Title: Nervous Tissue
1Nervous Tissue
2Functions of the Nervous System
- Along with the endocrine system, the nervous
system helps to maintain homeostasis in the body - Homeostasis is the state of physiological balance
that the body attempts to maintain - The nervous system regulates body activities by
responding rapidly using nerve impulses, or
action potentials
3Functions of the Nervous System (continued)
- The endocrine system responds slowly by releasing
chemical messengers called hormones - The specific nervous system functions can be
grouped as follows - Sensory functiondetects stimuli, changes in the
internal or external environment - Integrative functionprocesses and analyzes
sensory information so that an effective response
can be made - Motor functionresponds to the stimuli
4Organization of the Nervous System
- Central Nervous System (CNS)consists of the
brain and spinal cord - Peripheral Nervous System (PNS)includes all
nervous tissue outside the CNS, such as cranial
nerves (12 pair) and spinal nerves (31 pair)
5Organization of the Nervous System (continued)
- The PNS is subdivided further into the following
- Somatic Nervous System (SNS)consists of neurons
that cause skeletal muscles to respond voluntary - Autonomic Nervous System (ANS)consists of
neurons that cause smooth muscle, cardiac muscle,
and glands to respond involuntary - Enteric Nervous System (ENS)sometimes included
within the ANS consists of neurons that control
responses within the digestive, or
gastrointestinal tract involuntary
6Organization of the Nervous System (continued)
- The ANS is further subdivided into the following
- Sympathetic Divisioncontrols fight-or-flight
responses - Parasympathetic Divisioncontrols
rest-and-digest responses
7Cells within the Nervous System
- Neuronslike muscle cells, they are excitable
(can produce action potentials/nerve impulses in
response to stimuli) - Neurogliasupport, nourish, and protect the
neurons
8Neurons
- Cell Bodycontains the nucleus, cytoplasm, and
typical cellular organelles - Dendritesmultiple processes that receive
impulses and conduct them to the cell body - Axonsusually single processes that carry
impulses away from the cell body to an effector
(another neuron, a muscle fiber, or a gland)
9Types of Neurons (Structural)
- Multipolar Neuronsusually have several dendrites
and one axon most neurons in the brain and
spinal cord are this type have many processes - Bipolar Neuronshave one main dendrite and one
axon found in the retina of the eye, the inner
ear, and in the olfactory area of the brain have
two processes - Unipolar Neuronshave one main process
10Types of Neurons (continued)
- Myelinated neuronsneurons whose axons are
surrounded by a myelin sheath, a multilayered
lipid and protein covering the myelin insulates
the axon and increases the speed of impulse
conduction gaps in the myelin sheath are called
nodes of Ranvier - Unmyelinated neuronsneurons whose axons do not
have a myelin sheath
11Types of Neurons (Functional)
- Sensory neuronsneurons that carry sensory
information to the spinal cord and/or brain
afferent neurons - Interneuronsneurons within the CNS (brain and
spinal cord) that process sensory information and
decide the appropriate response most neurons in
the body are of this type have short axons also
called association neurons - Motor neuronsneurons that carry response
information from the brain and/or spinal cord to
effectors efferent neurons
12A Reflex Arc
- The most basic conduction pathway through the
nervous system is a reflex arc - It consists of 5 components
- Receptor
- Sensory neuron
- CNS integrating center (usually consisting of one
or more interneurons either in the brain or
spinal cord) - Motor neuron
- Effector
13Neuroglia
- Make up about half the volume of the CNS
- Nerve glue that holds nervous tissue together
- Generally smaller than neurons but more numerous
- Neuroglia are not able to transmit nervous
impulses - They can multiply and divide in the mature
nervous system, though - Most brain tumors involve neuroglia
14Types of Neuroglia
- Central Nervous System
- Astrocytes
- Oligodendrocytes
- Microglia
- Ependymal cells
- Peripheral Nervous System
- Schwann cells
- Satellite cells
15Astrocytes
- Star-shaped cells with many processes
- Help maintain appropriate chemical environment
for the generation of nerve impulses nourish
neurons take up excess neurotransmitters assist
in brain development help form the blood-brain
barrier
16Oligodendrocytes
- Smaller than astrocytes, with fewer processes
- Produce myelin sheaths within the CNS
17Microglia
- Small cells with few processes
- Protect CNS cells from disease by phagocytosis of
the invading microbes - Clean up dead or injured nerve tissue
18Ependymal Cells
- A single layer of epithelial cells
- Line the ventricles of the brain and the central
canal of the spinal cord, spaces filled with
cerebrospinal fluid - Produce cerebrospinal fluid
19Schwann Cells
- Flattened cells that surround PNS axons
- Form myelin sheaths around axons of the PNS by
wrapping around them many times - Help in regeneration of PNS axons
20Satellite Cells
- Flattened cells arranged around the cell bodies
of neurons in ganglia, groups of cell bodies
outside of the CNS - Support neurons in PNS ganglia
21Gray vs. White Matter
- Gray Matterregions of the brain and spinal cord
that appear gray in color consists of neuronal
cell bodies, dendrites, unmyelinated axons, and
neuroglia - White Matterregions of the brain and spinal cord
that appear white in color consists of many
myelinated axons and some unmyelinated axons
22Membrane Potentials
- The plasma membrane of each body cell has a
membrane potential, a voltage difference that
exists between the inside and the outside of the
membrane due to various concentrations of charged
atoms, or ions, in each location - Muscle cells and neurons are excitable these
cells have action potentials when responding to
stimuli and resting potentials when not responding
23Membrane Potentials (continued)
- A membrane potential occurs partly because the
plasma membrane contains ion channels (proteins)
which allow various ions, or charged atoms, to
pass into/out of a cell - Cells vary in their number and types of ion
channels - Ion channels may be open at some times and
closed at other times - Ions move into/out of open ion channels by the
process of diffusion - In diffusion, substances move from an area of
greater concentration to an area of lesser
concentration - Diffusion is a passive process, which means that
the cell does not use ATP energy when it occurs
24Membrane Potentials (continued)
- In addition to ion channels, the plasma membranes
of cells contain proteins that transport ions
into/out of cells by active transport, which does
require a cell to use ATP energy - The most common example of active transport is
the sodium-potassium pump, which consists of
proteins located in the plasma membrane - K ions are pumped into a cell and Na ions out
of a cell against the concentration gradient
(from lesser to greater concentration)
25Membrane Potentials (continued)
- Three Na ions are pumped out of the cell to
every two K ions that are pumped in - In addition, the cytoplasm of a cell contains
many negatively charged anions, such as those
attached to large proteins - As a result, the cytoplasm just inside the plasma
membrane of a cell is negative compared to the
extracellular fluid just outside the membrane,
which is positive - In neurons, the typical value of the resting
membrane potential is -70 millivolts (mV)
26Membrane Potentials (continued)
- A cell that has a membrane potential is said to
be polarized most body cells are polarized, but
the millivolt values vary - The membrane potential of excitable cells
(neurons and muscle cells) can change in response
to a stimulus - A stimulus may cause ion channels to open or
close differently than under normal (resting)
conditions
27Graded Potentials vs. Action Potentials
- A graded potential is a small change in membrane
potential in a localized area of the plasma
membrane - An action potential is a change in membrane
potential that propagates the length of the
plasma membrane also called a nerve impulse
28Action Potentials
- The events of an action potential occur in two
phases depolarization and repolarization - During depolarization, the negative membrane
potential decreases toward 0 mV and eventually
becomes positive (about 30 mV) - During repolarization, the resting membrane
potential is restored (back to its negative value
of -70 mV)
29Action Potentials (continued)
- During an action potential, ion channels open
first to allow Na ions to move into the cell - This causes depolarization as the membrane
potential changes from -70 mV to 30 mV - Next, ion channels open to allow K ions to move
out of the cell - This causes repolarization as the membrane
potential changes back from 30 mV to -70 mV - If K continues to flow out of the cell, the
membrane potential may reach -90 mV and
hyperpolarization occurs - Eventually, the resting membrane potential is
restored
30Action Potentials (continued)
- Neurons, like muscle cells, go through a
refractory period after generating an action
potential - The refractory period is a short period of time
during which an excitable cell cannot respond to
a second stimulus
31All-or-None Principle
- Action potentials arise according to the
all-or-none principle - When depolarization reaches threshold level,
which is about -55 mV, an action potential occurs - The action potential is always the same size, no
matter how strong the stimulus is - The frequency of a stimulus is the main factor
that varies in the conduction of a nerve impulse
32Continuous vs. Saltatory Conduction
- Continuous conductionthe step-by-step
depolarization and repolarization of the plasma
membrane described previously occurs in muscle
fibers and unmyelinated axons - Saltatory conductiona special type of nerve
impulse conduction that occurs in myelinated
axons the impulse leaps from one node of Ranvier
to another, resulting in much faster conduction
of an impulse
33Speed of Conduction
- Myelinated axons conduct impulses faster than
unmyelinated axons - Axons with large diameters conduct impulses
faster than axons with small diameters
34Transmission at Synapses
- When an action potential is propagated from a
neuron to a skeletal muscle cell, the signal must
be transmitted across a type of synapse called a
neuromuscular junction - In this situation, the impulse is transmitted
electrically along an axon and chemically
across a synapse (because of the neurotransmitter
acetylcholine)
35Transmission at Synapses (continued)
- As an action potential is transmitted from one
neuron to another, it usually must cross a
different type of synapse, but the basic process
is similar to that involving the neuromuscular
junction - The neuron that sends the impulse is called the
presynaptic neuron the neuron that receives the
impulse is called the postsynaptic neuron - The two neurons are separated by a synaptic
cleft, which is a tiny space filled with
interstitial fluid
36Transmission at Synapses (continued)
- When a nerve impulse arrives at a synaptic end
bulb of a presynaptic neuron, ion channels in
that neurons plasma membrane open and allow Ca2
ions to flow into the cell - This increase in Ca2 in the presynaptic neuron
triggers exocytosis of some of the synaptic
vesicles, causing the release of neurotransmitter
molecules into the cleft each vesicle contains
thousands of molecules of neurotransmitter
37Transmission at Synapses (continued)
- The neurotransmitter molecules diffuse across the
cleft and bind to receptors in the postsynaptic
neurons plasma membrane - This opens ion channels in the postsynaptic
neuron and allows specific ions to move into that
cell from the synaptic cleft
38Transmission at Synapses (continued)
- As the ions flow into the postsynaptic neuron,
the membrane potential there changes - If threshold level (-55 mV) is reached,
depolarization occurs hyperpolarization may also
occur - The neurotransmitter is then removed from the
synaptic cleft, so that it will not continuously
influence the postsynaptic neuron - This occurs through the action of enzymes,
reuptake by the neuron that released the
substance, or by the diffusion of the
neurotransmitter away from the cleft
39Excitatory and Inhibitory Postsynaptic Potentials
- If a neurotransmitter depolarizes the
postsynaptic membrane, an excitatory postsynaptic
potential (EPSP) results - If a neurotransmitter hyperpolarizes the
postsynaptic membrane, an inhibitory postsynaptic
potential (IPSP) results
40Summation of Postsynaptic Potentials
- Most CNS neurons receive input from many synapses
- Integration of these inputs is called summation
- When summation results from the buildup of
neurotransmitter released by several presynaptic
end bulbs at the same time, it is called spatial
summation - When summation results from the buildup of
neurotransmitter released by a single presynaptic
end bulb two or more times in rapid succession,
it is called temporal summation
41Summation (continued)
- A single postsynaptic neuron receives both
excitatory and inhibitory information at the same
time - If the total summation effect is excitatory and
threshold level is reached, an EPSP results and
creates a nerve impulse - If the total summation effect is inhibitory, an
IPSP results and no impulse is generated - If the total summation effect is excitatory but
threshold level is not reached, an EPSP results
however, an impulse is not generated until a
subsequent stimulus causes depolarization
42Neurotransmitters
- Many substances act as neurotransmitters
- Neurotransmitters can be excitatory or inhibitory
- A neurotransmitter can even be excitatory in some
locations and inhibitory in others - Examples include the following
- Acetylcholine (Ach)
- Gamma aminobutyric acid (GABA)inhibitory
- Norepinephrine (NE)
- Epinephrine
- Dopamine (DA)
- Serotonin
- Nitric oxide (NO)
- Endorphinsthe bodys natural painkillers
43Regeneration and Repair of Nervous Tissue
- Human neurons have very limited ability to repair
themselves or regenerate - In the PNS, repair may generally occur if the
cell body and Schwann cells are not damaged - In the CNS, little or no repair may occur