Download Chapter 7: Structure of Nervous System

Chapter 7: Structure of Nervous System
 Is divided into:
 Central nervous system (CNS) = brain and spinal cord
 Peripheral nervous system (PNS) = cranial and spinal nerves
 Consists of 2 kinds of cells:
 Neurons and supporting cells (glial cells).
 Neurons are ______________________ units of NS
 Glial cells maintain homeostasis. Are 5X more than neurons
Neurons: Gather and transmit information by responding to stimuli
Producing and sending electrochemical impulses. Releasing chemical messages
 Have a cell body, dendrites and axon. Cell body contains the nucleus
 Cell body is the ____________________ center and makes macromolecules
 Groups of cell bodies in CNS are called nuclei; in PNS are called ganglia
 Dendrites receive information, convey it to cell body
 Axons conduct impulses ___________________ from cell body
 Long axon length necessitates special transport systems:
 Axoplasmic flow moves soluble compounds toward nerve endings
 Via rhythmic contractions of axon
 Axonal transport moves large and insoluble compounds ____________________
along microtubules; very fast
 Anterograde transport moves materials away from cell body
 Uses the molecular motor kinesin
 ________________________ transport moves materials toward cell body.
Uses the molecular motor dynein. Viruses and toxins can enter CNS this way
Functional Classification of Neurons
 Sensory/Afferent neurons conduct impulses into CNS
 Motor/Efferent neurons carry impulses out of CNS
 Association/ Interneurons ___________ NS activity. Located entirely inside CNS
Structural Classification of Neurons
 Pseudounipolar: Cell body sits along side of single process. e.g. sensory neurons
 Bipolar: Dendrite and axon arise from opposite ends of cell body (retinal neurons)
 Multipolar: Have many dendrites and one axon. e.g. motor neurons
Supporting/Glial Cells
 PNS has Schwann and satellite cells. Schwann cells ______________ PNS axons
 CNS has oligodendrocytes, microglia, astrocytes and ependymal cells
 Each oligodendrocyte myelinates several CNS axons
 Ependymal cells appear to be neural stem cells.
 Other glial cells are involved in NS _______________________
Myelination
 In PNS each Schwann cell myelinates 1mm of 1 axon by wrapping round axon
 Electrically insulates axon
 Uninsulated gap between adjacent Schwann cells is called the node of Ranvier
Axon Regeneration
 Occurs much more readily in PNS than CNS
 Oligodendrocytes produce proteins that ____________________ regrowth
 And form glial scar tissue that blocks regrowth
Nerve Regeration
 When axon in PNS is severed: Distal part of axon degenerates
 Schwann cells survive; form regeneration tube
 Tube releases chemicals that attract growing axon
 Tube guides regrowing axon to synaptic site
Neurotrophins: Promote ___________________l nerve growth
 Required for survival of many adult neurons. Important in regeneration
Astrocytes: Most common glial cell
 Involved in: Buffering K+ levels. Recycling neurotransmitters
 Regulating adult neurogenesis.
 Releasing _______________________ that regulate neuronal activity
Blood-Brain Barrier: Allows only certain compounds to enter brain
 Formed by capillary specializations in brain; appear to be induced by astrocytes
 Capillaries are not as _________________ as those in body
 Gaps between adjacent cells are closed by tight junctions
Resting Membrane Potential
 At rest, all cells have a negative internal charge and unequal distribution of ions:
 Large cations being trapped inside cell
 Na+/K+ pump and limited permeability keep Na+ high outside cell
 K+ is very _________________________ and is high inside cell
Excitability
 Excitable cells can discharge their RMP quickly.
 By rapid changes in permeability to ions
 Neurons and muscles do this to generate and conduct impulses
Membrane Potential (MP) Changes
 Measured by placing 1 electrode inside cell and 1 outside
 Depolarization occurs when MP becomes more positive
 Hyperpolarization: MP becomes more _________________________ than RMP
 Repolarization: MP returns to RMP
Membrane Ion Channels
 MP changes occur by ion flow through membrane channels
 K+ leakage channels are always open
 Voltage-gated (VG) channels are opened by ________________________
 VG K+ channels are closed in resting cells
 Na+ channels are VG; closed in resting cells
The Action Potential(AP)
 Is a wave of MP change that sweeps along the axon from soma to synapse
 Wave is formed by rapid depolarization of the membrane by Na+ influx; followed
by rapid ___________________________by K+ efflux
Mechanism of Action Potential
 Depolarization:
 At threshold, VG Na+ channels open.
 Na+ driven inward, opens more channels
 Causes a rapid change in MP from ___________ to +30 mV
 Repolarization:
 VG Na+ channels close; VG K+ channels open
 Electrochemical gradient drives K+ outward. Repolarizes axon to RMP
 Depolarization and repolarization occur via diffusion
 After an AP, Na+/K+ pump extrudes Na+, recovers K+
APs Are All-or-None
 When MP reaches threshold an AP is ______________________ fired
 Positive feedback opens more and more Na+ channels
 Then Na+ channels close, inactivated until repolarization
How Stimulus Intensity is Coded
 Increased stimulus intensity causes more APs to be fired.
 ____________ of APs remains constant
Refractory Periods
 Absolute refractory period:
 Membrane cannot produce another AP because Na+ channels are
inactivated
 Relative refractory period occurs when VG K+ channels are ________________,
making it harder to depolarize to threshold
Cable Properties
 Axon’s properties affect its ability to conduct current
 High resistance of cytoplasm decreases as axon diameter increases
Conduction in an Unmyelinated Axon
 Axon _________ fires AP, its Na+ influx depolarizes adjacent regions to threshold
 Generating a new AP. Process repeats all along axon.
 So AP amplitude is always same. Conduction is slow
Conduction in Myelinated
 Ions can't flow across myelinated membrane. Thus no APs occur under myelin
 and no current leaks. This _______________________ current spread
 Gaps in myelin are called Nodes of Ranvier
 APs occur only at nodes. VG Na+ channels are present only at nodes
 Current from AP at 1 node can depolarize next node to threshold
 Fast because APs skip from node to node.
 Called __________________________ conduction
Synaptic Transmission: Synapse
 Is a functional connection between a neuron (presynaptic) and another cell
(postsynaptic)
 There are chemical and electrical synapses
 Synaptic transmission at ________ synapses is via neurotransmitters (NT)
 Electrical synapses are rare in NS
Electrical Synapse
 Depolarization flows from presynaptic into postsynaptic cell through channels
called ___________________________
 Found in smooth and cardiac muscles, brain, and glial cells
Chemical Synapse
 Synaptic cleft separates terminal bouton of presynaptic from postsynaptic cell
 _____________________________ are in synaptic vesicles
 Vesicles fuse with bouton membrane; release NT by exocytosis
 Amount of NT released depends upon frequency of APs
 NT (ligand) diffuses across cleft
 Binds to receptor proteins on postsynaptic membrane
 ______________________ chemically-regulated ion channels
 Depolarizing channels cause EPSPs (excitatory postsynaptic potentials)
 Hyperpolarizing channels cause IPSPs (inhibitory postsynaptic potentials)
 These affect VG channels in postsynaptic cell
 EPSPs and IPSPs summate
 If MP in postsynaptic cell reaches ________________________ at the axon
hillock, a new AP is generated
Acetylcholine (ACh): Most widely used NT.
 Used in brain and ANS; used at all neuromuscular junctions
 Has nicotinic and muscarinic receptor subtypes
 These can be excitatory or _________________________
Nicotinic ACh Channel
 2 subunits contain ACh binding sites. Opens when 2 AChs bind.
 Moves Na+ into and K+ out of postsynaptic cell. Produces EPSPs
Muscarinic ACh Channel
 Binding of 1 ACh activates _____________________ which
 Opens some gated K+ channels, causing hyperpolarization
 Closes others, causing depolarization
Acetylcholinesterase (AChE)
 ___________________________ ACh, terminating its action; located in cleft
Acetylcholine in the PNS
 Cholinergic neurons use acetylcholine as NT
 The large synapses on skeletal muscle are termed end plates or neuromuscular
junctions (NMJ)
 Produce large EPSPs called end-plate potentials.
 Open VG channels beneath end plate. Cause muscle contraction
 Curare _____________________ ACh action at NMJ
Monoamine NTs
 Include serotonin, norepinephrine and dopamine
 Serotonin is derived from tryptophan
 Norepinephrine and dopamine are derived from tyrosine. Called ______________
Serotonin
 Involved in regulation of mood, behavior, appetite and cerebral circulation
 LSD is structurally similar
Dopamine: is involved in motor control
__________________________ of this system causes Parkinson's disease
 is involved in behavior and emotional reward
 Most addictions activate this system
 Overactivity contributes to schizophrenia
Norepinephrine (NE)
 Used in PNS and CNS. In PNS is a _________________________ NT
 In CNS affects general level of arousal
Amino Acids NTs
 Glutamic acid and aspartic acid are major CNS excitatory NTs
 Glycine is an inhibitory NT
 GABA (gamma-aminobutyric acid) is most common NT in brain
 ____________________________
Polypeptide NTs (neuropeptides): Cause wide range of effects
 Involved in learning and neural plasticity
 Neuropeptide Y is most common neuropeptide
 Powerful stimulator of appetite
Gaseous NTs: NO and CO are gaseous NTs
 NO causes smooth muscle relaxation. __________________ increases NO
Synaptic Integration: EPSPs
 Graded in magnitude. Have no threshold. Cause depolarization
 Summate. Have no refractory period
Spatial Summation
 Cable properties cause EPSPs to _____________ quickly over time and distance
 Spatial summation takes place when EPSPs from different synapses occur in
postsynaptic cell at _____________ time
Temporal Summation
 Temporal summation occurs because EPSPs that occur closely in time can sum
before they fade
Synaptic Plasticity
 Repeated use of a synapse can increase or decrease its ease of transmission
 = synaptic facilitation or synaptic depression
 High frequency stimulation often causes enhanced excitability
 Called _______________________________
 Believed to underlie learning
Synaptic Inhibition
 Postsynaptic inhibition
 GABA and glycine produce IPSPs
 IPSPs dampen EPSPs
 Making it _____________________ to reach threshold
 Presynaptic inhibition:
 Occurs when 1 neuron synapses onto axon or bouton of another neuron,
inhibiting release of its NT
Chapter 8: The Central Nervous System
 Consists of brain and spinal cord
 Receives input from sensory neurons
 Directs activity of motor neurons
 _________________________ neurons integrate sensory and motor activity
 Perform learning and memory
 CNS composed of gray and white matter
 Gray matter consists of neuron bodies and dendrites
 White matter (myelin) consists of axon tracts
 Adult brain weighs 1.5kg. Contains 100 billion neurons
 Receives ____________ of blood flow to body
Embryonic Development
 Neural tube forms from groove in ectoderm by 20th day. Becomes the CNS
 Neural crest cells develop where tube fuses. Become _______________ of PNS
 During 4th week, 3 swellings form on neural tube
 These will become forebrain, midbrain and hindbrain
 During 5th week: Forebrain elaborates into telencephalon and diencephalon
 ________________________ does not subdivide
 Hindbrain forms metencephalon and myelencephalon
 Telencephalon grows disproportionately forming hemispheres of cerebrum
 Ventricles and central canal are remnants of hollow part of neural tube
 Contain _________________________________
Cerebrum
 Is largest part of brain (80% of mass). Is responsible for higher mental functions
 Its right and left hemispheres are interconnected by tract of the corpus callosum
Cerebral Cortex
 Is highly ________________________
 An elevated fold is called a gyrus.A depressed groove is called a sulcus
 Each hemisphere has 5 lobes: frontal, parietal, temporal, occipital and insula
 Frontal lobe is separated from parietal by central sulcus
 Precentral gyrus of frontal lobe is involved in motor control
 Postcentral gyrus of parietal lobe receives _____________________ info from
areas controlled by precentral gyrus
 Temporal lobe contains auditory centers; receives sensory info from cochlea
 Also links and processes auditory and visual info
 Occipital lobe is responsible for vision and coordination of eye movements
 Insula plays role in ______________________ encoding
 Integrates sensory info with visceral responses
 Coordinates cardiovascular response to stress
Visualizing the Brain
 X-ray computed tomography (CT) visualizes soft tissues
 Positron-emission tomography (PET) examines brain metabolism and blood flow,
drug distribution
 Magnetic resonance imaging (MRI) shows brain _____________________
 Functional MRI (fMRI) shows areas with increased neural activity by tracking
blood flow
Electroencephalogram
 Measures ______________________l activity of cerebral cortex
 Used to diagnose epilepsy and brain death
EEG Waves
 Alpha waves are recorded from parietal and occipital lobes with person awake,
relaxed, eyes closed
 ___________________________ are strongest from frontal lobes; evoked by
visual stimuli and mental activity
 Theta waves come from temporal and occipital lobes
 Common in newborns. In adults indicates severe emotional stress
 Delta waves are from cerebral cortex
 Common during adult sleep and in awake infants
 In awake adult indicates brain _______________________
Sleep: 2 types of sleep are recognized
 REM - rapid eye movement
 EEGs are similar to awake ones. Type when dreaming occurs
 Non-REM has delta waves
 For consolidation of short- into long-term memory
Basal Nuclei (basal ganglia)
 Are distinct masses of cell bodies located deep inside cerebrum
 Function in control of _________________________ movement
Cerebral Lateralization
 Specialization of each hemisphere for certain functions
 Each cerebral hemisphere controls movement on ________________ side of body
 And receives sensory info from opposite side of body
 Hemispheres communicate thru the corpus callosum
 Left hemisphere possesses language and _______________________ abilities
 Right hemisphere is best at visuospatial tasks
Language
 Language areas of brain are known mostly from aphasias
 (speech and language disorders due to brain damage)
 Broca’s area is necessary for speech
 Wernicke’s area is involved in ______________________ comprehension
Limbic System and Emotion
 The hypothalamus and limbic system are crucial for emotions
 Including aggression, fear, feeding, sex and goal-directed behaviors
Memory
 Includes ________- and long-term memory. Involves a number of regions in brain
 There are two types of long-term memory
 Non-declarative (explicit) includes memories of simple skills and
conditioning
 Declarative (implicit) includes verbal memories
 _____________________ have impaired declarative memory
 Hippocampus is critical for acquiring new memories
 And consolidating short- into long-term memory
 Amygdala is crucial for fear memories
 Storage of memory is in cerebral hemispheres
 Higher order processing and planning occur in ___________________________
Long-Term Potentiation (LTP)
 Is the increased excitability of a synapse after high frequency stimulation
 LTP is thought to be a form of synaptic learning
Thalamus and Epithalamus
 Are located at base of cerebral hemispheres
 Thalamus is a _______________ center thru which all sensory info (except
olfactory) passes to cerebrum
 And plays role in level of arousal
 Epithalamus contains the choroid plexus which secretes CSF
 Also contains _________________________ which secretes melatonin
 Involved in sleep cycle and seasonal reproduction
Hypothalamus
 Is most important structure for homeostasis
 Contains neural centers for hunger, thirst, body temperature
 Regulates sleep, emotions, sexual arousal, anger, fear, ______________________
 Controls hormone release from anterior pituitary
 Produces ADH and oxytocin
 Coordinates sympathetic and parasympathetic actions
Pituitary Gland
 Is divided into anterior and posterior lobes
 ________________ pituitary stores and releases ADH (vasopressin) and oxytocin
 Both made in hypothalamus and transported to pituitary
 Hypothalamus produces releasing and inhibiting hormones that control anterior
pituitary hormones
Circadian Rhythms
 Are body's daily rhythms
 Regulated by SCN (suprachiasmatic nucleus) of hypothalamus
 SCN is the __________________ clock. Adjusted daily by light from eyes
 Controls pineal gland secretion of melatonin which regulates circadian
rhythms
Midbrain contains:
 Superior colliculi -- involved in visual reflexes
 Inferior colliculi -- relay ______________________ information
 Red nucleus and substantia nigra -- involved in motor coordination
 S. nigra dopamine neurons degenerate in Parkinson’s
Hindbrain
 Contains pons, cerebellum and medulla
Pons
 Contains several nuclei of cranial nerves. And 2 ____________________ control
centers: Apneustic and pneumotaxic centers
Cerebellum
 2nd largest structure in brain
 Receives input from ___________________ (joint, tendon and muscle receptors)
 Involved in coordinatng movements and motor learning
Medulla
 Contains all tracts that pass between brain and spinal cord
 And several crucial centers for breathing and cardiovascular systems
Reticular Activating System (RAS)
 Is an ascending arousal system from the pons, midbrain reticular formation,
hypothalamus and basal forebrain
 These project to the cerebral cortex and control its level of ____________
 Activation of the RAS promotes wakefulness; inhibition promotes sleep
Spinal Cord Tracts
 Sensory info from body travels to brain in ____________________ spinal tracts
 Motor activity from brains travels to body in descending tracts
Ascending Spinal Tracts
 Ascending sensory tracts ____________________ (cross) so that brain
hemispheres receive info from opposite side of body
 Same for most descending motor tracts from brain
Descending Spinal Tracts: Are divided into 2 major groups:
 Pyramidal (or corticospinal) tracts descend from cerebral cortex to spinal cord
without synapsing
 Originate in __________________. Function in control of fine movements
 Extrapyramidal (or Reticulospinal) tracts descend with many synapses
 Influence movement indirectly
Peripheral System (PNS)
 Consists of nerves that exit from CNS and spinal cord, and their ganglia
(collection of cell bodies __________________ CNS)
Cranial Nerves: Consists of 12 pairs of nerves
 2 pairs arise from neurons in forebrain
 10 pairs arise from midbrain and hindbrain neurons
 Most are _____________ nerves containing both sensory and motor fibers
Spinal Nerves
 Are mixed nerves that separate next to spinal cord into dorsal and ventral roots
 Dorsal root composed of sensory fibers.
 Ventral root composed of motor fibers
 There are 31 pairs: _________ cervical, 12 thoracic, 5 lumbar, 1 coccygeal
Reflex Arc
 Is a simple sensory input, motor output circuit involving only peripheral nerves
and spinal cord
 Sometimes arc has an association neuron between sensory and motor neuron.
Chapter 9: TheAutonomic Nervous System
 Autonomic nervous system (ANS) manages our physiology
 Regulates organs and organ systems, and their smooth muscles and glands
 Smooth muscle maintains a _________________ in absence of nerve stimulation
 Smooth becomes more sensitive when ANS input is cut (denervation
hypersensitivity)
 Many smooth are spontaneously active and contract rhythmically without ANS
input. ANS input simply increases or decreases intrinsic activity
Autonomic Neurons
 ANS has 2 neurons in its efferent pathway
 1st neuron (____________________ neuron) has cell body in brain or spinal cord
 Synapses with 2nd neuron (postganglionic neuron) in an autonomic
ganglion
 Postganglionic axon extends from autonomic ganglion to target tissue
Divisions of the ANS
 ANS has sympathetic and parasympathetic divisions
 Which usually have ____________________________ effects
 These coordinate physiology with what’s going on in person's life
 Sympathetic mediates "fight, flight, and stress" reactions
 Parasympathetic mediates "rest and digest" reactions
Sympathetic Division
 Is also called thoracolumbar division because its preganglionics exit spinal cord
from T1 to L2
 Most then synapse on postganglionics in the _________________ ganglia
 Which form chain of interconnected ganglia paralleling spinal cord
 Is characterized by divergence and convergence which cause Symp to mostly act
as a unit (________________________________)
 Divergence: preganglionics branch to synapse with a number of postganglionic
neurons
 Convergence: postganglionics receive synaptic input from a large number of
preganglionics
 Some postganglionics do not synapse in paravertebral ganglion but go to outlying
_________________________ ganglion
Sympathoadrenal System
 The adrenal medulla appears to be a modified collateral ganglion
 Its secretory cells appear to be modified postganglionics
 That release 85% epinephrine (Epi) and 15% norepinephrine
(Norepi) into blood in response to preganglionic stimulation
 Adrenal is stimulated during mass activation
Parasympathetic Division
 Is also called the ______________________ division because its long
preganglionics originate in midbrain, medulla, pons, and S2 - S4
 These synapse on postganglionics in terminal ganglia located next to or
within a target organ
 Postganglionics have ______________ axons that innervate target
 The long vagus nerve carries most Parasymp fibers
 Innervates heart, lungs, esophagus, stomach, pancreas, liver, small
intestine, and upper half of the large intestine
 Preganglionic fibers from S2-4 innervate lower half of large intestine, rectum,
urinary and reproductive systems
ANS Neurotransmitters
 Both Symp and Parasymp preganglionics release ACh
 Parasymp postganglionics also release ACh. Called ________________ synapses
 Most Symp postganglionics release Norepi (noradrenaline)
 Called ______________________________ synapses. Few release ACh
 Postganglionics have unusual synapses called varicosities
 Which release NTs along a length of axon (synapses en passant)
Adrenergic Stimulation
 Causes both excitation and inhibition depending on tissue
 Because of different subtypes of _______________________for same NT
 Drugs that promote actions of a NT are agonists
 Drugs that inhibit actions of a NT are antagonists
Cholinergic Stimulation
 __________________________ is used at all motor neuron synapses on skeletal
muscle, all preganglionics, and Parasymp postganglionics
 Cholinergic receptors have 2 subtypes:
 Nicotinic which is stimulated by nicotine;
 And ____________________________ which is stimulated by muscarine
(from poisonous mushrooms)
Other ANS NTs
 Some postganglionics do not use Norepi or ACh
 Called nonadrenergic, noncholinergic fibers
 Appear to use ATP, VIP, or NO as NTs
 NO produces smooth muscle relaxation in many tissues
Organs With Dual Innervation
 Most visceral organs receive _______________ innervation (supplied by both
Symp and Parasymp)
 The 2 branches are usually antagonistic, e.g. in controlling heart rate
 But can be complementary (cause similar effects), e.g. in
controlling salivation
 Or ______________________ (produce different effects that work
together to cause desired effect) such as with micturition
 Regulation is achieved by increasing or decreasing firing rate
 E.g. adrenal medulla, arrector pili muscle, sweat glands, and most blood
vessels receive only ________________________ innervation
Control of the ANS by Higher Brain Centers
 The medulla oblongata most directly controls activity of ANS
 It has centers for control of cardiovascular, pulmonary, urinary,
reproductive, and digestive systems
 ___________________________ has centers for control of body temperature,
hunger, and thirst; and can regulate medulla
Chapter 10: Sensory Physiology
Sensory Receptors
 Transduce (change) environmental info into APs -- the common language of NS
 Each type responds to a particular _________________________ (form of info,
e.g. sound, light, pressure)
 Different modalities perceived as different because of CNS pathways they
stimulate
 Can be simple _____________________ endings of neurons
 Or specialized endings of neurons or non-neuronal cells
 Are grouped according to type of stimulus they transduce
 Chemoreceptors sense chemical stimuli
 Photoreceptors transduce ____________________
 Thermoreceptors respond to temperature changes
 Mechanoreceptors respond to deformation of their cell membrane
 Nociceptors respond to intense stimuli by signaling pain
 Proprioceptors signal ________________________ info of body parts
 Also can be categorized according to location:
 Cutaneous receptors are near an epithelial surface
 Respond to touch, pressure, temperature or pain
 ___________________sense receptors are part of a sensory organ
 Such as hearing, sight, equilibrium
Sensory Receptor Responses
 Tonic receptors respond at constant rate as long as stimulus is applied, e.g. pain
 __________________ receptors respond with burst of activity but quickly reduce
firing rate to constant stimulation (adaptation), e.g. smell, touch
Law of Specific Nerve Energies
 Stimulation of sensory fiber evokes only the sensation of its modality
 Adequate stimulus is normal stimulus
 Requires least energy to _____________________ its receptor
Generator Potential
 Are sensory receptor equivalents of EPSPs.
 Produced in response to adequate stimulus
 Are proportional to stimulus intensity
 After threshold is reached AP frequency is _______________________l
to amplitude of generator potential
 In phasic receptors the generator potential adapts to a constant stimulus and
quickly diminishes in amplitude
 In tonic receptors ______________ potential does not adapt to a constant stimulus
Cutaneous Sensations
 Include touch, pressure, heat, cold, pain
 Mediated by free and __________________________ nerve endings
 Free nerve endings mediate heat, cold, pain
 Cold receptors located in upper dermis
 Warm located deeper in dermis
 Pain mediated by nociceptors.
 Use glutamate and Substance P as NTs. Substance P called "pain NT"
 Heat elicits pain thru ____________________________ receptors.
 Capsaicin is "hot" chemical in chili peppers
 Ruffini endings and Merkel's discs are slow-adapting, expanded free nerve
endings that mediate touch
 Encapsulated nerve endings mediate touch, pressure. Adapt quickly
 Include ___________________________ and Pacinian corpuscles
Somathesthetic Senses
 Include sensations from cutaneous and proprioceptors
 Heat, cold and pain. Touch and pressure
Receptive Field
 Is area of skin whose stimulation results in changes in firing rate of sensory
neuron
 Area varies ____________________________ with density of receptors
 E.g. back, legs have low density of sensory receptors
 Receptive fields are large
 Fingertips have hi density of receptors
 Receptive fields are small
Two-Point Touch Threshold
 Is minimum distance at which 2 points of touch can be perceived as separate
 Measure _______________________ or distance between receptive fields
Lateral Inhibition
 Is CNS process that sharpens sensation
 Sensory neurons at center of stimulation area inhibit more lateral neurons
Taste and Smell
 Receptors are exteroceptors because respond to _________________________ in
external environment
 Interoceptors respond to chemicals in internal environment
Taste (Gustation)
 Detects sweet, sour, salty, bitter and amino acids (umami)
 Taste receptor cells are modified epithelial cells
 50-100 are in each _________________________
 Each bud can respond to all categories of tastants
 Salty and sour do not have receptors; act by passing thru channels
 Sweet and bitter have receptors; act thru G-proteins
Smell (Olfaction)
 Receptors are located in olfactory ________________________ at top of nose
 Olfactory apparatus consists of receptor cells, supporting cells and basal cells
 Receptor cells are bipolar neurons that send axons to olfactory bulb
 Basal cells are stem cells that produce new receptor cells every 1-2 months
 Supporting cells contain detoxifying enzymes
 _____________ molecules bind to receptors and act through G-proteins
 Olfactory receptor gene family is huge
Ears and Hearing: Vestibular Apparatus
 Provides sense of equilibrium (orientation to gravity)
 Vestibular apparatus and cochlea form inner ear
 V. apparatus consists of otolith organs (_____________________________) and
semicircular canals
 Sensory structures located with membranous labyrinth
 Which is filled with endolymph, And located within bony labyrinth
 Utricle and saccule provide info about __________________ acceleration
 Semicircular canals, oriented in 3 planes, give sense of angular acceleration
 Hair cells are receptors for equilibrium
 Each contains 20-50 hairlike extensions called stereocilia
 1 of these is a kinocilium
 When stereocilia are bent ______________________ kinocilium, hair cell
depolarizes and stimulates 8th nerve
 When bent away from kinocilium, hair cell hyperpolarizes
Utricule and Saccule
 Have a ________________ containing hair cells
 Hair cells embedded in gelatinous otolithic membrane
 Which contains calcium carbonate crystals (otoliths) that resist
change in movement
 Utricle sensitive to horizontal acceleration
 Hairs pushed backward during forward acceleration
 Saccule sensitive to vertical acceleration
 Hairs pushed ______________________ when person descends
Semicircular Canals
 Provide information about rotational acceleration
 Project in 3 different planes. Each contains a semicircular duct
 At base is crista ampullaris where sensory hair cells are located
 Hair cell processes are embedded in ____________________ of crista ampullaris
 When endolymph moves cupula moves
 Sensory processes bend in opposite direction of angular acceleration
Nystagmus and Vertigo
 Vestibular nystagmus is involuntary oscillations of eyes that occur when spinning
person stops
 ______________________ is loss of equilibrium. Often caused by viral infection
 Or anything that alters firing rate of 8th nerve
Ears and Hearing
 Sound waves travel in all directions from source
 Waves characterized by frequency and intensity
 Frequency is measured in hertz (cycles/sec).
 __________________ is directly related to frequency
 Intensity (loudness) is directly related to amplitude of waves
 Measured in decibels
Outer Ear
 Sound waves funneled by pinna (auricle) into external auditory meatus
 External auditory meatus channels sound waves to tympanic membrane
Middle Ear
 Middle ear is between tympanic membrane and _______________; holds ossicles
 Malleus (hammer) is attached to tympanic membrane
 Carries vibrations to incus (anvil)
 Stapes (stirrup) receives vibrations from incus, transmits to oval window
 ______________ muscle, attached to stapes, provides protection from loud noises
 Contract and dampen large vibrations, Prevents nerve damage in cochlea
Cochlea
 Consists of a tube wound 3 turns and tapered so looks like snail shell
 Tube is divided into 3 fluid-filled chambers
 Scala vestibuli, cochlear duct, scala tympani
 ______________ window attached to scala vestibuli (at base of cochlea)
 Vibrations at oval window induce pressure waves in perilymph fluid of scala
vestibuli
 Scalas vestibuli and tympani are continuous at apex
 So waves in vestibuli pass to tympani and displace round window (at base
of cochlea)
 Low frequencies can travel _________ way thru vestibuli and back in tympani
 As frequencies increase they travel less before passing directly thru vestibular and
basilar membranes to tympani
 ____________ frequencies produce maximum stimulation of Spiral Organ closer
to base of cochlea and lower frequencies stimulate closer to _______________
Spiral Organ (Organ of Corti)
 Is where sound is transduced
 Sensory _______________________ located on the basilar membrane
 1 row of inner cells extend length of basilar membrane
 Multiple rows of outer hair cells are embedded in tectorial membrane
 Pressure waves moving thru cochlear duct create shearing forces between basilar
and tectorial membranes, moving and bending _______________________
 Causing ion channels to open, depolarizing hair cells
 The greater the displacement, the greater the amount of NT released and
APs produced
Neural Pathway for Hearing
 Info from 8th nerve goes to medulla, then to inferior colliculus, then to thalamus,
and on to auditory cortex
 Neurons in different regions of cochlea stimulate neurons in corresponding areas
of auditory cortex. This is called ______________________ organization.
Hearing Impairments
 Conduction deafness occurs when transmission of sound waves to oval window is
impaired. Helped by hearing aids
 ________________________ (perceptive) deafness is impaired transmission of
nerve impulses. Helped by cochlear implants
 Which stimulate fibers of 8th in response to sounds
Vision
 Eyes transduce energy in small part of electromagnetic spectrum into APs
 Only wavelengths of 400 – 700 nm constitute visible light
Structure of Eye
 The sclera (white of eyes) is outermost layer
 The transparent cornea is continuous with sclera
 Light passes thru it into ___________________ chamber
 Then thru pupil which is formed by iris
 Then thru lens and vitreous to retina
 The iris (a pigmented muscle) controls size of pupil
 Pupil constricts by contraction of circular muscles, Under parasympathetic control
 Dilation is via contraction of radial muscles
 __________________________ are in retina.
 Retina absorbs some light. Rest is absorbed by the dark choroid layer
 Axons of retinal neurons gather at the optic disc (blind spot) and exit eye in
_______________________
Refraction
 Is bending of light as it passes thru media of different densities
 Refractive index is speed of light in a medium
 RI of air is 1; cornea 1.38; lens 1.40
 Depends on:
 Comparative density of 2 media
 Curvature of interface between 2 media
 Lens provides fine control of focusing because its curvature can be ___________
Visual Field
 Image projected onto retina is __________________________ and backward
 Cornea and lens focus right part of visual field on left half of retina
 Left half of visual field focuses on right half of each retina
Accommodation
 Is ability of eyes to keep image focused on ________________ as distance
between eyes and object varies
 Results from contraction of ciliary muscle
 At distances > 20 ft ciliary relaxation places tension on suspensory ligament
 Pulls lens taut; is least _________________
 As distance decreases ciliary muscles contract reducing tension on suspensory
ligament
 Lens becomes more convex
Visual Acuity
 Is ________________________ of vision
 Depends upon resolving power (ability to resolve 2 closely spaced dots)
 With myopia (nearsightedness) image is focused in front of retina because eyeball
is too long
 With ________________________ (farsightedness) image is focused behind
retina because eyeball too short
 With astigmatism cornea or lens is not symmetrical
 Light is bent unevenly, Causing uneven focus
Retina
 Is a multilayered epithelium consisting of neurons, pigmented epithelium, and
photoreceptors (____________________________)
 Neural layers are an extension of brain
 Light must pass through neural layers before striking rods and cones
 Rods and cones face away from pupil, send sensory info to bipolar cells
 ______________________ send electrical activity to ganglion cells
 Ganglion cells project axons thru optic nerve to brain
Rods and Cones
 Have inner and outer segments
 Outer segments contain stacks of photopigment discs
 New discs added at base and removed at tip
 Retinal __________________ epithelium phagocytizes old discs from tips
 also absorbs excess light
 delivers nutrients from blood to the photoreceptors
 suppresses potential immune attack on retina
 stabilizes ion levels for photoreceptors
Effects of Light on Rods
 Rods are activated when light produces chemical change in __________________
 dissociates into retinal and opsin (bleaching reaction)
 Causes changes in permeability, resulting in APs in ganglion cells
Dark Adaptation
 Is a gradual increase in photoreceptor sensitivity when entering a dark room
 Maximal sensitivity reached in 20 min
 ____________________ amounts of visual pigments produced in the dark
 Increased pigment in cones produces slight dark adaptation in 1st 5 min
 Increased rhodopsin in rods allows light ______________________ to
increase up to 100,000-fold
Electrical Activity of Retinal Cells
 Visual transduction is inverse of other sensory systems
 In dark, photoreceptors release inhibitory NT that ____________ bipolars
 Light inhibits photoreceptors from releasing inhibitory NT, stimulates
bipolars
 Rods and cones contain many Na+ channels that are open in dark
 This depolarizing Na+ influx is the _______________________________
 Light hyperpolarizes by closing Na+ channels
 cGMP keeps Na+ channels open
 Light converts cGMP to GMP and Na+ channels close
Cones and Color Vision
 Cones ____________ sensitive than rods to light
 Provide color vision and greater visual acuity
 In day, high light intensity bleaches out rods, and high acuity color vision is
provided by cones
 Humans have _________________________ color vision
 All colors created by stimulation of 3 types of cones. Blue, green, red
 Instead of opsin, cones have photopsins
 A different photopsin for each type of cone
 Causing each to absorb at different _______________________
Visual Acuity and Sensitivity
 Eyes oriented so that object of attention is focused on fovea centralis
 Pin-sized pit within yellow macula lutea
 Contain only _______________________
 Neural layers displaced to sides so light strikes cones directly
 In fovea each cone supplies 1 ganglion cell
 Allows high acuity
 Peripheral regions contain both rods and cones
 Degree of convergence of rods on ganglions is much ________________
 Allows high sensitivity, low acuity
Neural Pathway from Retina
 Right half of visual field projects to left half of retina
 Left half of visual field projects to right half of retina
 Left lateral geniculate nucleus receives input from right half of visual field of both
eyes
 Right Lat geniculate body receives input from both eyes from left half of visual
field
Eye Movement
 Superior colliculi coordinate:
 Smooth pursuit movements that track moving objects, keeping image
focused on fovea
 __________________________ eye movements that allow eyes to jump
from one object to another
 Such as when reading words
 Even during a fixed gaze there are tiny fixational movements that
prevent photoreceptor bleaching
 _______________reflex which constricts pupil in response to strong light
Ganglion Cell Receptive Fields
 Part of visual field that affects activity of a particular ganglion cell is its
_________________________________
 Some ganglion cells respond to light in center of their visual field (on-center
fields)
 Some are inhibited by light in center, stimulated by light in the surround (offcenter fields)
Cerebral Cortex
 Neurons in visual cortex can be simple, complex or hypercomplex
 Depending on their receptive fields
 Receptive fields of simples are rectangular