The Nervous System II: CNS

Question 1

How are sensory signals detected?

Answer 1

Specialised receptor cells in sensory organs (skin, eyes, etc) respond to changes in the environment
There are dozens of different types of receptors (e.g., touch, smell, heat, etc(
Mostly NOT neurons, but are directly connected to a sensory neuron
(Relevant: Change detection & Sensory adaptation)

Question 2

Define: Sensory Adaptation

Answer 2

Sensory adaptation: after a while if they don’t change we get used to the stimuli (clothing on body, etc)

Question 3

Define: Change Detection

Answer 3

We only sense when things are changing

Question 4

How do sensory signals undergo transmission?

Answer 4

Sensory signals from body (except head) enter CNS via spinal cord

Where in the spinal cord depends on the type of information (Precisely localised information vs Poorly localised information)

Sensory neurons from head send axons directly into brain via cranial nerves (e.g., optic nerve)
All signals (not just visual) are transmitted via several ‘relay stations’

Question 5

Describe: How do neurons carrying precisely localised information send sensory signals from body to CNS?

Answer 5

Neurons carrying precisely localised information (e.g., fine touch, proprioception) send axons to the top of the spinal cord (medulla)

Question 6

Describe: How do neurons carrying poorly localised information send sensory signals from body to CNS?

Answer 6

Neurons carrying poorly localised information (e.g., temperature, pain) synapse immediately with other neurons upon entering spinal cord

Question 7

Which pathways are used in transmission? (3)

Answer 7

1) Peripheral nerves: Sensory neurons send signals through peripheral nerves

2) Spinal cord: Signals enter the spinal cord via the dorsal root

3) Brain: Signals are relayed to the brain, where they reach specific sensory areas (e.g., the somatosensory cortex for touch)

Question 8

Describe: Transmission of sensory signals (Transgression & Perception)

Answer 8

Sensory signals from diencephalon relayed via thalamus to their appropriate primary sensory cortex

Visual signals –> visual cortex (occipital lobe)

Auditory signals –> auditory cortex (temporal lobe)

Signals from skin, muscles & joints –> somatosensory cortex (parietal lobe)

All signals are identical (electrical neural activity: “action potential”) - how a signal is interpreted depends entirely on its location.

Thalamus: acts as a relay station for incoming sensory signals, directing them to appropriate cortical areas. Receives downward-going input from higher areas, modulating relay of sensory signals.

Cortex: brain’s sensory cortex processes and interprets these signals, leading to perception

Question 9

Explain: The hindbrain (aka rhombencephalon)

Answer 9

-Medulla and pons - where the spine becomes the brain (thickening of the brain)

-Contains several nuclei (cluster of cell bodies) of the automatic NS

-Cerebllum (little brain), not part of the brainstem

-Function: balance, motor learning

-All develop during embryonic development from same piece of tissue

Question 10

Explain: The midbrain (aka mesencephalon)

Answer 10

• Between forebrain & midbrain, part of the brainstem, above the pons

Question 11

Describe: Functions of the hindbrain & midbrain

Answer 11

Functions: Combination of information from different sense modalities; direction of attention

Question 12

Explain: The Forebrain: Diencephalon

Answer 12

Thalamus: (relay station)
-massive structure on top of midbrain, deep in centre of the brain

Hypothalamus: Small structure in front of & below thalamus; directly connected to pituitary gland (‘master gland’ of the ES, controls activity of all other glands)
‘Gateway’ to ES: NS can influence ES via hypothalamus - pituitary connection

Question 13

Explain: The Forebrain: Telencephalon

Answer 13

Cerebral hemisphre
-from diencephalon, incoming signals go up to the cerebrum
-divided into 2 highly similar (not identical) hemispheres
-each covered in cerebral cortex (thin layer of neurons covering each hemisphere), also contains several groups of sub-cortical nuclei (tight cluster of neuron’s cell bodies)

Question 14

Explain: General processing principle (Reversal)

Answer 14

-Contralateral (on opposite side)

-Sensory input from right side of body (or right visual field) is processed in left half of the brain: VICE VERSA

-Motor output to right side of body is generated in left half of brain: VICE VERSA

Question 15

Explain: Signal transmission & Interpretation: Maps

Answer 15

Inside specific sensory organs, signals arrive at positions corresponding to the position of the receptor cells (topographic representation):

Somatotopic map: signals arrive in the hand area of somatosensory cortex, next to arm area, which is next to should area, etc

Retinotopic map: similarly, visual signals from neighbouring retinal positions arrive at neighbouring positions in primary visual cortex

Tonotopic map: auditory signals from adjacent areas of cochlea arrive at adjacent areas in primary auditory cortex

So, multiple, differing maps exist for each sense modality

Question 16

Explain: Basal Ganglia

Answer 16

-Group of nuclei surrounding thalamus

-Consists of: globus pallidus, putamen & caudate:
putament & caudate referred to as corpus striatum (striped body)

-Functions: involved in motor control process (modulate movements, particularly involved in selective inhibition of movements)

-Amygdala closely connected to this system, therefore sometimes described as being part of basal ganglia (but usually described in context of limbic system)

Question 17

Explain: Limbic system

Answer 17

-Several interconnected cortical & sub-cortical areas, playing a crucial role in memory & emotion

-Sub-cortical: almost complete circle formed by fornix & hippocampus, ending in mammillary body & amygdale

-Cortical: cingulate cortex directly above corpus callosum (evolutionary older, more ‘primitive’ than rest of cortex)

-Connected to hypothalamus (spectum) & olfactory system

Question 18

Explain: Cortex & Corpus callosum

Answer 18

-Cerebral cortex: thin layers of neurons covering whole hemisphere (i.e., not just outside, but inner (medial) surface too

-Highly folded, forming gyri & sulci

-longitudinal fissure: largest sulcus, separating left & right hemisphere
-smaller sulci used to define boundaries of cerebral lobes:

Cerebral cortex is divided into 4 lobes:
Occipital lobe (at the back): Visual perception
Temporal lobe (at the sides): Auditory perception
Parietal lobe (at the top): Somatosensory perception, inter-sensory & sensory-motor integration
Frontal lobe (at the front): planning & motor output

Question 19

Define: Gyrus/gyri & Sulci/sulcu

Answer 19

Gyri: (s. gyrus, outward folded areas)
Sulci: (s. sulcus, inward folded areas)

Question 20

Explain: Corpus Callosum

Answer 20

-Corpus callosum: thick bundle of axons connecting the 2 hemispheres

-Virtually all signals transfer between cortices of hemispheres done via cc

Question 21

Explain: Relationship between Cerebral cortex, lobes & corpus callosum

Answer 21

Lobes: specific regions of the cerebral cortex, each with specialised functions

Cortex as a whole is responsible for higher-order brain functions, and it encompasses the lobes

Corpus callosum: connects the two hemispheres of the cortex, allowing for the integration and coordination of information between the left and right sides of the brain.

Question 22

Explain motor output:

Answer 22

Cortical motor areas are massively interconnected with 2 sub-cortical structures, forming complex motor control circuits:

-Basal ganglia
-Cerebellum

Both receive input from motor cortex, sensory cortex & other sub-cortical structures

Motor signals then sent down spinal cord

Question 23

Describe: The Cortical motor areas

Answer 23

-Located in frontal cortex, at boundary to parietal cortex

-Supplementary motor cortex & premotor cortex: involved in planning, monitoring & sensory guidance of movements

-Primary motor cortex: final execution stage: its motor neurons send axons directly down spinal cord (the pyramidal tract)

Question 24

Explain: Cerebellum (function)

Answer 24

Functions: involved in maintaining posture & balance, timing of movements & motor learning

Question 25

Direction of signal transmission

Answer 25

Neurons transmit signals in only one direction: dendrites to cell body (soma) to the end of the axon, but receive signals from different sources

Some input comes from earlier (functionally ‘lower’) processing stages (‘bottom up’ or ‘feed-forward’) (means basic information/initial stages of processing)
Other input comes from functionally ‘higher; processing areas (‘top down’ or ‘feed back’)

Yet other input comes from neighbouring neurons in the same area (‘lateral’)

THUS: combination of feed-forward & feedback signal transmissions = signals are never just passively ‘forwarded’: every ‘input’ is already modified by everything else going on in the brain
We cannot perceive the world ‘objectively’