The nervous system:part IV

The Nervous System/Part IV
Dr. Fawaz A. Mustafa
PhD in Medical Physiology and Pharmacology
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work for educational, not for profit use by university staff or students

Transmission of sensation to the
brain
• All the sensations go upward in the spinal cord or
they go through trigeminal nerve, then the nerve
fibres ascend to the thalamus, there is crossing of
fibres to the opposite side either in spinal cord or
brainstem
• In each hemisphere, there are sulci and gyri
• The brain has four lobes: frontal, parietal, temporal
and occipital

brain
• In the middle, there is a central sulcus. Any gyrux
posterior to it is called “postcentral gyrux”, and
any one anterior to it is called “precentral gyrux”
• Anything behind the central sulcux is concerned
with sensory, and in front of, it is concerned with
motor
• The parietal lobe is concerned with the
proprioception, the occipital with vision, temporal
with hearing and some tastes, frontal with motor

brain
• The postcentral gyrux represents the body as
following:
1. The cerebral cortex of the postcentral gyrux
receives the sensations of the opposite side of the
body
2. The body, in this area, is represented upside down
3. Any part in the body is represented in the gyrux
according to the number of the receptors in that
part

brain
• The sensations should go to the postcentral gyrux
of the cerebral cortex because this region is the
final centre for:
a. Analysis
b. Sometimes, the sensation is stored or it discarded.
The storage is as memory
c. Localisation of the sensation
d. Production of synthetic sensation, such as
stereognosis or proprioception, which combine
many forms of sensations to produce body image.

brain
The more complicated sensations, the more need for
well-developed cerebral cortex. But, the question is:
Do we need the cerebral cortex for all sensations?

Motor system
• It is concerned with movement
• Human beings can do very skilled movement
(writing, drawing, sewing) with their fingers as
compared with animals which cannot do skilled
movements
• These skilled movements are not occurring
spontaneously (from infancy). They need practice
and experience to learn skilled movements

Motor system
• To perform skilled movements, we need the help of
other movements. These other movements are
called “postural movements”
• The postural movement is changeable according to
the skilled movement
• Skilled movement is highly organised, while the
postural movement is not. So, each is
complementary to each other

Motor system
• Who is doing the movement? Or by what
we can produce movement?

Motor system
• In terms of movements, there are agonist and
antagonist muscle.
• Agonist is a group of muscle trying to produce the
movement
• Antagonist is a group of muscle which opposes the
movement
• In any movement, we should stimulate the agonist
and inhibit the antagonist.

Motor system
• Movement to be produced needs 3 points:
1. Movement is produced by a group of muscles, and
this includes stimulation of agonist and inhibition
of antagonist.
2. The postural movement. i.e. proper positioning of
a group of joints.
3. Coordination of movement between agonist and
antagonist. Without coordination, movement can
be too fast or too slow but correction produce the
exact wanted movement.

Motor system
• Moreover, to produce movement, 4 elements are
needed:
1. Conception and understanding the stimuli whether
it is external or internal (which is the information
stored in the memory area).
2. Planning for a movement
3. The stimulation of many muscles (either those of
body position, or those of joints or those of doing
movement)

Motor system
4. Proper watching of the movement. There is actual
movement and intended movement. Sometimes,
the actual movement is as the intended
movement, and other times it goes too far beyond
what it is intended. If the difference between them
is zero, this is proper movement. This is done by
continuous watching of movement. When
movement goes on, it sends impulses from joints,
muscle spindles,… etc. and these impulses goes
to the cerebellum

Motor system
• The movement is so complicated. Therefore, there
is need of centre by which we can do all these
elements
• This centre is present in the frontal lobe in the
precentral gyrux (or primary motor area) where the
body (and not the muscles) is represented.

Motor system
• The characteristics of this representation are:
1. The body is represented upside down. the most superior
region of the gyrux is related to foot and lower parts, while
the movement of the head are controlled by the inferior
region the precentral gyrux.
2. The movement of the right side of the body is controlled by
the left precentral gyrux, and that of the left side is
controlled by the right precentral gyrux
3. The area representation of precentral gyrux is related to
the skilled movement. The more skilled movement, the
more surface area in the precentral gyrux

Motor system
• One of the area involved in the motor system is the
primary motor area. The cerebral cortex, regarding
the primary motor area, is considered as the top
level
• Its responsibility is the plan and the idea about
movement which are formulated or synthesised in
their regions: the primary motor area, premotor
area (in front of it) and motor association area,
which are the areas that take the information and
got an idea about movement (the plan for
movement)

Motor system
• Other two structures of brain involved in motor
system are the basal ganglia and the lateral
hemisphere of the cerebellum
• For movement, there is a need of planning which is
done by the previously mentioned three structures
or areas
• After planning, execution occurs by orders sent
downward for movement to start. Execution (or
performing) the movement is done by a group of
muscles which are supplied by motor nerve fibres

Motor system
• Any skeletal muscle below the head is supplied by
motor axons of cells in the anterior horn of the
spinal cord, while any muscle in the head is
supplied by motor axons of cells in the nuclei of the
motor cranial nerves
• These motor axons are always excitatory to the
muscles, and through these axons, action potential
goes to the muscles causing sliding of actin and
myosin on each other (i.e. contraction)

Motor system
• The contraction of these muscles does not occur
haphazardly, but there is a control of these motor
nerve by orders from above (i.e. primary motor area
and brainstem)
• The lower level of spinal nerves are not working by
themselves but they work under the control of
higher centre called “upper motor neuron”

Motor system
• The lower motor neurons are from the cells of the
anterior horn of the spinal cord or cells of the motor
nuclei of the cranial nerves to the muscles
• The lower motor neurons are always excitatory to
the skeletal muscles under the control of the upper
motor neurons
• The upper motor neurons are of two types: direct
and indirect
• The direct is either corticospinal tract or
corticonuclear tract, both of which are called
“pyramidal tract”

Pyramidal tract
• The corticospinal tracts begin at the precentral
gyrus then come down through the pons and
midbrain and when they reach medulla oblongata,
and they cross to the opposite side forming
pyramids of the medulla, then they descend in the
cervical and the thoracic, lumbar and sacral giving
branches ending in the white matter in the lateral
side of the spinal cord, and some continue to end in
the anterior horn cells at each segments

Pyramidal tract
• The corticonuclear tracts also begin from the
cerebral cortex to the nuclei of the motor cranial
nerves. Also, crossing to the opposite side occurs
(i.e. the right hemisphere affects the left side, and
vice versa).
• The direct pyramidal corticospinal and
corticonuclear tract are responsible for activity or
movement of the distal muscles of the body
(muscles of hand, foot, .. etc.)

Extrapyramidal tracts
• Other tracts are responsible for postural movement by
indirect pathway called “extrapyramidal tracts”
• The origin of these tracts is usually from the brainstem, in
which there are many nuclei that are motor in origin giving
rise to axons which run downward in the form of group or
bundles ending in the median region of the spinal cord, and
they have different names.
• Some of them come from the reticular formation of pons
region and they are called “pontine reticulospinal tracts”
till the reach the median region of the white matter of the
spinal cord. This tract activates the proximal extensor group
of muscles (such as triceps in the upper limbs, and the
quadriceps in the lower limbs).

Extrapyramidal tracts
• The other tract is the “medullary reticulospinal
tract” (that originates from red nucleus) which
activates the proximal flexor muscles (such as the
biceps, branchials, … etc. in the upper limbs, and
the hamstring in the lower limbs).
• Another tract is called “reticulospinal tract” which
originates in the superior colliculus in the midbrain,
then it runs downward, and these are responsible
for certain movements (such as turning the head
towards the stimulus).

Motor system
• The cell body of the lower motor neuron is present
in the anterior horn. These tracts come from the
above to the spinal cord to influence the lower
motor neurons (anterior horn cells).
• The direct and indirect tracts influence the anterior
horn cells by another called “intermediate neuron”
which is a small neuron that transmits the action
potential to the anterior horn cells.

Motor system
• The direct tracts as well as the rubrospinal tract
descend in the white matter in the lateral side of the
spinal cord, while the indirect tract descends in the
white matter at the median region.
• The direct and indirect tracts, some are excitatory
and others are inhibitory. So, the anterior cells
receive many influences, excitatory and inhibitory.
• The summation of these descending tracts, if it is
excitatory, the lower motor neurons are excited, if it
is inhibitory, the lower motor neurons are inhibited.
The lower motor neurons are always excitatory to
the skeletal muscles.

Motor system and movement
• The tracts that descend in the lateral portion of the
spinal cord concerned with the skilled movement
affecting the distal muscles of the body, while those
of the median portion is concerned with postural
movement affecting the proximal truncal muscles
• The final element in movement is the muscles
• “Motor unit” may be defined as nerve cell body
giving axon whose branches go to many muscle
fibres. Each muscle consists of many motor units

• The basic unit of lower motor neuron is the motor
unit. The lower motor unit neuron extends from the
cell body to the muscle, while the upper motor unit
is the tracts above the muscles.
• To make a muscle to contract, excitation of the
muscle with action potential from the cell body is
needed, conducted through axons to the muscle
yielding sliding phenomenon of the actin and
myosin filament (i.e. muscle contraction)

• There are two types of motor units:
1. Phasic motor units: they are those capable of
producing rapid contraction. They are wide and
myelinated nerve fibres conducting rapid action
potential. Phasic motor units cause rapid
contraction and relaxation (such as in running and
jumping), so they fatigue easily and not last for
long time.
2. Tonic motor units: they are motor units in which
muscle contraction is slow and continuous, thus,
they produce slow movements (such as postural
movements). They don’t fatigue easily.

How movement is produced
• First of all, it must be planned in the cerebral cortex
of the precentral gyrus (in the primary motor area,
premotor area and supplementary motor area).
This is the higher centre. The precentral gyrus is
the place where the body is presented in motor
system, this is why the is formed here.
• The plan comes from the higher centre via the
upper motor neuron in tracts that are either direct
(for skilled movements) or indirect (for postural
movements)

• The upper motor neurons control the lower ones.
The anterior horn cells of many levels of the spinal
cord are activated, not only one level. This is
because if we want to produce a movement, a
group of muscle should be activated to contract,
and other should be inhibited to relax.
• To eat, the plan comes from the higher centre and
is conducted through upper motor neurons which
end in the nuclei of the brainstem and not in the
spinal cord. Then, the lower motor neurons send
orders to the superficial muscles to do so and the
muscle of mastication to do so

• Talking involves movement of the muscles of lips,
tongue, jaw, larynx and the respiratory muscles,
which all are supplied by nerve fibres from nerve
cell in the brainstem nuclei. The nuclei don’t work
by themselves by the are controlled by the higher
centre. The relation between the upper motor
neurons and lower motor neurons is that the upper
ones command the lower ones to do so and so,
and lower ones are always supervised or over
checked by the upper motor neurons.

• Whenever there is no check from the higher centre,
dissociation results. Therefore, the movement is
always produced by interaction between upper and
lower motor neurons
• If a nerve is cut, there will be a defect in the muscle
and this may obstruct the movement. But, if there is
something wrong with the cerebral cortex (as in
cerebral infarction), the movement will be affected
despite the fact that there is nothing wrong with the
muscles. So, the upper motor neurons are
concerned with movement while the lower ones are
concerned with the muscles

The nervous system:part IV

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The nervous system:part IV