Nervous System
• The body’s primary communication and control system
• Divided according to structural and functional categories
Nervous System
Structural Organization• CNS
BrainSpinal cord
• PNSCranial nervesSpinal nervesGanglia
Functional Organization
Functional Organization• Sensory (Afferent) Division – receives sensory
information (input) from receptors and transmits this information to the CNS
• Motor (Efferent) Division – transmits motor impulses (output) from the CNS to muscles or glands
Sensory Division
• Somatic sensory components are the general somatic senses (touch, pain, pressure, vibration, temperature, proprioception)
• Visceral sensory components transmit nerve impulses from blood vessels and viscera to the CNS (temperature, stretch of the organ wall)
Motor Division
• Somatic motor component conducts nerve impulses from the CNS to skeletal muscles– Voluntary nervous system
• Autonomic motor component (ANS) innervates internal organs, regulates smooth muscle, cardiac muscle, and glands– Visceral motor system or involuntary nervous
system
Nervous Tissue
• Specialized tissue responsible for excitability and conduction of impulses
• Comprised of:– Neurons – nerve cells with processes – Neuroglia (glial cells) – cellular connective tissue
of the NS
Neuron
• Structural and functional unit of nervous tissue
• Excitable cells that initiate and transmit nerve impulses
Neuron
• Characteristics:– High metabolic rate– Extreme longevity– Typically non-mitotic
Neuron
• Size: varies from 4-20 um (motor neurons > sensory neurons)
• Nucleus:– Large, pale vesicular, usually central– Large prominent nucleolus – “Fish eye” or “owl’s eye”
Neuron
• Cytoplasm (neuroplasm)– Basophilic, contains usual cell organelles (Golgi,
mitochondria) and pigments (melanin, lipofuscin)– Centrosome – absent– Neurofibrils– Nissl granules
Nissl Granules
• Chromophilic bodies• Give a granular appearance• Rough endoplasmic reticulum• Degenerate due to fatigue or injury to the
neuron (chromatolysis)
Neurofibrils
• Thread-like structures • Form a plexiform pattern in the cell body• Arranged in parallel manner in both dendrites
and axon• Functions: provide internal support, play role
in intracellular transport of organelles and vesicles
Neurofibrils
1. Microtubules – thickest, contain a thin central filament with a hollow core
2. Neurofilaments – abundant in axon3. Microfilaments – thinnest
Neuron Structure
• Vary in shape and size• Cell body (perikaryon,
soma) + one or more cell processes
• Enclosed by neurolemma
Neuron Structure
• Cell body – serves as the neuron’s control center and is responsible for receiving, integrating, and sending nerve impulses
• Cell processes– Axons– Dendrites
Cell Processes
Axons DendritesOnly one axon is present in a neuron
May be absent, solitary, or numerousUsually multiple in a neuron
Thin long process of uniform thickness and smooth surface
Short, multiple processesThickness diminishes as they divide repeatedlyBranches are studded with spiny projections
Cell Processes
Axons DendritesBranches of axon are fewer and at right angles to the axon
Branch diffusely and are given off at acute angles
Contains neurofibrils and no Nissl granules
Contain both neurofibrils and Nissl granulesNo Golgi complexes
Forms the efferent component of the impulse
Afferent component
Axon
• a.k.a. axis cylinder• One long thin process that arises from a
conical elevation on the perikaryon (axon hillock)
• Cell membrane = axolemma• Carries impulse away from cell body• Axonal transport– Anterograde – perikaryon → axon terminal– Retrograde – axon terminal → perikaryon
Dendrites
• Single or multiple processes containing the extension of cytoplasm of neurons with its cell organelles
• Provide receptive surface for the neuron• Carry impulses towards the soma of the
neuron
Types of Neurons
• Based on number of processes:1.Unipolar2.Pseudounipolar3.Bipolar4.Multipolar
Types of NeuronsDescription Location
Unipolar Only 1 process (axon) Embryo, rare in adults
Pseudounipolar 1 axon bifurcates Dorsal root gangliaGanglia of some cranial nerves
Bipolar Dendrite and axon arise at opposite poles of cell body
Olfactory epitheliumVestibular and cochlear gangliaBipolar cells of retina
Multipolar Numerous dendritesMost common
Most neurons of cerebrum and cerebellum
Types of Neurons
Functional Classification Description
Sensory (Afferent) Transmit impulses toward the CNS
Motor (Efferent) Carry impulses toward the body surface
Interneurons (Association Neurons)
Any neurons between a sensory and a motor neuron
Neuroglia/Glial Cells
• The cellular connective tissue of the nervous tissue
• Functions:1. Provide support and nutrition2. Maintain homeostasis3. Form myelin 4. Participate in signal transmission
Neuroglia
• Found in both the CNS and PNS• Outnumber neurons• Smaller than neurons and capable of mitosis• Do not transmit nerve impulses
Neuroglia
• Astrocytes– Protoplasmic astrocytes– Fibrous astrocytes
• Oligodendrocytes• Schwann cells• Ependymal cells• Microglia
NeurogliaGlial Cell Type Origin Location Function
Astrocytes Neural tube
CNS Structural support, repair processes
Blood–brain barrier, metabolic exchanges
Oligodendrocytes Neural tube
CNS Myelin production, electric insulation
Schwann cells Neural tube
PNS Myelin production, electric insulation
Ependymal cells Neural tube
CNS Lining cavities of central nervous system
Microglia Bone marrow
CNS Macrophagic activity
Microglia
• Smallest glial cells• Small elongated cells
with short irregular processes
• Dense elongated nuclei (other glial cells: spherical nuclei)
Microglia
• Function: phagocytic (represent the mononuclear phagocytic system in nerve tissue)
Astrocytes
• Largest, most numerous• Star-shaped cells with
multiple radiating processes
• Large vesicular nuclei• Only astrocytes exhibit
vascular end-feet
Astrocytes
• Functions: – Form supporting network for neurons– Provide nourishment to the neurons
• Types:– Protoplasmic– Fibrous
Protoplasmic Astrocyte• Thick processes• Abundant granular
cytoplasm• Some processes
attached to neighboring capillaries by perivascular sucker feet (vascular end-feet)
• Found in grey matter
Fibrous Astrocyte• Have long and straight
processes seen only in the white matter
Ependymal Cells
• Low columnar (cuboidal) epithelial cells
• Lining the ventricles of the brain and central canal of the spinal cord
Ependymal Cells
• Ciliated in some locations (facilitate movement of CSF)
• Function: – Secrete CSF
Oligodendrocytes
• Smaller than astrocytes, have fewer and shorter processes
• Scanty cytoplasm• Nucleus usually ovoid or
spherical and more deeply staining than astrocytes
• Found mostly in white matter
Oligodendrocytes
• Produce the myelin sheath (provides the electrical insulation of neurons in the CNS)
• Processes wrap around axons (producing a myelin sheath)
Oligodendrocytes
• Function: provides insulation to the axon that allows electrical signals to propagate more efficiently
• 1 oligodendrocyte can extend to up to 50 axons
Schwann Cells
• Nucleus: flattened• Abundant cytoplasm• Function : same as
oligodendrocytes but are located around axons in the PNS
• Can only wrap around 1 axon
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