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
The Corticobulbar Pathway
Indirect Pathways
• Originate in the brain stem.
• Include:
–
–
–
–
Rubrospinal tract
Tectospinal tract
Vestibulospinal tract
Reticulospinal tract
Modulation of Movement from the Cerebellum
• The cerebellum coordinates and
smoothes contractions of skeletal
muscles during skilled movements
and helps maintain posture and
balance.
The Spinal Cord is More Than Just a Conduit
for Nerve Fibers
• Neuronal circuits for walking and
various reflexes are contained within the
spinal cord.
• Higher brain centers activate and
command these circuits.
– walking
– maintaining equilibrium
Motor Organization of the Spinal Cord
• Sensory fibers enter the cord and are
transmitted to higher centers, or they
synapse locally to elicit motor reflexes.
• Motor neurons are located in the
anterior portion of the cord.
– motor neurons are 50 - 100 % bigger than
other neurons
Anterior Motor Neurons
• Alpha motor neurons
– give rise to large type A alpha fibers (~14
microns).
– stimulation can excite 3 - 100 extrafusal
muscle fibers collectively called a motor unit
• Gamma motor neurons
– give rise to smaller type A gamma fibers (~5
microns)
– stimulation excites intrafusal fibers, a special
type of sensory receptor
Interneurons and Propriospinal Fibers
• Interneurons
– 30 times as many as anterior motor neurons
– small and very excitable
– comprise the neural circuitry for the motor
reflexes
• Propriospinal fibers
– travel up and down the cord for 1 - 2 segments
– provide pathways for multisegmental reflexes
Sensory Receptors of the Muscle
• Muscle Spindle
– sense muscle length and change in length
• Golgi Tendon Organ
– sense tendon tension and change in
tension
The Muscle Spindle
Figure 54-2; Guyton and Hall
Static Response of the Muscle Spindle
• When the center of spindle is stretched
slowly - the number of impulses
generated by the primary and
secondary endings increases in
proportion to the degree of stretch.
• This is the ‘static response’.
• Function of the static nuclear bag
and nuclear chain fibers.
Dynamic Response of the Muscle
Spindle
• When the center of the spindle is stretched
rapidly - the number of impulses generated by
the primary endings increases in proportion to
the rate of change of the length.
• This is the ‘dynamic response’.
• Function of the dynamic nuclear bag fiber.
Physiologic Function of the Muscle
Spindle
• Comparator of length between the
intrafusal and extrafusal muscle fiber.
• Opposes a change in length of the
muscle.
• When the muscle is stretched the
spindle returns it to its original length.
• Leads to the stretch reflex.
Control of the Gamma Motor System
(Fusimotor System)
• Gamma signal excited by the bulboreticular
facilatory area of the brain stem.
• Secondarily by areas that send impulses to
this area.
– cerebellum, basal ganglia, cortex
• Little is known about the precise control of
this system.
Clinical Application of the Stretch
Reflex
• Knee jerk reflex
– striking the patellar tendon with a hammer
stretches the quadriceps muscle.
– this initiates a stretch reflex which shortens the
muscle and causes the knee to move forward.
• Can be done with almost any muscle.
• Index of the facilitation of the gamma
efferents.
• Cortical lesions usually increase muscle
stretch reflexes.
Golgi Tendon Reflex
• Mediated by the golgi tendon organ receptor
located in the tendon.
• This receptor responds to tension.
• When the tension becomes too great the
reflex inhibits the motor fibers attached to the
tendon.
• Function is to equalize force among muscle
fibers.
Transmission of Stretch Information
to Higher Centers
• Muscle spindle and golgi tendon signals are
transmitted to higher centers.
• This informs the brain of the tension and
stretch of the muscle.
• Information is transmitted at 120 m/sec.
• Important for feedback control of motor
activity.
The Withdrawal Reflexes
• A painful stimulus causes the limb to
automatically withdraw from the stimulus.
• Neural pathways for reflex:
– nociceptor activation transmitted to the spinal cord
– synapses with pool of interneurons that diverge
the to the muscles for withdrawal, inhibit
antagonist muscles, and activate reverberating
circuits to prolong muscle contraction
– duration of the afterdischarge depends on strength
of the stimulus
Crossed Extensor Reflex
• Painful stimulus elicits a flexor reflex in affected
limb and an extensor reflex in the opposite limb.
• Extensor reflex begins 0.2 - 0.5 seconds after
the painful stimulus.
• Serves to push body away from the stimulus,
also to shift weight to the opposite limb.
Other Reflexes for Posture and Locomotion
• Pressure on the bottom of the feet
cause extensor reflex.
– more complex than flexor-crossed extensor
reflex
• Basic walking reflexes reside in the
spinal cord.
Reflexes that Cause Muscle Spasm
• Pain signals can cause reflex activation
and spasm of local muscles.
• Inflammation of peritoneum can cause
abdominal muscle spasm.
• Muscle cramps caused by painful
stimulus in muscle:
– can be due to cold, ischemia, of overactivity
– reflex contraction increases painful stimulus
and causes more muscle contraction
Motor Cortex
• Divided into 3 sub areas
– primary motor cortex
• unequal topographic representation
• fine motor movement elicited by stimulation
– premotor area
• topographical organization similar to primary
motor cortex
• stimulation results in movement of muscle
groups to perform a specific task
• works in concert with other motor areas
Motor Cortex (Cont.)
– supplemental motor area
• topographically organized
• simulation often elicits bilateral movements.
• functions in concert with premotor area to provide
attitudinal, fixation or positional movement for the
body
• it provides the background for fine motor control of
the arms and hands by premotor and primary motor
cortex
Motor Areas of the Cerebral Cortex
Figure 55-3; Guyton and Hall
Figure 55-2; Guyton and Hall
Specialized Areas of the Motor Cortex
• Broca’s area
– damage causes decreased speech capability
– closely associated area controls appropriate
respiratory function for speech
• eye fixation and head rotation area
– for coordinated head and eye movements
• hand skills area
– damage causes motor apraxia the inability to
perform fine hand movements
Transmission of Cortical Motor Signals
• Direct pathway
– corticospinal tract
– for discrete detailed movement
• Indirect pathway
– signals to basal ganglia, cerebellum, and
brainstem nuclei
Corticospinal Fibers
• 34,000 Betz cell fibers, make up only
about 3% of the total number of fibers
• 97% of the 1 million fibers are small
diameter fibers
– conduct background tonic signals
– feedback signals from the cortex to control
intensity of the various sensory signals to the
brain
Other Pathways from the Motor Cortex
• Betz collaterals back to cortex sharpen the
boundaries of the excitatory signal
• Fibers to caudate nucleus and putamen
• Fibers to the red nucleus, which then sends axons to
the cord in the rubrospinal tract
• Reticular substance, vestibular nuclei and pons then
to the cerebellum
• Therefore the basal ganglia, brain stem and
cerebellum receive a large number of signals from
the cortex.
Incoming Sensory Pathways to
Motor Cortex
• Subcortical fibers from adjacent areas of the
cortex especially from somatic sensory areas
of parietal cortex and visual and auditory
cortex.
• Subcortical fibers from opposite hemisphere
which pass through corpus callosum.
• Somatic sensory fibers from ventrobasal
complex of the thalamus (i.e., cutaneous and
proprioceptive fibers).
Incoming Sensory Pathways to
Motor Cortex (Cont.)
• Ventrolateral and ventroanterior nuclei of
thalamus for coordination of function between
motor cortex, basal ganglia, and cerebellum.
• Fibers from the intralaminar nuclei of
thalamus (control level of excitability of the
motor cortex), some of these may be pain
fibers.
Red Nucleus and the Rubrospinal Tract
• Substantial input from primary motor cortex
• Primary motor cortex fibers synapse in the
lower portion of the nucleus called the
magnocellular portion which contains large
neurons similar to Betz cells.
• Magnocellular portion gives rise to rubrospinal
tract.
• Magnocellular portion has somatotopic
organization similar to primary motor cortex.
Red Nucleus and the Rubrospinal Tract
• Stimulation of red nucleus causes relatively
fine motor movement, but not as discrete as
primary motor cortex.
• Accessory route for transmission of discrete
signals from the motor cortex.
Red Nucleus and the Rubrospinal Tract
Figure 55-5; Guyton and Hall
Sensory Feedback is Important for
Motor Control
• Feedback from muscle spindle, tactile
receptors, and proprioceptors fine tunes
muscle movement.
• Length mismatch in spindle causes auto
correction.
• Compression of skin provides sensory
feedback to motor cortex on degree of
effectiveness of intended action.
Excitation of Spinal Motor Neurons
• Motor neurons in cortex reside in layer V.
• Excitation of 50-100 giant pyramidal cells is needed
to cause muscle contraction.
• Most corticospinal fibers synapse with interneurons.
• Some corticospinal and rubrospinal neurons synapse
directly with alpha motor neurons in the spinal cord
especially in the cervical enlargement.
• These motor neurons innervate muscles of the
fingers and hand.
Lesions of the Motor Cortex
• Primary motor cortex - loss of voluntary
control of discrete movement of the distal
segments of the limbs.
• Basal ganglia - muscle spasticity from loss of
inhibitory input from accessory areas of the
cortex that inhibit excitatory brainstem motor
nuclei.
Control of Motor Function by the
Brainstem
• Brainstem as an extension of the spinal cord.
– performs motor and sensory functions for the
face and head (i.e., cranial nerves).
– similar to spinal cord for functions from the
head down.
• Contains centers for stereotypic movement
and equilibrium.
Support of the Body Against Gravity
• The muscles of the spinal column and the
extensor muscles of the legs support the
body against gravity.
• These muscles are under the influence of
brainstem nuclei.
• The pontine reticular nuclei excite the
antigravity muscles.
• The medullary reticular nuclei inhibit the
antigravity muscles.
Orientation of the
Pontine and
Medullary Reticular
Nuclei
Figure 55-7;
Guyton and Hall
Pontine Reticular Nuclei
• Transmit excitatory signals through pontine
reticulospinal tract.
• Pontine reticular nuclei have a high degree of
natural excitability.
• When unopposed they cause powerful
excitation of the antigravity muscles.
Medullary Reticular Nuclei
• Transmit inhibitory signals to the antigravity muscles
through the medullary reticulospinal tract.
• These nuclei receive collateral input from the
corticospinal tract, rubrospinal tract, and other motor
pathways.
• These systems can activate the inhibitory action of
the medullary reticular nuclei and counterbalance the
signals from the pons.
The Cerebellum (little brain)
• responsible for coordinating muscle activity
• sequences the motor activities
• monitors and makes corrective adjustments
in the activities initiated by other parts of the
brain
• compares the actual motor movements with
the intended movements and makes
corrective changes
Functional Organization of the
Cerebellum
• Functionally arranged along the longitudinal axis
• Vermis, located at the center, controls axial
movements of the neck, shoulders, and hips
• Intermediate zone controls motion of distal
portions of upper and lower limbs especially the
hands and feet
• Lateral zone controls sequencing movements of
the muscle. Important for timing and coordination
of movement.
Axial movements of neck,
shoulders and hips
Motion of distal limbs, especially
hands and feet
Sequencing of movements,
timing and coordination
Figure 56-2; Guyton & Hall
Afferent Pathways to the Cerebellum
• From the brain
– corticopontocerebellar pathway from motor
and premotor area, somatosensory cortex as
well as some pontine nuclei which join this
tract. Projects mostly to the lateral areas.
– olivocerebellar tract, vestibulocerebellar tract,
reticulocerebellar tract
• These pathways transmit information about
intended motion.
Afferent Pathways to the Cerebellum
• from the periphery
– dorsal spinocerebellar tract - transmits information
mostly from muscles spindle but also from Golgi
tendon organs, tactile, and joint receptors
• apprises the brain of the momentary status of
muscle contraction, muscle tension and limb
position and forces acting on the body surface
– ventral spinocerebellar tract - signals from anterior
horn, and interneurons
• transmits information on which signals have
arrived at the cord
Efferent Pathways from the Cerebellum
• fastigioreticular tract
– equilibrium control
• cerebellothalamocortical tract
– coordinates agonist and antagonist muscle
contractions
Neuronal Organization of the
Cerebellar Cortex
• organized in three layers
– molecular cell layer
– Purkinje cell layer
– granular cell layer
• output from the cerebellum comes from a
deep nuclear cell layer located below
these layers of cortex
Organization of the Cerebellum
Figure 56-7; Guyton & Hall
Neuronal Circuit of the Cerebellum
Deep nuclear cells receive excitatory and inhibitory inputs
Inhibitory from
Purkinje cells
Excitatory afferent
inputs from climbing
fibers and mossy fibers
Figure 56-7; Guyton & Hall
Copyright © 2006 by Elsevier, Inc.
Neuronal Circuit of the Cerebellum
climbing fibers send branches to
the deep nuclear cells before they
make extensive connections with
the dendrites of the Purkinje cell.
Causes complex spike output from
Purkinje cell.
Figure 56-7; Guyton & Hall
Copyright © 2006 by Elsevier, Inc.
all climbing fibers originate
from the inferior olive
Neuronal Circuit of the Cerebellum
mossy fibers relay all other afferent
input into the cerebellum, also send
branches to the deep nuclear cell
mossy fiber stimulation
causes a simple spike
output
Copyright © 2006 by Elsevier, Inc.
Figure 56-7; Guyton & Hall
mossy fibers terminate in the
granular cell layer.
Neuronal Circuit of the Cerebellum
granular cells send axons to the
molecular cell layer where they divide
and go a few mm in opposite directions
to become parallel fibers in the
molecular layer
500 - 1000 granule cells for every
Purkinje cell, anywhere from 80,000 to
200,000 parallel fibers synapse with
each Purkinje cell
Figure 56-7; Guyton & Hall
Deep Nuclear cell activity
Inhibited by Purkinje cell input
Stimulated by both climbing
and mossy fiber input
Normally the balance is in
favor of excitation
Deep nuclear cell at first
receives an excitatory input
from both the climbing fibers
and mossy fibers.
This is followed by an inhibitory
signal from the Purkinje cells
Deep Nuclear Cell Activity
• At beginning of motion there are excitatory
signals sent into motor pathways by deep
nuclear cells to enhance movement, followed
by inhibitory signals milliseconds later.
– Provides a damping function to stop
movement from overshooting its mark
– Resembles a delay-line type of electronic
circuit for negative feedback
The Turn-On / Turn-Off Function
• cerebellum contributes to the rapid turn-on
signals for agonist muscles and turn-off of
antagonist muscles at beginning of a motion
• then it times the opposite sequence at the
end of the intended motion
• direct motor pathway via corticospinal tract is
enhanced by cerebellum by additional signals
to the tract or by signals back to the cortex
The Turn-On / Turn-Off Function
• mossy fiber input also to Purkinje cells which
activates them after a few millisec., this
results in an inhibitory signal to the deep
nuclear cell
• this inhibits the agonist muscle which stops its
activity
Purkinje Cells Function to Correct
Motor Errors
• precise motor movement must be learned
• climbing fiber input adjusts the sensitivity of
the Purkinje cells to stimulation by parallel
fibers
• this changes the long-term sensitivity of the
Purkinje cell to mossy fiber input (i.e., from
muscle spindle, golgi tendon, proprioceptor)
• this adjusts the feedback control of muscle
movement
Correction of Motor Errors
• inferior olivary complex receives input from:
– corticospinal tract and motor centers of
the brain stem
– sensory information from muscles and
surrounding tissue detailing the
movement that actually occurs
• inferior olivary complex compares intent
with actual function, if a mismatch occurs
output to cerebellum through climbing fibers
is altered to correct mismatch
Motion Control by the Cerebellum
• most motion is pendular, therefore, there is
inertia and momentum
• to move a limb accurately it must be
accelerated and decelerated in the right
sequence
• cerebellum calculates momentum and inertia
and initiates acceleration and braking activity
Predictive and Timing
Function of the Cerebellum
• motion is a series of discrete sequential
movement
• the planning and timing of sequential movements
is the function of the lateral cerebellar hemisphere
• this area communicates with premotor and
sensory cortex and corresponding area of the
basal ganglia where the plan originates
• the lateral hemisphere receives the plan and
times the sequential events to carry out the
planned movement
Clinical Abnormalities of the Cerebellum
• ataxia and intention tremor
– failure to predict motor movement, patients will
overshoot intended target
• dysdiadochokinesia
– failure of orderly progression of movement
• dysarthria
– failure of orderly progression in vocalization
• cerebellar nystagmus
– intention tremor of the eyes
Basal Ganglia
• Consist of Four Nuclei
• striatum
– caudate and putamen
• globus pallidus
• substantia nigra
• subthalamus
The basal ganglia are the
principle subcortical components
of a family of parallel circuits
linking the thalamus with the
cerebral cortex
Figure 56-11
Figure 56-12; Guyton & Hall
Copyright © 2006 by Elsevier, Inc.
Motor Function of the Basal Ganglia
• control of complex patterns of motor activity
– writing
– using scissors
– throwing balls
– shoveling dirt
– some aspects of vocalization
Function of the Basal Ganglia?
• not much is known about the specific
functions of each of these structures
• thought to function in timing and scaling of
motion and in the initiation of motion
• most information comes from the result of
damage to these structures and the resulting
clinical abnormality
Caudate Circuit
Caudate extends into all
lobes of the cortex and
receives a large input from
association areas of the
cortex
Mostly projects to globus
pallidus, no fibers to subthalamus or substantia
nigra
Most motor actions occur
as a result of a sequence of
thoughts. Caudate circuit
may play a role in the
cognitive control of motor
functions
Figure 56-12; Guyton & Hall
Copyright © 2006 by Elsevier, Inc.
Putamen Circuit
Mostly from premotor and
supplemental motor cortex to
putamen then back to motor
cortex.
Figure 56-11; Guyton & Hall
Neurotransmitters in the Basal Gangelia
Figure 56-14; Guyton & Hall
Lesions of Basal Ganglia
• globus pallidus
– athetosis - spontaneous writing movements of
the hand, arm, neck, and face
• putamen
– chorea - flicking movements of the hands, face,
and shoulders
• substantia nigra
– Parkinson's disease - rigidity, tremor and
akinesia
– loss of dopaminergic input from substantia
nigra to the caudate and putamen
Lesions of Basal Ganglia
• subthalamus
– hemiballismus - sudden flailing movements of
the entire limb
• caudate nucleus and putamen
– huntington’s chorea - loss of GABA containing
neurons to globus pallidus and substantia nigra
Integration of Motor Control
• spinal cord level
– preprogramming of patterns of movement of all
muscles (i.e., withdrawal reflex, walking
movements, etc.).
• brainstem level
– maintains equilibrium by adjusting axial tone
• cortical level
– issues commands to set into motion the patterns
available in the spinal cord
– controls the intensity and modifies the timing
Integration of Motor Control (cont’d)
• cerebellum
– function with all levels of control to adjust cord
motor activity, equilibrium, and planning of
motor activity
• basal ganglia
– functions to assist cortex in executing
subconscious but learned patterns of movement,
and to plan sequential patterns to accomplish a
purposeful task
Overall scheme for
integration of
motor function
Figure 56-10;
Guyton & Hall
Physiologic Anatomy of Cerebral Cortex
• Each area of the cortex is connected to a
specific part of the thalamus.
• When thalamic connection is lost cortical
function stops.
• All sensory pathways pass through the
thalamus with the exception of the olfactory
tract.
Physiologic Anatomy of Cerebral Cortex
Figure 57-5; Guyton & Hall
Dominant and Non-Dominant
Hemisphere
• Wernicke’s area more developed in one
hemisphere, responsible for verbal symbolism
and related intelligence.
• 95% of population has a left dominant
hemisphere.
• Wernicke’s area can be as much as 50%
larger in the dominant hemisphere.
Dominant and Non-Dominant Hemisphere (Cont’d)
• Damage to dominant Wernicke’s area leads
to dementia.
• Non-dominant side related to other forms of
sensory intelligence (music, sensory
feelings).
Intellectual Functions of the Prefrontal
Association Area
• responsible for calling forth stored information
and using it to obtain a goal
• responsible for concerted thinking in a logical
sequence
– damage causes an inability to keep tract of
simultaneous bits of information, easily
distracted
Intellectual Functions of the Prefrontal
Association Area (Cont’d)
• elaboration of thought
– prognosticate, plan, consider
consequences of motor actions before
they are performed
– correlate widely divergent information,
control one’s activities
Sensory Aspects Communication
Wernicke's aphasia
Destruction of the
visual and auditory
association areas
results in an inability
to understand the
written or spoken
word.
Figure 57-7; Guyton & Hall
Motor Aspects of Communication
• Speech involves two things
– formation in the mind of thoughts to be
expressed and the choice of words
– motor control of vocalization and the act of
vocalization
• Formation of word, thought and choice of
words is function of Wernicke’s area.
• Broca’s area controls the motor coordination
required for speech.
Function of the Corpus Callosum
• connects the two hemispheres and allows
transfer of information
• interruption of these fibers can lead to bizarre
types of anomalies
– dominant hemisphere understands spoken word
– non dominant hemisphere understands written
word and can elicit motor response without
dominant side knowing why response was
performed
Thoughts and Memory
• Neural mechanism for thought is not known.
• Most likely a specific pattern of simultaneous
neural activity in many brain areas.
• Destruction of cerebral cortex does not
prevent one from thinking.
– However, depth of thought and level of
awareness may be less.
Learning and Memory
• Learning is the ability to acquire new
information or skills through instruction or
experience.
• Memory is the process by which
information acquired through learning is
stored and retrieved.
Memory
• Change in the capability of synaptic
transmission from neuron to neuron as a
result of prior stimulation.
• Memory trace is a specific pattern or pathway
of signal transmission.
• Once established they can be activated by
the thinking mind to reproduce the pattern
and thus the memory.
3 Types of Memory
• immediate memory
– lasts for seconds or minutes
• short-term memory
– lasts for days to weeks
• long-term memory
– lasts for years or for a lifetime
Mechanism of Memory
• Immediate memory may result from synaptic
potentiation through the accumulation of
calcium in the presynaptic membrane.
– would promote neurotransmitter release
• Short-term memory may result from a
temporary physical or chemical change in the
pre- or postsynaptic membrane.
Cellular Basis for Memory
• repetitive stimulation causes a progressive
decline in sensitivity called habituation
• results from progressive decline in the
number of active calcium channels
• less calcium entry less transmitter released
• stimulation of facilitator terminal prevents
habituation
Molecular Basis for Memory
Transmitter activates G protein
which in turn activates
adenylate cyclase resulting in
an increase
in cAMP.
cAMP activates a protein kinase
that phosphorylates a
component of the K+ channel
blocking its activity.
This prolongs the action
potential which increases
transmitter release.
Figure 57-9; Guyton & Hall
Long-Term Memory
• results from a structural change in the
synapse
• increase in the area for vesicular release
therefore, more transmitter is released
• during periods of inactivity the area
decreases in size
• enlargement of the release site area results
from synthesis of release site proteins
Consolidation of Memory
• converting immediate into short and longterm memory
• results from chemical, physical and
anatomical changes in the synapse
• requires time
• interruption of the process by electrical shock
or by anesthesia will prevent memory
development
• rehearsal enhances consolidation
Brain Centers and Memory
• Thalamic structures are important for
recalling memories.
• Damage to thalamus causes retrograde
amnesia or the inability to recall stored
experiences.
• Thalamus scans the cortex for the area
and the circuit for the stored memory.
Schematic of the Limbic System Components
Figure 58-5; Guyton & Hall
Location of the Limbic System
Figure 58-4; Guyton & Hall
Hypothalamus
• major output pathway of the limbic system
• vegetative functions:
– neurogenic control of arterial pressure
– regulation of body temperature
– regulation of fluid volume
– regulation of endocrine gland secretion
• growth hormone, thyroid hormone,
glucocorticoid secretion, sex hormones
Behavioral Functions of the
Hypothalamus and Related Areas
• lateral hypothalamus
– eating, thirst, general level of activity, rage
• ventromedial nucleus
– satiety, tranquillity
• periventricular nucleus
– fear, punishment reactions
• anterior and posterior hypothalamus
– sexual drive
Functional Areas of the Hypothalamus
Figure 58-6; Guyton & Hall
Behavior and its Control
• Almost all behaviors are associated with either reward or
punishment.
• Several limbic structures are concerned with the affective
nature of the sensory experience–is it pleasant or
unpleasant?
– Reward center - located around medial forebrain
bundle (the lateral and ventromedial hypothalamus).
– Punishment center - located in central gray around
the aqueduct and extending into periventricular zones
of hypothalamus and thalamus.
Punishment always takes precedent over reward
Function of Other Limbic Areas
• amygdala
– receives input from other areas of limbic system
as well as most areas of the cortex
– sends output back to cortex as well as into
hippocampus, septum, and hypothalamus
– functions in behavioral awareness at the
semiconscious level
– projects into limbic system one’s status with
respect to the surroundings and current thoughts
– helps pattern behavior appropriate for the each
occasion
Function of Other Limbic Areas
• hippocampus
– originated as part of the olfactory cortex
– in lower animals the sense of smell is an
important determinant of behavior (is it good to
eat, does it smell like danger, is it sexually
inviting)
– therefore, the early hippocampus was involved
in decision making by determining the
importance of the incoming information
Hippocampus
– memory and learning
– damage causes anterograde amnesia
– reward and punishment determine whether or
not information will be stored as memory
– a person becomes habituated to indifferent
stimuli but learns any sensory experience that
causes pain or pleasure
– hippocampus provides the drive to rehearse
and consolidate these sensory experiences
Limbic Cortex
• cerebral association area for control of
behavior
• stimulation of various portions of this area
can elicit almost all behaviors in an animal’s
repertoire
The role of Reticular Activating System
(RAS) in Awakening
• Consists of neurons
whose axons project
from the reticular
formation through
the thalamus to the
cerebral cortex.
• Increased activity of
the RAS causes
awakening from
sleep (arousal).
Activating Systems of the Brain
• Cerebrum requires a constant input to
remain active.
• Signals from the brainstem activate wide
areas of the cortex (background activation) or
specific areas to perform discrete tasks.
Excitatory Signals from the Brainstem
• Bulboreticular facilitory area
- sends excitatory signals to the antigravity muscles
- sends excitatory signals to the thalamus and from
here they are distributed to widespread areas of the
cortex
• Bulboreticular area is excited by signals from
the periphery, especially pain signals and also
signals from the cortex (positive feedback).
Inhibitory Signals from the Brainstem
• reticular inhibitory area
– sends inhibitory signals to the bulboreticular
area
– when the inhibitory area is excited, it will
decrease the activity of the excitatory area and
decrease the activity of the cortex
Location of excitatory
and inhibitory areas
of the brain
Figure 58-1;
Guyton & Hall
Neurohumoral Control of Brain Activity
Figure 58-3; Guyton & Hall
Sleep
• unconsciousness from which one can
be aroused by sensory stimulus
• different from coma from which one
cannot be aroused
• two types of sleep:
– slow wave or deep sleep
– REM sleep or paradoxical sleep
Fig. 15.11
Slow Wave Sleep
• restful sleep at the beginning of the
sleep period
• associated with a decrease in
vegetative functions
• usually not associated with dreaming;
dreams do occur but they are not
remembered
Rapid Eye Movement (REM) Sleep
• associated with active dreaming
• peripheral muscle tone is inhibited
• associated with an increase in cortical activity
and metabolism
• brain waves similar to wakefulness
• begin about 90 minutes after falling asleep
and reappear at 90 minute intervals
– last for progressively longer periods of time
each time they occur, a few minutes at first, 30
minutes toward the end of the sleep period
Why Do We Sleep?
• mechanism is unknown
• probably an active inhibitory process in which
the excitatory reticular neurons are inhibited
• stimulation of the raphe nuclei causes sleep
– these nuclei release serotonin which is thought
to induce sleep
– blockade of serotonin formation causes
prolonged wakefulness in animals, however,
blood levels of serotonin are lower during sleep
Why Do We Sleep?
• stimulation of other brain regions can also
induce sleep
• nucleus of the solitary tract
– solitary tract stimulation will not produce sleep
if the raphe nuclei are destroyed
– therefore, solitary tract may be stimulating
release of serotonin from the raphe nuclei
• suprachiasmatic area of the rostral
hypothalamus, diffuse thalamic nuclei
Why Do We Sleep?
• accumulation of sleep factors
– muramyl peptide - found in CSF and urine of
animals keep awake for prolonged periods, will
cause sleep when injected into third ventricle
– also a peptide isolated from the blood of
sleeping animals
– also substance from brain stem of animals keep
awake
• lesions of the raphe nuclei can prevent sleep
REM Sleep
• function of REM sleep is unknown
– lesions of the locus ceruleus prevent REM
sleep
– may be important for neural development
– testing the cortex to see if it can be brought to
activity
Sleep Cycle
• no explanation for the sleep - wakefulness
cycle
• however, there are many theories
– sleep cycle may be caused by fatigue of
excitatory areas to induce sleep and fatigue of
inhibitory areas of the lower brain to awaken.
– sleep probably is an active process driven by a
center below the midpontine level of the brain
stem.
Fig. 15.11
Physiological Effect of Sleep
• little on the body itself
– decrease in sympathetic tone, muscle tone, fall in
arterial pressure
• profound effect on the brain
– lack of sleep can lead to altered mental states
• paranoia, psychoses
• sleep probably functions to balance the activity
of the various areas of the brain, to reset/rezero/reboot neuronal circuits
Brain Waves
• electrical recordings from the surface of the
brain
• characterized as alpha, beta, theta and delta
depending on the frequency
• each functional state of the brain has a
characteristic pattern of brain waves (sleep,
wakefulness, epilepsy, psychoses, etc.)
Alpha and Beta Waves
• Alpha waves
– occur at 8 -13 Hz
– mostly from occipital cortex but can also be found in
frontal and parietal regions as well
– occur during quiet resting states of cerebration, they
disappear when there is a specific mental activity
(opening of the eyes, intense mental concentration or
stress) or during sleep
– will not occur without cortical connection to thalamus
•
Beta waves
– occur at 14 - 80 Hz
– occur during intense mental activity or stress
Theta and Delta Waves
• Theta waves
– occur at 4 - 7 Hz
– recorded from parietal and temporal regions in children
– occur during emotional stress in adults particularly in
response to disappointment or frustration
• Delta waves
– all waves below 3.5 Hz
– occur during deep sleep thought to be activity of the cortex
independent of signals from lower brain areas
Brain Waves
Figure 59-1; Guyton & Hall
Epilepsy
• uncontrolled excessive activity of a part or all
of the central nervous system
• grand mal
– spasmodic contraction of the muscles followed
by an alternating contraction and relaxation
– this is called a tonic-clonic convulsion which
causes violent jerky muscle contractions
– excessive activity lasts for 30 seconds to 3-4
minutes
Epilepsy
• grand mal
– caused by an overly excitable area of the brain
called a focus
– causes massive activation of reverberating
circuits which reactivate themselves many
times
• petit mal
– short periods of unconsciousness with twitching
contractions of the muscles of the face
Epilepsy
• petit mal (cont’d)
– characteristic electrical pattern called spike and
dome
• focal epilepsy
– localized organic lesion
– can be confined to a single area of the brain or
it can spread to adjacent regions
EEG Activity During Epilepsy
Figure 59-5; Guyton & Hall