265 terms

Neuroanatomy Block I: Nervous Tissue I and II, Neurohistology

What is the basic structural and functional unit of nervous system?
Neuron (nerve cell)
Is excitable/irritable
What is trophic metabolic center of nerve cell?
Cell body
What is another name for cell body?
Where does most protein synthesis occur in nerve cell?
Cell body
Where is receptive area of multipolar neurons?
Cell body
What structures are found in cell body of nerve cell?
Neurofilaments, microfilaments, microtubules
Lipid droplets
Melanin granules (in certain neurons--substantia nigra)
Axon hillock
What shapes can cell body of nerve cell take?
Spherical, ovoid or angular
What are basophilic clumb found in cytoplasm of nerve cells?
Nissl bodies--stacks of rER
(Free ribosomes also found in cytoplasm)
Can neurons form tumors in adults?
Do not undergo mitosis
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 axon hillock of nerve cell
Origin of axon, funnel-shaped
no rER--stains pale
What is chromatolysis?
Dissolution of Nissl substance following injury to nerve cell body
What processes radiate from cell body of nerve cell?
Branch and taper
What increases receptive area of neuron?
Describe structures found in dendrites
No Golgi apparatus
Mitochondria and rER may be present
Never myelinated
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?
No rER
Describe axon of neuron
Variable length (1mm-1m)
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 axolemma?
Axon cell membrane
What is axoplasm?
Axon cytoplasm
What is initial segment of axon?
Segment distal to axon hillock and proximal to where myelination begins
Where are collateral branches of axon located?
Near axon hillock
Where is action potential initiated in axon?
Dense undercoating of cell membrane
Describe regeneration of axon that occurs following injury?
Proximal axon regenerates
Distal axon degenerates
What are terminal arborizations of axon?
Terminal axonal branches
What are terminal buttons/axon terminals?
Dilated ends of arborizations
What is function of terminal buttons?
Form synapses
Some local protein synthesis
In which direction does anterograde transport occur?
Away from cell body
In which direction does retrograde transport occur?
Toward cell body
What microtubule-associated motor protein plays a role in anterograde transport?
What microtubule-associated motor protein plays a role in retrograde transport?
How do viruses and toxins that enter nerve endings enter CNS?
via retrograde transport
What substances are transported via slow anterograde transport system?
Tubulin molecules
Neurofilament proteins
Describe fast axoplasmic transport system
Requires ATP
What substances are transported via fast axoplasmic transport system?
Membranous organelles (sER, synaptic vesicles, mitochondria)
Sugars, amino acids, nucleotides
What is most common type of neuron?
Where are bipolar neurons found?
Inner ear
Describe bipolar neuron
2 processes--peripheral (dendrite) and central (axon)
Describe multipolar neuron
Numerous dendrites and single axon
Describe unipolar/pseudounipolar neuron
Single process arising from cell body which secondarily bifurates into 2 processes with axon characteristics
T shaped
How are impulses transmitted in pseudounipolar neurons?
Impulse transmitted down axon, bypassing cell body
What kind of neurons are motor (efferent) neurons?
What is function of sensory (afferent) neurons?
Receive sensory stimuli from environment or from within body
Where do skeletal motor neurons go?
To skeletal muscle fibers
Where do visceeral motor neurons go?
To smooth muscle, cardiac muscle or glands
Where are interneurons located?
Confined to CNS
99.9% of neurons
What kind of neurons are interneurons?
What is function of neuroglia?
Provide physical and physiological support, protecting nerve cells
How many neuroglia are present for every neuron?
Can neuroglia form tumors?
Yes, they can divide
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 are largest neuroglial cells?
Astrocytes (astroglia)
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?
Fibrous astrocytes
(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 type of cancer makes up 80% of primary brain tumors in adult?
Fibrous astrocytomas
What kind of neuroglia form rows in white matter?
Oligodendrocytes (oligodendroglia)
smaller than astrocytes
What organelles are found in oligodendrocyte?
Many mitochondria
Golgi apparatus
What kind of neuroglia have small cell bodies and elongated nuclei? (other gila have round nuclei)
Condensed chromatin
Where are microglia found?
In both grey and white matter
What kind of neuroglia are phagocytic (CNS housekeepers)?
(derived from monocytes)
What are cuboidal-columnar cells lining brain and spinal cord cavities, containing CSF?
Ependymal cells
What structures are found in ependymal cells?
Motile cilia which move CSF
Abundant mitochondria
apical Golgi apparatuses
sparse rER
Gap junctions and zonula adherens
Describe unmyelinated axons in CNS
not covered by glia
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
T/F There are no nodes of Ranvier along unmyelinated axons
What is function of myelin?
Increase impulse conduction and insulate neurons
What is mesaxon?
Fused cell membranes
From what structure are Schwann cells derived?
Neural crest cells
What is myelin?
Lipoprotein formed by concentric layers of cell membrane which have a high lipid content (~80%)
T/F Myelin is a substance that is secreted on the axon by a cell
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 name of uninsulated segment of axon?
Nodes of Ranvier
What is name of insulated segment of axon?
Internodal segment (internode)
What is delicate layer of loose connective tissue with collagen fibrils that covers individual axons?
What cells synthesize endoneurium?
Schwann cells
Describe layers of myelinated axon in PNS
Myelin sheath, basal lamina, endoneurium
What is specialized connective tissue, lined by layers of perineural cells, that surrounds a nerve bundle?
What inhibits passage of macromolecules between nerve cells?
Zonulae occludentes
What is enclosed by perineurium?
Schwann cell-ensheathed axons and endoneurium
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 optic neuritis?
Inflammation of optic nerve
What is most common cause of optic neuritis?
Multiple sclerosis
(or other demyelinating disease)
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 papilledema?
Swelling of optic disk
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 a bundle of nerve cell axons in PNS?
How many cranial nerves are present?
12 pairs
How many spinal nerves are present?
31 pairs
What are collections of nerve cell bodies in PNS?
What is example of sensory ganglia in PNS?
Cranial nerve sensory ganglia
Dorsal root ganglia
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 involved in "fight, firght, or flight" response?
What nervous system is referred to as "thoracolumbar outflow"?
(Connects with thoracic and lumbar regions of spinal cord-- T1-L2(3))
What nervous system maintains homeostasis?
What nervous system is referred to as "cranosacral outflow"?
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?
Another neuron
Muscle cell (smooth, cardiac, skeletal)
Gland cell
Where are electrical synapses found in human?
NOT common in mammals
Present in brainstem, retina, and cerebral cortex
Describe electrical synapse
Gap junctions allow ions (electrical current) to pass from cell to cell
Which type of synapse results in faster impulse transmission: electrical or chemical?
What is most common type of synapse?
What structures are contained within terminal bouton (presynaptic component)?
Presynaptic membrane
Synaptic vesicles
What are contained within synaptic vesicles?
What is presynaptic density?
Cone-shaped structures that represent active site of synapse
What is a narrow extracellular space between presynaptic and postsynaptic membranes?
Synaptic cleft
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
Where is choroid plexus located?
in ventricles (hollow cavities) of brain
Describe structure of choroid plexus
Folds of pia mater covered by simple cuboidal epithelium (ependymal cells held together by tight junctions/zonulae occludentes)
What kind of capillaries are found in choroid plexus?
Fenestrated (leaky) capillaries
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
Low protein
High Na+, K+, Cl-
Clear, low density
90% water and ions, a few lymphocytes
How often is CSF replaced?
4 times/day
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 is function of CSF?
Shock absorber around brain and spinal cord
What are 3 layers of cerebellar cortex?
Molecular layer (superficial)
Purkinje layer (middle)
Granular layer (deepest)
What cells are unique to cerebellum?
Purkinje cells
What are largest cells of nervous system?
Purkinje cells
What are smallest cells of nervous system?
Granule cells
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
(orthograde degeneration)
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?
Basal lamina
Schwann cells
Where are cell bodies in CNS located?
Grey matter
Cerebral cortex
Cerebellar cortex
Where are cell bodies in PNS located?
Where are axons in CNS located?
White matter
Whre are axons in PNS located?
What type of synapse (axo---) are most common?
What type of synapse (axo---) are least common?
How do sensory receptor vary?
Location in body
Velocity of conduction
Modality (sensation) which they carry--generally more than 1
What is transformation of stimulus into electrical signal?
Sensory transduction
What is generated following stimulation of receptor?
Receptor potential
What is nerve impulse?
Action potential
What is area/territory where receptor resides and where it transduces stimuli into receptor potentials?
Receptive field
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 is conduction velocity of Type Ia or A-alpha fibers?
80-120 m/s
What is conduction velocity of Type Ib or A-alpha fibers?
80-120 m/s
What type of receptor fibers are myelinated fibers from flower spray endings innervating muscle spindles?
Type II or A-beta
What type of fibers are cutaneous tactile receptors?
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
What is conduction velocity of type II or A-beta fibers?
35-75 m/s
What is conduction velocity of Type III or A-delta fibers?
5-30 m/s
What type of receptor fibers are unmyelinated fibers that relay pain and temperature?
Type IV or C
What is conduction velocity of Type IV or C fibers?
0.5-2 m/s
Where are exteroreceptors located?
Close to body surface
To what kind of stimuli do exteroreceptors respond?
Sensory stimuli from environment--olfactory, visual, gustatory, auditory, tactile
What are 2 classifications of exteroreceptors?
Contact receptors
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 are proprioceptors?
Position sense receptors
Where are proprioceptors located?
Vestibular apparatus
Joint capsules
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?
Bipolar neurons
Pseudounipolar neurons
What is sensory receptive end of neuron?
Dendritic terminal
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
Where are bipolar neurons located?
Olfactory epithelium
Vestibular apparatus
With what modality are bipolar neurons associated?
Special senses
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 of vision?
Bipolar neurons synapse with rods and cones in neural retina
What are receptors in cochlea and vestibular apparatus?
Bipolar neurons in cochlear and vestibular ganglia synapse with hair cells
Where is cell body of pseudounipolar neuron located?
CN sensory ganglia
Where are dendrites of pseudounipolar neurons located?
Muscle, tendons, ligaments
Joint capsules
Describe nerve endings/receptors
Can be naked/free or encapsulated
Give examples of encapsulated nerve cell endings
Meissner's corpuscles
Pacinian corpuscles
Rufini's end organs
Muscle spindles
Golgi tendon organs
T/F Multipolar neurons do not have sensory receptors at dendritic terminals
Instead, they have NT receptors
What modalities are carried only by cranial nerves?
What nerve carries olfaction?
Olfactory nerve
What nerve carries vision?
Optic nerve
What nerve carries gustation?
Facial, glossopharyngeal, vagus nerves
What nerve carries proprioception?
Vestibular nerve
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)
What can impair proprioceptors?
Alcohol or other drugs
Where are interoceptors located?
Deep within body
Hypothalamus, carotid body, carotid sinus, urinary bladder
What type of receptor are gustatory receptors?
Contact 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 kind of receptor is found in urinary bladder?
Baroreceptor (interoreceptor)
What are 2 classifications of interoreceptors?
What do baroreceptors detect?
What do chemoreceptors detect?
pO2 and pCO2
What kind of receptors respond to noxious stimuli?
(free nerve endings)
What type of fibers make up nerve endings of nociceptors?
Type A-delta and C
What are 3 classifications of nociceptors?
What type of fibers make up terminals of mechanosensitive nociceptors?
What type of fibers make up terminals of temperature-sensitive nociceptors?
What type of fibers make up terminals of polymodal nociceptors?
C fibers
To what do mechanosensitive nociceptors respond?
excessive mechanical stimulation or tissue injury
To what do polymodal nociceptors respond?
Various noxioius stimuli (mechanical, thermal, chemical)
What type of fibers make up nerve endings of thermoreceptors?
A-delta and C
(free nerve endings)
What are 3 classifications of thermoreceptors?
Cold receptors
Warmth receptors
Temperature-sensitive nociceptors
What type of fibers make up nerve endings of cold receptors?
What type of fibers make up nerve endings of warmth receptors?
C fibers
To what do temperature-sensitive nociceptors respond?
Intense heat or cold
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?
mucous membranes
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?
Meissner's corpuscles
Pacinian corpuscles
Rufini's end organs
Muscle spindles
Golgi tendon organs
Where are peritrichial nerve endings (nonencapsulated mechanoreceptor) found?
At base of hair follicle
Sense movement of strands of hair
Where are Meissner's corpuscles located?
Dermal papillae
What kind of receptor would be used to read Braille?
Meissner's corpuscle (encapsulated mechanoreceptor)
Where are Pacinian corpuscles located?
Dermis, hypodermis
ligaments, interosseous membranes, joint capsules
external genitalia
What stimulates Pacinian corpuscles?
Where are Rufini's end organs found?
joint capsules
hypodermis of hairy skin
What is function of Rufini's end organs?
Proprioceptive input
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 are extrafusal muscle fibers?
Typical skeletal muscle cells (fibers)
What occurs when extrafusal fibers are stimulated?
Muscle cells contract to produce movement
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
How are muscle spindles oriented?
Parallel to longitudinal axis of extrafusal muscle fibers
Describe structure of muscle spindle
2-12 intrafusal fibers (each covered by a thin capsule) enclosed by a fibrous capsule
What are the 2 distinct types of intrafusal fibers?
Nuclear bag fibers
Nuclear chain fibers
Describe intrafusal fibers
Noncontractile region with nuclei in center
Skeletal muscle contractile portion with myofibrils at 2 polar ends
What are 2 types of nuclear bag fibers?
What stimulates static nuclear bag fibers?
Changes in muscle length
What stimulates dynamic nuclear bag fibers?
Rate of change in muscle length
What stimulates nuclear chain fibers?
Changes in muscle length
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 provides motor innervation to intrafusal fibers?
Gamma motor neurons
What fibers become activated at onset of muscle stretch/tension to innervate intrafusal fibers?
Annulospiral (primary) endings--Type Ia
What fibers become activated when stretch is in progress?
Flower spray (secondary) endings--Type II
Where are golgi tendon organs located?
Where muscle inserts into tendon
How are golgi tendon organs arranged?
In series
What innervates golgi tendon organs?
Receives sensory innervation
NO motor innervation (do not have contractile portion)
What do golgi tendon organs monitor?
Amount of tensioin placed on skeletal muscle
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
T/F Neurologic deficits following nerve cell injury in CNS are permanent
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)
What cells are activated in CNS to clean up debris following nerve cell injury?