2.9 - Central Nervous System

Question 1

What are the components of the nervous system?

Answer 1

• central nervous system (CNS) - composed of brain and spinal cord
• peripheral nervous system (PNS) - composed of nerves (cranial and spinal) and ganglia outside brain and spinal cord

Question 2

What is the brain comprised of?

Answer 2

• forebrain - cerebral hemispheres, diencephalon (thalamus and hypothalamus)
• midbrain
• hindbrain - pons, medulla, cerebellum

Question 3

What are the lobes of the brain?

Answer 3

• frontal
• parietal
• temporal
• occipital
• (limbic lobe)
• (insular cortex)

Question 4

What is the frontal lobe responsible for?

Answer 4

• regulating and initiating motor function
• language
• cognitive functions (executive function e.g. planning, attention, memory)

Question 5

What is the parietal lobe responsible for?

Answer 5

• sensation (touch, pain)
• sensory aspects of language
• spatial orientation
• self-perception

Question 6

What is the temporal lobe responsible for?

Answer 6

Processing auditory information

Question 7

What is the occipital lobe responsible for?

Answer 7

Processing visual information

Question 8

What is the limbic lobe?

Answer 8

• includes amygdala, hippocampus, mamillary body and cingulate gyrus
• concerned with learning, memory, emotion, motivation and reward

Question 9

What is the insular cortex (lobe)?

Answer 9

• lies deep within lateral fissure
• concerned with visceral sensations, autonomic control, interoception (sensations inside of body e.g. hunger), auditory processing, visual-vestibular integration (integration between sensations from eyes and from balance organs)

Question 10

What makes up the meninges?

Answer 10

• dura mater - thick, composed of two layers - periosteal (layer of periosteum), meningeal (durable, dense, fibrous membrane) - at various points in the brain, meningeal layer peels away from periosteal and forms subdural venous sinuses
• arachnoid mater - thin, transparent, fibrous membrane
• pia mater - thin (two cell thick), translucent, mesh-like

Question 11

Where is cerebrospinal fluid produced and where does it circulate?

Answer 11

• produced by modified epithelial cells in choroid plexus of lateral, third and fourth ventricles
• occupies ventricular system and sub-arachnoid space
• 125ml volume at any one time and 500ml produced each day
• reabsorbed via arachnoid villi (granulations) into superior sagittal sinus
• often tapped during lumbar punctures to check for infections in CSF

Question 12

What does CSF do?

Answer 12

• shock absorbing - protection from trauma
• removes waste products from cellular metabolism
• supplies neurones with nutrients (secondary to blood)
• contains immune cells

Question 13

How do the constituents of CSF differ from plasma?

Answer 13

• lower pH
• less glucose
• less proteins
• less K+
• same Na+

Question 14

Spinal cord anatomy

Answer 14

• dorsal = back/posterior, ventral = front/anterior
• grey matter (middle of cord) and white matter (around outside)
• dorsal horn and ventral horn (back and front ends of ‘butterfly’ shape)
• dorsal and ventral rootlets
• dorsal root and ventral root
• dorsal root has a dorsal root ganglion (ventral does not) - cell bodies of sensory neurones
• mixed spinal nerve (two from each segment) - mixed as it has sensory and motor neurones - sensory into back of cord, motor out from front)
• rootlets –> root –> mixed spinal nerve

Question 15

What is the spinal cord comprised of?

Answer 15

• segments - each gives rise to a pair of mixed spinal nerves
• cervical (8), thoracic (12), lumbar (5), sacral (5), coccygeal (1)
• nerves emerge through intervertebral foramina
• relationship between nerves and foramina changes between cervical and thoracic regions
• nerves C1-C7 emerge above vertebrae
• nerves C8-Co1 emerge below vertebrae - there is no 8th cervical vertebra but there is C8 nerve

Question 16

How does spinal cord anatomy change throughout it?

Answer 16

• cervical enlargement at upper part of cord (C5) because of huge innervation of upper limbs going in and out
• lumbar enlargement at lower part of cord (L2) because of increase in innervation of lower limbs
• shape also changes
• segments get more out of alignment as you go down the cord

Question 17

What are the major descending pathways for voluntary movement?

Answer 17

• corticospinal tract
• two neurones in simplest of cases:
• composed of upper motor neurones in primary motor cortex and lower motor neurones in brainstem (if going out to head and neck) and spinal cord (if going out to rest to body)

Question 18

How do the lateral and anterior corticospinal tracts differ in decussation?

Answer 18

• the lateral corticospinal tract decussates in the medulla (this tract makes up 85% of fibres) - this is why left brain controls right body etc
• the ventral corticospinal tract does not decussate - they stay on the same side as the side the upper motor neurones have come from

Question 19

What muscles do the lateral and ventral corticospinal tracts supply?

Answer 19

• lateral corticospinal tract supplies limb muscles
• anterior corticospinal tract supplies trunk muscles

Question 20

What are the major ascending pathways for sensation?

Answer 20

• dorsal column pathway - mechanical - fine touch, vibration, pressure and proprioception (position) from the skin and joints (situated at back of cord)
• spinothalamic pathway - mechanical, chemical and thermal - pain and temperature (lateral) and crude touch (ventral) from the skin
• both descending and ascending pathways are present in both sides of the cord

Question 21

Where do the primary motor and somatosensory cortexes sit?

Answer 21

• primary motor cortex sits in pre-central gyrus (frontal lobe)
• primary somatosensory cortex sits in post-central gyrus (parietal lobe)
• pre- and post-central gyrus separated by central sulcus

Question 22

What is somatotopy?

Answer 22

• mapping out what parts of the body different parts of the brain control
• organisation of body within brain

Question 23

What is the corticobulbar tract?

Answer 23

• pathway from primary motor cortex to muscles of face (not to limbs or trunk muscles)
• voluntary control of muscles from neck up
• upper motor neurones are in primary motor cortex
• lower motor neurones are in cranial cavity in brainstem, in nuclei which give rise to cranial nerves - oculomotor, trochlear, trigeminal motor, abducens, facial, hypoglossal nuclei
• collections of cell bodies that send their nerve fibres out to muscles of the face

Question 24

What are the extrapyramidal tracts?

Answer 24

• they supply muscles that are more automated in their responses - not under voluntary control
• originate from brainstem –> muscles
• vestibulospinal - provides information about head movement and position and mediates postural adjustments
• tectospinal - orientation of head and neck during eye movements
• reticulospinal - preparatory and movement-related activities, postural control
• rubrospinal - innervate lower motor neurones of upper limb

Question 25

Where do fibres enter the dorsal column pathway?

Answer 25

- fibres enter via the dorsal horn and enter ascending dorsal column pathways - dorsal horn is where first order axons have their cell bodies in the dorsal root ganglia

Question 26

What is the dorsal column pathway through the body - gracile tract?

Answer 26

- information conveyed from lower limbs and body (below T6) travel ipsilaterally along the gracile tract into the gracile fasciculus (fibre bundle) - first synapse of the gracile tract is with cell bodies in the gracile nucleus (just below medulla in brainstem)

Question 27

What is the dorsal column pathway through the body - cuneate tract?

Answer 27

- information conveyed from upper limbs and body (above T6) travel ipsilaterally along the cuneate tract into the cuneate fasciculus - first synapse of the cuneate tract is with cell bodies in the cuneate nucleus (just below medulla in brainstem) --> second order neurone

Question 28

Where do second order axons of the dorsal column pathway decussate?

Answer 28

- second order axons for both gracile and cuneate tracts decussate in the caudal medulla - this forms the contralateral medial lemniscus tract which then synapses in the thalamus --> third order neurone

Question 29

Where do third order neurones project to?

Answer 29

- third order neurones from the thalamus (where third order neurone cell bodies are) project to the somatosensory cortex - size of somatotopic areas in the cortex is proportional to density of sensory receptors in that body region (somatosensory homunculus)

Question 30

Will a spinal cord injury lead to loss of function (dorsal column functions) on the same side (ipsilaterally) or on the opposite side (contralaterally)?

Answer 30

- same side - ipsilaterally - because they decussate in the medulla (brain) - after spinal cord, so injury affects same side (eg injury in right SC is carrying info from right side of body) - not about where it is projecting to but where it is from

Question 31

Outline the pathways of the anterior and lateral spinothalamic tracts

Answer 31

- first order neurones in dorsal root ganglion - primary afferent neurones (with cell bodies in dorsal root ganglion) terminate upon entering the spinal cord and synapse onto second order neurones - second order neurones (with cell bodies in spinal cord) decussate immediately in the spinal cord and form the spinothalamic tract - second order neurones terminate in the thalamus (where tertiary cell bodies are) - third order neurones from the thalamus project to the somatosensory cortex

Question 32

Will a spinal cord injury lead to loss of function (pain and temperature) on the same side (ipsilaterally) or on the opposite side (contralaterally)?

Answer 32

- opposite side - because they decussate in the spinal cord (e.g left side of cord carrying info from right side of body)

Question 33

Will a spinal cord injury lead to loss of function (motor function - corticospinal tract) on the same side (ipsilaterally) or on the opposite side (contralaterally)?

Answer 33

- ipsilateral - same side - as they decussate in the medulla = already crossed (eg injury in right SC, projects to right side as already crossed = ipsilateral) - brain injury - opposite side affected