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
Integration
To process and interpret sensory input and decide if action is needed
Motor output
A response to integrated stimuli
The response activates muscles or glands

3. Structural Classification
Central nervous system (CNS)
Brain
Spinal cord
Peripheral nervous system (PNS)
Nerves outside the brain and spinal cord

4. Functional Classification: PNS
Sensory (afferent) division
Nerve fibers that carry information to the central nervous system
Motor (efferent) division
Nerve fibers that carry impulses away from the central nervous system

5. Subdivisions of Motor division
Somatic nervous system = voluntary
Somatic motor division carries information to the skeletal muscles
Autonomic nervous system = involuntary
Efferent division of ANS carries information to the autonomic or visceral effectors
Sympathetic division: prepares the body to deal with immediate threats to the internal environment; produces fight-or-flight response
Parasympathetic division: coordinates the body’s normal resting activities; sometimes called the rest-and-repair division
Visceral sensory division carries feedback information to autonomic integrating centers in the CNS

7. Organization of the Nervous System

8. Nervous Tissue: Support Cells (Neuroglia)
Astrocytes
Abundant, star-shaped cells
Brace neurons
Form barrier between capillaries and neurons
Control the chemical environment of the brain

9. Nervous Tissue: Support Cells
Microglia
Spider-like phagocytes
Dispose of debris
Ependymal cells
Line cavities of the brain and spinal cord
Circulate cerebrospinal fluid
Some produce cerebrospinal fluid

10. Nervous Tissue: Support Cells
Oligodendrocytes
Produce myelin sheath around nerve fibers in the central nervous system

11. Nervous Tissue: Support Cells
Satellite cells
Protect neuron cell bodies
Schwann cells
Form myelin sheath in the peripheral nervous system

13. 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

14. Neuron Anatomy Cell body
Nissl bodies – specialized rough endoplasmic reticulum
Produce proteins
Neurofibrils – intermediate cytoskeleton that maintains cell shape

15. Neuron Anatomy
Cell body
Nucleus
Large nucleolus

16. Neuron Anatomy Extensions outside the cell body
Dendrites – conduct impulses toward the cell body
Axons – conduct impulses away from the cell body

17. Nerve Fiber Coverings
Schwann cells – produce myelin sheaths in jelly-roll like fashion
Nodes of Ranvier – gaps in myelin sheath along the axon

18. Connective Tissue Layers
Endoneurium- delicate layer of fibrous connective tissue surrounding each nerve
Perineurium- connective tissue holding together bundles of fibers (fascicles)
Epineurium- fibrous coat surrounding numerous fascicles and blood vessels to form a complete nerve

19. Neuron Cell Body Location
Most are found in the central nervous system
Gray matter – cell bodies and unmyelinated 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

20. Neuron Cell Body Location
White matter
PNS- myelinated nerves
CNS- myelinated tracts
In the CNS nerve fibers are referred to as tracts

21. CYCLE OF LIFE: NERVOUS SYSTEM CELLS
Nerve tissue development
Begins in ectoderm
Occurs most rapidly in womb and in first 2 years
Nervous cells organize into body network
Synapses
Form and re-form until nervous system is intact
Formation of new synapses and strengthening or elimination of old synapses stimulate learning and memory
Aging causes degeneration of the nervous system, which may lead to senility

22. Functional Classification of Neurons
Sensory (afferent) neurons
Carry impulses from the sensory receptors
Cutaneous sense organs
Proprioceptors – detect stretch or tension
Motor (efferent) neurons
Carry impulses from the central nervous system

23. Functional Classification of Neurons
Interneurons (relay neurons)
Found in neural pathways in the central nervous system
Connect sensory and motor neurons
Mixed Nerves
Contain both sensory and motor neurons

24. Neuron Classification

25. Structural Classification of Neurons
Multipolar neurons – many extensions from the cell body
One axon and many dendrites
Most of the neurons in the brain and spinal cord

26. Structural Classification of Neurons
Bipolar neurons – one axon and one dendrite
Least numerous
Found in retina, inner ear and olfactory pathways

27. Structural Classification of Neurons
Unipolar neurons – have a short single process leaving the cell body
Always sensory neurons, conduct towards the CNS

28. Repair of Nerve Fibers
Mature neurons can not undergo mitosis, damage is permanent
If damage is not extensive, it can be repaired…only if
Cell body and neurilemma intact and there is no scarring
In the CNS, repair is unlikely

29. Repair of Nerve Fibers
Following injury, distal portion of axon and myelin sheath degenerates
Macrophages remove debris
The neurilemma and endoneurium form a tunnel from the point of injury to the effector

30. Repair of Nerve Fibers
New Schwann cells grow in the tunnel to maintain the path for the regrowth of the axon
Cell body reorganizes its Nissl bodies to provide proteins
Axon “sprouts” and begins to fill tunnel
Skeletal muscle atrophies until nervous connection reestablished

31. Axons and Nerve Impulses
Axons end in axonal terminals
Axonal terminals contain vesicles with neurotransmitters
Acetylcholine, amines, amino acids, nitric oxide, neuropeptides, etc.
Axonal terminals are separated from the next neuron by a gap
Synaptic cleft – gap between adjacent neurons
Synapse – junction between nerves

32. The Reflex Arc
Reflex – rapid, predictable, and involuntary responses to stimuli
Reflex arc – direct route from a sensory neuron, to an interneuron, to an effector

33. Simple Reflex Arc

34. Types of Reflexes and Regulation
Autonomic reflexes
Smooth muscle regulation
Heart and blood pressure regulation
Regulation of glands
Digestive system regulation
Somatic reflexes
Activation of skeletal muscles

35. Functional Properties of Neurons
Irritability – ability to respond to stimuli
Conductivity – ability to transmit an impulse
Action potential refers to the membrane potential of a neuron that is conducting an impulse
The plasma membrane at rest is polarized
Fewer positive ions are inside the cell than outside the cell

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

37. 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

38. The Action Potential
The membrane potential of a nonconducting neuron’s plasma membrane is -70 mV
Depolarization allows the membrane potential to move towards zero

39. Nerve Impulse Propagation
Impulses travel faster when fibers have a myelin sheath

40. 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 of the next neuron

41. How Neurons Communicate at Synapses

42. Synaptic Transmission
Two types of synapses (junctions)
Electrical synapse occurs where cells joined by gap junctions allow an action potential to simply continue along the postsynaptic membrane
Chemical synapse occurs where presynaptic cells release neurotransmitters across a tiny gap to the postsynaptic cell, inducing an action potential

44. Synaptic Transmission
Action potential reaches a synaptic knob, causing release of calcium ions to diffuse into the knob
Increased calcium concentrations trigger the release of neurotransmitters via exocytosis
Neurotransmitters diffuse across the synaptic cleft and bind to receptor molecules causing ion channels to open
This causes postsynaptic potential

45. Synaptic Transmission
Neurotransmitter’s action is quickly terminated by returning neurotransmitters back into synaptic knob, or metabolized into inactive compounds

46. NEUROTRANSMITTERS
Neurotransmitters: means by which neurons communicate with one another; more than 30 compounds are known to be neurotransmitters, and dozens of others are suspected
Common classification of neurotransmitters:
Function
Chemical structure

48. NEUROTRANSMITTERS Acetylcholine Amines Amino acids
Present at various locations, sometimes in an excitatory role and other times inhibitory
Amines
Synthesized from amino acid molecules
Two categories: monoamines and catecholamines
Found in various regions of the brain, affecting learning, emotions, motor control
Amino acids
Believed to be among the most common neurotransmitters of the CNS
In the PNS, amino acids are stored in synaptic vesicles and used as neurotransmitters
Other small-molecule transmitters