Chapter 12: Neural Tissue
Learning Outcomes
• 12-1 Describe the anatomical and functional divisions of the nervous system.
Compare/contrast various divisions of the nervous system
• 12-2 Sketch and label the structure of a typical neuron, describe the functions of each
component, and classify neurons on the basis of their structure and function.
• 12-3 Describe the locations and functions of the various types of neuroglia.
• 12-4 Explain how the resting potential is created and maintained.
• 12-5 Describe the events involved in the generation and propagation of an action
potential.
• 12-6 Discuss the factors that affect the speed with which action potentials are
propagated.
• 12-7 Describe the structure of a synapse, and explain the mechanism involved in
synaptic activity.
• 12-8 Describe the major types of neurotransmitters and neuromodulators, and discuss
their effects on postsynaptic membranes. Identify how neurotransmitters discussed in
class affect the brain.
• Correctly list the 18 steps that occur in order for the contraction of a skeletal muscle to
take place.
• Describe Axoplasmic transport.
• Describe the steps of Wallerian degeneration and identify how the neuron is repaired
• Describe what happens to the brain when it is exposed to various drugs (cocaine, THC,
Ecstasy, alcohol, etc.)
• Describe what happens to dopamine in the brain of individuals with Parkinson’s disease.
• Be able to answer questions similar to clinical applications completed in class.
Chapter 12: Neural Tissue
I.
An Introduction to the Nervous System
A. The Nervous System
1. Includes all neural tissue in the body
2. Neural tissue contains two kinds of cells
a. Neuron: perform all of the communication, info processing and control
functions of nervous system
b. Neuroglia: support/protect neurons
3. Organs of nervous system: brain, spinal cord, receptors of sense organs, nerves
II.
12-1 Anatomical Divisions of Nervous System
A. Central Nervous System (CNS)
1. Composed of brain, spinal cord plus BVs and CTs
2. Functions to process and coordinate:
a. Sensory data from inside/outside body
b. Motor commands
c. Higher functions of brain including intelligence, memory, learning,
emotion
B. Peripheral Nervous System (PNS)
1. Includes neural tissue outside of CNS (cranial and spinal nerves)
2. Functions to:
a. Deliver sensory information to the CNS
b. Carry motor commands to peripheral tissues and systems
3. Functional Divisions of the PNS
a. Afferent Division
i.
ii.
Afferens = to bring to
Brings sensory info from PNS receptors to CNS
iii.
Receptors:
•
Detect changes in environment or respond to stimuli
•
Include neurons, specialized cells, sensory organs (eye
and ear)
b. Efferent Division
i.
Effero = to bring out
ii.
Brings motor commands from CNS out to PNS in muscles, glands
iii.
Effectors:
•
•
Cells and organs which respond to efferent signals
Examples: muscle fibers, secretory cells
Chapter 12: Neural Tissue
iv.
12-1 Two Components of Efferent Division
•
Somatic Nervous System (SNS)
•
•
•
•
Autonomic Nervous System (ANS)
•
•
•
III.
Controls voluntary and involuntary (reflex)
skeletal muscle contractions
Voluntary: under conscious control
Involuntary: controlled at subconscious level
Automatic regulation of smooth/cardiac
muscle, glands
Sympathetic division has a stimulating effect
(example: accelerating heart rate)
Parasympathetic division has a relaxing effect
(example: slowing heart rate)
12-2 Neurons
A. Structure of Neurons
1. Cell body
a. perikaryon: cytoplasm around nucleus
b. Contains nucleus, mitochondria (produce ATP) and Rough Endoplasmic
Reticulum (produces neurotransmitters)
c. Rough endoplasmic reticulum + ribosomes = Nissl bodies
i.
Give perikaryon a coarse, grainy appearance
ii.
Make up gray matter of brain and spinal cord
d. Cytoskeleton
i.
Neurofilaments and neurotubules
ii.
Proteins similar to intermediate filaments and microtubules
iii.
Bundles of neurofilaments = neurofibrils
2. Dendrites: slender, sensitive processes which receive information
3. Axons
a. Long cytoplasmic process extending from cell body
b. Carry impulse (AP) away from cell body
c. Axoplasm: cytoplasm of axon
d. axolemma: plasma membrane of axon
e. Initial segment: base of axon
f. Axon hillock: where initial segment joins cell body
g. telodendria: fine extensions at the end of an axon which end at synaptic
terminals
Chapter 12: Neural Tissue
4. Synapse
a.
b.
c.
d.
Specialized site where neuron communicates with another cell
Presynaptic cell: cell which sends a message
Postsynaptic cell: cell which receives the message
Synaptic cleft: narrow space separating presynaptic and postsynaptic
cell
e. Neurotransmitters: chemicals which affect activity of postsynaptic cell
f. Neuromuscular junction: synapse between neuron and muscle fiber
g. Neuroglandular junction: synapse between neuron and secretory cell
5. Neurons lack centrioles so typical CNS neurons cannot divide
a. Stem cells present but mostly inactive after age 4
b. Exception: stem cells active in nose and hippocampus
6. Axoplasmic transport
a. Movement of materials between cell body and synaptic terminal
b. Materials pulled by kinesin and dynein powered by ATP
c. Allows neurotransmitters to travel to synaptic terminal and materials to
return to cell body for reassembly of neurotransmitters
d. anterograde flow:
i.
ii.
iii.
Antero = forward
kinesin carries materials from cell body to synaptic terminal
How some toxins (like heavy metals), bacteria and viruses
bypass CNS defenses
7. Retrograde flow:
i.
ii.
iii.
Retro = backward
dynein carries materials toward cell body from synaptic
terminal
How rabies virus moves to CNS
Chapter 12: Neural Tissue
B. The Structural Classification of Neurons
Classification
Description/Function
Location
anaxonic


Small, all processes look alike
Function poorly understood
brain and sense organs
bipolar

special sensory organs

Small, one dendrite/one axon (2 distinct
processes)
Relay information about sight, smell and
hearing from receptors to other neurons
unipolar

Long axons, cell body to one side
sensory neurons of PNS
multipolar


Most common in CNS
very long single axon, multiple dendrites
Include all skeletal muscle motor neurons
C. The Functional Classification of Neurons
Classification
Sensory Neurons
(afferent neurons
of PNS)
Structure/Location
•
•
•

Motor Neurons
(efferent neurons
of PNS)
Unipolar
Cell bodies grouped in sensory ganglia
Afferent fibers extend from sensory
receptors to CNS
•
•
Contain axons traveling away from CNS
(efferent fibers)

•


Interneurons
(association
neurons)
Function
Found in brain and spinal cord

Deliver information from receptors to CNS
Visceral sensory neurons: monitor internal
environment
Somatic sensory neurons: monitor external
environment
Somatic motor neurons: innervate muscle
fibers allowing for contraction
Visceral motor neurons: innervate smooth
muscle, cardiac muscle, glands allowing for
involuntary control
Coordinate motor activity, distribute
sensory info, memory, planning, learning
Chapter 12: Neural Tissue
D. Types of Sensory Receptors
1. Interoceptors (inside) monitor:
a. internal systems (digestive, respiratory, cardiovascular, urinary,
reproductive)
b. Internal senses (deep pressure, pain, distension)
2. Exteroceptors (outside) monitor:
a. External senses (touch, temperature, pressure)
b. Distance senses (sight, smell, hearing)
3. Proprioceptors monitor position/movement of skeletal muscles/joints
IV.
12-3 Neuroglia
A. Supporting cells of neural tissue
B. Functions:
1. Separate/protect neurons
2. Provide framework for neural tissue
3. Regulate composition of interstitial fluid
4. Act as phagocytes
C. Neuroglia of CNS
1. Ependymal cells
a. Line central canal of spinal cord and ventricles (chambers) of brain
b. Form epithelium known as ependyma
c. Assist in producing, circulating and monitoring cerebrospinal fluid
2. Astrocytes
a. Largest and most numerous
b. Functions:
i.
Maintain blood-brain barrier
ii.
Provide structural support
iii.
Regulate ion, nutrient and dissolved gas concentrations
iv.
Absorb and recycle neurotransmitters
v.
Form scar tissue after injury
Chapter 12: Neural Tissue
3. Oligodendrocytes
a. Provide structural framework
b. Myelinate CNS axons
i.
myelination: wrapping of oligodendrocyte plasma membrane
around axolemma
ii.
iii.
Increases speed at which AP travels along axon
Nodes:
• gaps between internodes that are unmyelinated
•
aka nodes of Ranvier
iv.
internodes: large areas of axons wrapped in myelin
c. Myelinated vs. Unmyelinated Axons
i.
Myelinated
• Axons surrounded by myelin
• White appearance due to lipids in myelin
• Regions dominated by myelinated axons = white matter
ii.
Unmyelinated
• Axons not completely covered by processes of neuroglia
• Gray color = Gray matter
4. Microglia
a. Smallest, least numerous
b. Remove cell debris, wastes and pathogens via phagocytosis
D. Neuroglia of PNS
1. Satellite cells
a. Surround neuron cell bodies in ganglia
b. Ganglia: clustered mass of cell bodies of neurons in PNS
c. Regulate oxygen and carbon dioxide, nutrients and neurotransmitter
levels around neurons in ganglia
2. Schwann cells
a. Surround all axons in PNS
b. Responsible for myelination
c. Participate in repair after injury
V.
12-3 Neural Responses to Injuries
A. Wallerian degeneration
1. Process in PNS where axon distal to injury degenerates
2. Schwann cells
a. Form path for new growth
b. Wrap new axon in myelin
Chapter 12: Neural Tissue
B. Steps of peripheral nerve regeneration after injury
1. Axon distal to injury site degrades
2. Schwann cells form cord, grow into cut and unite stumps and macrophages
clean up debris
3. Axon sends buds into network of Schwann cels and grows into site of injury
4. Axon continues to grow and is enclosed by Schwann cells
C. Nerve Regeneration in CNS
1. Limited because:
a. More axons generally damaged
b. Astrocytes produce scar tissue
c. Astrocytes release chemicals that block growth of axons
VI.
12-4 Transmembrane Potential
A. Ion Movements and Electrical Signals
1. All plasma (cell) membranes produce electrical signals by ion movements
2. Transmembrane potential is particularly important to neurons
3. Five Main Membrane Processes in Neural Activities
a. Resting potential: transmembrane potential of resting cell
b. Graded potential: temporary, localized change in resting potential
caused by a stimulus
c. Action potential: electrical impulse produced by graded potential
propagated along axon to synapse
d. Synaptic activity: releases neurotransmitters producing graded
potentials in postsynaptic cell membrane
e. information processing: response of postsynaptic cell
B. Changes in Transmembrane Potential
1. Resting potential exists because
a. Cytosol and extracellular fluid differ in ion composition
b. Plasma membrane is selectively permeable
Chapter 12: Neural Tissue
2. Types of membrane channels
a. Passive channels or leak channels are always open and their
permeability varies
b. Active channels (aka gated channels) open or close in response to
specific stimuli
i.
Chemically gated channels open/close when bind to specific
chemicals. Example: ACh receptors at neuromuscular junction
ii.
Voltage- gated channels: open/close in response to changes in
transmembrane potential
iii.
Mechanically gated channels: open/close in response to
physical distortion of membrane surface by touch, pressure or
vibration
VII.
12-5 Action Potential
A. Action Potentials
1. Propagated changes in transmembrane potential
2. Affect an entire excitable membrane
3. Link graded potentials at cell body with motor end plate actions
B. Steps of an Action Potential
1. Dendrites receive information, voltage gates open
2. Na+ goes in, gates close
3. Potassium exits, gates close (NIKE)
4. Continues until generates impulse at trigger zone
5. Impulse travels down axon, opens Ca2+ voltage channels
6. Ca2+ goes in and fuses with synaptic vesicles
7. Triggers release of ACh into synaptic cleft
C. Propagation of Action Potentials
1. Continuous propagation
a. Occurs in unmyelinated axons
b. Action potential moves across membrane in series of tiny steps
2. Saltatory propagation
a. More rapid than continuous propagation and requires less energy
b. Occurs in myelinated axons
c. Action potential “jumps” from node to node, rather than moving in
series of tiny steps
Chapter 12: Neural Tissue
VIII.
IX.
12-6 Axon diameter and myelin affects propagation speed
A. Axons can be classified into three groups according to relationships among diameter,
myelination and propagation speed
B. Type A Fibers
1. Largest Myelinated axons
2. Fastest propagation speed
3. Carry sensory information about position, balance, delicate touch/pressure from
skin to CNS
4. Also include motor neurons that control skeletal muscles
C. Type B Fibers
1. Smaller Myelinated axons
2. Carry information to and from CNS
3. Also carry information to smooth and cardiac muscle and glands
D. Type C Fibers
1. Unmyelinated, small axons
2. Slowest
3. Carry information to and from CNS
4. Also carry information to smooth and cardiac muscle and glands
12-7 Synapses
A. Synaptic Activity
1. Action potentials (nerve impulses)
a. transmitted from presynaptic neuron to postsynaptic neuron (or other
postsynaptic cell) across a synapse
b. Most are chemical synapses, transmitting signal across a gap (synaptic
cleft) by chemical neurotransmitters
B. Two Classes of Neurotransmitters
1. Excitatory neurotransmitters: promote action potentials
2. Inhibitory neurotransmitters: suppress action potentials
Chapter 12: Neural Tissue
C. Examples of Neurotransmitters
Neurotransmitter
Action
Acetylcholine (ACh)
•
•
Changes permeability of postsynaptic cell membrane
Allows AP to move from one cell to another by stimulating sodium ion channels to
open
Norepinephrine (NE)
•
•
Distributed in brain and portions of ANS
Has an excitatory effect on postsynaptic cell
•
•
CNS neurotransmitter in brain
Inhibitory effect: release in one portion of brain prevents overstimulation of
neurons controlling muscle tone (helps control movements)
Excitatory effect: promote action potentials
Dopamine
•
Serotonin
Gamma-aminobutyric
acid
(GABA)
•
•
•
CNS neurotransmitter
Affects attention and emotional state
Antidepressants inhibit reabsorption of serotonin = increased serotonin
concentrations = relieve symptoms of depression
•
•
•
CNS neurotransmitter
Inhibitory effect, reduces anxiety
Some antianxiety drugs enhance GABA effect of reducing anxiety
D. Alcohol, drugs and Parkinson’s disease
1. Alcohol: increases GABA and decreases glutamate = increase in dopamine
2. Cocaine: inhibits removal of dopamine from synapses = “high”
3. THC: stimulates release of dopamine
4. Parkinson’s: damage/degeneration of dopamine producing neurons
5. Ecstasy: targets serotonin receptors
Chapter 12: Neural Tissue
E. Neuromodulators
1. Other chemicals released by synaptic terminals
2. Similar in function to neurotransmitters
3. Alter rate of neurotransmitter release or change postsynaptic cell’s response to
neurotransmitters
4. Generally have long-term effects that are slow to appear
Examples of Neuromodulators
Neuromodulator
Action
Neuropeptides
•
Bind to receptors and activate enzymes
Opioids
•
•
•
Primary function: relieve pain
Bind to same receptors as opium/morphine
Four classes: endorphins, enkephalins, endomorphins and
dynorphins