chapter 12 the central nervous system: the brain and spinal cord j.f. thompson, ph.d. & j.r....
TRANSCRIPT
Chapter 12
The Central Nervous System:The Brain and Spinal Cord
J.F. Thompson, Ph.D. & J.R. Schiller, Ph.D. & G. Pitts, Ph.D..
The Brain General
100 billion neurons
about 1.6 kg in males/1.45 kg in females proportional to body size
divided into hemispheres and lobes
its size is not representative of intelligence
complexity dictates processing power
Major Subdivisions of the Brain
Cerebral hemispheres
Diencephalon thalamus hypothalamus epithalamus
Brain stem midbrain pons medulla oblongata
Cerebellum
Distribution of Gray and White Matter
Gray matter: mostly unmyelinated processes and neuron cell bodies
White matter: myelinated fiber tracts cerebrum & cerebellum
gray matter mostly superficial (cortex)
white matter deep brain stem
variable spinal cord
white matter superficial gray matter deep
Brain Ventricles
Fluid filled spaces in brain 2 lateral ventricles
C-shaped chambers located deep in cerebral hemispheres
connected to 3rd ventricle
3rd ventricle a slit between and inferior to the
right and left halves of thalamus connects to lateral ventricles connects to 4th ventricle
4th ventricle lies between brain stem and
cerebellum connects to central canal of
spinal cord
Cerebral Hemispheres of Brain ~80% of the brain’s
mass During development,
gray matter grows faster than white matter gyrus - elevated ridges sulcus - shallow grooves fissure - deep grooves that
separate major regions Longitudinal fissure -
separates R and L hemispheres
Transverse fissure - separates cortex from cerebellum
Cerebral Lobes (5/hemisphere) Frontal, parietal,
temporal, occipital lobes Central sulcus separates
frontal from parietal lobe precentral gyrus postcentral gyrus
Lateral sulcus separates frontal from temporal lobe
Parieto-occipital sulcus separates parietal from occipital lobe
Insula: deep to portions of the temporal, parietal, and frontal lobes
Cerebral Cortex ~40% of brain’s mass Only 2-4 mm thick Center of
consciousness Contains neuron cell
bodies, dendrites, unmyelinated axons, glial cells
Folds greatly increase its surface area
A rich capillary blood supply is nearby
Cortex: General Functional OrganizationThree types of activity (areas)
motor sensory association
Each hemisphere primarily controls the opposite side of the body
Although roughly equal in structure, the hemispheres are not equal in function
No functional area of the brain works alone
Consciousness involves all areas of the brain
Primary motor cortex (4*) [* Brodmann areas] precentral gyrus of frontal lobe primarily involved in voluntary motor control with more
area devoted to skilled muscles (e.g., controls fingers, face)
Motor Areas of the Cerebral Cortex
Map of the Primary Motor Cortex Motor homunculus – shows
the locations on the precentral gyrus which control the skeletal muscles of each body region
The “size” of the illustrated body part indicates the number of neurons dedicated to that region
Control is contralateral
Note: areas of specialization for communication (large size of tongue, face) and manipulation (hands)
Motor Areas of the Cerebral Cortex
Premotor cortex Anterior to the
primary motor cortex
Involved in learned repetitious or patterned movements, e.g., playing a piano, typing
Also important in planning movements
Homeostatic Imbalance of Motor Cortex
Damage to the primary motor cortex effects the opposite side of body, e.g., stroke, trauma only voluntary control of skeletal muscle is lost reflexes remain -- controlled by the spinal cord
Damage to the premotor cortex loss of programmed motor skills muscle strength and the ability to perform
tasks remain one can still make finger movements to type, etc. not automatic
need to re-learn fine motor control
Motor Areas of the Cerebral Cortex
Language areas (Broca's area, 44, 45) only found in one hemisphere - left? a motor center for speech, controlling the muscles of the
tongue, throat, and lips also involved in planning some voluntary motor activities
Frontal eye field (8) - voluntary movements of eyes
Broca's areaBroca's area
Sensory Areas of the Cerebral Cortex
Primary somatosensory area Receives inputs directly from peripheral somatic sensory
receptors Localizes points of the body where sensations originate
primary areas are directly wired to the peripheral sensory receptors or motor effectorssecondary areas receive input from primary areas
Note:
Sensory Areas of the Cerebral Cortex
Primary somatosensory area distribution of input areas for cutaneous sensationscutaneous sensations spatial discriminationspatial discrimination - identifies the areas of the
body being stimulated
Motor Sensory
Compare motor and sensory homunculi:
Somatosensory association area Gets input from primary somatosensory association
area Integrates and analyzes information relative to size,
texture for identification of objects Uses memories and experiences for object
identification without visual input
Sensory Areas of the Cerebral Cortex
Posterior to the primary somatosensory area
Sensory Areas of the Cerebral Cortex
Visual area Medial surface of occipital lobe Impulses from the eyes are routed through the thalamus
Sensory Areas: Visual Cortex Sensory fibers cross over
to the opposite side -- 75%/25% at the optic chiasm lateral geniculate nucleus
(visual area) of the thalamus
occipital lobe primary sensory area association areas
Processing different areas for
different functions monocular vs binocular color, form, movement
Motion Aftereffect
Pinwheel
Stare directly into the center of the pinwheel for 60 seconds. Then immediately look away from the screen and at the back of your hand. Try looking at other things in the immediately vicinity as well!
Color Afterimage
Stare directly at the center of the next screen for 60 seconds.
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What did you see?
Let’s repeat it.
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green red
yellow blue
Reviews of Neural Processing Different levels
Neuron to neuron communication
Specific hardwired pathways route information in predictable directions to specific locations
Association areas permit integration and interpretation of different types of sensory information
Motor areas issue appropriate commands to effector organs
Sensory Cortex: Auditory Area Superior part of the temporal lobe Primary auditory cortex (anterior arrow) for pitch,
rhythm, loudness Auditory association area (posterior arrow)
identifies/perceives sounds using memories as references
Sensory Cortex: Olfactory Cortex
Located above the orbits and in the medial portion of the temporal lobes
Conscious awareness of different smells
Multimodal Association Areas Anterior association area (prefrontal cortex)
anterior frontal lobe intellect, complex learning, recall and personality judgement, & planning matures slowly – influenced by environment
Frontal Lobotomy
sever the frontal lobes from the rest of the brain stops all strong emotional reactions once a popular medical/psychiatric procedure with a
long history obsolete: patients had problems with planning and
performing socially appropriate behaviors despite seeming to know what those behaviors would be
films: Frances and One Flew Over the Cuckoo’s Nest
1500’s
1950’s
Multimodal Association Areas Posterior association area
Temporal, parietal and occipital lobes Pattern recognition, localizing position Receives input from motor and other sensory association areas and
interprets it dropping an acid bottle (sound, sight, touch, smell, memory, learning) many sensory inputs, but the dominant feeling is of danger
Multimodal Association Areas Limbic association area
Cingulate and parahippocampal gyri, & hippocampus Provides emotional impact and sense of danger
Association: Language Areas Wernicke’s area - involved in pronouncing and
interpreting words Broca’s area - speech production lateral prefrontal cortex - language
comprehension, word analysis lateral, ventral temporal lobe - auditory, visual
aspects (naming objects, reading) the right side is more
involved in body language
Brain Lateralization both hemispheres participate in every activity,
but one hemisphere is dominant for most activities
e.g.: the left hemisphere is dominant for language skills in most people (90%)
the left is also dominant for math abilities and logic
the right hemisphere is usually dominant for “creative” skills: visual-spatial skills intuition emotion appreciation of art and music
most left-hemisphere-dominant people are right-handed
Brain Lateralization (or Not)
Hemispheric dominance is reversed or lacking in 10% of people
Most right-hemisphere-dominant people are left-handed and male
Equal hemispheric function may result in ambidexterity and/or dyslexia
Beware of the many “pop psychology” interpretations of the significance and meaning of hemispheric dominance
Cerebral White Matter
Myelinated fibers provide 3 types of connections within the CNS: commissural fibers
connect the hemispheres (right left)
ex: corpus callosum association fibers –
connect neurons within one hemisphere
projection fibers connect cerebral
hemispheres to other parts of the CNS
ex: internal capsule
Deep Cerebral Gray Matter: Basal Nuclei
Diffuse masses of gray matter deep within the cerebral hemispheres
Involved in regulating slow, sustained motor movements – ex: arm swinging
Also inhibit unnecessary movements (stabilize and smooth primary movements)
this area is affected in Parkinson’s disease
results in tremors and slow, unsteady movements
Brain Regions: Diencephalon
Composed of the thalamus, hypothalamus, and epithalamus
Surrounded by the cerebral hemispheres
Encloses the third ventricle
An egg shaped collection of nuclei serving as major “switching station” as impulses transfer from one neuron to the next
Forms the lateral walls of the third ventricle
Receives input from: all ascending pathways afferent impulses from all
senses except smell
Processes sensory information
crude recognition of sensation (cerebral processing required for precise localization and conscious awareness)
Thalamus (“Gateway” to the Cortex)
Hypothalamus (below Thalamus)
Forms the bottom of the third ventricle many nuclei infundibulum – the
stalk connecting the hypothalamus and pituitary gland
Pituitary gland endocrine gland – “the
master gland” releases its several
hormones in response to chemical regulation factors from the hypothalamus
Functions of the Hypothalamus
1. Autonomic Nervous System (visceral) control center – important in homeostasis
2. a center for emotional responses and behaviors
3. body temperature regulation4. regulation of food intake5. regulation of water balance and thirst6. regulation of sleep-wake cycles7. controls many endocrine system functions
neuroendocrine feedback control
Epithalamus (upon the Thalamus)
Dorsal portion of the diencephalon
Pineal gland (body) melatonin involved in sleep-
wake cycles Location of one of
the choroid plexus sites for production of cerebrospinal fluid (CSF)
Brain Regions: Brain Stem
midbrain
pons
medulla
Composed of the midbrain, pons and medulla
Involved in automatic, unconscious behaviors needed for survival
Provides pathways (fiber tracts) for neurons which are communicating up or down
Brain Stem: Midbrain Pons to the lower
portion of the diencephalon with the cerebral aqueduct passing through it
Main connecting routes for all parts of the brain and spinal cord
Connections between the cerebellum and the brainstem (cerebellar peduncles)
Brain Stem: Pons
above the medulla and anterior to the cerebellum
contains both gray matter nuclei and white fibers tracts
primarily conduction pathways
site of origin for several cranial nerves
cerebral peduncles
Brain Stem: Medulla Oblongata most inferior part of the
brain; merges into the spinal cord inferiorly
involved in maintaining internal homeostasis cardiovascular center respiratory center other centers for:
vomiting hiccuping swallowing coughing sneezing
Brain Regions: Cerebellum Second-largest brain
region (cerebellum = “small brain”)
Separated from the cerebrum by the transverse fissure
Its surface is the cerebellar cortex (gray matter) with folds (folia)folia); its white matter fiber tracts are located in the interior (arbor arbor vitaevitae = “tree of life”)
Cerebellar Structure and Function
Shaped like a butterfly central vermis (“worm”) cerebellar hemispheres
Functions to compare an intended movement (directed from the cortex) with what movement is actually happening
Constantly receiving sensory input from muscle, tendon, and joint proprioceptors, and visual and equilibrium receptors
Homunculi: maps of the functional areas
arbor vitae
Cerebellar Structure and Function
Purkinje neurons play a major role in control over the refinement of motor activities initiated by the frontal motor cortex
Functional Systems of the Brain Limbic System
encircles the brain stem the “emotional” center different regions of gray matter, including part
of the hypothalamus and the olfactory bulbs
Limbic System (cont.) Functions in emotional aspects of behavior
related to survival Also functions with the cerebrum in memory
olfactory centers are near the limbic system individuals, objects and experiences which initiate
strong emotional responses or are associated with smells are committed to memory more easily
Memory impairment results from damage to the limbic system
Also associated with pleasure and pain electrical stimulation elicits different responses includes defensive posturing (rage); others inspire
timidity
Brain Systems: Reticular Formation
Gray matter (nuclei) distributed within the medulla, pons, and midbrain
Axonal connections to many other areas of the brain
Structural and functional areas sensory, integrative
and motor functions receives input from
higher centers for skeletal muscle actions
Reticular Formation Reticular Activating
System functions to alert the cerebral
cortex to important incoming signals
filters signal “noise” = repetitive stimuli (LSD interferes with this)
e.g., studying in a noisy room maintenance of consciousness
and waking from sleep (sudden stimuli)
sends a constant stream of information to the cortex, maintaining arousal
the RAS is inhibited by sleep centers in the hypothalamus
the RAS is depressed by alcohol, sleep-inducing drugs (hypnotics) and anti-anxiety drugs
LSD
Protection of the Brain Soft tissue which needs to be protected Several different protective mechanisms
scalp hair to prevent sunstroke? bones – the cranium (“brain case”) of the skull meninges
three connective tissue membranes wrapping the CNS
cerebrospinal fluid (CSF) a fluid “shock absorber” which cushions and
nourishes the brain blood-brain barrier
the physical and physiological separation of the CNS from the bloodstream
Functions of the Meninges Covers and protects
brain and spinal cord
Protect blood vessels and enclose venous sinuses
Confine the cerebrospinal fluid in the subarachnoid space
Form major connective tissue partitions for brain regions within skull falx cerebri, falx cerebelli,
tentorium cerebelli
Meninges: Dura Mater Outermost layer Dense, irregular fibrous connective tissue Strong, protective wall around the brain
and spinal cord (dura mater = “tough/hard mother”)
Meninges: Arachnoid Membrane
Loose connective tissue layer deep to the dura mater Subdural space
separates the arachnoid from the dura contains interstitial fluid
Arachnoid villi extend into the subdural space CSF is reabsorbed back into the blood here
Subarachnoid space separates arachnoid from pia mater contains CSF
CSF
Meninges: Pia mater Deepest layer A thin, tight transparent fibrous connective
tissue supporting a network of many tiny blood vessels
Pia mater extends into the sulci and follows the large blood vessels into the brain
Pia mater = “gentle/little mother”
Protection of the Brain: Cerebrospinal Fluid
CSF protects against chemical & physical injury; it serves as a second circulatory system and nourishes the CNS
Found in the four ventricles and subarachnoid space
80-150 ml of CSF is normal for an adult CSF composition differs slightly from plasma Clear, colorless plasma filtrate containing:
H2O, glucose, other nutrients, proteins, lactic acid, urea cations (Na+, K+, Ca2+, Mg2+) anions (Cl-, HCO3
-) some lymphocytes (white blood cells)
Formed by the choroid plexuses; reabsorbed by the arachnoid villi and returned to the plasma
Functions of Cerebrospinal Fluid
Mechanical protection shock absorbing fluid the brain “floats” in this fluid
Chemical protection provides a constant chemical environment the pH of the CSF is important in the control
of breathing CSF composition is important for regulating
cerebral blood flow Circulation for the exchange of nutrients
and waste products between the blood and nervous tissue
Cerebrospinal Fluid: Choroid Plexuses
Special capillary networks in certain places in the ventricular walls
Ependymal cells Fluid from plasma
passes through the ependymal cells at choroid plexuses
cells have ion pumps modify CSF regulate and maintain
the blood-brain barrier Protect the brain from
harmful substances in the blood
Blood-Brain Barrier Penetration of molecules from the blood
into the brain is regulated by: tight junctions between capillary cells thick basal lamina (connective tissue layer) astrocytes pressed against capillaries
The barrier is a selective membrane some substances, particularly if lipid-soluble,
pass easily from blood to the brain (water, glucose, O2, CO2, alcohol, caffeine, nicotine, heroin, most anesthetics)
most charged ions do not pass easily proteins and most antibiotics do not pass at
all
Blood-Brain Barrier
Permeability is variable depending on the site choroid plexus – CSF production vomiting center in brain stem - monitors the
blood for toxic molecules and poisons hypothalamus
has no blood-brain barrier monitors blood composition for water balance,
temperature, pH, osmolarity and many other homeostatic metabolic functions
Homeostatic Imbalances of the Brain
Traumatic Brain Injuries concussion
a blow to head the skull stops, but the brain keeps moving the brain bounces off the inside of the skull
possibly, there is no visible external damage a variety of cognitive problems follow
contusion breaks in small vessels, some bleeding, visible
bruising effect depends on the location
laceration tearing of the brain knife and gunshot wounds, other major traumas
Homeostatic Imbalances of the Brain
Traumatic Brain Injuries (cont.) epidural or subdural or subarachnoid hemorrhage
bleeding from ruptured vessels into that space a person is normal immediately after the injury, but
deteriorates as the bleeding continues hemorrhage increases intracranial pressure effects vary with the location of the hematoma surgical intervention
drill holes remove clots install drainage tubes
subarachnoid hemorrhage
Homeostatic Imbalances of the Brain
Cerebrovascular Accidents (CVA’s) Stroke
third leading cause of death in the United States ischemia anemia caused by reduced or blocked blood
flow) hemorrhages and blood clots increase intracranial
pressure brain tissue dies (infarct) risk factors: high blood pressure, high cholesterol,
heart disease, narrowed carotid arteries, diabetes, smoking, obesity, excessive alcohol intake
Transient ischemic attack (TIA/ministroke) may last minutes flow is reduced and brain tissue suffers temporarily blood flow is re-established
Homeostatic Imbalances of the Brain
Degenerative brain diseases Alzheimer’s disease
about 11% of population over age 65, 4 million people suffer, 100,000 die annually; hereditary component
widespread cognitive deficits - (short term) memory loss, shortened attention span and disorientation, loss of language skills
death from secondary causes, e.g., due to being bedridden
diagnosis is difficult since there is no definitive test; only after death can it be confirmed by autopsy:
significant loss of neurons in specific regions abnormal proteins are deposited in brain tissue tangled nerve masses
generally, the damage is limited to the cerebral cortex
Alzheimer’sDisease
Homeostatic Imbalances of Brain
Degenerative brain diseases (cont.) Parkinson’s disease
progressive disorder of the CNS which typically affects victims at age 60 or so
cause(s) unknown; hereditary component sometimes
characterized by degeneration of dopamine-releasing neurons
characterized by tremor (shaking) and rigidity (continuous contraction)
motor performance impaired by bradykinesia (slow motion) and hypokinesia (reduced range of motion)
treatments try to increase dopamine and decrease ACh with therapeutic drugs or by experimental implantation of fetal brain cells
Homeostatic Imbalances of Brain
Traumatic brain diseases Cerebral Palsy
damage to motor areas of brain during fetal life, at birth, or during infancy, usually transient O2
deprivation poor control and coordination of voluntary muscle
activities but usually little impact on intellect irreversible, but not progressive 70% of victims appear to be mentally retarded
often due to their inability to hear or speak well generally, they are more aware and understanding of their
situation and surroundings than they appear
Concussion, Contusion, Sudural or Subarachnoid Hemorrhage, Cerebral Edema, CVAs
Not Everybody is an Einstein!
The Spinal Cord Spinal cord is located within the vertebral
column Passes through the vertebral foramina
Anatomy and Protection of Spinal Cord Bone of vertebral arch CSF Spinal meninges
dura mater, arachnoid, pia mater
meninges cover spinal cord and spinal nerves
epidural space space between the dura
mater and wall of the vertebral canal
filled with adipose and loose connective tissue
nerves exit through intervertebral foramina
External Spinal Cord Anatomy Roughly cylindrical but slightly
flattened dorsi-ventrally
From foramen magnum to second lumbar vertebra (L2)
About 2 cm wide and 42-45 cm long
Cervical enlargementCervical enlargement and lumbar enlargementlumbar enlargement are conspicuous cervical enlargement - nerves
for upper extremities lumbar enlargement - nerves for
lower extremities
External Anatomy (cont.)
Spinal cord tapers, ends in the conus medullarisconus medullaris between L1 and L2
Filum terminaleFilum terminale (pia mater) extends from the conus to attach the spinal cord to the coccyx
Some nerves exit the vertebral column below the level of their exit from the spinal cord
Cauda equinaCauda equina – “horse's tail” at end of the cord are the last few pairs of spinal nerves
Cross-Sectional Anatomy of the Spinal Cord
“H” shaped gray matter – “butterfly” surrounded by white matter
Anterior median fissure Posterior medial sulcus Gray commissure forms
the cross bar of the 'H' Central canal
small space in middle of gray commissure
extends length of spinal cord
at superior end continuous with the 4th ventricle
contains CSF
Cross-Sectional Anatomy of the Spinal Cord
Anterior to the gray commissure is the anterior white commissure
The gray matter of the spinal cord is divided into horns
closer to front are the anterior (ventral) gray horns
closer to back are the posterior (dorsal) gray horns
lateral gray horns between anterior and
posterior horns present only in thoracic, upper
lumbar, and sacral segments gray matter also has some
named nuclei
dorsal
lateral
ventral
Gray Matter Anterior horn – visceral & somatic motor neurons Ventral root - efferent (motor) nerves to skeletal
muscles and to the visceral organs (effectors) Posterior horn – somatic & visceral sensory
neurons Dorsal root - afferent nerves from skin, skeletal
muscles, connective tissues, visceral organs
Gray matter In Chapter 13 we will examine the basic
connections in the spinal cord, such as the reflex arc
The simplest connection is a two cell reflex connecting a sensory neuron directly to a motor neuron
More complicated reflexes have one or more intervening interneurons
White Matter of the Spinal Cord Conduction tracts in the spinal
cord Named by
where each is coming from where each is going to
White Matter of the Spinal Cord Fasciculi cuneatus and gracilis - fine touch and pressure Lateral and anterior spinothalamic tracts - pain,
temperature, deep pressure and coarse touch Two paths for similar functions
The Spinal Cord
In Chapter 13 we will examine the routes and means by which the Central Nervous Connection interacts with and controls the rest of the body.
Those routes form the Peripheral Nervous System.
End Chapter 12
Some slides of specific spinal cord tracts appearafter this slide. You are not responsible for those specific tracts for the exam.
Specific and Posterior Spinocerebellar Tracts
• Specific ascending pathways within the fasciculus gracilis and fasciculus cuneatus tracts, and their continuation in the medial lemniscal tracts
• The posterior spinocerebellar tract
Nonspecific Ascending Pathway
Nonspecific pathway for pain, temperature, and crude touch within the lateral spinothalamic tract
The Direct (Pyramidal) System Direct pathways originate
with the pyramidal neurons in the precentral gyri
Impulses are sent through the corticospinal tracts and synapse in the anterior horn
Stimulation of anterior horn neurons activates skeletal muscles
Parts of the direct pathway, called corticobulbar tracts, innervate cranial nerve nuclei
The direct pathway regulates fast and fine (skilled) movements
Indirect (Extrapyramidal) System Includes the brain stem, motor
nuclei, and all motor pathways not part of the pyramidal system
This system includes the rubrospinal, vestibulospinal, reticulospinal, and tectospinal tracts
These motor pathways are complex and multisynaptic, and regulate: Axial muscles that maintain
balance and posture Muscles controlling coarse
movements of the proximal portions of limbs
Head, neck, and eye movement
Extrapyramidal (Multineuronal) Pathways
Reticulospinal tracts – maintain balance
Rubrospinal tracts – control flexor muscles
Superior colliculi and tectospinal tracts mediate head movements
End Chapter 12