1. Nervous Tissue
Chapter 9

2. Structures of Nervous System
Brain- Neurons enclosed in skull
Spinal cord– connects to brain & enclosed in spinal cavity
Nerves- bundles of neuronal axons
Cranial emerge from brain; spinal nerves- emerge from spinal
cord
Ganglia- groups of cell bodies outside brain & spinal cord
Enteric plexuses- networks in digestive tract
Sensory receptors- monitor changes in internal or external environments

3. Figure 9.1

4. Function Sensory Receptors & afferent nerves
Carry information into brain & spinal cord
Integration- information processing
Perception = awareness of sensory input
Carry by short interneurons
Motor activity- efferent nerves
Signals to glands and muscles (effectors)

5. Organization Central Nervous System (CNS)
Peripheral Nervous System (PNS)
Subdivided: Somatic (SNS) & Autonomic (ANS) nervous systems
Also
INPUT-Afferent or Sensory division
OUTPUT- Efferent or Motor division

6. Figure 9.2

7. Nervous System Neuron= nerve cell
Specialized for signal carrying & information processing
Neuroglia cells-support, nourish & protect neurons
Neuroglia critical for homeostasis of interstitial fluid around neurons

8. Neuronal Structure
Cell body- nucleus, cytoplasm with typical organelles
Dendrites- highly branched input structures emerging from cell body
Axon- conducts away from cell body toward another neuron or effector
Emerges at cone-shaped axon hillock
Axon terminals -at end of axon with synaptic bulbs

9. Figure 9.3

10. Myelination Axons covered with a myelin sheath
Many layered lipid & protein creating insulations
Increases speed of nerve conduction.
Nodes of Ravier= gaps in the myelin
Nodes are important for signal conduction
Some diseases destroy myelin E.g. multiple sclerosis & Tay-Sachs

11. Gray and White Matter White matter- primarily myelinated axons
Gray matter- cell bodies, dendrites, unmyelinated axons, axon terminals & neuroglia
spinal cord gray matter is centrally located

12. Other terms
Gray matter in brain covers surface of cerebrum & cerebellum – cortex
deep cluster of neuronal cell bodies = nucleus
Bundle of white matter in CNS= Tract

13. Neuroglia ~ half the volume of CNS Cells smaller than neurons
Can multiply and divide and fill in brain areas
Do not conduct nerve impulses

14. Neuroglia-support Astrocytes- blood brain barrier
Oligodendrocytes- myelin in CNS
microglia - defense
Ependymal cells- CSF production
Schwann- PNS cell support
Satellite cells- in PNS ganglia

15. Action Potentials Action potentials = nerve impulses
Require a membrane potential
electrical charge difference across cell membrane – like a Battery
Ion Channels- allow ions to move by diffusion = current
If no action potential then resting cell has resting membrane potential

16. Ion Channels Allow specific ions to diffuse across membrane
Move from high concentration to low
or toward area of opposite charge
Leakage channels
Gated channels- require trigger to open
Voltage- Gated channels respond to a change in membrane potential

17. Resting Membrane Potential
Leakage channels
Cytosol high in K+ & interstitial fluid high in Na+ (sodium –potassium pumps)
Leakage lets K+ through easily and Na+ poorly
inside is negative relative to outside
actual value depends on the relative leakage channel numbers

18. Figure 9.4

19. Action Potential (AP) Series of active events
Channels actively open & close
Some initial event is required to reach a voltage threshold (~ = - 55 mv)
Stimulus = any event bringing membrane to threshold

20. Action Potential Then Depolarizing phase- Repolarizing phase-
membrane potential rises and becomes positive
Repolarizing phase-
potential restored to resting value
May overshoot =hyperpolarizing phase
Then recovery to rest.

21. Active Events Stimulus to reach threshold Na+ channel opens=>
Na+ ions enter=>
positive potential=>
Causes K+ channel opening =>
repolarization

22. All- or -None This sequence is always the same
If threshold then the same size of changes occur no larger or smaller APs
Stimulus must reach threshold to start
After one AP there is a short period before next can be triggered= refractory period

23. Figure 9.5

24. Conduction of Nerve Impulses
Each section triggers next locally
Refractory period keeps it going the right direction
unmyelinated fiber- continuous conduction
With myelin- saltatory conduction
Can only be triggered at Nodes of Ranvier
Myelinated fibers faster & larger neurons faster

25. Figure 9.6a

26. Figure 9.6b

27. Synaptic Transmission
Sequence of events at synapse
Triggered by voltage change of the Action Potential
Sending neuron = presynaptic
Receiving neuron = postsynaptic
Space between = synaptic cleft
Neurotransmitter carries signal across cleft

28. Events at Synapse AP arrives at presynaptic end bulb=>
Opens voltage gated Ca2+ channels=>
Ca2+ flows into cell
increased Ca2+ concentration =>
exocytosis of synaptic vesicles=>
Neurotransmitter released into cleft
Diffuse across and bind to receptors in postsynaptic cell membrane

29. Synaptic Transmission
Binding at receptors
Chemical trigger of ion channels
May depolarize or hyperpolarize postsynaptic cell membrane
If threshold reached at axon hillock then postsynaptic cell action potential results

30. Synaptic Transmission
Finally the neurotransmitter must be removed from the cleft-
Diffusion away
Destroyed by enzymes in cleft
Transport back into presynaptic cell
Neuroglia destruction

31. Figure 9.7

32. Neurotransmitters AcetylCholine (Ach)- common in PNS Amino Acids-
May be stimulatory or inhibitory
Amino Acids-
Glutamate, Aspartate, gamma aminobutyric acid (GABA), glycine
Modified amino acids-
Norepinephrine (NE), Dopamine (DA), serotonin
Neuropeptides – endorphins
Nitric oxide (NO)