As the name suggests, the ascending tracts of the spinal cord ascend from the spinal cord and connect it to the brain. These tracts are named based on their origin and termination. They are found running along the dorsal, lateral, and ventral columns of the white matter.

1. FUNCTION OF THE SPINAL CORD AND
ASCENDING TRACT
FATIMA WAHID MANGRIO

2. FUNCTIONS OF THE SPINAL CORD AND SPINAL NERVES
1. The white matter of the spinal cord contains sensory and motor
tracts, the “highways” for conduction of sensory nerve impulses
toward the brain and motor nerve impulses from the brain toward
effector tissues.
2. The spinal cord gray matter is a site for integration (summing) of
excitatory postsynaptic potentials (EPSPs) and inhibitory postsynaptic
potentials (IPSPs).
3. Spinal nerves and the nerves that branch from them connect the
CNS to the sensory receptors, muscles, and glands in all parts of the
body.

5. 1 Sensory receptors detect a sensory stimulus.
2 Sensory neurons convey this sensory input from
sensory receptors into the spinal nerve and then into
the posterior root. From the posterior root, axons of
sensory neurons may proceed along three possible
paths .
MECHANISM OF SENSORY INPUT AND
MOTOR OUTPUT BY THE SPINAL CORD

6. 3 Axons of sensory neurons may extend into the
white matter of the spinal cord and ascend to the
brain as part of a sensory tract.
4 Axons of sensory neurons may enter the
posterior gray horn and synapse with interneurons
whose axons extend into the white matter of the
spinal cord and then ascend to the brain as part of
a sensory tract.
5 Axons of sensory neurons may enter the
posterior gray horn and synapse with interneurons
that in turn synapse with somatic motor neurons
that are involved in spinal reflex pathways.

7.  The pathways to the cerebral cortex
consist of thousands of sets of three
neurons:
ASCENDING TRACTS OF THE CORD

8. 1. First-order neurons
 Conduct impulses from somatic receptors into
the brain stem or spinal cord. From the face,
mouth, teeth, and eyes, somatic sensory
impulses propagate along cranial nerves into the
brain stem. From the neck, trunk, limbs, and
posterior aspect of the head, somatic sensory
impulses propagate along spinal nerves into the
spinal cord.

9. 2. Second-order neurons
 Conduct impulses from the brain stem and
spinal cord to the thalamus. Axons of second-
order neurons decussate in the brain stem or
spinal cord before ascending to the ventral
posterior nucleus of the thalamus. Thus, all
somatic sensory information from one side of
the body reaches the thalamus on the opposite
side.

10. 3. Third-order neurons
 conduct impulses from the thalamus to
the primary somatosensory area of the
cortex on the same side.

11.  Somatic sensory impulses ascend to the cerebral
cortex via three general pathways:
 (1) the posterior column–medial lemniscus
pathway,
 (2) the anterolateral (spinothalamic) pathway, and
 (3) the trigeminothalamic pathway.
 Somatic sensory impulses reach the cerebellum
via the spinocerebellar tracts.

12. Posterior Column–Medial Lemniscus Pathway to the
Cortex
 Nerve impulses for touch, pressure, vibration, and conscious
proprioception from the limbs, trunk, neck, and posterior head
ascend to the cerebral cortex along the posterior column–medial
lemniscus pathway. The name of the pathway comes from the
names of two white-matter tracts that convey the impulses: the
posterior column of the spinal cord and the medial lemniscus of the
brain stem.

13.  First-order neurons in the posterior column–medial lemniscus
pathway extend from sensory receptors in the limbs, trunk, neck,
and posterior head into the spinal cord and ascend to the medulla
oblongata on the same side of the body.
 The cell bodies of these first-order neurons are in the posterior
(dorsal) root ganglia of spinal nerves. In the spinal cord, their
axons form the posterior (dorsal) columns, which consist of two
parts: the gracile fasciculus and the cuneate fasciculus .

14.  The axons synapse with the dendrites of second-order neurons
whose cell bodies are located in the gracile nucleus or cuneate
nucleus of the medulla. Nerve impulses for touch, pressure,
vibration, and conscious proprioception from the upper limbs,
upper trunk, neck, and posterior head propagate along axons in
the cuneate fasciculus and arrive at the cuneate nucleus. Nerve
impulses for touch, pressure, and vibration from the lower limbs
and lower trunk propagate along axons in the gracile fasciculus
and arrive at the gracile nucleus.

15.  The axons of the second-order neurons cross to
the opposite side of the medulla and enter the
medial lemniscus, a thin ribbon like
projection tract that extends from the medulla
to the ventral posterior nucleus of the
thalamus. In the thalamus, the axon terminals
of second-order neurons synapse with third-
order neurons, which project their axons to the
primary somatosensory area of the cerebral
cortex.

17. Anterolateral Pathway to the Cortex
 Nerve impulses for pain, temperature, itch, and
tickle from the limbs,trunk, neck, and posterior
head ascend to the cerebral cortex along the
anterolateral (spinothalamic) pathway. Like the
posterior column–medial lemniscus pathway, the
anterolateral pathway is composed of three-
neuron sets.

18.  The first-order neurons connect a receptor of the
limbs, trunk, neck, or posterior head with the
spinal cord. The cell bodies of the first order
neurons are in the posterior root ganglion.
 The axon terminals of the first-order neurons
synapse with second-order neurons,whose cell
bodies are located in the posterior gray horn of
the spinal cord.

19.  The axons of the second-order neurons cross to
the opposite side of the spinal cord. Then, they
pass upward to the brain stem as the
spinothalamic tract.
 The axons of the second-order neurons end in
the ventral posterior nucleus of the thalamus,
where they synapse with the third-order
neurons. The axons of the thirdorder neurons
project to the primary somatosensory area on
the same side of the cerebral cortex as the
thalamus.

21. Trigeminothalamic Pathway to the Cortex
 Nerve impulses for most somatic sensations
(tactile, thermal, and pain) from the face, nasal
cavity, oral cavity, and teeth ascend to the cerebral
cortex along the trigeminothalamic pathway.
 Like the other somatosensory pathways just
described, the trigeminothalamic pathway consists
of threeneuron sets
 First-order neurons extend from somatic sensory
receptors in the face, nasal cavity, oral cavity, and
teeth into the pons through the trigeminal (V)
nerves.

22.  The cell bodies of these first-order neurons
are in the trigeminal ganglion.
 The axon terminals of some first-order
neurons synapse with second-order neurons
in the pons. The axons of other first order
neurons descend into the medulla to synapse
with second order neurons.

23.  The axons of the second-order neurons cross to
the opposite side of the pons and medulla and
then ascend as the trigeminothalamic tract to
the ventral posterior nucleus of the thalamus.
 In the thalamus, the axon terminals of the
second order neurons synapse with third-order
neurons, which project their axons to the
primary somatosensory area on the same side of
the cerebral cortex as the thalamus.