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General Organization of the
Nervous System
Parts
• Central Nervous System: Brain and
spinal cord
• Peripheral Nervous System: peripheral
nerves, autonomic ganglia and dorsal root
ganglia of the spinal cord.
The Peripheral Nervous System
• Somatovisceral sensory pathways
• Somatic motor pathways to skeletal
muscle
• Autonomic motor pathways to visceral
organs, skin and circulatory system.
We are going to use the somatovisceral pathway as our
initial way of looking at the peripheral NS
In order to process sensory information, the
CNS must have answers to 3 questions:
• What? Sensory receptors respond to specific
stimulus modalities (touch, pressure, vibration,
pain, temperature, light, chemicals, etc.
• Where? CNS is organized as a set of maps
showing the location of receptors – somatotopic
organization. Each receptor, and each higher
order neuron, has a discrete receptive field on
the body surface, etc.
• How much? In the neural code, stimulus
intensity is encoded in action potential frequency
Basic Principles of Sensory Physiology
• 1. a threshold level of intensity is required to
initiate a sensory signal
• Steps:
• Stimulus causes an intensity-graded receptor
(generator) potential within the sensory cell’s
input zone
• Receptor potential spreads to initial segment of
axon by decremental conduction
• Depending on the intensity of the stimulus, RP
initiates no AP, 1 AP, or a burst or train of APs at
the initial segment
2. Many receptors show adaptation: a
decrease in responsiveness with continued
stimulation
Adaptation biases the receptor, making it more
responsive to change in intensity versus
steady unchanging stimuli.
3. The relationship between stimulus intensity
and action potential frequency may be linear
or logarithmic or some mixture of the two.
Logarithmic responses are typical of receptors
that address large variations in stimulus
intensity
The somatovisceral sensory system
• Processes information from the body
surface, musculoskeletal system and body
interior.
5 types of Cutaneous Receptors
• Merkel touch domes
• Ruffini’s corpuscles
• Steady pressure
• Steady pressure to
low freq. vibration
• Meissner’s corpuscles • 30-40 Hz vibration
• Hair follicle receptors • Hair bending
• Pacinian corpuscles
• High frequency
vibration (50-500 Hz)
Receptive Field
= area on the
body surface
within which an
appropriate
stimulus evokes
a response.
Notice the large
difference in the
sizes of the
receptive fields of
the 4 types of
cutaneous
receptors shown
here. Arrows
indicate directionspecificity
Other somatovisceral receptors
• Thermoreceptors (2-3 types with different
temp ranges)
• Chemoreceptors (O2, CO2, osmolarity,
etc)
• Nociceptors (pain, discomfort)
• Baroreceptors (blood pressure)
• Muscle spindles (contain rapidly adapting
and slowly adapting stretch receptors)
• Golgi tendon organs
Organization of the Somatic Sensory System
• Embryonic body segments (somites) are each
served by a pair of spinal nerves from the
corresponding spinal segment, or by a pair of
cranial nerves from the corresponding head
somite.
• Each somite generates a dermatome that gives
rise to skin covering and a myotome that gives
rise to muscle.
• Receptors that arise in the somite project to the
corresponding spinal segment and its nearest
neighbors.
•The outcome of the body’s
segmental organization is that
each of the 30 pairs of spinal
nerve roots serves an adjacent
stripe of skin surface and a roughly
corresponding block of skeletal
muscle.
•A similar process operates for the
3 branches of the trigeminal nerve
versus the face.
The somatic sensory pathways to the spinal
cord
In the spinal cord, axons (white
matter) are superficial to the
cell bodies (gray matter).
Spinal nerves are mixed
nerves containing both
afferent and efferent
axons. The afferents
and efferents get
separated near the cord
so that the afferents
pass through the dorsal
root, where their cell
bodies are, and the
efferents pass out from
the cord through the
ventral horn.
Spinal afferents are divisible into 4 groups
based on size and myelination
• Group I: 13-20 mu diameter, myelinated, conduct. velocity 70-110
m/sec, carry muscle length information
• Group II 6-12 mu diameter, myelinated, 25-70 m/sec conduction
velocity, carry info from muscle, tendon receptors; Ruffini and
Pacinian corpuscles
• Group III 1-5 mu diameter, myelinated, 3.5-20 m/sec conduction
velocity, slowly adapting touch/pressure receptors; “fast”
nociceptors.
• Group IV 1mu or less in in diameter, not myelinated, <1 m/sec
conduction; “slow” pain, temperature, itch, tickle, etc.
Some authorities use a different form of
categorization based on conduction velocity
• Aβ: large, myelinated: mechanical stimuli
• Aδ: small, myelinated: cold, fast pain,
mechanical stimuli
• C: small, unmyelinated: slow pain,
temperature, mechanical stimuli
Spinal nerves are mixed nerves that contain both afferent and efferent axons. At
the spinal cord most afferent axons split off dorsally and pass through the
dorsal root ganglion where they connect with their cell bodies. Motor axons
turn ventrally and enter the spinal cord through the ventral horn of the spinal
segment. Note that unlike the brain, where the cell bodies are on the surface,
the spinal gray matter is in the center and the white matter consisting of
axons is on the outside.
Generally, each half-brain controls the
contralateral body half
• Spinal pathways decussate – a decussation occurs
when a fiber tract crosses the midline while ascending or
descending
• A commissure is a connection between homologous
structures on the two sides of the brain – so the corpus
callosum is a bundle of fibers that represents the main
route of communication between the two halves of the
cortex. If this connection is severed, the patient can be
shown to possess two minds that can communicate only
by watching each other’s actions.
Two main spinal pathways carry
somatovisceral sensations to the brain
• 1. The dorsal column pathway: ascends
on ipsilateral side of the spinal cord;
decussates in the medulla of the
brainstem; passes through thalamus to the
contralateral somatosensory cortex. Good
somatotopic organization is preserved
throughout this pathway.
Note that the afferents in the dorsal
column pathway don’t synapse on
interneurons until they reach the
dorsal column nuclei of the
medulla. These interneurons then
decussate to the contralateral
medulla and ascend to the
thalamus, where they synapse on
3rd order cells.
Sensory modalities carried in
dorsal column: cutaneous
vibration, pressure, joint position,
muscle stretch; visceral distention.
Note that higher
order cells in the
dorsal column
system typically
have annular or
bar-shaped
receptive fields
characterized by
central excitatory
zones surrounded
by inhibitory zones.
This arrangement is
called lateral
inhibition. It
sharpens the
system’s ability to
detect stimulus
location.
In lateral inhibition,
more intense
stimuli can cancel
out the effects of
less intense stimuli
that arrive nearby.
This biases the
map of stimulus
location so that
edges of the
stimulus show up
much more than
areas of no
intensity contrast.
The pathways that
mediate pain and
temperature sense
enter anterolateral
fiber tracts.
These pathways:
Synapse on 2nd order
neurons in the
ipsilateral substantia
gelatinosa of the
spinal gray.
The 2nd order neurons
decussate in the same
or adjacent segments
of the spinal cord and
thus ascend to the
brain on the
contralateral side of
the cord.
As pain information
ascends through the
medulla, it provides
important excitatory input to
the brain’s reticular
activating system, which
has widespread
connections throughout the
brain and controls alertness
and awareness.
Positional information is more highly conserved in
the dorsal column system than in the anterolateral
system
As axons enter the dorsal columns, the ones from
each spinal segment tend to stay together, so that
positional information is preserved and the
somatosensory cortex has an accurate map of the
input from those receptors.
In contrast, the axons that enter anterolateral
pathways do not preserve positional information nearly
as well, so sensations of pain and irritation tend to be
more diffuse and poorly localized.
Information carried in ventrolateral tracts is filtered
as it ascends the spinal cord.
• Centrifugal (descending) pathways synapse on
ascending neurons, releasing neuromodulators
that inhibit release of excitatory transmitters in
the ascending pathways – frequently these are
endogenous opioids, accounting for the
analgesic effect of opioid drugs and for stressinduced analgesia.
Spinal anatomy has
diagnostic
significance:
Unilateral lesions of
the spinal cord will
interrupt specific
types of information
from each side of
the body inferior to
the lesion.
Identification of the
specific sensory
deficits can be used
to localize the spinal
lesion.
The Brain and its functional
subdivisions
Functional specialization of brain regions
• Cerebrum
– Cerebral cortex – perception, control of voluntary movement,
consciousness
– Basal nuclei – control of skeletal muscles
– Limbic system – emotion and memory consolidation
• Diencephalon –
– Thalamus – relay station for sensory and motor information
– Hypothalamus – homeostasis and behavioral drives
• Cerebellum – coordinated body movements
• Brainstem
– Midbrain – ocular reflexes
– Pons-medulla – involuntary functions including breathing,
cardiovascular control, control of thoracic-abdominal organs
The cortex is divided into 4 lobes
In some – but not all mammals,
the cortex is thrown into folds
called gyri (gyrus) separated by
crevices called sulci (sulcus).
Sagittal
section
reveals
some
internal
structures
of the brain
Different brain regions have
different evolutionary histories:
The brainstem and cerebellum
of mammals correspond fairly
closely to those of reptiles.
The limbic system – sometimes
called the rhinencephalon or
smelling brain – attained its
present state relatively early in
evolution of the mammalian line.
Cortical structures are present to
some extent in all mammals, but
the volume and complexity of
the cortex reaches a maximum
in primates, in which the frontal
parts of the cortex are referred
to as neocortex because of their
evolutionary newness – the
olfactory cortex is older and has
closer affinities with the limbic
system.
The cortex typically has a columnar
organization
• For example, each of the whiskers
(vibrissae) on the muzzle of a rat maps to
a corresponding vertical cortical column on
the face part of the map of the
contralateral sensory cortex. Within this
column are neurons that are activated
specifically by vibrations of different
frequencies, or by directional bending of
the vibrissa.
This figure shows how receptors in the face of a rat map to neurons in the
brainstem, the thalamus, and the somatosensory cortex. Because the positional
arrangement of the afferent neurons closely follows the arrangement of receptive
fields, the cortex has a very good representation of the face. This is another
example of somatotopic organization.
Cortical organization is plastic
(sometimes)
• Plasticity refers to the power of the environment
to shape the brain’s organization
• Surgical ablation of individual vibrissae in a fetal
animal results in the loss of the corresponding
cortical column
• Even in adulthood, loss of an appendage results
in contraction of the corresponding part of the
sensory cortical map, and increased use of a
particular appendage results in an enlargement
of its representation in the sensory cortex.
The cortex has a laminar organization
• The cortex, an evolutionarily new part of the brain, is like
a 6-story building with specific neuron types located on
each floor. In the sensory cortex, sensory information
arrives from the thalamus at the ground floor, where it is
sorted out into submodalities. It then passes upward in
columns that are each devoted to a single submodality,
and is analyzed at increasingly greater detail as it
passes to higher floors. Within this column are pyramidal
cells, the main carriers of excitatory output from the
cortex to other parts of the CNS. Synaptic inputs from
interneurons in the cortical layers determine the ultimate
output carried by the pyramidal cells.
Cortical neurons have complex input segments
On the left, some typical pyramidal
cells from different cortical regions.
The purple bodies are the nuclei of
other cells that didn’t get stained with
the silver Golgi stain: some are
neurons, some are glia.
The primary
somatosensory cortex
occupies a band
posterior to the
Fissure of Rolando
and superior to the
Sylvian Gyrus.
Keep in mind that,
because all
somatosensory
pathways arising
below the neck
decussate at some
point, the left
somatosensory cortex
receives input from
the contralateral side
of the body and viceversa.
The somatosensory cortex is organized as a
map of the body surface
The map, called
the somatosensory
homunculus, is
distorted because
areas of the body
surface
characterized by
high receptor
density are served
by more cortical
neurons
See the next slide for a
more up-to-date version
It turns out that common textbook versions of the somatosensory
homunculus are characterized by some amount of prudery
Each sensory modality has primary and
secondary (association) cortical regions
The primary motor cortex also
has a somatotopic organization
Notice the large amount
of motor cortex that is
devoted to the hand and
face – these are the
parts of the body over
which we have the most
sophisticated control.
The situation would be
different in quadrupeds
or in animals in which
facial expressions are
less important.
There are additional,
specialized motor cortical areas
Many cortical functions are lateralized
Wernicke’s A. and Broca’s
A. are the language areas
of the cortex
Damage to either one of these
areas or to their connections
leads to some form of aphasia.
In the case of damage to
Wernicke’s Area, the result is
fluent but largely meaningless
language. Such patients also
have difficulty interpreting
language. Damage to Broca’s
area leads to halting, but
logically intact speech.
Language interpretation is not
affected.
Interior Structures of the Brain
• Limbic System
• Thalamus
• Basal Nuclei – we will learn about these in the
context of somatic motor control
• Hypothalamus
The limbic system consists of all of those structures shown in blue. In
particular, the hippocampus is involved in formation and recovery of
memory traces. In general, the limbic system plays a major role in
emotional states such as anger and fear.
The hypothalamus is responsible for many
homeostatic functions
•
•
•
•
•
•
•
•
•
•
Thermoregulation
Cardiovascular regulation
respiration
Body fluid volume/composition
Growth
Sexual differentiation and function
Motivation (“pleasure centers”)
Rage
Food intake and nutrient balance
Circadian rhythm
The thalamus is a relay station for sensory
pathways leading to the cortex
This is a repeat
of an earlier
slide
Cranial Nerves
• connect to specialized sensory structures
(eye, nose, ear, gustatory receptors): I=
olfactory, II=optic, VIII=vestibuloauditory,
gustatory = VII, IX, X)
• Carry parasympathetic efferents to
thorax/abdomen and visceral afferents to
brainstem (X = vagus)