Nervous Tissue
Nervous System
• CNS (brain and spinal cord)
• PNS (peripheral nervous system)
Types of Cells
• Neurons – structural units
• Supporting cells – protect and myelinate
neurons
Structure of Neurons
• Cell body – contains nucleus and
cytoplasm
Structure of Neuron
• Dendrites – receptive regions
Structure of Neuron
• Axons – nerve impulse generators
Structure of Neuron
• Axoplasm – cytoplasm of neuron
Structure of Neuron
• Axolema – plasma membrane
Structure of Neuron
• Collaterals – axon branches
Structure of Neuron
• Axon hillock – an enlarged cell body
structure, which is the beginning of the
axon
Structure of Neuron
• Axon terminals – The axon and its
collaterals end divide into fine processes
Structure of Neuron
• Synaptic end bulbs – tips of axon
terminals that contain the synaptic
vessicles
Structure of Neuron
• Synaptic cleft – gap between two neurons
Structure of Neuron
• Supporting cells wrap themselves around
the axons of some neurons like a jelly role
Structure of Neuron
• Myelin sheath – A tight core of plasma
membrane material around the axon
Structure of Neuron
• Neurilemma – The peripheral part of the
schwann cell and the exposed plasma
membrane
Structure of Neuron
• Nodes of Ranvier – Since the myelin
sheath is composed of several schwann
cells there are gaps or indentations called
nodes of ranier
Classification by Structure
1. Unipolar
2. Bipolar
3. Multipolar
Unipolar
• Have only one process that extends from
the cell body
Unipolar
• Conduct impulses toward the CNS
Bipolar
• Have two processes that extend from the
cell body (one axon and one dendrite)
Bipolar
• Found in the eye, ear, and olfactory
mucosa
Multipolar
• Have several processes that extend from
the cell body (several dendrites and one
axon)
Multipolar
• Most neurons in CNS and neurons that
carry impulses away from the CNS
Classification by Function
1. Sensory
2. Motor
Sensory or Afferent Neurons
• Carry impulses away from sensory
receptors in the skin, skeletal muscle, and
internal organs.
Sensory or Afferent Neurons
• Most are unipolar
Sensory or Afferent Neurons
• Cell bodies lie outside the CNS
Motor or Efferent Neurons
• Carry impulses away from the CNS
Motor or Efferent Neurons
• Most are multipolar
Motor or Efferent Neurons
• Cell bodies lie within the CNS
Neuroglia
• Supporting cells
Neuroglia
• Half the volume in the CNS
Neuroglia
• Smaller than neurons
Neuroglia
• Five to fifty times more numerous
Neuroglia
• Can divide
Neuroglia
• Do not generate or propogate action
potentials
Brain Tumors
• Derived from glia called gliomas
• Highly malignant
• Grow fast
Myelin sheath
• Some axons are covered by a myelin
sheath produced by neuroglia
Myelin sheath
• Increases speed of nerve impulse
production
Neuroglia
• Two types of neuroglia produce myelin
sheath
1. Schwann cells (PNS)
2. Oligodendrocytes (CNS)
Schwann Cells
• Helps in regeneration of PNS axons
Oligodendrocytes
• Inhibits axon regrowth after injury
Gray and White Matter
• In the brain and spinal cord some parts
appear gray while other appear white
White Matter
• Contains the myelinated and unmyelinated
axons of neurons
Gray Matter
• Contains cell bodies, dendrites, and
unmyelinated axons of neurons
• Also contains neuroglia
Organization of Nervous System
1. CNS
2. PNS
Organization of Nervous System
PNS
• Somatic nervous system
• Autonomic nervous system
Somatic Nervous System
1. Sensory neurons convey information
from receptors in the body wall, limbs,
head, and special senses
Somatic Nervous System
2. Motor neurons conduct impulses to
skeletal muscles
Autonomic Nervous System
1. Sensory Neurons convey information
from organs such as the lungs and
stomach to the CNS
Autonomic Nervous System
2. Motor Neurons conduct impulses from
the CNS to smooth muscle, cardiac
muscle, and glands
Autonomic Nervous System
• The motor part can be subdivided further
into the sympathetic and
parasympathetic divisions
Synapses
• The functional junction between a neuron
and an effector or between two neurons.
Synapses
• Presynaptic neuron – The neuron sending
the signal
Synapses
• Postsynaptic neuron – the neuron
receiving the message
Types of Synapses
1. Electrical
2. Chemical
Electrical Synapses
• Action potentials or nerve impulses are
conducted directly between adjacent cells
through gap junctions.
Electrical Synapses
• Ions flow from one cell to the next
through the connexons thus the action
potential is spread from one cell to the
next.
Chemical Synapses
• Nerve impulses cannot be conducted
across the synaptic cleft
Chemical Synapses - Summary
• The presynaptic neuron converts an
electrical signal (nerve impulse) into a
chemical signal (neurotransmitter release).
Chemical Synapses - Summary
• Then the postsynaptic neuron converts the
chemical signal back into an electrical
signal (postsynaptic potential).
Chemical Synapses – 7 Steps
1. An action potential arrives at a synaptic
end bulb of a presynaptic neuron
Chemical Synapses – 7 Steps
2. Voltage-gated calcium channels open
and calcium flows into the synaptic end
bulb of the presynaptic neuron
Chemical Synapses – 7 Steps
3. The increase in calcium concentration
inside the synaptic end bulb signals the
synaptic vessicles to release the
neurotransmitters via exocytosis into the
synaptic cleft
Chemical Synapses – 7 Steps
4. The neurotransmitters bind to receptors
on the postynaptic neuron
Chemical Synapses – 7 Steps
5. Once the neurotransmitters bind to the
receptors, ions flow across the membrane
Chemical Synapses – 7 Steps
6. As the ions flow across the membrane
the voltage across the membrane
changes (postsynaptic potential)
Chemical Synapses – 7 steps
• If sodium flows across the membrane it
causes depolarization
Chemical Synapses – 7 steps
• If chlorine flows across the membrane it
causes hyperpolarization
Chemical Synapses – 7 steps
7. Once a depolarizing postsynaptic
potential reaches threshold, an action
potential is triggered
Chemical Synapses
• Synaptic delay – The length of time
between the arrival of the action potential
at a presynaptic axon terminal and the
membrane potential change on the
postsynaptic membrane.
Chemical Synapses
• Due to the synaptic delay electrical
synapses are faster than chemical
synapses
Excitatory postsynaptic potential
• Depolarizing postsynaptic potential caused
by the influx of sodium, potassium, and
calcium
Excitatory postsynaptic potential
• Once it reaches threshold an action
potential is triggered
Inhibitory postsynaptic potential
• A hyperpolarizing postsynaptic potential
caused by the influx of chlorine or efflux
of potassium
Inhibitory postsynaptic potential
• Makes it difficult for the generation of an
action potential
Neurotransmitters
1. Small-Molecule Neurotransmitters
2. Neuropeptides
Small-Molecule Neurotransmitters
1. Acetylcholine
 Released by PNS and CNS neurons
 Can act as an excitatory (neuromuscular
junction) or inhibitory neuron (heart)
Small-Molecule Neurotransmitters
2. Amino acid
 CNS
 Glutamate has excitatory effects
(important for memory)
 Gamma aminobutyric (GABA) has
inhibitory effects
Small-Molecule Neurotransmitters
3. Biogenic amines
 Amino acids that are modified and
decarboxylated
 Can be inhibitory or excitatory depending
on the receptor
 Norepinephrine, epinephrine, dopamine,
and serotonin
Small-Molecule Neurotransmitters
3. Biogenic Amines continued
 NE and Epinephrine play roles in arousal,
dreaming, and regulating mood
Small Molecule-Neurotransmitters
3. Biogenic Amines continued
 Dopamine are active during emotional
responses, addictive behaviors, and
pleasurable experiences
Small Molecule-Neurotransmitters
3. Biogenic Amines continued
 Serotonin involved in sensory reception,
temperature regulation, control of mood,
appetite, and the induction of sleep
Parkinson disease
• Due to degeneration of neurons that
release dopamine
Schizophrenia
• Due to accumulation of excess dopamine
Small-Molecule Neurotransmitters
4.
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ATP and other Purines
CNS and PNS
Excitatory
Usually released with another
neurotransmitter
Small-Molecule Neurotransmitters
5.
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Nitric Oxide
Simple gas
Plays a role in memory and learning
Causes vasodilation
Viagra
• Enhances the effect of NO
Neuropeptides
• 3-40 amino acids linked by peptide bonds
• CNS and PNS
• Excitatory and Inhibitory actions
• Substance P enhances pain
• Enkephalins and Endorphins inhibit pain
(also morphine and heroin receptors)