The nervous system unraveled!
Three major divisions of the brain
The Forebrain
1. Cerebral hemispheres
2. Thalamus
3. Hypothalamus
The convoluted outer surface
“cerebral cortex”
Only millimeters thick but lots of surface area
Convolutions related to intelligence
Composed mostly of unmyelinated
cell bodies (nuclei)… gray matter
“neocortex”
Composed mostly of myelinated
axons (tracts)… white matter
Gyrus = a ridge
Small groove = suclus
large groove = fissure
Side note: most sensory & motor information from the right side of the body goes to the
left hemisphere and vs. versa (i.e. contralateral connection)
Ipsilateral = when info from one side of the body goes to the same hemisphere
The cerebral hemispheres
Postcentral gyrus: sensory motor
Precentral gyrus: motor cortex
Prefrontal cortex
Broca’s area
Auditory cortex
Wernicke’s area
Motor Cortex
The amount of space
allotted to a given
body part is
proportional to the
degree of fine motor
control we have over it
Somatosensory Cortex
The amount of space
allotted to a given body
part is directly
proportional to how
sensitive that body
part is
Frontal cortex
prefrontal cortex: higher cognitive functions
precentral gyrus: motor control
Broca’s area: motor movements in speech &
grammatical structure
Parietal Lobe
Postcentral gyrus: somatosensory
Association areas: to further combine & analyze
info from other areas
In general, associates visual with sensory input
used to identify objects and locate them in space
lesions can produce neglect
Temporal Lobes
hearing, vision, memory & language
Wernicke’s Area: understanding language &
speaking meaningfully
Occipital Lobes
processes visual information
the visual cortex contains a map of your
visual field
The Diencephalon of the Forebrain
Thalamus: receives info from all the sense except smell; sends
this info to cortical projection areas & other cortical sites
Also alerts the rest of the brain to incoming stilumi
…functions much like the old switchboard operators…
Hypothalamus: motivated behaviors (The 4 F’s), endocrine control
Midbrain
&
Secondary roles in vision, hearing
movement
Hindbrain
Circadian rhythms
Includes the pons, medulla,
reticular formation, & the
cerebellum
Part of the “reward center”
(see next slide)
The Brainstem
Also includes thalamus & hypothalamus
(not shown here)
Sleep & arousal
Breathing, HR
Arousal +
A filter
Movement & some types of
learning
The Spinal Cord
About as thick as a thumb and 17 inches
long, your spinal cord is actually an
extension of your brain. Through your
spinal cord, your brain can keep in
constant contact with your whole body.
Sensory nerves:
Nerves located
in the skin and
other sensory
organs. These
nerves receive
stimuli and
send impulses
to your spinal
cord and brain.
Motor nerves: Nerves that carry impulses from your brain
and spinal cord to your muscles, glands and other organs.
Spinal Cord Functional Circuitry
Sensory information
enters the cord via the
dorsal roots
Motor signals exit the
cord through the ventral
roots
In a simple reflex, neural
impulses carrying sensory info
synapse on motor neurons
without communicating
with the brain first.
Information ascends and
descends the cord via
axons in the “white
matter” (outer) part of
the cord
Protecting the Brain and Spinal Cord
1. Meninges: 3 layers of protective membranes
2. CSF: cushions neural tissue
3. Blood Brain Barrier: heterogeneous & selectively permeable
A glial cell
The Peripheral Nervous System
Includes all the nerves (except those in the spinal cord & brain)
which innervate the rest of the body: these are the cranial nerves
and the spinal nerves
The PNS is divided into two branches: somatic & autonomic
The somatic NS takes sensory information from the outside
world and brings it to the spinal cord & brain for processing
Once processed, the motor nerves of the somatic NS allow
you to interact with the outside world
Activity in the Parasympathetic & Sympathetic NS is NOT
Heads or Tails
How does the NS grow & develop?
Once the neural tube forms, neural development proceeds in
4 stages
Stage 1: cell proliferation
250 000 new cells every minute!
Takes place in the ventricular zone: the area surrounding the
hollow tube – which later becomes the ventricles & central canal
Stage 2: cell migration
Using specialized radial glial cells
as scaffolding, neurons move out
to their ultimate destinations.
The functional role a neuron will assume depends on the time
of its birth and its location
Fetal brain cells are functionally flexible: fetal cells transplanted
into adult brains will assume the role of the tissue they’re
transplanted into.
Ethical Considerations?
Stage 3: circuit formation
Axons form growth cones – using chemical & molecular signposts,
the cones are pushed and pulled toward their final destination,
forming new synaptic connections
Stage 4: circuit pruning
Elimination of excess synapses, death to lost or late arrivals
Hebb’s Postulate
“When an axon of cell A is near enough to excite cell
B or repeatedly or consistently takes part in firing it,
some growth or metabolic change takes place in one
or both cells such that A's efficiency, as one of the
cells firing B, is increased” (D.O. Hebb, 1949)
Myelination completes NS system development
Begins late in the 3rd trimester & continues into adulthood
Development of myelinated axons in the central nervous
system, as seen by electron microscopy in transverse section.
(a) newborn. Occasional axons are loosely ensheathed by
primitive, undifferentiated glial cells, g, but myelin is not yet
present. × 40, 000. (b) Adult. The axons are fully myelinated.
× 20, 000.
Experience Modifies the Nervous System
Reorganization synaptic connections change and as a result,
the function of the affected brain area change
Examples:
the visual cortex of blind people is activated
by touch
The cortical area representing the index finger
(brail finger) is larger relative to the areas
representing the remaining fingers
Rats reared in an enriched environment show more synapses,
dendritic branches, and heavier brains compared to rats reared
in standard laboratory conditions
Lab
reared
Enriched
environment
Damage & Recovery of the Nervous System
There is neural REGENERATION in the PNS: the growth of
severed axons
Myelination by glial cells provide guide tubes for axons to
grow through
In the CNS, growth is inhibited:
chemical & molecular cues to guide growth are gone
scar tissue from glial cells blocks growth
glial cells produce growth inhibitors
immune cells inhibit growth
Neurogenesis the growth of new neurons does occur in the CNS
Limited to the hippocampus and near the lateral ventricles
Despite this, recovery of function does occur – how?
swelling goes down & dead neurons are cleared away
Compensation occurs as healthy tissue takes over the
function of damaged tissue
this occurs as a result of reorganization
All of this is testimony that the brain is highly PLASTIC
Current Directions in CNS Repair
• using neuron growth enhancers
• counteracting the forces that inhibit regrowth
• providing guide tubes or scaffolding for sprouting axons to
follow
• tissue transplants using embryonic stem cells
stem cell research in the US is restricted for ethical
considerations as these cells are taken from aborted
fetuses ….
For those interested in the history and current state of research
on stem cells, visit:
http://en.wikipedia.org/wiki/Stem_cell#Controversy_surroundi
ng_human_embryonic_stem_cell_research
CLONES: Australia's government issued its
first licence allowing creation of cloned
human embryos to obtain embryonic stem
cells. Frozen embryos in the lab, and inset, an
early stage embryo called a blastocyst.