The Central Nervous System

Question 1

What is the CNS composed of?

Answer 1

CNS is composed of brain and spinal cord – brain is composed of forebrain (cerebral hemispheres and diencephalon), midbrain and hindbrain (pons, medulla and cerebellum)

Question 2

What are the lobes of the brain and their functions?

Answer 2

Frontal: Regulating and initiating motor function, language, cognitive functions, executive functions like planning, attention and memory.
Parietal: Sensation (touch, pain), sensory aspects of language, spatial orientation and self-perception
Temporal: Processing auditory information
Occipital: Processing visual information
Limbic lobe: Amygdala, Hippocampus, Mamillary body and Cingulate Gyrus. Concerned with learning, memory, emotion, motivation and reward.
Insular cortex: Lies deep within lateral fissure. Concerned with visceral sensations, autonomic control and interoception, auditory processing, visual-vestibular integration.

Question 3

What are the meninges and their properties?

Answer 3

Meninges – three layers: Dura, Arachnoid and Pia Mater
Dura – thick, composed of two layers: Periosteal (layer of periosteum) and Meningeal (durable, dense fibrous membrane)
Arachnoid – thin, transparent, fibrous membrane
Pia – thin, translucent and mesh-like

Question 4

Where is CSF made and how does it circulate?

Answer 4

Produced in choroid plexus of lateral, 3rd and 4th ventricles. Occupies ventricular system and sub-arachnoid space. 125ml volume 500ml produced each day. Reabsorbed via arachnoid villi (granulations) into superior sagittal sinus. Lower pH, less glucose, protein and potassium than plasma.

Question 5

Describe spinal cord anatomy

Answer 5

Dorsal and ventral horns are part of the grey matter of the spinal cord. Axons relay sensory information into dorsal roots. White matter surrounds grey matter and filled with axons travelling superiorly + inferiorly to spinal cord. Ventral root takes fibres away from spinal cord. Mixed spinal nerve contains sensory and motor axons. Enlargement in the dorsal root is known as the dorsal root ganglion which contains cell bodies of primary sensory neurones (have endings in skin and joints, provide info about general sensation).

Question 6

Describe spinal cord and nerve relationship

Answer 6

The spinal cord is composed of segments each of which gives rise to a pair of mixed spinal nerves. 8 cervical, 12 thoracic, 5 lumbar, 5 sacral and 1 coccygeal. Nerves emerge from intervertebral foramina: C1-C7 emerge above vertebrae but C8-Co1 emerge below.

Question 7

Describe how spinal cord changes in different regions

Answer 7

Shape + proportion of grey and white matter changes through the spinal cord. The upper part of the spinal cord is enlarged – cervical enlargement is due to innervation of upper limbs. Lumbar enlargement is due to innervation of lower limbs.

Question 8

What is the role of the corticospinal tract?

Answer 8

Major pathway of voluntary movement – composed of upper motor neurons in primary motor cortex and lower motor neurons in brainstem and spinal cord.

Question 9

What pathways bring sensation from body to brain?

Answer 9

Dorsal Column pathway and Spinothalamic tract. Dorsal column pathway is for fine touch, vibration and proprioception (position) from the skin and joints. Spinothalamic pathway is for pain, temperature (and crude touch) from the skin.

Question 10

Describe internal anatomy of spinal cord

Answer 10

1. Descending Tracts (Motor) – Lateral corticospinal tract and ventral corticospinal tract
2. Ascending Tracts (Sensory) – Dorsal columns (fine touch, proprioception, vibration), Lateral Spinothalamic Tract (pain, temperature) and Ventral Spinothalamic Tract (crude touch)

Question 11

Describe location of cortices around central sulcus

Answer 11

Anterior to central sulcus is pre-central gyrus. Primary motor cortex is located in the pre-central gyrus. Somatotopy can be observed if a coronal section is taken of the primary motor cortex, different regions associated with different body parts. Upper motor neurones housed in primary motor cortex but lower motor neurones housed in brainstem, neck muscles or spinal cord.

Question 12

How is the contralateral brain effect created?

Answer 12

85% of fibres from the brain decussate (cross) in the medulla and hence contralateral brain effect. Anterior corticospinal tracts stay on the same side as the upper motor neurone they descend from. Lateral corticospinal tracts have a lot of crossing over – supply limb muscles in general. ACT supplies trunk muscles.

Question 13

What is the corticobulbar tract?

Answer 13

Pathways that go from primary motor cortex to muscles of the face. Have their upper motor neurones in the primary cortex but LMN are in cranial cavity in brain stem in nuclei of cranial nerves.

Question 14

What muscles do the different cranial nerve nuclei supply?

Answer 14

Oculomotor nucleus and Trochlear nucleus = Extraocular muscles
Trigeminal motor nucleus = Muscles of mastication
Abducens nucleus = Extraocular muscles
Facial nucleus = Muscles of facial expression
Hypoglossal nucleus = Muscles of the tongue

Question 15

What is the internal capsule?

Answer 15

Somatotopy can be observed as fibres bundle together descending from the primary motor cortex in upper motor neurones. As tract descends, forms a structure called internal capsule.

Question 16

What are the different brainstem motor tracts?

Answer 16

Vestibulospinal: provides information about head movement and position and mediates postural adjustments

Tectospinal: orientates head and neck during eye movements

Reticulospinal: control of breathing and emotional motor function

Rubrospinal: Only really functional in humans if corticospinal tract damaged as functions mostly taken over by that. Innervates lower motor neurones of the upper limb.

Question 17

How is the dorsal column pathway organised?

Answer 17

Fibres enter via dorsal horn and enter ascending dorsal column pathway. Info conveyed from lower limbs and body below T6 travel ipsilaterally to gracile tract. Info conveyed from upper limbs and body above T6 travel ipsilaterally to cuneate tract. Synapses in the medulla where it descends contralaterally up to the thalamus, synapsing onto a third neurone which projects up onto relevant part of sensory cortex.

Question 18

Where are the cell bodies of the three neurones of DCP?

Answer 18

First has root body in dorsal root ganglion, second has cell body in medulla (gracile or cuneate) and third is in thalamus.

Question 19

How is the spinothalamic tract organised?

Answer 19

Pain and temperature sensations ascend within the lateral spinothalamic tract whereas crude touch ascends within the anterior spinothalamic tract. Primary neurone comes into the dorsal horn and rather than going up on the same side, synapses onto second neurone in spinal cord at level that it comes into spinal cord. This second order neurone crosses onto contralateral side of body and travels all the way up to the thalamus where it synapses onto third order neurone which goes to part of sensory cortex corresponding to that body region.