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The Central Nervous System
Chapter 9
Evolution of the Nervous System
Figure 9.1a (1 of 6)
Nerve net of jellyfish
Nervenet
Figure 9.1b (2 of 6)
The flatworm nervous system has aprimitive brain.
Primitive brain
Nerve cords
Figure 9.1c (3 of 6)
The earthworm nervous system has asimple brain and ganglia along a nerve cord.
Esophagus
Primitivebrain
Mouth
Subpharyngealganglion Ventral nerve cord
with ganglia
Figure 9.1d (4 of 6)
The fish forebrain is smallcompared to remainder of brain.
Forebrain
Figure 9.1e (5 of 6)
The goose forebrain is larger.
ForebrainCerebellum
Figure 9.1f (6 of 6)
The human forebrain dominates the brain.
Forebrain
Cerebellum
Figure 9.2a ESSENTIALS – Development of the Human Nervous System
Day 20
In the 20-day embryo (dorsal view), neural plate cells(purple) migrate toward the midline. Neural crest cellsmigrate with the neural plate cells.
Neural crest
Neural plate
Figure 9.2b ESSENTIALS – Development of the Human Nervous System
Day 23By day 23 of embryonic development, neural tube formation isalmost complete.
Anterior openingof neural tube
Posterior openingof neural tube
Neural crestbecomes peripheral
nervous system.
Neural tubebecomes CNS.
Dorsal bodysurface
Figure 9.2c ESSENTIALS – Development of the Human Nervous System
4 Weeks
A 4-week human embryo showingthe anterior end of the neural tubewhich has specialized into threebrain regions.
Forebrain Midbrain
Hindbrain
Spinalcord
Lumen of neural tube
Figure 9.2d ESSENTIALS – Development of the Human Nervous System
6 Weeks
At 6 weeks, the neural tube has differentiatedinto the brain regions present at birth. The centralcavity (lumen) shown in the cross section willbecome the ventricles of the brain (see Fig. 9.4).
Hindbrain
Forebrain
Midbrain
Medullaoblongata
Cerebellumand Pons
Diencephalon
Cerebrum
Diencephalon
Cerebrum
Eye Midbrain
Medullaoblongata
Spinalcord
Figure 9.2e ESSENTIALS – Development of the Human Nervous System
11 Weeks
By 11 weeks of embryonic development, thegrowth of the cerebrum is noticeably more rapidthan that of the other divisions of the brain.
Cerebrum
Diencephalon
Midbrain
Cerebellum
Pons
Medullaoblongata
Spinal cord
Figure 9.2f ESSENTIALS – Development of the Human Nervous System
40 Weeks
At birth, the cerebrum has coveredmost of the other brain regions.Its rapid growth withinthe rigid confines ofthe cranium forcesit to develop aconvoluted,furrowedsurface.
Cerebrum
PonsCerebellum
Medullaoblongata
Spinal cordCranialnerves
Figure 9.2g ESSENTIALS – Development of the Human Nervous System
Child
The directions “dorsal” and“ventral” are different in thebrain because of flexionin the neural tube duringdevelopment.
Dorsal (superior)
Rostral Caudal
Rostral
Caudal
Ventral(inferior)
Ventral(anterior)
Dorsal(posterior)
Figure 9.3 ANATOMY SUMMARY – The Central Nervous System
ANATOMY SUMMARY
CNS: Gray and White Matter
• Gray matter– Unmyelinated nerve cell bodies
– Clusters of cell bodies in the CNS are nuclei– Dendrites– Axon terminals
• White matter– Myelinated axons
– Axon bundles connecting CNS regions are tracts– Contains very few cell bodies
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CNS: Bone and Connective Tissue
• Brain is encased in bony skull, or cranium• Spinal cord runs through vertebral column• Meninges lie between bone and tissues to stabilize neural
tissue and protect from bruising– Dura mater: thickest and most superficial – Arachnoid membrane: cobweb like, middle
– Subarachnoid space contains cerebrospinal fluid secreted by choroid plexus
– Pia mater: deepest, thin that adheres to surface of brain or spinal cord, arteries that supply blood to brain.
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Figure 9.3-1 ANATOMY SUMMARY – The Central Nervous System
Posterior View of the Central Nervous System Sectional View of the Meninges of the Brain, showinghow they cushion and protect delicate neural tissue
Cranium
Cerebralhemispheres
Cerebellum
Cervical spinalnerves
Cranium
DuramaterVenous sinus
Arachnoidmembrane
Pia mater
Brain
Subduralspace
Subarachnoidspace
Moving from the cranium in,name the meninges that formthe boundaries of the venoussinus and the subdural andsubarachnoid spaces.
FIGURE QUESTION
Figure 9.3b ANATOMY SUMMARY – The Central Nervous System
ANATOMY SUMMARY
Sectional View of the Meninges of the Brain, showinghow they cushion and protect delicate neural tissue
Cranium
Duramater
Venous sinus
Arachnoidmembrane
Pia mater
Brain
Subduralspace
Subarachnoidspace
Cerebrospinal Fluid
The Ventricles of the Brain
Lateral ventricles
Third ventricle
Fourth ventricle
Cerebellum
Central canal
Spinal cordFrontal viewLateral view
The lateral ventricles consist ofthe first and second ventricles.The third and fourth ventriclesextend through the brain stemand connect to the centralcanal that runs through thespinal cord. Compare thefrontal view to the crosssection in Fig. 9.10a.
– Cerebrospinal fluid (CSF): solution secreted by choroid plexus (region on walls of ventricles)
– Choroid plexus is similar to kidney tissue (capillaries and transporting epithelium from the ependyma). Pump sodium and solutes from plasma into ventricles,creating an osmotic gradient that draw water along water along with the solutes.
– CSF flows from ventricles into subarachnoid space, flows around body and absorbed back by special villi on arachnoid membrane
– 2 purposes of CSF: physical and chemical protection
Figure 9.4b-d ANATOMY SUMMARY – Cerebrospinal Fluid
ANATOMY SUMMARY
Cerebrospinal Fluid Secretion
Cerebrospinal fluid is secreted into the ventriclesand flows throughout the subarachnoid space,where it cushions the central nervous system.
Choroid plexusof third ventricle
Pia mater
Arachnoidmembrane
Arachnoidvilli
Sinus
Spinal cord
Central canal
Subarachnoidspace
Arachnoidmembrane
Dura mater
Cerebrospinal Fluid Reabsorption
Cerebrospinal fluid is reabsorbed into the bloodat fingerlike projections of the arachnoidmembrane called villi.
Cerebrospinal fluid
Bone of skull
Dura mater
Endotheliallining
Blood invenous sinus
Fluid movementArachnoidvillus
Dura mater(inner layer)
Subduralspace
Arachnoidmembrane
Subarachnoidspace
Piamater
Cerebralcortex
FIGURE QUESTIONS
1. Physicians may extract a sample of cerebrospinal fluid when they suspect an infection in the brain. Where is the least risky and least difficult place for them to insert a needle through the meninges? (See Fig. 9.4b.)2. The aqueduct of Sylvius is the narrow passageway between the third and fourth ventricles. What happens to CSF flow if the aqueduct becomes blocked by infection or tumor, a condition known as aqueductal stenosis {stenos, narrow}? On a three-dimensional imaging study of the brain, how would you distinguish aqueductal stenosis from a blockage of CSF flow in the subarachnoid space near the frontal lobe?
Choroid plexusof fourth ventricle
Figure 9.4c ANATOMY SUMMARY – Cerebrospinal Fluid
ANATOMY SUMMARYThe Choroid Plexus
The choroid plexustransports ions andnutrients from the bloodinto the cerebrospinal fluid.
Capillary
Ependymalcells
Ions, vitamins,nutrients
WaterCerebrospinalfluid in third ventricle
FIGURE QUESTIONS
Physicians may extract a sample ofcerebrospinal fluid when they suspectan infection in the brain. Where is theleast risky and least difficult place forthem to insert a needle through themeninges? (See Fig. 9.4b.)
1.
2.The aqueduct of Sylvius is the narrowpassageway between the third and fourthventricles. What happens to CSF flowif the aqueduct becomes blocked byinfection or tumor, a condition known asaqueductal stenosis {stenos, narrow}?On a three-dimensional imaging studyof the brain, how would you distinguishaqueductal stenosis from a blockage ofCSF flow in the subarachnoid space nearthe frontal lobe?
Blood-Brain Barrier
– Blood-brain barrier serves as functional protection by protecting interstitial fluid and blood
– prevents harmful chemicals (ions, hormones, etc.) and pathogens
– endothelial cells form tight junctions with one another
– astrocyte foot processes secret substance that promote tight junctions
Figure 9.5a (1 of 2)
Astrocyte
Astrocyte foot processessecrete paracrines that
promote tightjunction formation.
Tight junction preventssolute movement
between endothelial cells.
Figure 9.5b (2 of 2)
Astrocyte foot processessecrete paracrines that
promote tightjunction formation.
Tight junction preventssolute movement
between endothelial cells.
Astrocyte footprocesses
Tightjunction
Basallamina
Capillary lumen
CNS: Neural Tissue – Metabolic Needs
• Neurons need a constant supply oxygen and glucose to make ATP for active transport of ions and neurotransmitters
• Oxygen– Passes freely across blood–brain barrier– Brain receives 15% of blood pumped by heart
• Glucose– Membrane transporters move glucose from plasma into the
brain interstitial fluid– Brain responsible for about half of body’s glucose
consumption– Progressive hypoglycemia leads to confusion,
unconsciousness, and death© 2013 Pearson Education, Inc.
Figure 9.3a ANATOMY SUMMARY – The Central Nervous System
Posterior View of the Central Nervous System
Cranium
Cerebralhemispheres
Cerebellum
Cervical spinalnerves
Thoracic spinalnerves
Lumbar spinalnerves
Sacral spinal nerves
Sectionedvertebra
Coccygealnerve
Figure 9.6a (1 of 5)
One segment of spinal cord, ventral view,showing its pair of nerves.
White matter
Gray matter
Dorsal root:carries sensory
(afferent)information
to CNS.Ventral root: carries motor (efferent)information to musclesand glands.
Figure 9.6b (2 of 5)
Gray matter consists of sensory and motor nuclei.
Visceral sensory nuclei
Dorsalroot
ganglion
Lateralhorn
Ventralroot
Ventralhorn
Dorsalhorn
Somaticsensorynuclei
Autonomicefferentnuclei
Somaticmotornuclei
Figure 9.6c-1 (4 of 5)
KEY
Ascending tractscarry sensoryinformation to the brain.
Descending tractscarry commands tomotor neurons.
White matter in the spinal cord consists of tracts ofaxons carrying information to and from the brain.
To the brain
Figure 9.6c-2 (5 of 5)
KEY
Ascending tractscarry sensoryinformation to the brain.
Descending tractscarry commands tomotor neurons.
White matter in the spinal cord consists of tracts ofaxons carrying information to and from the brain.
From the brain
Figure 9.7
SPINAL REFLEXES
In a spinal reflex, sensory information entering the spinal cord isacted on without input from the brain. However, sensoryinformation about the stimulus may be sent to the brain.
StimulusSpinalcord
Integratingcenter
Sensoryinformation
Interneuron
Response
Command tomuscles or
glandsA spinal reflex initiatesa response without input
from the brain.
Figure 9.8 ANATOMY SUMMARY – Central Nervous System
The Brain: The Brain Stem
• 11 of 12 cranial nerves originate – Cranial nerves can include sensory fibers, efferent fibers, or both
(mixed nerves)
• Many nuclei are associated with reticular formation (basic processes, sleep/wake, muscle tone, stretch reflexes, breathing, blood pressure, pain
• Medulla– Somatosensory and corticospinal tracts : convey info about cerebrum to
spinal cord– Pyramids: tracts cross– coordinates breathing, blood pressure, hiccups, swallowing, vomitting
• Pons: info transfer between cerebellum and cerebrum• Midbrain: eye movement, audio and visual reflexes
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Lateral View of Brain Stem
Thalamus
Cut edge ofascending
tracts tocerebrum
Cranialnerves
Optic tract
Midbrain
Pons
Cut edgesof tracts leading tocerebellum
Medullaoblongata
Spinal cord
Functions of the Brain Stem
Midbrain
Pons
• Eye movement
• Relay station between cerebrum and cerebellum• Coordination of breathing
• Control of involuntary functions
• Arousal• Sleep• Muscle tone• Pain modulation
Medulla oblongata
Reticular formation (not shown)See Figure 9.16
Figure 9.8f ANATOMY SUMMARY – Central Nervous System
ANATOMY SUMMARY
Table 9.1 The Cranial Nerves
Cerebellum
– second largest brain structure– process sensory information– control coordination of movement– receives motor input from cerebrum
Figure 9.8a-b ANATOMY SUMMARY – Central Nervous System
Lateral view of the CNS
Cerebrum
Spinalcord
Vertebrae
Lateral View of Brain
Medulla oblongata
Pons Cerebellum
Occipitallobe
Temporallobe
Parietallobe
Frontallobe
Figure 9.9
The DIENCEPHALON
The diencephalon lies between the brain stem and the cerebrum. Itconsists of thalamus, hypothalamus, pineal gland, and pituitary gland.
Thalamus
Hypothalamus
Anteriorpituitary
Posterior pituitary
Pinealgland
Corpus callosum
Diencephalon
– thalamus: sensory gate keeper, receives sensory input (optic, ears, and spinal cord) and motor information from the spinal cord and cerebellum, it projects fibers to the cerebrum for processing
– hypothalamus: center for homeostasis, hunger, thirst– pituitary gland: anterior (endocrine gland) and
posterior pituitary (neurohormones)– pineal glands: melatonin
Table 9.2 Functions of the Hypothalamus
Figure 9.8c ANATOMY SUMMARY – Central Nervous System
ANATOMY SUMMARYMid-Sagittal View of Brain
Parietallobe
Occipitallobe
Frontallobe
Corpuscallosum
Temporallobe
Cingulate gyrus
Medulla oblongata
Cerebellum
Pons
Figure 9.10-1The cerebral cortex and basal ganglia are two of the three regions ofgray matter in the cerebrum. The third region, the limbic system, isdetailed in Figure 9.11. The frontal view shown here is similar to thesectional view obtained using modern diagnostic imaging techniques.
FIGURE QUESTIONThe section through thisbrain is a section throughthe ––––––––– plane.
(a) coronal(b) lateral(c) frontal(d) transverse(e) sagittal
Section throughthe brain showingthe basal ganglia
Basalganglia
Corpuscallosum
Lateral ventricle
Tracts ofwhite matter
Tip of lateralventricle
Gray matter ofcerebral cortex
Cerebrum– Gray matter most superficial, white matter most deep (interior)
– Largest and most distinctive part of human brain, associated with reasoning
– 2 hemispheres connected by corpus callosum (axons passing from one side of the brain to the other side (communication)
– each hemisphere divided into 4 lobes (named for cranial bones)
– sulci (groves) and gyri (ridges)
– Gray matter:1. cerebral cortex: most superficial, neurons arranged in layers
2. basal ganglia: movement
3. limbic system: link btw high cognitive functions and more primitive emotional responses (fear)
1. amygdala: memory, deacon making, emotion
2. cingulate gyrus: emotional memory
3. hippocampus: learning and memory
Figure 9.10b (2 of 2)
Cell bodies in the cerebral cortex form distinct layersand columns.
Outer surface of the cerebral cortex
1
2
3
4
5
6
Graymatter
Whitematter
Layers
Figure 9.11
THE LIMBIC SYSTEM
The limbic system includes the amygdala, hippocampus, andcingulate gyrus. Anatomically, the limbic system is part of thegray matter of the cerebrum. The thalamus is shown fororientation purposes and is not part of the limbic system.
Cingulate gyrusplays a rolein emotion.
Thalamus
Hippocampus is involved in learningand memory.
Amygdala is involved in emotionand memory.
Figure 9.12
SIMPLE AND COMPLEX PATHWAYS IN THE BRAIN
A simpleneural reflex
Behavioral state and cognitioninfluence brain output.
Sensoryinput
Sensorysystem(reflex)
Cognitivesystem
(voluntary)
CNSbehavioral
statesystem
Motorsystemoutput
Physiologicalresponse or
behavior
Integration
Output
Response
Feedback
Brain Function: Cerebral Cortex
• From a functional viewpoint, it can be divided into three specializations. Information passing along a pathway is usually processed in one or more of these areas:– Sensory areas
– Sensory input translated into perception (awareness)– Motor areas
– Direct skeletal muscle movement– Association areas
– Integrate information from sensory and motor areas– Can direct voluntary behaviors
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Figure 9.13
FUNCTIONAL AREAS OF THE CEREBRAL CORTEX
The cerebral cortex contains sensory areas for perception, motor areas that direct movement,and association areas that integrate information.
FRONTAL LOBE PARIETAL LOBE
OCCIPITAL LOBE
TEMPORAL LOBE
Primary motor cortex
Motor associationarea (premolar cortex)
Skeletalmusclemovement
Prefrontalassociationarea
Coordinatesinformation fromother association
areas, controlssome behaviors
Taste
Smell
Hearing
Vision
Sensoryinformationfrom skin,musculoskeletalsystem, viscera,and taste buds
Gustatory cortex
Olfactory cortex Auditorycortex
Auditoryassociation area
Visualcortex
Visualassociationarea
Primary somatic sensory cortex
Sensory association area
http://trifini.com/mind-tricks-images.html
Figure 9.14
CEREBRAL LATERALIZATION
The distribution of functional areas in the two cerebral hemispheres is not symmetrical.
LEFT HAND RIGHT HAND
Prefrontalcortex
Speechcenter
Writing
Auditorycortex
(right ear)
Generalinterpretive center
(language andmathematical
calculation)
Visual cortex(right visual field)
Prefrontalcortex
Analysisby touch
Auditorycortex(left ear)
Spatialvisualizationand analysis
Visual cortex(left visual field)
LEFTHEMISPHERE
RIGHTHEMISPHERE
FIGURE QUESTIONS
1. What would a person see if a stroke destroyed all function in the right visual cortex?2. What is the function of the corpus callosum?3. Many famous artists, including Leonardo da Vinci and Michelangelo, were left-handed. How is this related to cerebral lateralization?
CORPUS CALLOSUM
Brain Function: Sensory Information
• Primary somatic sensory cortex (parietal lobe)– Termination point of pathways from skin, musculoskeletal
system, and viscera– Somatosensory pathways
– Touch– Temperature– Pain– Itch – Body position
– left side of brain controls right side of body, (vice versa)
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Brain Function: Sensory Information
• Special senses have devoted regions– Visual cortex– Auditory cortex– Olfactory cortex– Gustatory cortex
• Neural pathways extend from sensory areas to association areas, which integrate stimuli into perception
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Figure 9.15
PERCEPTION
The brain has the ability to interpret sensory information to createthe perception of (a) shapes or (b) three-dimensional objects.
What shape do you see? What is this object?
Brain Function: Motor System, Efferent Division
• Three major types– Skeletal muscle movement
– Somatic motor division– Neuroendocrine signals
– Hypothalamus and adrenal medulla– Visceral responses
– Autonomic division, smooth and cardiac muscle edocrine and exocrine glands
• Voluntary movement originate:– Primary motor cortex– Motor association areas– receive sensory input from cerebellum and basal ganglia
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Brain Function: Behavioral State
• Modulator of sensory and cognitive processes• Neurons collectively known as diffuse modulatory
systems – Originate in reticular formation in brain stem– Project axons to large areas of the brain– 4 modulatory systems: noradrenergic, serotonergic,
dopaminergic, and cholinergic– influences attention, motivation, wakefulness, memory,
motor control, mood, and metabolic homeostasis
• Reticular activating system controls consciousness and a role in keeping the “conscious brain” awake
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Figure 9.17a (1 of 2)
Figure 9.17b (2 of 2)
Brain Function: Sleep
• Four stages with two major phases – Slow-wave sleep (deep sleep)
– Adjusts body without conscious commands– REM sleep
– Brain activity inhibits motor neurons to skeletal muscle, paralyzing them
– Dreaming takes place
• Circadian rhythm: 24 Hour light dark cycle– Suprachiasmatic nucleus (hypothalamus)
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Figure 9.18
EMOTIONS AFFECT PHYSIOLOGY
The association between stress and increased susceptibility toviruses is an example of an emotionally linked immune response.
Sensorystimuli
Cerebralcortex
Integrationoccurs within theassociationareas of thecerebral cortex
Feedback createsawareness of emotionsIntegrated information
Limbic systemcreates emotion
Hypothalamusand brain stem
KEY
Interneuron
initiate
Somaticmotor
responses
Autonomicresponses
Endocrineresponses
Immuneresponses
(both voluntaryand unconscious)
Brain Function: Motivation
• Defined as internal signals that shape voluntary behaviors• Some states known as drives• Work with autonomic and endocrine responses• Motivated behaviors stop when a person has reached a
certain level of satiety
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Brain Function: Moods
• Similar to emotions but longer-lasting• Mood disorders
– Fourth leading cause of illness worldwide today– Depression
– Sleep and appetite disturbances– Alterations of mood – May affect function at school or work or in personal
relationships– Antidepressant drugs alter synaptic transmission
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Brain Function: Learning and Memory
• Learning has two broad types– Associative learning: 2 things are associated with each other– Nonassociative learning: change in behavior after repeated
exposure– Habituation (decreased response to repeated stimulation) and
sensitization (enhanced response to repeated stimulation)
• Memory has several types– Short-term and long-term
– Working memory and consolidation– implicit (reflexive) and explicit (declarative)– Stored in memory traces near sensory association areas– Antrograde amnesia is inability to remember new information
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Figure 9.19
MEMORY PROCESSING
New information goes into short-term memory but is lost unlessprocessed and stored in long-term memory.
Output
Long-termmemory
Locate andrecall
Short-termmemory
Informationinput
Processing(consolidation)
Table 9.3 Types of Long-Term Memory
Brain Function: Language
• Integration of spoken language involves two regions • Wernicke’s area (temporal lobe)• Broca’s area (frontal lobe)
• Sensory input from either visual or auditory• Output from Broca’s initiates spoken or written
– Damage to Broca’s area causes expressive aphasia
• Output from Wernicke’s involved in understanding– Damage to Wernicke’s area causes receptive aphasia
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Figure 9.20a (2 of 4)
Speaking a written word
Motorcortex
Broca’sarea
Wernicke’sarea
Readwords
Visualcortex
Figure 9.20b (3 of 4)
Speaking a heard word
Broca’sarea
Motorcortex
Hearwords
Auditorycortex
Wernicke’sarea
Figure 9.20c (4 of 4)
PET scan of the brain at work
FIGURE QUESTIONIn the image above, the brain area activein seeing words is in the _________ lobe,and the brain area active during wordgeneration is in the _______ lobe.
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Sensory Physiology
Chapter 10
Sensory Division
– Special senses: vision, hearing, taste, smell, and equilibrium
– Somatic Sense: (think of mainly our sense of “touch” or feeling)touch temperature, pain, itch, and proprioception
– Proprioception: awareness of body movement and position (conscious or unconscious), mediated by muscle and joint proprioceptors
Sensory Pathways
• Stimulus as physical energy → sensory receptor– Receptor acts as a transducer
• Intracellular signal → usually change in membrane potential
• Stimulus → threshold → action potential to CNS• Integration in CNS → cerebral cortex or acted on
subconsciously
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Figure 10.1a (1 of 3)
Free nerve endings
Unmyelinatedaxon
Cell body
Stimulus
Simple receptors are neuronswith free nerve endings. Theymay have myelinated orunmyelinated axons.
Figure 10.1b (2 of 3)
Stimulus
Enclosed nerveending
Layers of connectivetissue
Myelinated axon
Cell body
Complex neural receptors have nerveendings enclosed in connective tissue capsules.This illustration shows a Pacinian corpuscle,which senses touch.
Figure 10.1c (3 of 3)
Stimulus
Myelinated axon
Cell body ofsensory neuron
Synapse
Synaptic vesicles
Specialized receptorcell (hair cell)
Most special senses receptors are cellsthat release neurotransmitter onto sensoryneurons, initiating an action potential. Thecell illustrated is a hair cell, found in the ear.
Types of Sensory Receptors
Sensory Transduction
• Transduction: stimulus energy converted into information processed by CNS– Ion channels or second messengers initiate membrane
potential change
• Adequate stimulus: Form of energy to which a receptor is most responsive (Ex. thermoreceptors are most sensitive to temperature)
• Threshold: Minimum stimulus required to active a receptor
• Change in sensory receptor membrane potential is a graded potential called a receptor potential
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Convergence creates large receptive fields.
Compass with pointsseparated by 20 mm
Primarysensoryneurons
Secondarysensoryneurons
Skin surface
The receptive fields of threeprimary sensory neuronsoverlap to form one largesecondary receptive field.
Convergence of primaryneurons allows simultaneoussubthreshold stimuli to sumat the secondary sensoryneuron and initiate anaction potential.
Two stimuli that fall within the samesecondary receptive field are perceivedas a single point, because only onesignal goes to the brain. Therefore,there is no two-point discrimination.
Figure 10.2a (1 of 2)
Compass with pointsseparated by 20 mm
Primarysensoryneurons
Secondarysensoryneurons
Skin surface
Small receptive fields are found in more sensitive areas.
The two stimuli activate separatepathways to the brain. The twopoints are perceived as distinctstimuli and hence there istwo-point discrimination.
When fewer neurons converge,secondary receptive fields aremuch smaller.
Figure 10.2b (2 of 2)