Central Nervous System

2. Tracts
Ascending
Descending
Transverse

3.  The ascending tracts (somatosensory pathways or
systems) refer to the neural pathways by which
sensory information from the peripheral nerves is
transmitted to the cerebral cortex.
 The ascending tracts can be divided into the type of
information they transmit – conscious or unconscious:
 Conscious tracts – comprised of the dorsal column-
medial lemniscal pathway and the anterolateral
system.
 Unconscious tracts – comprised of the
spinocerebellar tracts.

5. The Dorsal Column-Medial
Lemniscal Pathway
 The dorsal column – medial lemniscal pathway (DCML)
carries the sensory modalities of fine touch (tactile
sensation) vibration and proprioception.
 Its name arises from the two major structures that
comprise the DCML. In the spinal cord, information
travels via the dorsal (posterior) columns. In the
brainstem, it is transmitted through the medial
lemniscus.

7.  There are three groups of neurones involved in this
pathway – first, second and third order neurones.
First Order Neurones
 The first order neurones carry sensory information
regarding touch, proprioception or vibration from the
peripheral nerves to the medulla oblongata. There are two
different pathways which the first order neurones take:
 Signals from the upper limb (T6 and above) – travel in the
fasciculus cuneatus (the lateral part of the dorsal
column). They then synapse in the nucleus cuneatus of the
medulla oblongata.
 Signals from the lower limb (below T6) – travel in the
fasciculus gracilis (the medial part of the dorsal column).
They then synapse in the nucleus gracilis of the medulla
oblongata.

8. Second Order Neurones
 The second order neurones begin in the cuneate
nucleus or gracilis. The fibres receive the information
from the preceding neurones, and delivers it to the
third order neurones in the thalamus.
 Within the medulla oblongata, these
fibres decussate (cross to the other side of the CNS).
They then travel in the contralateral medial lemniscus
to reach the thalamus.

9. Third Order Neurones
 The third order neurones transmit the sensory
signals from the thalamus to the ipsilateral primary
sensory cortex of the brain. They ascend from the
ventral posterolateral nucleus of the thalamus, travel
through the internal capsule and terminate at the
sensory cortex.

10. The Anterolateral System
 The anterolateral system consists of two separate tracts:
 Anterior spinothalamic tract – carries the sensory
modalities of crude touch and pressure.
 Lateral spinothalamic tract – carries the sensory modalities
of pain and temperature.
 Much like the DCML pathway, both tracts of the anterolateral
system have three groups of neurones.
First Order Neurones
 The first order neurones arise from the sensory receptors
in the periphery. They enter the spinal cord, ascend 1-2
vertebral levels, and synapse at the tip of the dorsal horn –
an area known as the substantia gelatinosa.

11. Lateral spinothalamic
tract
 Pain & temp pathways
 1st-order neurons
 Pain conducted by A-type
fibres & C-type fibres
 2nd-order neurons
 decussate to the opposite
side
 ends in thalamus (ventral
posterolateral nucleus
 3rd-order neurons
 ends in sensory area in
postcentral gyrus

12. Anterior (ventral)
spinothalamic tracts
 Light (crude) touch and
pressure pathways

13. Second Order Neurones
 The second order neurones carry the sensory information from the
substantia gelatinosa to the thalamus. After synapsing with the first
order neurones, these fibres decussate within the spinal cord, and then
form two distinct tracts:
 Crude touch and pressure fibres – enter the anterior spinothalamic
tract.
 Pain and temperature fibres – enter the lateral spinothalamic tract.
 Although they are functionally distinct, these tracts run alongside each
other, and they can be considered as a single pathway. They travel
superiorly within the spinal cord, synapsing in the thalamus.
Third Order Neurones
 The third order neurones carry the sensory signals from the thalamus
to the ipsilateral primary sensory cortex of the brain. They ascend from
the ventral posterolateral nucleus of the thalamus, travel through the
internal capsule and terminate at the sensory cortex.

14. The Spinocerebellar Tracts –
Unconscious Sensation
 The tracts that carry unconscious
proprioceptive information are collectively known as
the spinocerebellar tracts.
 Function:-
 Brain co-ordination and refining of motor movements.

15.  They transmit information from
the muscles to the cerebellum.
 The spinocerebellar tracts have
two pathways:
 Posterior spinocerebellar
tract – Carries proprioceptive
impulses from the lower limbs to
the ipsilateral cerebellum.
 Anterior spinocerebellar
tract – Carries proprioceptive
impulses from the lower limbs.
The fibres decussate twice – and
so terminate in the ipsilateral
cerebellum.

16. Posterior (Dorsal)
spinocerebellar tract
 Muscle joint sense pathways
to cerebellum
 Unconscious proprioception
 Muscle joint info from
muscle spindles, Golgi
tendon organ (GTO) -
a proprioceptive sensory
receptor organ that senses
changes in muscle tension,
joint receptors of the trunk &
lower limbs
 Info is used by the
cerebellum in the
coordination of movements
& maintenance of posture

17. Anterior (Ventral)
spinocerebellar tract
 Majority of 2nd-order
neurons cross to the
opposite side
 Enter cerebellum
through superior
cerebellar peduncle
 Info from trunk, upper &
lower limbs
 Also carries info from
skin & subcut tissue

19. The descending tracts
 The descending tracts are the pathways by which motor signals are
sent from the brain to lower motor neurones. The lower motor
neurones then directly innervate muscles to produce movement.
 The motor tracts can be functionally divided into two major groups:
Pyramidal tracts
These tracts originate in the cerebral cortex, carrying motor fibres to the
spinal cord and brain stem.
 Functions
 The voluntary control of the musculature of the body and face.
Extrapyramidal tracts
These tracts originate in the brain stem, carrying motor fibres to the
spinal cord.
 Functions
 The involuntary and automatic control of all musculature, such as
muscle tone, balance, posture and locomotion

20. Upper motor neurones.
 There are no synapses within the descending
pathways. At the termination of the descending tracts,
the neurones synapse with a lower motor neurone.
Thus, all the neurones within the descending motor
system are classed as upper motor neurones.

21. The pyramidal tracts
 The pyramidal tracts derive their name from
the medullary pyramids of the medulla oblongata,
which they pass through.
 These pathways are responsible for the voluntary
control of the musculature of the body and face.
 Functionally, these tracts can be subdivided into two:
 Corticospinal tracts – supplies the musculature of the
body.
 Corticobulbar tracts – supplies the musculature of the
head and neck.

22. Corticospinal Tracts
 The corticospinal tracts
begin in the cerebral
cortex, from which they
receive a range of inputs:
 Primary motor cortex
 Premotor cortex
 Supplementary motor
area
 somatosensory area

23.  After originating from the
cortex, the neurones
converge, and descend
through the internal
capsule.
 After the internal capsule, the
neurones pass through
the crus cerebri of the
midbrain, the pons and into
the medulla.
 In the most inferior (caudal)
part of the medulla, the tract
divides into two:
 Lateral corticospinal tract
 Anterior corticospinal tract

24. Lateral corticospinal tract
 Lateral corticospinal
tract decussate. They
then descend into the
spinal cord, terminating
in the ventral horn. From
the ventral horn, the
lower motor neurones go
on to supply the muscles
of the body.

25. Anterior corticospinal tract
 Anterior corticospinal
tract remains ipsilateral,
descending into the
spinal cord. They then
decussate and terminate
in the ventral horn of
the cervical and upper th
oracic segmental levels.

27. The corticobulbar tracts
 The corticobulbar tracts arise from
the lateral aspect of the primary
motor cortex. They receive the same
inputs as the corticospinal tracts.
The fibres converge and pass
through the internal capsule to
the brainstem.
 The neurones terminate on the
motor nuclei of the cranial nerves.
 Facial nerve
 Hypoglossal
 Here, they synapse with lower
motor neurones, which carry the
motor signals to the muscles of
the face and neck.

28. Extrapyramidal Tracts
 The extrapyramidal tracts originate in the brainstem,
carrying motor fibres to the spinal cord.
 Functions
 The involuntary and automatic control of all
musculature, such as muscle tone, balance, posture and
locomotion.
 There are four tracts.
 Vestibulospinal tracts
 Reticulospinal tracts
 Rubrospinal tracts
 Tectospinal tracts

29. Reticulospinal tract
 The two recticulospinal
tracts have differing
functions:
 The medial reticulospinal
tract arises from the pons.
It facilitates voluntary
movements, and increases
muscle tone.
 The lateral reticulospinal
tract arises from
the medulla. It inhibits
voluntary movements,
and reduces muscle tone

30. Tectospinal tract
 This pathway begins at
the superior colliculus of the
midbrain. The superior
colliculus is a structure that
receives input from the optic
nerves. The neurones then
quickly decussate, and enter
the spinal cord. They
terminate at the cervical levels
of the spinal cord.
 The tectospinal tract
coordinates movements of the
head in relation to vision
stimuli.

31. Rubrospinal tract
 The rubrospinal tract
originates from the red
nucleus, a midbrain
structure. As the fibres
emerge, they decussate
(cross over to the other side
of the CNS), and descend
into the spinal cord. Thus,
they have
a contralateral innervation.
 Its exact function is unclear,
but it is thought to play a
role in the fine control of
hand movements

32. Vestibulospinal Tracts
 There are two vestibulospinal
pathways; medial and lateral.
They arise from the vestibular
nuclei, which receive input
from the organs of balance. The
tracts convey this balance
information to the spinal cord,
where it remains ipsilateral.
 Fibres in this pathway
control balance and posture by
innervating the ‘anti-gravity’
muscles (flexors of the arm,
and extensors of the leg), via
lower motor neurones.