Introduction to the Nervous System to be copied
Feb 23, 2016
Introduction to the Nervous System
to be copied
Nervous Tissue & Homeostasis
• excitable characteristic of nervous tissue allows for generation of nerve impulses (action potentials) that provide communication & regulation of most body tissue.
• together with endocrine system: responsible for maintaining homeostasis
Differences in Nervous & Endocrine Control of HomeostasisNERVOUS ENDOCRINE• rapid responder• action potentials
• slow, prolonged response
• releases hormones
Structures of the Nervous System• total mass of 2 kg (~3% of total body
mass)• Skull• Spinal Cord• Spinal Nerves• Cranial Nerves• Ganglia• Enteric Plexus• Special Senses & other Sensory Receptors
Functions of the Nervous System
• 3 basic functions:1. Sensory2. Integrative3. Motor
Sensory Function• sensory receptors detect internal &
external stimuli• sensory (afferent) neurons carry this
sensory information to spinal cord & brain thru cranial & spinal nerves
Integrative Function• integrate: process• nervous system takes information from
sensory neurons & processes that information, analyzes it, stores some of it & makes decisions for appropriate responses
• served by interneurons (connect 1 neuron to another neuron
• Perception:– conscious awareness of sensory stimuli– occurs in brain
Motor Function• served by motor (efferent) neurons• carry info from brain/spinal cord
effectors (muscle or gland) thru cranial or spinal nerves
• results in muscles contraction or gland secreting
Organization of the Nervous System
Histology of the Nerrvous System
• 2 cell types1. Neurons2. Neuroglia
Neurons • nerve cells that possess electrical
excitability:– ability to respond to a stimulus &
convert it into an action potential
– stimulus: any change in environment that is strong enough to initiate an action potential
Action Potential• electrical signal that propagates
along surface of neurolema (membrane)– begins & travels due to movement of
ions between interstitial fluid & inside of neuron thru specific ion channels
– once begun it travels rapidly @ constant strength
Parts of a Neuron
Parts of Neuron: Cell Body• contains nucleus, cytoplasm, typical organelles,
• + Nissl bodies clusters of RER–make materials for:• growth of neuron• regenerate damaged axons in PNS
Nerve Fiber• general term for any neuronal
process or extension that emerges from cell body
• most neurons have 2:1. Dendrites2. Axons
Dendrites • “little trees”• input portion of neuron• usually, short, tapering, highly
branched• their cytoplasm contains Nissl
bodies, mitochondria
Axon • propagates action potentials – another neuron–muscle fiber– gland cell
Parts of an Axon• joins cell body @ cone-shaped
elevation: axon hillock• part of axon closest to hillock =
initial segment• jct of axon hillock & initial segment
where action potential arises so is called the trigger zone
Parts of an Axon• axoplasm: cytoplasm of an axon• axolemma: plasma membrane of
axon• axon collaterals: side branches along
length of axon (most @ 90°)• axon terminals: axon divides into
many fine processes
Synapse • site of communication between 2
neurons or between a neuron & effector cell
• synaptic end bulbs: tips of some axon terminals swell into bulb-shaped structures
• synaptic vesicles: store neurotransmitter– many neurons have >1 neurotransmitter,
each with different effects on postsynaptic cell
Axonal Transport• 2 types:– for moving materials from cell body axon
terminals1. slow– 1-5 mm/d– replenishes new axoplasm to developing or
regenerating axons2. fast– 200 – 400 mm/d– moves materials to/from cell body• organelles or membranes needed in axon terminal
Types of NeuronsFunctional Classification Structural Classification
• Sensory• Interneurons• Motor
• use # processes extending from cell body
1. Multipolar neurons2. Bipolar neurons3. Unipolar neurons
Multipolar Neurons• several dendrites with 1 axon• includes most neurons in brain &
spinal cord
Bipolar Neuron• 1 main dendrite & 1 axon• retina, inner ear, olfactory area of
brain
Unipolar Neuron• are sensory neurons that begin in embryo
as bipolar• during development axon & dendrite fuse
then divide into 2 branches (both have characteristic structure & function of an axon)
• 1 branch ends with dendrites (out of CNS)• 2nd branch ends in axon terminal (in CNS)• cell bodies of most found in ganglia
Unipolar Neuron
Pyramidal Cells• in cerebral cortex of brain
Neuroglia (Glia)• ~50% vol of CNS• “glue”• do not generate or propagate action
potentials• multiply & divide in mature nervous
systems• glioma:– brain tumors derived from glial cells– very malignant, grow rapidly
Glial Cells of the CNS1. ASTROCYTES2. OLIGODENDROCYTES3. MICROGLIA4. EPENDYMAL CELLS
Astrocytes • star-shaped• largest & most numerous of glial cells• functions:1. physically support neurons2. assist in blood-brain-barrier (bbb)3. in embryo: regulate growth, migration,
&interconnections between neurons4. help maintain appropriate chemical
environment for propagation of action potentials
Oligodendrocytes • “few trees”• smaller & fewer branches than
astrocytes• Functions:1. form & maintain myelin sheath on
axons in CNS2. 1 oligo. myelinates many axons
Microglia • small cells with slender processes
giving off many spine-like projections• function:1. phagocytes– remove cellular debris made during
normal development– remove microbes & damaged nervous
tissue
Ependymal Cells• single layer of cuboidal to columnar
cells• ciliated & have microvilli• function:1. line ventricles of brain & central canal
of spinal cord2. produce, monitor, & assist in
circulation of cerebrospinal fluid (CSF)3. form bbb
Neuroglial Cells of the PNS• Schwann cells• Satellite cells
Schwann Cells• functions:1. myelinate axons in PNS– 1 Schwann cell myelinates 1 axon
2. participate in axon regeneration
Satellite Cells• flat cells that surround cell bodies of
neurons in PNS ganglia• functions:1. structural support2. regulate exchange of materials
between neuronal cell bodies & interstitial fluid
Myelination • myelin sheath: made up of
multilayered lipid & protein (plasma membrane) covering
• function:1. electrically insulates axon2. increases speed of nerve impulses
Myelinated & Unmyelinated Axons
Nodes of Ranvier• gaps in myelin sheath• 1 Schwann cell wraps axon between
nodes of Ranvier
Myelin • amount increases from birth to
maturity • infant‘s responses slower & less
coordinated as older child or adult in part because myelination is a work in progress thru infancy
Demyelination • loss of myelin sheath• see in disorders:–multiple sclerosis– Tay-Sachs– side effect of radiation therapy &
chemotherapy
Gray Matter of the Nervous System
• contains:– neuronal cell bodies– dendrites– unmyelinated axons– axon terminals– neuroglia
White Matter of the Nervous System
• composed of:–myelinated axons