neuron cell bodies • Schwann cells • Form myelin sheath in the peripheral nervous system Figure 7.3e

15. Nervous Tissue: Neurons • Neurons = nerve cells • Cells specialized to transmit messages • Major regions of neurons • Cell body – nucleus and metabolic center of the cell • Processes – fibers that extend from the cell body

16. Neuron Anatomy • Cell body • Nissl substance – specialized rough endoplasmic reticulum • Neurofibrils – intermediate cytoskeleton that maintains cell shape Figure 7.4a

17. Neuron Anatomy • Cell body • Nucleus • Large nucleolus • Lacks centrioles – confirms amitoticnature of most neurons Figure 7.4a–b

18. Neuron Anatomy • Extensions outside the cell body – frommicroscopic to 3-4feet in length • Dendrites – conduct impulses toward the cell body – may number in the 100’s • Axons – conduct impulses away from the cell body – only one Figure 7.4a

19. Axons and Nerve Impulses • Axons end in axon terminals • Axon terminals contain vesicles with neurotransmitters – impulses stimulate their release • Axon terminals are separated from the next neuron by a gap • Synaptic cleft – gap between adjacent neurons • Synapse – junction between nerves

20. Nerve Fiber Coverings • Schwann cells – produce myelin sheaths in jelly-roll like fashion on axons outside the CNS • Neurilemma – part of Schwann cell external to myelin sheath • Nodes of Ranvier – gaps in myelin sheath along the axon Figure 7.5

21. Nerve Fiber Coverings • Oligodendrocytes form CNS myelin sheaths • CNS sheaths lack a neurilemma • Because the neurilemma remains intact (for the most part) when a peripheral nerve fiber is damaged, it plays an important role in fiber regeneration, an ability that is largely lacking in the CNS

22. Neuron Cell Body Location • Most are found in the central nervous system • Gray matter – cell bodies and unmylenated fibers • Nuclei – clusters of cell bodies within the white matter of the central nervous system • Ganglia – collections of cell bodies outside the central nervous system • Bundles of nerve fibers in the CNS are called tracts; whereas in the PNS they are called nerves

23. Functional Classification of Neurons • Sensory (afferent) neurons • Cell bodies always found in the ganglion outside the CNS • Carry impulses from the sensory receptors to CNS • Cutaneous sense organs (skin) • Pain receptors – bare dendrite endings – most numerous • Proprioceptors – detect stretch or tension (muscles & tendons) – help maintain balance & normal posture

24. Functional Classification of Neurons • Motor (efferent) neurons • Carry impulses from the CNS to viscera, &/or muscles & glands • Cell bodies of motor neurons are always located in the CNS

25. Functional Classification of Neurons • Interneurons (association neurons) • Found in neural pathways in the central nervous system • Connect sensory and motor neurons • Cell bodies are always located in the CNS

26. Neuron Classification Figure 7.6

27. Structural Classification of Neurons • Multipolar neurons – many extensions from the cell body • All motor & interneurons (association neurons) are multipolar • Most common Figure 7.8a

28. Structural Classification of Neurons • Bipolar neurons – one axon and one dendrite • Rare in adults - found only in some special sense organs (eye, nose) Figure 7.8b

29. Structural Classification of Neurons • Unipolar neurons – have a short single process leaving the cell body Figure 7.8c

30. Structural Classification of Neurons • Short, divides almost immediately into proximal (central) & distal (peripheral) processes • Dendrite = small process at end of peripheral process; remainder = axon • Axon conducts impulses both away from & toward cell body • Sensory neurons found in PNS ganglia are unipolar

31. Functional Properties of Neurons • Irritability – ability to respond to stimuli • Conductivity – ability to transmit an impulse • The plasma membrane at rest is polarized • Fewer positive ions are inside the cell than outside the cell

32. Starting a Nerve Impulse • Depolarization – a stimulus depolarizes the neuron’s membrane • A deploarized membrane allows sodium (Na+) to flow inside the membrane • The exchange of ions initiates an action potential in the neuron Figure 7.9a–c

33. The Action Potential • If the action potential (nerve impulse) starts, it is propagated over the entire axon • Potassium ions rush out of the neuron after sodium ions rush in, which repolarizes the membrane • The sodium-potassium pump restores the original configuration • This action requires ATP

34. Nerve Impulse Propagation • The impulse continues to move toward the cell body • Impulses travel faster when fibers have a myelin sheath Figure 7.9d–f

35. Continuation of the Nerve Impulse between Neurons • Impulses are able to cross the synapse to another nerve • Neurotransmitter is released from a nerve’s axon terminal • The dendrite of the next neuron has receptors that are stimulated by the neurotransmitter • An action potential is started in the dendrite

36. How Neurons Communicate at Synapses Figure 7.10