Major ascending and descending pathways

Question 1

Do spinal reflexes involve any connections to the brain?

Answer 1

No.

Question 2

What is a classic example of a spinal reflex. (most well known one)

Answer 2

Patella tendon reflex aka “Knee jerk reflex”

Question 3

In a transverse section of the spinal cord what is the characteristic shape of the grey matter arranged in?

Answer 3

a “H” shape

Question 4

What would happen if a physician hit the patella tendon with a reflex hammer?

(Full LONG physiological answer include neuron pathways)

Answer 4

• 1st this results in the stretch of the quadriceps muscles, and the quadriceps muscle contains stretch receptors, when they’re triggered an action potential fires along the axon of the sensory neuron, the cell body of the sensory neurone is found in the dorsal root ganglion.
• The action potential is then conveyed into the dorsal horn of the grey matter of the spinal cord.
• Then the axon projects into the ventral horn of the grey matter, here it synapses onto a motor neuron, this neurone projects an axon that exists the spinal cord on the ventral side, which projects back to the quadriceps muscle and its activation results in the kicking out of the leg.
• Simultaneously the sensory neuron that’s conveyed the stretch from the quad fem muscle, also has a collateral axon that projects directly or indirectly via a interneuron onto another motor neuron in the ventral horn.
• This motor neuron also projects out the spinal cord ventrally but innervates the hamstring muscle, and this relaxes the muscle as the interneuron is an inhibitory interneuron.

Question 5

Do all spinal reflexes work in the same way as the knee jerk reflex, in the sense that they both causes a muscle to contract and the agonist muscle to relax?

Answer 5

Yes.

Question 6

What do the Ascending pathways contain/include?

& What are their roles in sensation?

Answer 6

General somatic afferents, (GSAs) these are fibres that convey general sensation from the periphery to the brain.

GSAs from the periphery can involve different modalities they can convey the sensation of;

• Pain
• Temperature
• Crude and Fine touch - - Conscious proprioception (awareness of your position in space, e.g. lying down with eyes closed its conscious proprioception that makes you aware that you’re lying down and not standing up.)
• Subconscious proprioception, conveys by specific spinal cord pathways, it involves constant feedback between state of tension in the muscles and the cerebellum.

General visceral afferents (GVAs) these are sensory fibres from the autonomous nervous system that convey sensation from the internal organs.

The brain also receives input from Special senses these are well known (sight, tastes etc) which are typically conveyed by cranial nerves which attach to the brainstem rather than the spinal cord.

Question 7

What is the role of the Cerebellum?

Answer 7

Regulates a constant adjustment of muscle tone that prevents us from falling over and allows us to execute smooth movement.

Question 8

What is the Cerebellum?

Answer 8

The back of your brain (looks like a mini brain underneath the occipital lobe) its the lowest part of the brain, and the spinal cord extends from it.
Its a highly folded structure and often referred to as a mini brain.

Question 9

What do the Descending pathways contain/include?

& What are their roles in Motor execution?

Answer 9

General somatic efferents (GSEs) they convey motor execution programmes to the periphery.

General visceral efferents (GVEs) they are part of the autonomous nervous system, and they either convey subconscious motor execution in the organs and innervate some glands for endocrine functions.

Question 10

What is the general role of the ascending pathways to the brain?

Answer 10

Sensation.

Question 11

What is the general role of the descending pathways from the brain?

Answer 11

Motor execution.

Question 12

Spinal cord is connected to the periphery via how many pairs of spinal nerves?

Answer 12

31

Question 13

How are spinal nerves connected to the spinal cord?

Answer 13

Via a Ventral root and a Dorsal root.

Question 14

Which root (Dorsal or Ventral) contain afferent sensory fibres?

Answer 14

Dorsal

Question 15

Which root (Dorsal or Ventral) contain efferent motor fibres?

Answer 15

Ventral

Question 16

What is the dorsal root ganglion?

Answer 16

Its the cell body of the sensory fibres of the spinal nerve.

Question 17

What is distal to the dorsal root ganglion?

Answer 17

This is where the spinal nerve splits into, a much smaller posterior ramus and a larger anterior ramus.

Question 18

What do the anterior rami innervate?

broadly speaking

Answer 18

most of the trunk

Question 19

What do the posterior rami innervate?

broadly speaking

Answer 19

only a small part of the back.

Question 20

What are the sympathetic chain ganglion?

Answer 20

The spinal nerves are linked to 2 sympathetic chain ganglion on either side of the vertebral column.
They’re part of the autonomous nervous system, and the spinal nerves are linked to them via the grey and white rami communicants.

Question 21

For the sake of the following FC, what is the organisation of the CNS (Top –> Down) starting from the cerebral cortex then going down and ending with the spinal nerves?

Answer 21

• Forebrain: Cerebral cortex –> Thalamus
• Then the Brain stem:
  Midbrain –> Pons –> Medulla oblongata
• Then: Spinal cord –> Dorsal root ganglion –> spinal nerves.

Question 22

What are the 3 major Ascending pathways of the brain, and what are they each responsible for?

Answer 22

• Spinothalamic tract; responsible for crude touch, temperature, pain.
• Dorsal column-medial lemniscus pathway (aka Dorsal column pathway); responsible for conscious proprioception.
• Spinocerebellar tract; responsible for subconscious proprioception.

Question 23

What is the pathway for the Spinothalamic tract?

Answer 23

• Sensory nerve ending in the periphery becomes excited.
• Sending an action potential down the axon that enters the spinal cord on the dorsal side.
• This axon synapses onto a second order neurone then projects an axon that crosses over to the contralateral side of the cord.
• This neuron then takes a sharp turn up and projects towards the brain.
• It projects to the thalamus, and in the thalamus it synapses to a 3rd order neurone.
• From there it sends projections to the somatosensory cortex.

Question 24

What is the pathway for the Dorsal column pathway?

Answer 24

• Sensory nerve ending is activated in the periphery, e.g. a stretch receptor.
• the axon of the spinal nerve enters the spinal column via the dorsal root.
• It then projects immediately up into the medulla oblongata.
• Here it synapses onto a second order neurone that is in one of the dorsal column nuclei, found dorsally in the medulla oblongata.
• the 2nd order neuron then projects an axon that decussates (crosses over) to the contralateral side ventrally in the medulla oblongata.
• Then it makes a sharp turn up and projects up to the contralateral thalamus.
• Then a 3rd order neuron in the thalamus projects to the somatosensory cortex.

Question 25

What is the pathway for the spinocerebellar tract?

Answer 25

• Sensory nerve ending are activated in the periphery, e.g. a muscle spindle/ Golgi tendon organs.
• The afferent axon of the sensory neurone enters the spinal cord via the dorsal root.
• And synapses to a 2nd order neurone the cell body of which is located in the dorsal horn.
• Then it doesn’t decussate it stays ipsilateral it projects to the lateral side of the spinal cord, where it makes a sharp turn and projects upwards towards the cerebellum.
• The cerebellum forms reciprocal connections with the cerebral cortex, and there efferent fibres from the cerebellum that are relayed via the thalamus, and then enter the cortex and then their reciprocal connections from the motor cortex, back to the cerebellum that are relayed via nuclei in the brainstem including the pontine nuclei and the inferior olivary nuclei and from there back to the cerebellum.
• And finally, there are also connections back from the cerebellum to the spinal cord that are indirect and are relayed by a number of nuclei in the brainstem.

Question 26

Why is the cerebellum considered like a computer?

Answer 26

It constantly regulates harmonious interactions between different systems; between higher command centres centrally and between muscle executers peripherally.

Question 27

What are the effects of a cerebellar defect?

Answer 27

• Muscle hypertonia; inrceased tone making muscles stiff and hard to move.

Question 28

What are the effects of a cerebellar defect?

Answer 28

• Muscle hypertonia; increased tone making muscles stiff and hard to move.
• Changes to posture and gait.
• Intention tremor; tremor when voluntary movements are executed
• decomposition of movements; complex movements are broken down into individual movements.
• Disturbed reflexes, e.g. altered knee jerk reflex.
• Dysarthria; problem swallowing.

Question 29

What are the causes of a cerebellar defect?

Answer 29

• Alcohol poisoning; The alcohol inhibits GABA receptors (mainly inhibitory neurotransmitters in CNS)
• Heavy metal poisoning.
• Congenital disorders; problems in the forming of the cerebellum.
• Degenerative disorders e.g. Friedreich’s Ataxia.
• Trauma.
• Infection.
• Tumours e.g. Medulloblastoma
• MS
• Thrombosis (blood clot) of cerebellar arteries.
• Aging.

Question 30

What is the pathway for the Corticospinal tract?

Answer 30

• The 1st order neurons that control this process are found in the primary motor cortex, of the cerebral cortex hemispheres.
• These axons run through a flat sheet of fibres between the thalamus and the basal ganglia known as the internal capsule.
• And from there they enter the brainstem, run through the pons and eventually the medulla oblongata.
• On the ventral surface of the medulla oblongata there are two “pyramids” that descend from it, and contain the fibres of the Corticospinal tract.
• At the base of the pyramids the fibres cross over this area is known as the pyramidal decussation, here 85% of the fibres decussate and 15% stay ipsilateral.
• Both sets of fibres then descend down the spinal cord, the 15% that stayed ipsilateral will then decussate more closely to their target of innovation.
• So ultimately all fibres end up on the contralateral side, and they then synapse onto a lower motor neuron, a motor neuron in the ventral horn of the spinal cord.
• And this lower motor neuron then projects a 2nd order projection to muscles.

Question 31

What is the pathway for the Reticulospinal tract?

Answer 31

• The first order axons originate from a network in the centre of the brainstem known as the reticular formation.
• The first order axons projects an axon that goes into the spinal cord and these axons then synapse onto lower motor neurons, either ipsilaterally or contralaterally.
• These then project into the periphery and innervate muscles.

The fibres from the reticulospinal tract are around the ventral horn of the grey matter of the spinal cord.

Question 32

What is the pathway for the Vestibulospinal tract?

Answer 32

• Originates from neurons in the vestibular nuclei that sends axons into the spinal cord.
• they eventually synapse onto lower motor neurons, these will innervate muscles in the periphery.

Question 33

What are the 3 major Descending pathways of the brain, and what are they each responsible for?

Answer 33

• Corticospinal tract; responsible for voluntary control of skilled movements e.g. playing piano.
• Reticulospinal tract; responsible for Posture and locomotion.
• Vestibulospinal tract; responsible for Balance, posture, “anti-gravity” (keep us upright against gravity)

Question 34

What are the other 2 descending pathways, and what are their pathways?

Answer 34

• Tectospinal tract; From superior colliculus (dorsal midbrain) to contralateral side of the spinal cord –> coordination of head and eye movements.
• Rubrospinal tract: from red nucleus (ventral midbrain) to cervical segments of the spinal cord –> voluntary motor control of upper limbs (facilitates flexion)