Download Neural Tissue – Chapter 12

Neural Tissue – Chapter 12
Nervous System - Includes all neural tissue in the body
Two kinds of cells:
Neuron
Neuroglia
Organs of the nervous system:
There are two major anatomical divisions of the nervous system:
1. Central Nervous System (CNS)
2. Peripheral Nervous System (PNS)

Nerve – bundles of axons with blood vessels and connective tissue
 Cranial nerves – nerves connected to the brain
 Spinal nerves – nerves connected to the spinal cord

Two divisions:
a. Afferent division
b. Efferent division

Two components:
1. Somatic Nervous System (SNS)

Also controls reflexes – an involuntary contraction controlled at the subconscious level to
remove the body from harm
 Ex) Jerking your hand off a hot pot
2. Autonomic Nervous System (ANS)
Two Divisions:
Sympathetic division
Ex) accelerates heart rate
Parasympathetic division
Ex) slows heart rate
Neurons



Basic functional unit of the nervous system
Most neurons have a large cell body (soma) that is connected to a single, elongate axon and several short,
branched dendrites.
Most neurons lack centrioles and cannot divide.
Structure of a Neuron (multi-polar neuron)
1. The Cell Body (soma)
 Consists of:



Perikaryon contains neurofilaments and neurotubules.
Contains organelles that provide energy and synthesize organic materials such as the neurotransmitters
necessary for cell-to-cell communication
Nissl Bodies
2. Dendrites
3.
Axons

Structure of the Axon

Axon hillock

Initial Segment

Collateral

Telodendria

Synaptic terminal
The Synapse

Involves
o Presynaptic cell –
o
Postsynaptic cell –
o
Synaptic cleft –
Synaptic Knob (synaptic terminal)
 The communication between cells at a synapse usually involves chemicals called neurotransmitters that are
released by the synaptic terminal.

Neurotransmitters:
– are chemical messengers
– are released at presynaptic membrane
– affect receptors of postsynaptic membrane
– are broken down by enzymes
– are reassembled at synaptic knob
Types of synapes
 Neuromuscular junction –

Neuroglandular junction –
A relatively round synaptic knob occurs where the postsynaptic cell is another neuron. At a synapse, a narrow synaptic
cleft (space) separates the presynaptic membrane (where neurotransmitters are released) from the postsynaptic
membrane (which bears receptors for the neurtransmitters)
The Classification of Neurons
Neurons can be grouped by structure or by function.
1. Structural Classification
Neurons are classified structurally on the basis of the relationship of the dendrites to the cell body and the axon.
This includes: ( I strongly suggest drawing an example of these as well…)
a. anaxonic
b. bipolar neurons
c. unipolar neuron
d. multipolar neuron
2. Functional Classification
Neurons can also be categorized by function as sensory neurons, motor neurons, and interneurons.
a. sensory neurons
b. motor neurons
c. Interneurons
Neuroglia (supporting cells)
Account for roughly half the volume of the nervous sytem
Neuroglia of the Central Nervous System
The central nervous system has four types of neuroglia: ependymal cells, astrocytes, oligodendrocytes, and microglia
1. ependymal cells
2. astrocytes
3. oligodendrocytes
Myelination
Internode
Nodes of Ranvier
White matter
Gray Matter
4. microglia
Neuroglia of the Preipheral Nervous System
 The cell bodies of neurons in the PNS are clustered in masses called ganglia.
 There are two types of neuroglia in the PNS:
a. satellite cells
b. Schwann cells
Ion Movements and Electrical Signals
1. Transmembrane Potential (voltage difference btwn inside and outside of cell)
Resting potential
Three important concepts regarding the transmembrane potential:
a. Extracellular fluid contains high concentrations of sodium and chloride ions while the cytosol has high
concentrations of potassium ions and negatively charged particles.
b. Equilibrium does not exist across cell membranes due to the fact that they have selectively permeable
membranes. At the resting potential ion movement occurs through leak channels (channels that are always
open).
c. Since membrane permeability varies by ions, there is not an equal distribution of charges across the
membrane. As a result, the inner surface has an excess of negative charges with respect to the outer surface.
Forces acting across the membrane
The passive forces acting across the membrane are both chemical and electrical in nature.
A. Chemical Gradients
B. Electrical Gradient
The size of a potential difference is measured in millivolts (mV). The average resting potential for most cells is
-70mV. The minus sign signifies that the inner surface of the cell membrane is negatively charged with respect to the
exterior.
Chemical Gradient + Electrical Gradient = Electrochemical Gradient
Changes in the Transmembrane Potential
A. Passive channels (Leak channels)
B. Active channels (gated channels)

Each gated channel can be in one of three states:
1. closed but capable of opening
2. open (activated)
3. closed an incapable of opening (inactivated)
Three classes of gated channels exist:
a. chemically gated channels
b. voltage-gated channels
c. mechanically gated channels
2. Graded Potentials
What happens to the resting membrane when chemicals open sodium channels?
Step One: Sodium ions enter the cell and are attracted to the negative charges along the inner surface of the
membrane. The arrival of positive charges shifts the transmembrane potential toward 0 mV. This is called
depolarization.
Step Two: At the resting potential, sodium ions are drawn to the outer surface of the cell membrane, attracted by the
excess of negative ions on the inside of the membrane. As the cell membrane depolarizes, sodium ions are released
from its outer surface. These ions move toward the open channels, replacing ions that have already entered the cell.
This is called local current. (As sodium ions move into the cell, other sodium ions fill in the gaps.) This causes
adjacent portions of the cell membrane to become depolarized.
The degree of depolarization decreases with distance away from the stimulation site and because some of the sodium
ions move back across the membrane through the sodium leak channels.
Step Three: When a chemical stimulus is removed and normal membrane permeability is restored, the transmembrane
potential soon returns to resting levels. This is called repolarization.
What happens to the resting membrane when chemical open potassium channels?
Opening a gated potassium channel would have the opposite effect on the transmembrane potential as opening a gated
sodium channel.
Step One: The rate of potassium outflow would increase, and the interior of the cell would lose positive ions. This is
called hyperpolarization. When this happens an increase in the negativity of the resting potential occurs.
Step Two: The local current will distribute the effect to adjacent portions of the cell membrane, and the effect would
decrease with distance from the open channel or channels.
Step Three: Repolarization occurs when the chemical stimulus is removed.
Graded potentials are often the trigger for specific cell functions. It is the graded depolarization of the motor end plate
by ACh that triggers an action potential in adjacent portions of the sarcolemma.
3. Action Potentials
 Propagated changes in the transmembrane potential that, once initiated, affect an entire excitable membrane.
 The first step is the opening of voltage-regulated sodium ion channels at one site, usually the initial segment of the
axon. The movement of the sodium ions into the cell depolarizes adjacent sites, triggering the opening of
additional voltage-regulated channels. The result is a chain reaction that spreads across the surface of the
membrane, propagating the signal down the length of the axon toward the synaptic terminals.
All-or-None Principle
 The stimulus that initiates an action potential is a depolarization large enough to open voltage-regulated
sodium channels. That opening occurs at a transmembrane potential known as the threshold. The threshold
for a typical axon is between -60mV and -55 mV.
 If the stimulus does not shift the resting membrane potential at least 10-15mV you will only produce a graded
depolarization and not an action potential.
 The depolarization of the initial segment of the axon is caused by local currents resulting from the graded
depolarization of the axon hillock.
 All-or-None principle states that either an action potential is generated or it is not.
Generation of Action Potentials
At the normal resting potential, the activation gates of the voltage-regulated sodium channels are closed.
The steps to generate an action potential are as follows:
Step One: Depolarization to Threshold.
Step Two: Activation of Sodium Channels and Rapid Depolarization.
Step Three: Inactivation of Sodium Channels and the Activation of Potassium Channels.
Step Four: The Return to Normal Permeability.
Refractory Period

Two portions:
a. absolute refractory period
b. relative refractory period
The Role of the Sodium-Potassium Exchange Pump
Propagation of Action Potentials
An action potential is relayed from one location to another in a series of steps. At each step, the message is repeated.
Because the same events take place over and over, the term propagation is used. Action potentials travel down an axon
by continuous propagation (unmyelinated axons) or by salutatory propagation (myelinated axons).
A. Continuous Propagation
 The basic mechanism by which an action potential is propagated along an unmyelinated axon
 It occurs at a speed of 1 m/s
 The action potential begins at the initial segment causing the transmembrane potential to become positive. A local
current develops as sodium ions begin moving in the cytosol and the extracellular fluid. The local current spreads
in all directions, depolarizing adjacent portions of the membrane. The axon hillock cannot respond with an action
potential because it lacks voltage-gated channels, but they develop there. Each time a local current develops, the
action potential moves forward, but not backward, because the previous segment of the axon is still in the absolute
refractory period. As a result, action potentials always proceed away from the site of generation and cannot
reverse direction.
B.



Saltatory Propagation
The basic mechanism by which an action potential is propagated along a myelinated axon
Carries nerve impulses faster than continuous propagation
Continuous propagation cannot occur along a myelinated axon, because myelin increases resistance to the flow of
ions across the membrane. Ions can readily cross the cell membrane just at the nodes. As a result, only the nodes
can respond to a depolarizing stimulus.
 When an action potential appears at the initial segment of a myelinated axon, the local current skips the internodes
and depolarizes the closest node to threshold. The action potential jumps from node to node rather than moving
along the axon in a series of tiny steps.
Axon Diameter and Propagation Speed
 The larger the diameter the lower the resistance, and the faster the propagation speed. This is because cytoplasm
offers less resistance than the cell membrane.

Axons are classified into three groups according to the relationships among the diameter, myelination, and
propagation speed:
1. Type A fibers
2. Type B fibers
3. Type C fibers
All axons can not be type A fibers because your nerves would be the size of garden hoses. Messages in the nervous
system are routed according to priority. The most important messages (messages that deal with survival and motor
commands) move along Type A fibers. Less urgent messages travel along either Type B or C fibers depending on their
importance.
Synaptic Activity
 Synapses can be electrical or chemical:
1. Electrical Synapses
2. Chemical Synapses

There are two types of neurotransmitters:
a. Excitatory neurotransmitters
b. Inhibitory neurotransmitter
Closer look at a synapse that releases Acetylcholine
Cholinergic Synapses
 Synapses that release ACh
 This includes:
o All neuromuscular junctions involving skeletal muscle fibers
o Many synapses in the CNS
o All neuron-to-neuron synapses in the PNS
o All neuromuscular and neuroglandular junctions within the parasympathetic division of the ANS
Events at a Cholinergic Synapse
Step One: An action potential arrives and depolarizes the synaptic knob
Step Two: Extracellular calcium ions enter the synaptic knob, triggering the exocytosis of ACh.
 The depolarization of the synaptic knob opens voltage-regulated calcium channels.
 Calcium rushes into the knob and triggers exocytosis and the release of ACh.
 The calcium is very quickly removed from the cytoplasm by active transport
Step Three: ACh binds to receptors and depolarizes the postsynaptic membrane
 The ACh released through exocytosis diffuses across the synaptic cleft toward receptors on the postsynaptic
membrane.
 The receptors are chemically regulated ion channels that increase the permeability to Na+ which produces a
depolarization that lasts about 20 msec.
 The depolarization is a graded potential: The greater the ACh released, the larger the depolarization. An action
potential will occur if threshold is reached.
Step Four: ACh is removed by AChE
Acetylcholinesterase is an enzyme that breaks down ACh into acetate and choline.
The choline is actively absorbed by the synaptic knob to synthesize more ACh.
Synaptic Delay
Synaptic Fatigue
The Activities of Other Neurotransmitters
Major categories of neurotransmitters include: biogenic amines, amino acids, neuropeptides, dissolved gases, and a
variety of other compounds.
Some of the most important neurtransmitters include:

Norepinephrine (NE)

Dopamine

Serotonin

Gamma aminobutyric acid (GABA)
Postsynaptic Potentials
Graded potentials that develop in the postsynaptic membrane in response to a neurotransmitter
Two major types develop at neuron-neuron synapses: excitatory and inhibitory
1. Excitatory Postsynaptic Potential (EPSP)
2. Inhibitory Postsynaptic Potential (IPSP)
Inhibition
Summation
 An individual EPSP typically produces a depolarization of 0.5 mv. Unfortunately, the initial segment must be
depolarized by at least 10mV before an action potential will occur.
 Individual EPSPs combine through the process of summation which integrates the effects of all the graded
potentials that affect one portion of the cell membrane.
 There are two types of summation: temporal and spatial.
1. Temporal Summation
2. Spatial Summation