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 tumors are gliomas - tumors formed by neuroglia
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 amitotic nature of most neurons
Figure 7.4a–b

18. Neuron Anatomy
Extensions outside the cell body – from microscopic to 3-4 feet 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