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Nervous System
Chapter 9
Pages 211-257
Chapter 9 Wordbytes
1. af- = toward 11. -ferrent = carried2. arachn- = spider 12. gangli- = swelling3. astro- = star 13. -glia = glue4. auto- = self 14. mening- =membrane 5. dendro- = tree 15. micro- = small6. di- = 2, through 16. neuro- = nerve7. ef- = away from 17. –oid = similar to8. encephalo- = brain 18. oligo- = few9. enter- = intestines 19. peri- = around10. epen- = above 20. somat- = body
Nervous System Overview
Master controller and communicator for the body
Responsible for all behavior 3 functions:
Sensory input monitors changes inside/outside of body
Integration processes and interprets, then decides what should be done
Motor output causes a response in effector organs
Organization—2 main parts:
1. Central Nervous System (CNS) = brain and spinal cord
Interprets incoming sensory info. and dictates motor responses
2. Peripheral Nervous System (PNS) = nerves from brain & in spinal cord
INPUT-Afferent or Sensory division OUTPUT- Efferent or Motor division Subdivided: Somatic (SNS—from CNS to
skeletal muscles=voluntary) & Autonomic (ANS—regulate smooth & cardiac muscle, glands=involuntary)
Major structures:
Histology
Highly cellular—densely packed & tightly intertwined
2 types of cells:1. Neuron= nerve cell
• Specialized for signal carrying & information processing
2. Neuroglia cells support, nourish & protect neurons• Neuroglia critical for homeostasis of interstitial fluid
around neurons
Supporting cells (Neuroglia)
~ half the volume of CNS Cells smaller than neurons Can multiply and divide and fill in
brain areas Do not conduct nerve impulses
Supporting Cells in CNS
Astrocytes most abundant and most versatile; blood-brain barrier
Oligodendrocytes have fewer branches; produce insulating myelin sheath in CNS
Microglia ovoid cells with thorny processes; provide defense (because immunity cells not allowed in CNS)
Ependymal cells squamous/columnar cells with cilia; produce cerebrospinal fluid (CSF)
Supporting Cells in PNS
Schwann cells PNS cell support; produce & maintain myelin sheath, regenerate PNS axons
Satellite cells in PNS ganglia; support neurons in ganglia, regulate exchange of materials between neurons and interstitial fluid
Neuron Characteristics
They conduct nerve impulses from one part of the body to another
They have extreme longevity live/function for a lifetime
They are amitotic lose their ability to divide
They have a high metabolic rate = need O2 and glucose
Neuronal Structure Cell body nucleus, cytoplasm with typical
organelles; most within CNS = protected by cranial bones & vertebrae
Dendrites short, highly branched input structures emerging from cell body = high surface area to receive signals
Axon may be short or long, only one per neuron; 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
Figure 9.3
Pg. 391
(Neurilemma)
= impulse direction
Myelination
Axons covered with a myelin sheath Many layered lipid & protein creating insulations Increases speed of nerve conduction. Formed by:
• Schwann cells in PNS (pg. 393 fig. 11.5)• Oligodendrocytes in CNS
Nodes of Ranvier= gaps in the myelin Nodes are important for signal conduction
Some diseases destroy myelin multiple sclerosis & Tay-Sachs
Gray and White Matter
White matter- primarily myelinated axons Gray matter- nerve cell bodies, dendrites,
unmyelinated axons, axon terminals & neurogliaSpinal cord gray matter is centrally located
Classification of Neurons
1. Structural according to # of processes:
Multipolar 3 or more; most common, especially in CNS
Bipolar 2 processes (an axon and a dendrite) that extend from opposite sides; found in special sense organs
Unipolar 1 process that divides like a T; found in ganglia in PNS
2. Functional according to the direction impulse travels (Table 11.1)
Sensory (afferent) neurons transmit impulses from sensory receptors toward or into the CNS; mostly unipolar, with cell bodies in ganglia outside CNS
Motor (efferent) neurons carry impulses away from CNS to muscles and glands; multipolar, usually with cell bodies in CNS
Interneurons (association neurons) between motor & sensory neurons; most in CNS; 99% of neurons in body; mostly multipolar
Neurophysiology
Neurons are highly irritable = responsive to stimuli
When stimulated, an electrical impulse (action potential) is conducted along its axonAction potential underlies all functional
activities of the nervous system
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
Ion Channels
Allow specific ions to diffuse across membraneMove 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
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
Figure 9.4
Graded Potentials
Short-lived, local changes to membrane potential
Cause current flows that decrease with distance
Magnitude varies with strength of stimulus
Action Potential (AP)
Generated by neurons and muscle cells
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
Action Potential
Resting statevoltage-gated channels closed
Depolarizing phase- membrane potential rises and becomes
positive Repolarizing phase-
potential restored to resting value ( PNa, PK)
UndershootPotassium permeability continues
Figure 9.5
Active Events
Stimulus to reach threshold Na+ channel opens=> Na+ ions enter=> positive potential=> Causes K+ channel opening => repolarization
All- or –None Phenomenon
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= absolute refractory period each AP is a separate, all-or-none event; enforces one-way transmission of AP
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
Figure 9.6a
Figure 9.6b
The Syanpse
Synapse (to clasp or join)- junction that mediates information transfer from 1 neuron to another or from a neuron to an effector cell
Axodendritic or axosomatic synapses – most synapses occur between the axonal ending of a neuron and the dendrites or cell body of other neurons
Synaptic Transmission – Electrical synapse
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
Events at Synapse – Chemical 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
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
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
Figure 9.7
Neurotransmitters
AcetylCholine (Ach)- common in PNS Biogenic amines - Norepinephrine (NE), Dopamine (DA),
serotonin, Histamine
Amino Acids- Glutamate, Aspartate, gamma aminobutyric
acid (GABA), glycine Neuropeptides – endorphins Novel Messengers - ATP/ Nitric oxide (NO)/
Carbon monoxide (CO)
Development of Neurons
P. 422-424 Neuroblasts Growth cone Programmed cell death
Web sites:
http://www.sciencecases.org/chin/chin.asp http://www.pbs.org/wgbh/nova/sciencenow/3204/01.html http://www.getbodysmart.com/ap/nervoussystem/menu/menu.h
tml http://www.bbc.co.uk/science/humanbody/body/interactives/3dji
gsaw_02/index.swf?startPosition=nervous http://learn.genetics.utah.edu/units/addiction/reward/madneuro
n.cfm http://www.gpc.edu/~bbrown/peril/neurons/level1.htm