Download Central Nervous System Anatomy and Organization The Brain Has

Central Nervous System
Anatomy and Organization
The Brain Has Been Called a 3 Pound Universe



The brain weighs about 3 lbs (~1400 gm)
All sensation and consciousness originates in the brain
We know a lot about the anatomy, chemistry & electricity of the brain, but almost
nothing about what sensations are
The Nervous System Has Peripheral and Central Units





The central nervous system (CNS) is the brain and spinal column
The peripheral nervous system (PNS) consists of nerves outside of the CNS
There are 31 pairs of spinal nerves (mixed motor & sensory)
There are 12 pairs of cranial nerves (some are pure sensory, but most are mixed)
The pattern of innervation plotted on the skin is called a dermatome
The Central Nervous System Has Several Patterns of Organization


Sensory/motor:
o Sensory nerves enter the spinal cord by the dorsal root, their cell bodies
are in ganglia outside the spinal cord (dorsal root ganglia)
 Sensory nerves, which go to the brain, are referred to as afferent
o Motor nerves leave the spinal cord by the ventral root
 Motor nerves, which come from the brain, are referred to as
efferent
White matter/gray matter:
o White matter consists of the myelinated axons of nerves, usually going up
and down
o Gray matter contains the cell bodies (containing the nucleus) , dendrites
with synapses and blood vessels
o In both the spinal cord and brain cell bodies are clustered into ganglia and
nuclei
1

Decussations & commisures:
o There is a tendency for sensory and motor nerves and association fibers to
cross from one side of the brain to the other
o Most motor nerves cross in the medulla oblongata; a few do not cross
o Most sensory nerves also cross in the medulla (touch, pressure,
proprioception); others cross in the spinal cord (pain, temperature)
o The corpus callosum is a major tract connecting the left and right
hemispheres of the brain
Spinal Cord Injuries Disconnect Parts of the Body from the Brain



Cutting the spinal cord "disconnects" the body from the brain below the cut
o Sensory impulses from areas below cut cannot reach the brain: loss of
sensation
o Motor impulses from the brain cannot reach muscles whose nerves are
below the cut: voluntary control of these muscles is lost (paralysis)
After the spinal cord recovers from the trauma of the wound, reflex contraction of
muscles below the cut reappears
o Reflex activity may be more vigorous than in a normal person
o Nerves from brain control the sensitivity of spinal reflexes
If the spinal cord is cut above cervical nerves 3, 4 & 5 respiration will be lost
o The respiratory rhythm is generated in the brain and is sent to the
diaphragm through the phrenic nerve
o Loss of the phrenic nerve will cause death unless artificial respiration is
used
Glial Cells Support the Brain in Many Ways


About 50% of the weight of the brain is glia cells (several types)
Glial cells do not conduct: support brain in other ways
2


Some glial cells produce the myelin sheaths of nerves (oligodendroglia, Schwann
cells)
Other glial cells secrete cerebrospinal fluid, defend against bacteria and regulate
ions
The Brain is Immersed in Cerebrospinal Fluid





The brain is a hollow tube with bulges: has 4 interconnected fluid-filled reservoirs
(ventricles) filled with cerebrospinal fluid (CSF)
Total volume ~150 mL
CSF circulates: secreted into ventricles by the choroid plexus (about 500
mL/day)-> passes from 4th ventricle into subarachnoid space -> then absorbed
into veins
Some CSF passes into central canal of spinal cord
Brain floats in CSF; acts as a cushion and reduces injury
There is a Special Barrier Between the Brain and the Blood


There is a "blood brain barrier"- brain capillaries are tighter and less permeable
than those in the rest of the body; protects brain from many chemicals and
bacteria
Hydrophobic compounds cross the blood brain barrier more readily than
hydrophilic ones
o Example: people with Parkinson's disease have low levels of the
neurotransmitter, dopamine (DOPA)
o DOPA is too hydrophilic to cross the blood brain barrier
o Parkinson's patients are treated with levadopa (L-DOPA) instead
o L-DOPA is hydrophobic and crosses the blood brain barrier
o Brain later converts L-DOPA into DOPA
The Brain is Covered by Tough Meninges and Protected by Bone


Three tough membranes cover the brain:
o 1) dura mater: outermost- firmly attached to skull
o 2) arachnoid: middle layer
o 3) pia mater: bottom layer, firmly attached to brain, contains many blood
vessels
Inflammation of brain meninges = meningitis
The Brain Has a Hierarchical Organization



Upper centers control lower centers
Cortex tends to dominate the spinal cord
But many basic life support functions (i.e., respiration, blood pressure) are under
control of "lower" centers
The Brain Uses Energy at a Rate of 10 Watts
3



The brain functions at about 10 watts - this is equivalent to a "dim bulb"
But 10 watts is a fairly high proportion of the total body energy consumption rate
of 80 watts
o Brain is 2% of body weight but uses 12% of body energy
o 14% of the blood flow goes to the brain
o The blood flow per kilogram is equal to that of a muscle doing heavy
exercise
Thinking does not significantly raise the energy consumption of the whole brain,
but circulation increases to specific areas being used (this can be seen with PET
scans); presumably these areas have higher levels of energy use
Lecture 25: Central Nervous System
Special Centers
Fissures Divide the Brain Into Several Distinct Regions





This diagram is modified from one in The Sourcebook of Medical Illustration,
edited by Peter Cull (Park Ridge, NJ: Parthenon, 1989).
o Most of the midbrain is covered by the other lobes and cannot be seen
Medulla, pons, midbrain = brain stem
The cortex is split by fissures (sulci: singular = sulcus) into different regions
The bulges between sulci are called gyri (singular = gyrus)
Two important landmarks on the cortex:
o Central sulcus: separates frontal from parietal lobe
4
o

Lateral sulcus (Sylvanian fissure): separates temporal lobe from parietal &
frontal lobes
To learn brain anatomy & function mark these areas on blank copies of brain
drawings.
The Medulla Oblongata Has Centers for Basic Life Support


The medulla oblongata is the section of brain stem just above the spinal cord
(sometimes called the myelencephalon)
Major features:
o Pyramids: bulges of ventral surface- major site for crossing of both
sensory and motor nerves
o Centers for regulation of blood pressure and breathing are found here
o Swallowing and vomiting centers
o Olives: nuclei which send sensory information to the cerebellum
o Reticular formation (continues to midbrain)- involved in consciousness,
attention, sleep
o Four cranial nerves (9-12) leave the medulla
The Pons Connects the Cerebellum to the Brain Stem


The pons is the region with a prominent ventral bulge, just above the medulla
Pons means bridge- connected to R and L cerebellum by fiber tracts in peduncles
o Contains many nuclei, including the raphe nuclei and locus coeruleus (part
of reticular formation)
 Raphe nuclei are involved in control of pain sensation
o Reticular formation of pons is involved is sleep (both rapid-eye-movement
and slow wave types)
o Involved in control of breathing
o Most of the 4th ventricle is between the pons and cerebellum
o Four cranial nerves (5-8) leave the pons.
The Cerebellum Coordinates Body Movements


The cerebellum is a large bulge on posterior side of brain- name means "little
brain"
o Required for:
 posture
 balance
 smooth, coordinated movements (especially simultaneous
movements of different body parts)
o Cerebellar lesions cause clumsy movement, ataxia (unsteady gait),
overshooting of movements, poor balance
The cerebellum and the pons together are called the metencephalon
The Midbrain Has Centers for Optic and Auditory Reflexes
5


The midbrain (or mesencephalon) is just above the pons
Features:
o Dorsal side is the tectum (roof)
 Superior and inferior colliculi (corpora quadrigemini) are 4 bulges
easily seen on dorsal surface
 The superior colliculus is involved in visual reflexes (i.e., pupil
reflex, tracking objects)
 The inferior colliculus is involved in auditory sensation & reflexes
(loud sound -> startle reflex )
o Two cranial nerves (3-4) leave the midbrain (the optic, nerve 2, sends
fibers both to the superior colliculi and the thalamus)
o Cerebral aqueduct, connecting 3rd and 4th ventricles passes through
midbrain
o Other nuclei
 Periaqueductal gray matter- involved in pain control
 Red nucleus- involved in muscle reflexes
 Substantia nigra- connects to basal ganglia, important in
Parkinson's disease
The Diencephalon is the Thalamus, Hypothalamus & Pineal


Region just below the cortex is the diencephalon = thalamus + hypothalamus +
epithalamus
o Thalamus is a sensory relay center (means "bedroom")
o Hypothalamus (under the thalamus) is a center for control of body
functions
o Epithalamus (above the thalamus) is mainly the pineal gland: secretes
hormone melatonin- involved in sleep and attention
Most of 3rd ventricle is in the diencephalon
The Thalamus is a Sensory Relay Station



The right and left thalamus are a groups of nuclei controlling the flow of sensory
information
o Joined in the middle by the massa intermedia
o Form the lateral walls of the 3rd ventricle
o Form the floors of the lateral ventricles
All sensory information to the brain synapses in the thalamus before going to the
cortex (smell is a partial exception)
o Cranial nerve 2 (optic) connects directly to the thalamus
o Other cranial nerves send fibers to the thalamus from lower nuclei
Different uses of sensory information
o Sensory impulses are routed to the cortex for conscious sensation
o Impulses also sent to reflex centers such as the cerebellum and basal
ganglia for fine control of movement & balance
o Sensory input to the reticular formation is involved in consciousness
o Emotions are also affected by sensation
6
The Hypothalamus is the Major Center for Control of the Internal
Environment



The hypothalamus is a small, but very important region just below the thalamus
o Monitors the internal environment
o Causes responses that maintain homeostasis
 Nerve responses: mainly through the autonomic nervous system
 Hormonal responses: hypothalamus controls the pituitary gland
(master endocrine gland)
o Works with many other brain centers (i.e., medulla, autonomic nervous
system) and organs (i.e., pituitary)
Some of the homeostatic functions controlled by the hypothalamus
o Body temperature: controls blood supply to skin, sweating, shivering
o Blood pressure: controls heart rate & stroke volume, dilation of arteriolesconnections to medulla
o Blood glucose: mostly hormonal control
o Blood pH: controls respiratory rate, kidney functions
o Blood osmotic pressure: controls secretion of ADH, thirst mechanisms
Other functions:
o Sleep cycle: suprachiasmatic and preoptic nuclei
o Control of reproductive functions
 Sex drive
 Menstrual cycle: involves anterior pituitary
 Uterine contraction at parturition: involves posterior pituitary
 Milk release: involves posterior pituitary
o Light reflex: constriction of pupil by bright light
o Food drive: controls hunger- affects supply of body nutrients
o Controls pituitary gland: hormonal control of many body functions
The Cerebrum Serves Higher Mental Functions



The cerebrum is the largest and most complex part of the brain
o Controls most of the lower centers
o Many folds (form gyri & sulci) increase surface area
Corebrum is involved in consciousness, thinking, learning, emotions
Three general types of areas:
o Primary Sensory Areas
 Postcentral gyrus: skin sensations, taste
 Occipital lobe: vision (cranial nerve 2: optic)
 Temporal lobe: hearing (cranial nerve 8: auditory)
 Frontal lobe: smell (cranial nerve 1: olfactory)
o Motor Areas
 Precentral gyrus: primary motor area
 Frontal lobe: premotor area
 Broca's area: frontal lobe, speech motor
o Association Areas
7



Many regions: makes connections between sensations and motor
activity
The central and lateral sulci are important brain landmarks
o Central sulcus separates the frontal from the parietal lobes.
o Lateral sulcus separates the temporal lobe from those above it (frontal and
parietal).
Lobes are named for the skull bones which cover them: frontal, parietal, occipital,
temporal
The Corpus Callosum Connects the Right and Left Hemispheres of the Cortex


The corpus callosum is the major set of axons connecting the L and R brain
Tracts connecting the 2 sides of the brain are called commisures: other
commisures:
o Anterior commisure
o Massa intermedia: connects 2 sides of thalamus
Functions are Localized in Different Regions of the Cortex
This diagram is modified from one in The Sourcebook of Medical Illustration, edited by
Peter Cull (Park Ridge, NJ: Parthenon, 1989).
o
Sensory:
8

o
Somatic (skin & muscle) Senses:
Postcentral gyrus (parietal lobe). This area senses touch, pressure,
pain, hot, cold, & muscle position. The arrangement is upsidedown (head below, feet above) and is switched from left to right
(sensations from the right side of the body are received on the left
side of the cortex). Some areas (face, hands) have many more
sensory and motor nerves than others. A drawing of the body parts
represented in the postcentral gyrus, scaled to show area, is called
a homunculus .
 Vision:
Occipital lobe, mostly medial, in calcarine sulcus. Sensations from
the left visual field go to the right cortex and vice versa. Like other
sensations they are upside down. The visual cortex is very
complicated because the eye must take into account shape, color
and intensity.
 Taste:
Postcentral gyrus, close to lateral sulcus. The taste area is near the
area for tongue somatic senses.
 Smell:
The olfactory cortex is not as well known as some of the other
areas. Nerves for smell go to the olfactory bulb of the frontal
cortex, then to other frontal cortex centers- some nerve fibers go
directly to these centers, but others come from the thalamus like
most other sensory nerves
 Hearing:
Temporal lobe, near junction of the central and lateral sulci.
Mostly within the lateral sulcus. There is the usual crossover and
different tones go to different parts of the cortex. For complex
patterns of sounds like speech and music other areas of the cortex
become involved.
Motor:
 Primary Motor ( Muscle Control):
Precentral gyrus (frontal lobe). Arranged like a piano keyboard:
stimulation in this area will cause individual muscles to contract.
Like the sensory cortex, the arrangement is in the form of an
upside-down homunculus. The fibers are crossed- stimulation of
the right cortex will cause contraction of a muscle on the left side
of the body.
 Premotor (Patterns of Muscle Contraction):
Frontal lobe in front of precentral gyrus. This area helps set up
learned patterns of muscle contraction (think of walking or running
which involve many muscles contracting in just the right order).
 Speech-Muscle Control:
Broca's area, frontal lobe, usually in left hemisphere only. This
area helps control the patterns of muscle contraction necessary for
speech. Disorders in speaking are called aphasias.
9
o
Perception:
 Speech- Comprehension:
Wernicke's area, posterior end of temporal lobe, usually left
hemisphere only. Thinking about words also involves areas in the
frontal lobe.
 Speech- Sound/Vision Association:
Angular gyrus, , makes connections between sounds and shapes of
words
10