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Cranial Nerves, source: training.seer.cancer.gov
Nervous System Overview
BIOL241 - Lecture 12a
Topics
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Divisions of the NS: CNS and PNS
Structure and types of neurons
Synapses
Structure and function of glia in the CNS
and PNS
The Nervous System
• Includes all neural tissue in the body
General Overview
Neural Tissue
• Contains 2 kinds of cells:
– neurons:
• cells that send and receive signals
– neuroglia (glial cells):
• cells that support and protect neurons
Organs of the Nervous System
• Brain and spinal cord
• Sensory receptors of sense organs (eyes,
ears, etc.)
• Nerves connect nervous system with other
systems
INTERCONNECTEDNESS
Anatomical Divisions
of the Nervous System
1. Central nervous system (CNS)
2. Peripheral nervous system (PNS)
Nervous System
CNS
PNS
Somatic
NS
Autonomic
NS
Sympathetic
Parasympathetic
The Central
Nervous System (CNS)
• Consists of the spinal cord and brain
• Contain neural tissue, connective tissues,
and blood vessels
• INTERCONNECTEDNESS
Functions of the CNS
• Are to process and coordinate:
– sensory data:
• from inside and outside body (“inputs”)
– motor commands:
• control activities of peripheral organs (e.g., skeletal
muscles) [“controls’]
– higher functions of brain:
• intelligence, memory, learning, emotion (“outputs”)
The Peripheral
Nervous System (PNS)
• Includes all neural tissue outside the CNS
(nerves that control muscles and glands)
Functions of the PNS
1. Deliver sensory information to the CNS
2. Carry motor commands to peripheral
tissues and systems
Nerves
• Also called peripheral nerves:
– bundles of axons with connective tissues and
blood vessels
– carry sensory information and motor
commands in PNS:
• cranial nerves—connect to brain
• spinal nerves—attach to spinal cord
Functional Divisions of the PNS
• Afferent division:
– carries sensory information
– from PNS sensory receptors to CNS
• Efferent division:
– carries motor commands
– from CNS to PNS muscles and glands
SAME: Sensory is Afferent, Motor is Efferent
PNS divisions
• Somatic nervous system (SNS)
• Autonomic nervous system (ANS)
• More “layers”
The Somatic
Nervous System (SNS)
• Controls skeletal muscle contractions:
– voluntary muscle contractions
– involuntary muscle contractions (reflexes)
The Autonomic
Nervous System (ANS)
• Controls subconscious actions:
– contractions of smooth muscle and cardiac
muscle
– glandular secretions
Divisions of the ANS
• Sympathetic division:
– has a stimulating effect
• Parasympathetic division:
– has a relaxing effect
Nervous System
CNS
PNS
Somatic
NS
Autonomic
NS
Sympathetic
Parasympathetic
Neurons
• The basic functional units of the nervous
system
The Structure of Neurons
Figure 12–1
Major Organelles
of the Cell Body
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Large nucleus and nucleolus
Cytoplasm (perikaryon)
Mitochondria (produce energy)
RER and ribosomes (produce
neurotransmitters)
• Cytoskeleton
• Nissl Bodies: RER and ribosomes
Dendrites
• Highly branched
• Dendritic spines:
– many fine processes
– receive information from other neurons
– 80–90% of neuron surface area
The Axon
• Are long
• Carries electrical signal (action potential)
to target
• Axon structure is critical to function
Structures of the Axon
• Axon hillock:
– thick section of cell body
– attaches to initial segment
• Initial segment:
– attaches to axon hillock
• Collaterals:
– branches of a single axon
• Synaptic terminals with knobs:
– tips of axon (knobs at ends)
Movie
Neurophysiology: Neuron Structure
The Synapse
Figure 12–2
Synapse
• Synapse: Area where a neuron
communicates with another cell
• Presynaptic cell:
– neuron that sends message
• Postsynaptic cell:
– cell that receives message
• Synaptic cleft The small gap that
separates the presynaptic membrane and
the postsynaptic membrane
• Area of terminal containing synaptic
vesicles filled with neurotransmitters
Neurotransmitters
• Chemical messengers (like ACh)
• Released at presynaptic membrane
• Affect receptors of postsynaptic
membrane
• Broken down by enzymes and, or, taken
up into presynaptic cell
• Are reassembled at synaptic knob
Soma
• What does soma mean?
Major Structural
Classifications of Neurons
• Unipolar neurons:
– found in sensory neurons of PNS
• Multipolar neurons:
– common in the CNS
– include all skeletal muscle motor neurons
– cell body (soma)
– short, branched dendrites
– long, single axon
Unipolar Neurons
• Have very long axons
• Dendrites fused to axon
• Cell body to 1 side
Figure 12–3 (3 of 4)
Multipolar Neurons
• Often have long axons
• Multiple dendrites, 1 axon
Figure 12–3 (4 of 4)
3 Functional
Classifications of Neurons
• Sensory neurons:
– afferent neurons of PNS
• Motor neurons:
– efferent neurons of PNS
• Interneurons:
– association neurons
Sensory (Afferent) Neurons
• Monitor internal environment (visceral
sensory neurons)
• Monitor effects of external environment
(somatic sensory neurons)
Motor (Efferent) Neurons
• Signals from CNS motor neurons to
visceral effectors pass synapses at
autonomic ganglia dividing axons into:
– preganglionic fibers
– postganglionic fibers
Interneurons
• Most are located in brain, spinal cord, and
autonomic ganglia:
– between sensory and motor neurons
• Are responsible for:
– distribution of sensory information
– coordination of motor activity
• Are involved in higher functions:
– memory, planning, learning
Neuroglia are supporting
cells
Neuroglia
• Half the volume of the nervous system
• Many types of neuroglia in CNS and PNS
Volume
• What is volume?
Neuroglia of the
Central
Nervous
System
Figure 12–4
4 Types of Neuroglia in the CNS
1.
2.
3.
4.
Ependymal cells
Astrocytes
Microglia
Oligodendrocytes
1. Ependymal Cells
• Form epithelium called ependyma
• Line central canal of spinal cord and
ventricles of brain:
– secrete cerebrospinal fluid (CSF)
– have cilia or microvilli that circulate CSF
– monitor CSF
– contain stem cells for repair
Blood-Brain Barrier
• What is this?
2. Astrocytes
• Maintain blood–brain barrier (isolates
CNS)
• Create 3-dimensional framework for CNS
• Repair damaged neural tissue
• Guide neuron development
• Control interstitial environment
3. Microglia
• Migrate through neural tissue
• Clean up cellular debris, waste products,
and pathogens
• Not of neural origin; related to
macrophages (like osteoclasts)
4. Oligodendrocytes
• Processes contact other neuron cell bodies
• Wrap around axons to form myelin sheaths
Myelination
• Increases speed of action potentials
• Myelin insulates myelinated axons
• Makes nerves appear white
Nodes and Internodes
• Internodes:
– myelinated segments of axon
• Nodes:
– also called nodes of Ranvier
– gaps between internodes
– where axons may branch
White Matter and Gray Matter
• White matter:
– regions of CNS with many myelinated nerves
• Gray matter:
– unmyelinated areas of CNS
2 Neuroglia of the
Peripheral Nervous System
1. Satellite cells (amphicytes)
2. Schwann cells (neurilemmacytes)
1. Satellite Cells
• Surround ganglia
• Regulate environment around neuron
Ganglia vs Nuclei
• Masses of neuron cell bodies
– Called ganglia in the PNS and are surrounded
by satellite cells
– Called nuclei in the CNS
1. Schwann Cells
• Form myelin sheath around peripheral
axons (nerves)
• 1 Schwann cell sheaths 1 segment of
axon:
– many Schwann cells sheath entire axon
Schwann
Cells
Figure 12–5a
Schwann Cells
Figure 12–5b
“Summary”
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Divisions of the NS: CNS and PNS
Structure and types of neurons
Synapses
Structure and function of glia in the CNS
and PNS