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Nervous system
Afferent: Sensory Receive
Efferent: Motor
Integrative:
Functions of nervous system
Receive: receptors, sensory neurons
Store: spinal cord, brain
Integrate: spinal cord, brain
Create new complexes: brain
Respond: motor neurons, muscles, glands
Regulate
Protect
Sensory, Motor and Integrative
Systems
Cerebral Blood Flow, Cerebral Spinal Fluid
BloodFlow
Flowto
tothe
theBrain
Brain
Blood
Blood
FlowFlow
to the Brain
Blood
to the Brain
• brain is highly dependent on blood flow
• cessation of flow for 5-10 seconds results
in a loss of consciousness
• highly dependent on oxygen delivery
Cerebral Blood Flow
Blood Flow to the Brain
• 15% of the resting cardiac output (750-900
ml/min)
• related to the metabolic rate of the brain
• 3 metabolic factors have potent effects on blood
flow
- CO2
- H+
- O2
Cerebral Blood Flow
Cerebral Blood Flow
• An increase in CO2 increases blood flow.
• This is thought to work through an increase in
the H+ ion concentration.
– CO2 + H2O
→
H2CO3
→
H+ + HCO3-
• An increase in H+ concentration depresses
neuronal activity.
• Normal PO2 35 - 40 mm Hg.
• Below 30 mm Hg begin to see increase in
flow, below 20 mm Hg coma.
Cerebral
Blood
Cerebral
Blood Flow
Flow
• cerebral blood flow is autoregulated
– nearly constant over a pressure range of
60 to 140 mm Hg
– below 60 mm Hg blood flow begins to
fall rapidly
– above 140 mm Hg the blood flow
increases but most importantly the blood
vessels begin to stretch, which can lead to
damage and eventual rupture (stroke)
Cerebrospinal fluid (CSF)
Site
Composition
Circulation
Functions
Cerebrospinal Fluid (CSF)
• Clear fluid.
• Circulates through cavities in the brain
(ventricles) and the spinal cord (central canal)
and also in the subarachnoid space.
• Absorbs shock and protects the brain and the
spinal cord.
• Helps transport nutrients and wastes from the
blood and the nervous tissue.
Formation and Circulation of CSF in the
Ventricles
• Choroid plexuses- networks of capillaries
in the walls of the ventricles.
• Ventricles are lined by ependymal cells.
• Plasma is drawn from the choroid
plexuses through ependymal cells into the
ventricles to produce CSF.
Cerebrospinal fluid (CSF)
volume: 100-150 ml (wt in pounds)
contains: (glucose, proteins, H+, Na+, k+, Ca++, Mg++, ClHCO3-,
Cerebrospinal fluid (CSF)
Mechanical Protection: shock absorber
Chemical: homeostasis
Circulation
Cerebrospinal fluid (CSF)
HYDROCEPHALUS
Circulation of CSF
• CSF from the lateral
ventricles →
interventricular
foramina → third
ventricle → cerebral
aqueduct → fourth
ventricle →
subarachnoid space
or central canal.
• CSF is reabsorbed
into the blood by
arachnoid villi.
Brain Metabolism
• highly metabolic organ
– 2% of total body mass, yet has 15% of
total metabolism of the body
• limited anaerobic metabolism
– anaerobic breakdown of glycogen cannot
supply the energy requirements of the
brain
– most neuronal activity depends on the
second by second delivery of oxygen to
the brain
Brain Metabolism
• mostly glucose dependent
• uptake of glucose into the brain is not
dependent on insulin
• blood glucose must be maintained for the
brain to receive the proper amount of this
substrate
Nervous system
Afferent: Sensory Receive
Efferent: Motor
Integrative:
Functions of nervous system
Receive: receptors, sensory neurons
Store: brain , spinal cord
Integrate: brain, spinal cord
Create new complexes
Respond: Motor neurons, muscles, glands
Regulate
Protect
Classification of Sensory Receptors Based on the
Location
• Exteroceptors
• Interoceptors
• Proprioceptors
Sensation
• Conscious and subconscious awareness
of changes in the external or internal
environment.
• Components of sensation: Stimulation of
the sensory receptor → transduction of the
stimulus → generation of nerve impulses
→ integration of sensory input.
Classification of Sensory Receptors
based on the type of Stimulus
•
•
•
•
•
•
Mechanoreceptors
Thermoreceptors
Nociceptors
Photoreceptors
Chemoreceptors
Osmoreceptors
Classification of Sensory Receptors
• General senses: somatic and visceral.
Somatic- tactile, thermal, pain and
proprioceptive sensations.
Visceral- provide information about
conditions within internal organs.
• Special senses- smell, taste, vision,
hearing and equilibrium or balance.
Sensation
• Each of the principle types sensation; touch,
pain, sight, sound, is called a modality of
sensation.
• Each receptor is responsive to one type of
stimulus energy. Specificity is a key property of a
receptor, it underlines the most important coding
mechanism, the labeled line.
• How the sensation is perceived is determined by
the characteristics of the receptor and the central
connections of the axon connected to the
receptor.
Sensory Receptors in the Skin
Types of Sensory Receptors
• Free nerve endings: pain and
thermoreceptors.
• Encapsulated nerve endings: pacinian
corpuscles.
• Separate cells: hair cells, photoreceptors
and gustatory receptor cells.
Generator Potential and Receptor
Potential
• Generator potential is produced by free
nerve endings, encapsulated nerve
endings, and olfactory receptors. When it
reaches a threshold, it triggers one or
more nerve impulses in the axon of a firstorder sensory neuron.
• Receptor potential triggers the release of
neurotransmitter → postsynaptic potential
→ action potential.
Sensory Receptors and their Relation-ship to FirstOrder Sensory Neurons
Somatic Sensations
• Sensory receptors in the skin (cutaneous
sensations), muscles, tendons and joints
and in the inner ear.
• Uneven distribution of receptors.
• Four modalities: tactile, thermal, pain and
proprioceptive.
Receptor Excitation
Figure 46-3; Guyton & Hall
Relationship between Receptor
Potential and Action Potentials
Figure 46-2; Guyton & Hall
Adaptation of Sensory Receptors
• Rapidly adapting receptors: receptors that
detect pressure, touch and smell.
• Slowly adapting receptors: receptors that
detect pain, body position, and chemical
composition of the blood.
Adaptation of Receptors
• When a continuous stimulus is applied, receptors
respond rapidly at first, but response declines
until all receptors stop firing.
Adaptation
• Rate of adaptation varies with type of
receptor.
• Therefore, receptors respond when a change
is taking place (i.e., think of the feel of
clothing on your skin.)
Mechanism of Adaptation
• varies with the type of receptor
• photoreceptors change the amount of light
sensitive chemicals
• mechanoreceptors redistribute themselves to
accommodate the distorting force (i.e., the
pacinian corpuscle)
• some mechanoreceptors adapt slowly, some
adapt rapidly
Slowly Adapting (Tonic) Receptors
• continue to transmit impulses to the brain for
long periods of time while the stimulus is
present
• keep brain apprised of the status of the body
with respect to its surroundings
• will adapt to extinction as long as the stimulus
is present, however, this may take hours or
days
• these receptors include: muscle spindle, golgi
tendon apparatus, Ruffini’s endings, Merkels
discs, Macula, chemo- and baroreceptors
Rapidly Adapting (Phasic) Receptors
• respond only when change is taking place
• rate and strength of the response is related to
the rate and intensity of the stimulus
• important for predicting the future position or
condition of the body
• very important for balance and movement
• types of rapidly adapting receptors: pacinian
corpuscle, semicircular canals
Fig. 13.05
Transmission of Receptor Information
to the Brain
• the larger the nerve fiber diameter the faster
the rate of transmission of the signal
• velocity of transmission can be as fast as 120
m/sec or as slow as 0.5 m/sec
• nerve fiber classification
– type A - myelinated fibers of varying sizes, generally
fast transmission speed
• subdivided into a, b, d, g
– type C - unmyelinated fibers, small with slow
transmission speed
Importance of Signal Intensity
• Signal intensity is critical for interpretation of
the signal by the brain (i.e., pain).
• Gradations in signal intensity can be
achieved by:
1) increasing the number of fibers stimulated,
spatial summation
2) increasing the rate of firing in a limited
number of fibers, temporal summation.
Signal Intensity
An example of spatial
summation
Figure 46-7;
Guyton & Hall
Relaying Signals through Neuronal
Pools
Neuronal Pools
• groups of neurons with special
characteristics of organization
• comprise many different types of neuronal
circuits
– converging
– diverging
– reverberating
Neuronal Pools
Modified from figures 46-11 and 46-12; Guyton & Hall
Neuronal Pools
Lateral inhibition
Reverberating Circuits
Figure 46-14;
Guyton & Hall
Classification of Somatic Sensations
• Mechanoreceptive - stimulated by mechanical
displacement
– tactile
•
•
•
•
touch
pressure
vibration
tickle and itch
– position or proprioceptive
• static position
• rate of change
Classification of Somatic Sensations
• Thermoreceptive
– detect heat and cold
• Nociceptive
– detect pain and are activated by any factor that
damages tissue
Tactile Sense Transmission
• Meissner’s corpuscles, hair receptors,
Pacinian corpuscles and Ruffini’s end organs
transmit signals in type Ab nerve fibers at 3070 m/sec.
• Free nerve endings transmit signals in type
Ad nerve fibers at 5-30 m/sec, some by type
C unmyelinated fibers at 0.5-2 m/sec.
• The more critical the information the faster
the rate of transmission.
Pathways for the Transmission
of Sensory Information
• Almost all sensory information enters
the spinal cord through the dorsal roots
of the spinal nerves.
• Two pathways for sensory information
– dorsal column-medial lemniscal system
– anterolateral system
Dorsal Column System
• Contains large myelinated nerve fibers for
fast transmission (30-110 m/sec).
• High degree of spatial orientation maintained
throughout the tract.
• Transmits information rapidly and with a high
degree of spatial fidelity (i.e., discrete types of
mechanoreceptor information).
• Touch, vibration, position, fine pressure
The Dorsal Column
System
The Posterior Column-Medial Lemniscus
Pathway
• Conveys nerve
impulses for touch,
pressure, vibration
and conscious
proprioception from
the limbs, trunk,
neck, and posterior
head to the cerebral
cortex.
The Somatosensory Cortex
Figure 47-6; Guyton and Hall
Anterolateral System
• Smaller myelinated and unmyelinated fibers
for slow transmission (0.5-40 m/sec)
• Low degree of spatial orientation
• Transmits a broad spectrum of modalities
• Pain, thermal sensations, crude touch and
pressure, tickle and itch, sexual
sensations.
The Anterolateral (spinothalamic) pathway
• Conveys nerve
impulses for pain,
cold, warmth, itch,
and tickle from the
limbs, trunk, neck,
and posterior head
to the cerebral
cortex.
Figure 47-13;
Guyton and Hall
Somatic Sensory Cortex
• Located in the postcentral gyrus
• Highly organized distinct spatial orientation
• Each side of the cortex receives information
from the opposite side of the body
• Unequal representation of the body
– lips have greatest area of representation
followed by the face and the thumb
– trunk and lower body have the least area
Figure 47-7; Guyton and Hall
Cellular Organization of the Cortex
• Six separate layers of neurons with layer I
near the surface of the cortex and layer VI
deep within the cortex.
• Incoming signals enter layer IV and spread
both up and down.
• Layers I and II receive diffuse input from
lower brain centers.
Cellular Organization of the Cortex
• Layer II and III neurons send axons to closely
related portion of the cortex presumably for
communicating between similar areas.
• Layer V and VI send axons to more distant
parts of the nervous system, layer V to the
brainstem and spinal cord, layer VI to the
thalamus.
Diffuse lower input
Related brain areas
Incoming signals
To brainstem and cord
To thalamus
Figure 47-08; Guyton and Hall
Cellular Organization of the Cortex
• Within the layers the neurons are also arranged in
columns.
• Each column serves a specific sensory modality (i.e.,
stretch, pressure, touch).
• Different columns interspersed among each other.
– interaction of the columns occurs at different cortical
levels which allows the beginning of the analysis of the
meaning of the sensory signals
Function of the Somatic Sensory Cortex
• Destruction of somatic area I results in:
–
–
–
–
loss of discrete localization ability
inability to judge the degree of pressure
inability to determine the weight of an object
inability to determine the shape or form of
objects, called astereognosis
– inability to judge texture
Somatic Association Areas
• Located behind the somatic sensory cortex in
the parietal area of the cortex.
• Association area receive input from somatic
sensory cortex, ventrobasal nuclei of the
thalamus, visual and auditory cortex.
• Function is to decipher sensory meaning.
• Loss of these areas results in the inability to
recognize complex objects and loss of self.
Special Aspects of Sensory Function
• Thalamus has some ability to
discriminate tactile sensation.
• Thalamus has an important role in the
perception of pain and temperature.
Special Aspects of Sensory Function
• Corticofugal fibers
– fibers from the cortex to the sensory relay areas
of thalamus, medulla and spinal cord
– these fibers are inhibitory, they can suppress the
sensory input
– function to decrease the spread of a signal and
sharpen the degree of contrast and adjust the
sensitivity of the system
Pain
Pain
•
•
•
•
•
occurs whenever tissue is being damaged
protective mechanism for the body
causes individual to remove painful stimulus
two types of pain, fast pain and slow pain
fast pain felt within 0.1 sec of the stimulus
and is sharp in character
• slow pain begins after a second or more and
is throbbing or aching in nature
Pain Receptors and Their Stimulation
• all pain receptors are free nerve endings
• can be stimulated by:
– mechanical (stretch)
– thermal
– chemical
• bradykinin, serotonin, histamine, potassium ions,
acids, acetylcholine and proteolytic enzymes
• prostaglandins and substance P enhance the
sensitivity of pain endings but do not directly excite
them
Pain Receptors and Their Stimulation
• pain receptors do not adapt to the stimulus
• the rate of tissue damage is the cause of pain (most
individuals feel pain at 450 C)
• extracts from damaged tissue cause pain when
injected under the skin
• bradykinin causes the most pain and may be the
single agent most responsible for causing the tissue
damage type of pain
– also the local increase in potassium ion concentration
and action of enzymes can contribute to pain
Dual Pain Pathways
• Fast pain is transmitted by type Ad fibers
(velocity 6-30 m/sec).
• Slow pain is transmitted by type C fibers
(0.5 - 2 m/sec).
• Fast pain fibers are transmitted in the
neospinothalamic tract.
• Slow pain fibers are transmitted in the
paleospinothalamic tract.
Neospinothalamic
tract
Paleospinothalamic
tract
Figure 48-2; Guyton & Hall
Copyright © 2006 by Elsevier, Inc.
Neospinothalamic Tract
• On entering the cord, pain fibers may travel
up or down 1-3 segments and terminate on
neurons in the dorsal horn.
• 2nd neuron crosses immediately to the
opposite side and passes to the brain in the
anterolateral columns.
• Some neurons terminate in the reticular
substance but most go all the way to the
ventrobasal complex of the thalamus.
• 3rd order neurons go to the cortex.
Neospinothalamic Tract (cont’d)
• Fast-sharp pain can be localized well.
• However, fast pain fibers must be
stimulated with other tactile receptors
for the pain to be highly localized.
Paleospinothalamic Tract
• Type C pain fibers terminate in laminae II and III of
the spinal cord and make one or two local
connections before giving rise to 2nd order neurons
which cross immediately and pass to the brain in the
anterolateral columns.
• Only 10 to 25 % of the fibers terminate in the
thalamus.
• Most terminate diffusely in the:
– reticular nuclei of the medulla, pons and
mesencephalon
– tectal area of the mesencephalon
– periaqueductal gray region.
Paleospinothalamic Tract
• lower terminations important to appreciate the
suffering type of pain
• from the lower reticular areas of the brain
stem neurons project to the intralaminar
nuclei of the thalamus, hypothalamus and
other basal brain regions
• poor localization of slow pain, often to just the
affected limb or part of the body
Paleospinothalamic
tract
Neospinothalamic
tract
Figure 48-3; Guyton & Hall
The Appreciation of Pain
• Removal of the somatic sensory areas of the
cortex does not destroy the ability to perceive
pain.
• Pain impulses to lower areas can cause
conscious perception of pain.
• Therefore, cortex probably important for
determining the quality of pain.
• Stimulation of the reticular areas of the brain
stem and intralaminar nuclei of thalamus
(where pain fibers terminate) causes
widespread arousal of the nervous system.
Analgesia System of the Brain and Spinal
Cord
• The brain has the capability to suppress pain
fibers.
• Periaqueductal gray area neurons send
axons to the nucleus raphe magnus and the
nucleus paragigantocellularis.
• Raphe magnus and paragigantocellularis
neurons send axons to the dorsal horns of
the spinal cord.
• These neurons activate a pain inhibitory
complex in the spinal cord.
Analgesia system of
the brainstem
and spinal cord
Figure 48-4;
Guyton & Hall
Analgesia System of the Brain and Spinal
Cord
• Higher brain levels, the periventricular
nuclei of the hypothalamus and the medial
forebrain bundle can activate the
periaqueductal gray region and suppress
pain.
Pain Suppression Mechanism
• Nerve fibers in the periventricular nucleus
and the periaqueductal gray secrete
enkephalin at their nerve endings.
• Nerve fibers from the raphe magnus secrete
serotonin at their nerve endings.
• The serotonin causes the local neurons to
secrete enkephalin.
• Enkephalin is believed to cause both pre- and
post-synaptic inhibition of type C and type Ad
pain fibers where they synapse in the dorsal
horns.
Endogenous Opiate Systems
• In the early 1970’s it was discovered that an
injection of minute quantities of morphine into
the area around the third ventricle produced a
profound and prolonged analgesia.
• This started the search for “morphine
receptors” in the brain.
• Several “opiate-like” substances have been
identified. All are breakdown products of three
large molecules; proopiomelanocortin,
proenkephalin, and prodynorphin.
Endogenous Opiate Systems
• The major opiate substances; bendorphin, met-enkephalin, leuenkephalin, dynorphin
• The enkephalins and dynorphin are
found in the brain stem and spinal cord.
• The b-endorphin is found in the
hypothalamus and the pituitary.
Function of the Opiate System
• pain suppression during times of stress
• an important part of an organism’s
response to an emergency is a
reduction in the responsiveness to pain
– effective in defense, predation,
dominance and adaptation to
environmental challenges
Pain and Tactile Fibers
• Stimulation of large type Ab sensory
fibers from peripheral tactile receptors
can depress the transmission of pain
signals, “the gate control hypothesis”.
• Mechanism is a type of lateral inhibition
of the pain fiber by the sensory fiber.
• Mechanism of action of massage,
liniments, electrical stimulation of the
skin
Referred and Visceral Pain
• Pain from an internal organ that is perceived
to originate from a distant area of the skin.
• Mechanism is thought to be intermingling of
second order neurons from the skin and the
viscera.
• Viscera have few sensory fibers except for
pain fibers.
• Highly localized damage to an organ may
result in little pain, widespread damage can
lead to severe pain.
Referred and Visceral Pain
• localized to the dermatome of
embryological origin
• heart localized to the neck, shoulder and
arm
• stomach localized to the above the
umbilicus
• colon localized to below the umbilicus
Referred Pain From the Viscera
Figure 48-6;
Guyton & Hall
Causes of Visceral Pain
•
•
•
•
ischemia
chemical irritation
spasm of a hollow viscus
over distension of a hollow viscus
Headache Pain
• Brain tissue itself is insensitive to pain
• Pain sensitive structures of the brain are
the membranes that cover the brain and
the blood vessels:
– dura
– blood vessels of the dura
– venous sinuses
– middle meningeal artery
Origin of Headache Pain
Figure 48-9; Guyton & Hall
Intracrainial Headache
• meningitis
– inflammation of the meninges resulting in a
severe headache
• migraine
– results from abnormal vascular phenomenon
– vasospasm followed by prolonged vasodilation
– vasodilation causes stretching of the coverings
of the blood vessels
• hangover
– irritation of the meninges by alcohol
breakdown products and additives
Extracranial Headache
• muscular spasm, tension headache
– emotional tension may cause tension of muscles
attached to the neck and scalp which causes
irritation of scalp coverings
• sinus headache
– irritation of nasal structures
• eye strain
– excessive contraction of the ciliary muscle in an
attempt to focus, contraction of facial muscles
Thermal Sensations
• many more cold receptors than warm
receptors
• density of cold receptors varies
– highest on the lips, lowest on the trunk
• freezing cold and burning hot are the
same sensation because of stimulation
of pain receptors
Stimulation of Thermal Receptors
• Cold receptors respond from 7 to 44o C with
the peak response at 25o C.
• Warm receptors respond from 30 to 49o C
with the peak response at 44o C.
• The relative degree of stimulation of the
receptors determines the temperature
sensation.
• Thermal receptors adapt to the stimulus but
not completely.
Stimulation of Thermal Receptors
Figure 48-10; Guyton & Hall
Mechanism of Stimulation
• Cold or warm is thought to change the
metabolic rate of the receptor.
• This changes the rate of intracellular
reactions.
Somatic Motor Pathways
• Upper motor neurons → lower motor
neurons → skeletal muscles.
• Neural circuits involving basal ganglia and
cerebellum regulate activity of the upper
motor neurons.
Organization of the Upper Motor
Neuron Pathways
• Direct motor pathway- originates in the
cerebral cortex.
Corticospinal pathway: to the limbs and trunk.
Corticobulbar pathway: to the head.
• Indirect motor pathway- originates in the
brain stem.
Mapping of the Motor Areas
• Located in the
precentral gyrus of
the frontal lobe.
• More cortical area is
devoted to those
muscles involved in
skilled, complex or
delicate movements.
The Corticospinal Pathways