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What is the basic structural and functional unit of nervous system?
Neuron (nerve cell)
What structures are found in cell body of nerve cell?
Neurofilaments, microfilaments, microtubules
Melanin granules (in certain neurons--substantia nigra)
What are basophilic clumb found in cytoplasm of nerve cells?
Nissl bodies--stacks of rER
(Free ribosomes also found in cytoplasm)
Describe nucleus of cell body of nerve cell
Large, clear, euchromatic
Encloses spherical dense nucleolus
How can neurons in adult regnerate?
Neurons do not divide
Adult brain may have neural stem cells that retain capacity to regenerate--migrate to site of injury where they differentiate into nerve cells
What is a yellowish-brown pigment found in cell body of nerve cells?
Lysosomal enzymatic residue
Describe structures found in dendrites
No Golgi apparatus
Mitochondria and rER may be present
What is function of dendrite?
Receive stimuli from epithelial receptor cells or other neurons and transmit info to cell body of neuron
What structures are found in axon of neuron?
What is function of axon?
Relay action potentials away from cell body to axon terminales and then transmit them to other neurons, smooth/skeletal/smooth muscle, gland cells
What is initial segment of axon?
Segment distal to axon hillock and proximal to where myelination begins
Describe regeneration of axon that occurs following injury?
Proximal axon regenerates
Distal axon degenerates
What substances are transported via slow anterograde transport system?
What substances are transported via fast axoplasmic transport system?
Membranous organelles (sER, synaptic vesicles, mitochondria)
Sugars, amino acids, nucleotides
Describe unipolar/pseudounipolar neuron
Single process arising from cell body which secondarily bifurates into 2 processes with axon characteristics
How are impulses transmitted in pseudounipolar neurons?
Impulse transmitted down axon, bypassing cell body
What is function of sensory (afferent) neurons?
Receive sensory stimuli from environment or from within body
Where is connective tissue found in CNS?
CNS (nerve tissue of brain and spinal cord) does NOT contain connective tissue
Only connective tissue found in brain and spinal cord is associated with blood vessels
What kind of neuroglia exhibit processes with mat-like vascular feet (pedicels) that ensheathe all blood vessels covered by a basal lamina?
What is function of astrocytes?
Assist in formation of blood-brain barrier
Form scar tissue following nerve tissue injury
Monitor ionic and chemical composition in extracellular space around nerve cells
What kind of neuroglia have processes that surround periphery of brain and spinal cord, forming a layer deep to pia mater?
What kind of astrocytes are found primarily in grey matter?
Protoplasmic astrocytes (granular cytoplasm)
Where are processes of protoplasmic astrocytes located?
Processes cover synapses, neurons and blood vessels
What kind of astrocytes are found primarily in white matter?
(long thin processes, light staining)
With what structures are fibrous astrocytes associated?
Associated with blood vessels and pia mater
Cover nodes of Ranvier and synapses
What kind of neuroglia form rows in white matter?
smaller than astrocytes
What kind of neuroglia have small cell bodies and elongated nuclei? (other gila have round nuclei)
What are cuboidal-columnar cells lining brain and spinal cord cavities, containing CSF?
What structures are found in ependymal cells?
Motile cilia which move CSF
apical Golgi apparatuses
Gap junctions and zonula adherens
Describe unmyelinated axons in PNS
Occupy a groove of Schwann cell; forms longitudinal uninterrupted sheath along length of axon
One Schwann cell envelops many axons
What is myelin?
Lipoprotein formed by concentric layers of cell membrane which have a high lipid content (~80%)
Describe myelination of nerve cell axons in PNS
Schwann cell wraps its membrane repeatedly around one internode of a single axon, forming myelin sheath
Each myelinated axon is covered by a series of Schwann cells
What is delicate layer of loose connective tissue with collagen fibrils that covers individual axons?
What is specialized connective tissue, lined by layers of perineural cells, that surrounds a nerve bundle?
What is dense irregular connective tissue with elastic fibers that covers several nerve bundles to form gross nerve?
What are satellite cells?
Support cells that surround cell bodies of neurons housed in ganglia
Function analogous to Schwann cell but do NOT form myelin
What is affected by MS?
White matter (myelinated axons) of CNS
Any myelinated tract (bundle of axons) may be affected
What causes MS?
Autoimmune attack of myelin sheath formed by oligodendrocytes--demyelination of CNS nerve cell axons
What cranial nerves are susceptible to demyelination during MS?
Only optic nerve (CN II)--is tract of CNS
The other 11 CNs are components of PNS
What are typical symptoms of optic neuritis?
Visual loss affecting one eye (sometimes both)
Pain when moving eye
Papilledema (depending on segment of optic nerve affected)
Pupil of affected eye will not respond to light normally (if visual acuity severely impaired)
What is treatment of optic neuritis?
IV coritcosteroids over 3-5 days
Most cases completely within weeks or months
What is ependymoma?
Tumor that grows into (4th) ventricle, compresses surrounding structures as it expands
5% of primary brain tumors
What are clefts of Schmidt-Lanterman?
Schwann cell cytoplasm trapped in concentric layers of cell membrane myelinating an axon
What is example of autonomic ganglia in PNS?
Sympathetic trunk, visceral ganglia
Parasympathetic--ganglia associated with CNs and intramural ganglia within visceral walls
What is voluntary nervous system?
Somatic motor system
Influences skeletal muscle to produce movement
What is involuntary nervous system?
Autonomic nervous system/visceral motor system
Influences smooth muscle, cardiac muscle and glands
What nervous system is referred to as "thoracolumbar outflow"?
(Connects with thoracic and lumbar regions of spinal cord-- T1-L2(3))
What is cranial outflow portion of PANS?
PANS connects with brain through CNs (oculomotor, facial, glossopharyngeal, vagus)
What is sacral outflow portion of PANS?
PANS connects with spinal cord through sacral spinal nerves S2-S4
What is a junction between a neuron and another cell where action potential (nerve impulse) is transmitted from cell to cell?
What kind of cell can postsynaptic cell be?
Muscle cell (smooth, cardiac, skeletal)
Where are electrical synapses found in human?
NOT common in mammals
Present in brainstem, retina, and cerebral cortex
What structures are contained within terminal bouton (presynaptic component)?
What structures are contained with postsynaptic component?
Postsynaptic membrane (which displays postsynaptic density)
Receptors for neurotransmitter
Ligand-gated Na+ channels
What happens as action potential reaches presynaptic membrane?
Voltage-gated Ca++ channels on presynaptic membrane open
Ca++ ions enter presynaptic terminal
What occurs after Ca++ ions enter presynaptic terminal?
Synaptic vesicles approach and attach to inner surface of presynaptic membrane
The 2 membranes rupture at point of contact, releasing NT into synaptic cleft via exocytosis
What happens after NT is released from presynaptic terminal?
NT diffuses across synaptic cleft and binds to receptor sites on postsynaptic membrane
What happens after NT binds receptor sites on postsynaptic membrane?
Ligand-gated Na+ channels on postsynaptic membrane open
Na+ enters postsynaptic membrane, causing depolarization of membrane
What results from depolarization of postsynaptic membrane?
Voltage-gated Na+ channels open, generating nerve impulse
Describe structure of choroid plexus
Folds of pia mater covered by simple cuboidal epithelium (ependymal cells held together by tight junctions/zonulae occludentes)
What is function of choroid plexus?
Produce cerebrospinal fluid which is released into ventricular system
Where is CSF found?
Central canal of spinal cord
Subarachnoid space around brain and spinal cord
Describe composition of CSF
High Na+, K+, Cl-
Clear, low density
90% water and ions, a few lymphocytes
How does brain get rid of its metabolic waste?
Brain's metabolic waste products diffuse into CSF of subarachnoid space
Brain does NOT contain lymphatics
Where is CSF reabsorbed?
Reabsorbed by arachnoid villi in superior sagittal sinus
Passes into bloodstream
What are 3 layers of cerebellar cortex?
Molecular layer (superficial)
Purkinje layer (middle)
Granular layer (deepest)
What kind of changes occur distal to site of injury in nerve cell?
Anterograde changes (includes degeneration and elimination of debris)
What is Wallerian degeneration?
Segment of axon distal to nerve cell injury degenerates
What kind of cells clean up debris following nerve cell injury?
Phagocytic cells derived from Schwann cells
Blood-derived monocytes (form macrophages)
What occurs in anterograde changes (after debris is eliminated) following nerve cell injury?
Schwann cells proliferate and along with their external lamina, form tubes/tunnels
What kind of changes occur proximal to site of injury in nerve cell?
Retrograde reaction and regeneration
What happens during chromatolysis of nerve cell following injury?
Cell body swells
Loss of Nissl bodies
Nucleus moves away from center of cell body
What occurs during retrograde reaction and regeneration following nerve cell injury?
Formation of free ribosomes and protein synthesis
Axon grows sprouts
Schwann cells guide axon growth toward target cell
Axon grows into endoneurium
What structures are essential for regeneration following nerve cell injury?
How do sensory receptor vary?
Location in body
Velocity of conduction
Modality (sensation) which they carry--generally more than 1
What is area/territory where receptor resides and where it transduces stimuli into receptor potentials?
What type of receptor fibers are myelinated fibers from annulospiral endings innervating muscle spindles?
Type Ia or A-alpha
What type of receptor fibers are myelinated fibers innervating Golgi tendon organs?
Type Ib or A-alpha
What type of receptor fibers are myelinated fibers from flower spray endings innervating muscle spindles?
Type II or A-beta
What type of receptor fibers are lightly myelinated fibers that relay pain, temperature and crude touch sensation?
Type III or A-delta
To what kind of stimuli do exteroreceptors respond?
Sensory stimuli from environment--olfactory, visual, gustatory, auditory, tactile
To what kind of stimuli do teloreceptors respond?
Activated by stimulus from a distance (contact NOT required)
To what kind of stimuli do contact receptors respond?
Contact is required
What is a sensory receptor?
Free nerve ending or specialized terminal at peripheral process of sensory neuron
Where are sensory receptors located?
Scattered throughout body:
Skin, subcutaneous connective tissue, skeletal muscle, tendons, ligaments, periosteum, viscera, conjunctiva, dura mater
What kind of sensory neurons are associated with sensory receptors?
What is sensory receptive end of neuron?
Picks up sensory info which is relayed via peripheral process toward cell body
Eventually neural impulses reach central process (axon) and go to CNS/brain
Describe sense of smell
When inspire air, odorous substances dissolve in mucus blanket that covers olfactory epithelium
Then travel to receptors at dendritic terminals of bipolar neurons
Causes neurons to fire and send info to CNS (olfactory bulb)
What are receptors in cochlea and vestibular apparatus?
Bipolar neurons in cochlear and vestibular ganglia synapse with hair cells
Where are dendrites of pseudounipolar neurons located?
Muscle, tendons, ligaments
Give examples of encapsulated nerve cell endings
Rufini's end organs
Golgi tendon organs
T/F Multipolar neurons do not have sensory receptors at dendritic terminals
Instead, they have NT receptors
What modalities are carried by cranial and spinal nerves?
Tactile sensation--touch (light/crude, discriminative/fine, detailed), pressure (deep touch), vibration
Describe sensation of brain
Brain processes all thypes of sensory info but has NO sensation itself
During brain surgery, have to anesthetize skin of scalp, bone, meninges, dura mater but not brain
What type of info do proprioceptors relay?
Static position sense (when certain body part is immobile)
Kinesthetic position sense (when body part is moving)
Where are interoceptors located?
Deep within body
Hypothalamus, carotid body, carotid sinus, urinary bladder
What type of receptor are gustatory receptors?
(taste buds on tongue and epiglottis)
What type of receptor is found in carotid body?
Chemoreceptor (interoreceptor) that senses pCO2 in bloodstream
What type of receptor is found in carotid sinus?
Baroreceptor (interoreceptor) that senses stress/stretch of wall as result of BP
When nerve terminals of glossopharyngeal nerve stretched, sends info to CNS
When are proprioceptors stimulated?
Sense our weight--when muscles stretched in response to weight, receptors fire and send info to CNS
What are 3 classifications of thermoreceptors?
What are 3 types of nonencapsulated mechanoreceptors?
Free nerve endings
Peritrichial nerve endings (of A-beta)
Tactile receptors (of A-beta)
Where are nonencapsulated mechanoreceptors with free nerve endings found?
somatic structures (muscles, tendons, ligaments, joint capsules)
To what do nonencapsulated mechanoreceptors with free nerve endings respond?
What kind of receptor is Merkel's disc?
Tactile receptor (nonencapsulated mechanoreceptor)
Expanded dendritic terminals that ends in stratus spinosum of skin
To what do Merkel's discs respond?
Fine, detailed touch sensation
(shape, texture, edges of objects)
What are examples of encapsulated mechanoreceptors?
Rufini's end organs
Golgi tendon organs
Where are peritrichial nerve endings (nonencapsulated mechanoreceptor) found?
At base of hair follicle
Sense movement of strands of hair
What kind of receptor would be used to read Braille?
Meissner's corpuscle (encapsulated mechanoreceptor)
Where are Pacinian corpuscles located?
ligaments, interosseous membranes, joint capsules
What is function of muscle spindle?
Monitor skeletal muscle length
Proprioception--respond to stretch
When are golgi tendon organs stimulated?
When load is placed on muscle, creating tension on tendon, GTOs are stimulated
What kind of neurons innervate extrafusal fibers?
Motor-- alpha motor neurons
Sensory-- pseudounipolar neurons
What are muscle spindles?
Specialized mechanoreceptor (proprioceptor) that monitors skeletal muscle length
Describe structure of muscle spindle
2-12 intrafusal fibers (each covered by a thin capsule) enclosed by a fibrous capsule
Describe intrafusal fibers
Noncontractile region with nuclei in center
Skeletal muscle contractile portion with myofibrils at 2 polar ends
What provides sensory innervation to intrafusal fibers?
Sensory--pseudounipolar neurons with Type Ia (annulospiral primary endings) and Type II (flower spray secondary endings) fibers
What fibers become activated at onset of muscle stretch/tension to innervate intrafusal fibers?
Annulospiral (primary) endings--Type Ia
What innervates golgi tendon organs?
Receives sensory innervation
NO motor innervation (do not have contractile portion)
What happens if injury occurs to axons in CNS?
Minimal or no regeneration of tract axons
If minimal regeneration occurs--does NOT lead to recovery of function
Why do axons in CNS not regenerate following injury?
Basal lamina and Schwann cell tunnels formed in PNS following injury are non-existent in CNS
Oligodendrocytes/myelin may release inhibitory factor, impeding regrowth of axons
Astrocytes may not release enough GF to support sprouting axons
What happens following nerve cell injury in CNS?
Neuron forms sprouts
Astrocytes proliferate--hypertrophy, forming scar tissue to replace damaged tissue
Guiding tunnels not formed
Scar tissue gets in way/prevents axon sprouts from reaching target (axon sprouts pull back)
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