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Section 9Section 9
The Nervous System
2
LIU Chuan Yong
刘传勇
Institute of Physiology
Medical School of SDU
Tel 88381175 (lab)
88382098 (office)
Email: [email protected]
Website: www.physiology.sdu.edu.cn
Reference – Website and Reference – Website and TextbookTextbook
Chapter 29. Cells in Nervous Chapter 29. Cells in Nervous System and Their FunctionSystem and Their Function
NeurocyteNeuroglia (Glia cell)
The Nervous System
• Components– Brain, spinal cord, nerves, sensory receptors
• Responsible for– Sensory perceptions, mental activities,
stimulating muscle movements, secretions of many glands
• Subdivisions– Central nervous system (CNS)– Peripheral nervous system (PNS)
Central Nervous System
• Consists of– Brain– Spinal cord
• Brain and spinal cord– Continuous with
each other
Peripheral Nervous System
• Two subcategories– Sensory or afferent
– Motor or efferent• Divisions
– Somatic nervous system
– Autonomic nervous system (ANS)
» Sympathetic
» Parasympathetic
» Enteric
Nervous System Organization
Cells of Nervous System
• Neurons or nerve cells– Receive stimuli and
transmit action potentials
– Organization• Cell body or soma
• Dendrites: Input
• Axons: Output
• Neuroglia or glial cells– Support and protect
neurons
Part 1 Neuron
Typical Neuron has 4 Regions
• Cell Body • Dendrites• Axon• Presynaptic Terminals
Each region is specialized for its particular function
Information flows in a single direction
1. Structure and Function
Neuron Cell Body LocationNeuron Cell Body Location
Most are found in the central nervous system
Gray matter – cell bodies and unmylenated fibers
Nuclei – clusters of cell bodies within the white matter of the central nervous system
Ganglia – collections of cell bodies outside the central nervous system
Functional Classification of Functional Classification of NeuronsNeurons
Sensory (afferent) neurons
Carry impulses from the sensory receptors
Cutaneous sense organs
Proprioceptors – detect stretch or tension
Motor (efferent) neurons
Carry impulses from the central nervous system
Functional Classification of Functional Classification of NeuronsNeurons
Interneurons (association neurons)
Found in neural pathways in the central nervous system
Connect sensory and motor neurons
By function (connections)
Interneuron
Sensory Motor
Neuron ClassificationNeuron Classification
Neuron AnatomyNeuron Anatomy
Extensions outside the cell body Dendrites –
conduct impulses toward the cell body
Axons – conduct impulses away from the cell body
Dendrites of Motor Neurons
• Short, tapering, and diffusely branched processes
• The receptive, or input, regions of the neuron
• Electrical signals are conveyed as graded potentials (not action potentials)
Axons: Structure • Slender processes of uniform diameter arising from the hillock
• Long axons are called nerve fibers
• Usually only one unbranched axon per neuron
• Rare branches, if present, are called axon collaterals
• Axonal terminal – branched terminus of an axon
Axons: Function
• Generate and transmit action potentials
• Secrete neurotransmitters from the axonal terminals
Myelin Sheath
• Whitish, fatty (protein-lipid), segmented sheath around most long axons
• It functions in:– Protection of the axon
– Electrically insulating fibers from one another
– Increasing the speed of nerve impulse transmission
Myelin
• CNS: oligodendrogliaor
oligodendrocytes
• PNS: Schwann cells ~
Nodes of Ranvier • Gaps in the
myelin sheath between adjacent Schwann cells
• The sites where collaterals can emerge
• Saltatory conduction
Myelin Sheath and Neurilemma: Formation
• Formed by Schwann cells in the PNS• A Schwann cell:
– Encloses the axon with its plasma membrane
– Concentric layers of membrane make up the myelin sheath
• Neurilemma (神经鞘)– remaining nucleus and cytoplasm of a Schwann cell
Myelin Sheath and
Neurilemma: Formation
Figure 11.5a-d
Unmyelinated Axons• A Schwann cell surrounds nerve fibers but
coiling does not take place
• Schwann cells partially enclose 15 or more axons
Axons of the CNS• Both myelinated and unmyelinated fibers are
present
• Myelin sheaths are formed by oligodendrocytes
• Nodes of Ranvier are widely spaced
• There is no neurilemma
2. Function of Nerve Fibers2. Function of Nerve Fibers
• conducting action potential
• 1) Characteristic
physiological integration (anesthetic and
tetrodotoxin, TTX)
insulation
two direction
no fatigue
. Conducting Action PotentialConducting Action Potential
2). velocities of AP propagation:
– 0.5~120m/s
– The factors that influence the AP propagation Diameter
Myelin sheath
Temperature
3). Classification of NF3). Classification of NF
Efferent nerve
– A, A, A, A; B, C.
Afferent nerve
– I, II, III, IV.
4) Axoplasmic transport4) Axoplasmic transportThe concept: Various organelles and materials is
moved from the cell body to the axon and its terminals in order to maintain the structure and function of the cell axon.
The mechanisms: Cytoskeletal filaments in the axon and cell body, which serve as the rails along which the transport occurs, are linked by proteins to the substances and organelles being moved.
Axoplasmic transportAxoplasmic transport
Anterograde axoplasmic transport– ~ fast axoplasmic transport: 410mm/day,
mitochondria, vesicles, secretory granule.
– ~ slow axoplasmic transport:1-12 mm/d, cytoskeletal elements & soluble proteins
Retrograde axoplasmic transport:– 205mm/d,
– NGF, tetanic toxin, horseradish peroxidase (HRP).
Fig. Axopasmic transport Fig. The method of horseraidish peroxidase
5) Trophic action of the nerve 5) Trophic action of the nerve to the targetto the target
The motor nerve release some substance that has trophic action on the skeletal muscle
The denervated muscle does not receive nerve signals and due to this, muscle atrophy begins. – After two months, the muscle fibers degenerate
and denervation atrophy follows. – Fibrous tissue replaces the muscle
II Supporting Cells: Neuroglia
Ependymal Cell
Microglia
Oligodendrocyte
Astrocyte
Martini, F. Fundamentals of Anatomy and Physiology, Fig 12-6.
Neuroglia of CNS
• Astrocytes– Regulate extracellular brain fluid composition– Promote tight junctions to form blood-brain barrier
• Ependymal Cells– Line brain ventricles and spinal cord central canal– Help form choroid plexuses that secrete cerebrospinal fluid (CSF)
Neuroglia of CNS
• Microglia– Specialized macrophages
• Oligodendrocytes– Form myelin sheaths if surround axon
Neuroglia of PNS
• Schwann cells or neurolemmocytes– Wrap around portion of only one axon to form myelin sheath
• Satellite cells– Surround neuron cell bodies in ganglia, provide support and
nutrients