CNS sensory and motor (Lecture 2):
Brain anatomy: coronal slice
Grey matter: Outside layer. 2-
3 mm of neurons make up this
cerebral cortex and grow
specimens on the outside.
White matter: composed of
trillions of myelinated axons
connecting different areas.
Basal nuclei (ganglia): one of
the areas embedded inside the
cerebral hemispheres. Contains
a collection of cell bodies.
Involved in motor control.
Thalamus: relay station, also siting inside the white matter.
Limbic system: contains all areas mentioned above and other ones embedded inside and
involved in memory and emotions.
Ventricles: openings inside our brain that contain cerebral spinal fluid.
Divisions of the spinal cord (a part of the CNS):
Signals can go up and down
the spinal cord. We will later
talk about the “circuitry” of
the spinal cord.
Broken up into segments.
Each spinal segment
corresponds to spinal nerves
coming out on either side.
Spinal nerves carry motor
signals and somatosensory
Cervical nerves (top 8 pair):
Innervate neck, shoulders,
arms, and hands.
Thoracic nerves (next 12 pairs): Shoulders, chest, upper abdominal wall.
Lumbar nerves (next 5 pairs): located in the lower abdominal wall, hips, and legs.
Messier in terms of clear cut segments. Spinal nerves start running down before they exit.
Sacral nerves (next 5 pairs): Genitals and lower digestive track.
Coccygeal nerves (1 pair)
Total number of segments: 31.
What happens if you cut the spinal nerve at the top of the thoracic segment?
o Loss of somatosensation and motor control solely in a thin strip innervating the
upper shoulders and chest of the patient. Spinal cord anatomy:
Spinal segment: nerves and corresponding chunk of spinal cord in between them
Gray matter: cell bodies + circuitry
White matter: axons going up and down the spinal cord carrying information to the
brain and motor signals from the brain.
Central Canal: has CFF
Dorsal & Ventral horn: important anatomical parts of the gray matter
o Dorsal horn: where sensory information goes in
o Ventral horn: where motor information go out
Every spinal nerve, as it approaches the spinal cord, it divides.
o It has a dorsal root (back) – contains the sensory input
o And has a ventral root (front)
– consists of the motor output
If in a surgical situation, you cut the
dorsal root in the thoracic region, the
patient loses somatic sensation on a
thin strip corresponding to where that
thoracic segment innervates.
But the ventral root remains
unaffected. They both carry different
Where are the cell bodies for the
sensory information entering the
o They are located outside the
CNS: in the dorsal root ganglion (group of cell bodies)
Where are the cell bodies for the axons composing the ventral root?
o They are found in the ventral horn.
o In fact, motor neurons’ cell bodies are located inside the CNS.
Recall: a nerve is a collection of axons all bumbled together.
Another way the sensory and
motor information get in and out
of the brain, other than through
the spinal nerves, are the 12
cranial nerves, located underneath
Each cranial nerve has a different
shape and carries different
Important: 10/12 innervate the
brain stem, mid… pons and
medulla. So, 10 cranial nerves are
coming in and out of the brain
stem. 2 of them go to other places, and we’ll discuss them later.
o Olfactory nerve (coming from the retina and hits the thalamus): carrying our sense
of smell. Projects to many areas of the brain simultaneously.
o Optic nerve: carrying our visual inputs
5 cranial nerve (in the middle, to the right) that innervates somatosensation and motor
control of your face.
Early development of the nervous system:
We’ll talk shortly about neural development now.
We all start of as a fertilized egg (ovum), a single cell.
Over the course of the week, this cell divides into a ball of cells.
Week 1: Blastocyst (collection of cells that become “us”)
Over the week or so, this inner cell mass starts to divide and differentiate.
By week 3, it has a couple cavities. Actually, this ball of cells is no longer a ball. Starts
stretching out. There is a
big layer of cells in the
middle, called the
o If we cut where the
dashed line is and
take of the top part
and look down, we
see something that
looks like the last
image to the right:
the embryonic disk
with a neural
plate at the very
Development: the neural tube
It’s between weeks 3 and 4 that interesting things start to happen in our development.
o Neural plate: collection of cells in the embryonic disk called the ectoderm.
o Ectoderm: becomes the CNS and the PNS.
This is our focus, since we’re interested in the nervous system.
o Mesoderm: chunk of cells underneath the ectoderm
Becomes muscle, skin, bones.
o Endoderm: collection of cells at the very bottom.
Folds into a tube for our digestive system.
o In the neural plate/ectoderm, a groove starts to form, called the neural groove.
o The mesodermal layer becomes dura – the covering of the CNS.
o This groove pinches together forming the neural tube. Ectodermal cells have
lined the neural tube, becoming the CNS and part of the PNS. A few of these ectodermal cells have branched off to form the neural
crest, which becomes part of the PNS.
Our brain starts off as a tube in week 4.
o Vesicles/enlargements develop in the neural tube during week 4. These bulges
eventually become your brain.
9 MONTHS LATER… The neural tube starts to get longer, enlarges and finally
becomes the CNS.
o Forebrain: develops into the cerebral hemispheres and the thalamus.
o Midbrain: develops into the midbrain
o Hindbrain vesicle: develops into the remaining structures ->
o The rest of the neural tube becomes the spinal cord. o The cavity in the spinal cord becomes the central canal and the openings in these
bulges and other structures become part of the ventricles, which contain cerebral
spinal fluid. These are the openings inside the brain.
o Here concludes the simplified overview of neural development.
Ventricles contain 150 ml of cerebral spinal fluid (CSF).
There are 4 ventricles, which correspond to openings inside our brain.
CSF cushions the brain, provides the brain with important glucose and takes away some
Top and front view: we are looking at the lateral ventricles, a part of the green structure,
which are two large structures, extending in both hemispheres.
Lining the inside of the ventricles is
the choroid plexus, which produces
the cerebral spinal fluid (CSF).
Each ventricle has choroid plexus, but
the later ventricles, being the biggest
ones, produce most of the CSF.
If we cut the brain along the dashed
line (aka coronal cut/plane), we see the
top openings of the lateral ventricles
and a bit of their bottom tail.
In the middle is the third ventricle
(bottom, left), much smaller, sitting in
the middle of the thalamus. Shown in
the gross specimen (middle, left, on
the right image). Below the third ventricle is the fourth ventricle, sitting behind the pons and in front of
Cerebrospinal spinal fluid:
1. Formation: Produced by the choroid plexus (in the four ventricles, but mainly the two
lateral) at a rate of 500 ml/day.
1. The capacity in the brain is 150 ml, so the CSF has to get out of the ventricles and
back into the blood supply. We will see how this happens later.
1. Supports and cushions the CNS. Specific gravity of CSF and the brain are equal.
Brain “floats” in the CSF in our skull. Brain tissue is pretty soft.
2. Provides nourishment to the brain (in addition to the blood supply)
3. Removes metabolic waste through absorption at the arachnoid villi.
Composition: Sterile, colorless, acellular fluid that contains glucose.
Circulation: Passively diffuses between the ventricles (and not pumped through the head).
How do we get between the ventricles? CSF circulation
The CSF passes between the ventricles through what’s called the foramen (opening). rd
o Foramen of Monro: where the two lateral ventricles empty out into the 3
ventricle, which sits in the middle, inside the thalamus.
o From the third ventricle, we squeeze in through the midbrain, called the cerebral
aqueduct, and empty out into the fourth ventricle (sitting behind the pons and in
front of the cerebellum).
o From there, the CSF works its way down [diffuses passively down] the central
canal (opening in the middle of the spinal cord). CSF circulation:
CSF has to get out. It can’t produce in the ventricles, because it would start expanding
and would squish our brain against the skull.
What happens is that CSF enters the subarachnoid space, through three openings at the
bottom: Foramens of Lushka (two lateral, on the side) and Foramen of Magendie (in
Subarachnoid space is part of the lining/covering of the brain. It surrounds the brain and
the spinal cord.
At the top of our head, in the midline, the dura (one of the coverings of the brain) opens
up to form a sinus, which is connected to venous blood supply (w