Functional Organisation of the CNS

Question 1

What makes up the CNS and PNS?

Answer 1

CNS - Brain & Spinal Cord

PNS - Nerves and Ganglia

Question 2

What is the basic structure of the brain?

Answer 2

Basic Brain Structure:

• -> Cerebrum (surface of the top of the brain), cerebellum, brain stem (midbrain, pons, medulla), spinal cord, hypothalamus, thalamus, basal ganglia, limbic system
• -> When the brain is cut into, there is obviously two types of matter: Grey and White
• -> Modern imaging techniques are able to visualise grey and white matter

Question 3

What is the difference between white matter and grey matter?

Answer 3

Grey Matter: neuron somata and glia; where the neurons are with dentrites and synapses
White Matter: myelinated axons and glia; connections between different parts of the brain

Question 4

What are white matter tracts and how are they visible?

Answer 4

–> Modern imaging (MRI etc) can be adjusted so as to only see the fibres of axon tracts in the white matter showing the relationship of the these tracts to different areas of the brain = diffusion weighted MRI

Question 5

Describe the Anatomical Directions in NS of primates?

Answer 5

–> Brain has a major kink between the midbrain and diencephalon
–> The brainstem runs vertically along body axis and this means that due to development of human brain ‘dorsal’ is both at the back of the neck and the top of the head
CAN BE CONFUSING!!

Question 6

Give some features of the cerebrum?

Answer 6

Cerebrum:
• Two cerebral hemispheres; Split by longitudinal fissure
–> These two hemispheres are joined deep in the fissue by the corpus collosum
• Brain is narrower at the front (rostral) and wider at the back (ventral)
• Ridges - gyri/gyrus
• Grooves - sulci/sulcus
• Surface is cortex
• Site of language, memory, emotions, self awareness
• Organised into four lobes basically but this has limited functional significance because of the mapping of the regions and the functional regions of the cerebral cortex not correlating (occipital, parietal, frontal, temporal)
• You can further divide it by giving names to gyri and sulci however this too has limited functionality

Question 7

How is the cerebrum generally divided?

Answer 7

• Organised into four lobes basically but this has limited functional significance because of the mapping of the regions and the functional regions of the cerebral cortex not correlating (occipital, parietal, frontal, temporal)
• You can further divide it by giving names to gyri and sulci however this too has limited functionality
• Instead, the Brodmann Map is used to divide the brain and is the result of micrscopic analysis of the whole brain looking for differences in function and organisation; it is still referred to today
• Today, we look at functionality based on experimental data and functional assessment (strokes etc) however all of these methods have
both benefits and disadvantages
• Functional imaging is also used to look at living brains and look at activity while performing specific tasks

Question 8

What is the cellular structure of the cortex?

Answer 8

•   Superficial layer of grey matter over a core of white matter
•   Layered structure
•   Pyramidal neurons + interneurons
•   Left and right cortices linked at corpus callosum

Question 9

Give some features of the cerebellum?

Answer 9

Cerebellum:
• Contains 70% of the neurons in the brain but a far smaller volume and so it serves an important purpose
• Tucked in behind the occipital lobe
•  Key element in motor control
•  Compares what you want to do with want you are doing
–> Tight feed back between what you want to do and what you are actually doing to ensure that you are graceful
–> otherwise appear clumsy
•  Refines execution of motor program
• May also be used in language and other functions

Question 10

Give some features of basal ganglia?

Answer 10

Basal Ganglia:
• Group of nuclei
•  In telencephalon; live in the core of the brain
•  Also important in motor control
•  Selects and initiates voluntary movements
•  Forms loops with cortex; critical for the formation of motor programs
•  Damage leads to movement disorders such as Parkinson’s disease and Huntington’s disease

Question 11

Give some features of the thalamus?

Answer 11

Thalamus:
•  Part of diencephalon; there are two (one on each side)
• Around the size of quail eggs
•  Major sensory relay to cortex; relays sensory information from periphery and processes it before it goes to the cortex.
• Contains Subnucelei

Question 12

Thalamic nuclei project in three ways, what are they?

Answer 12

1.   Relays sensory information to specific areas of cortex
2.   Relays of non-sensory information from cortex and basal ganglia to specific areas of cortex
3.   Project globally to cortex; arousal, sleep; thalamus allows you to switch between the two states

Question 13

Give some features of the hypothalamus?

Answer 13

Hypothalamus:
•  Part of diencephalon; only a few mm in size below the thalamus
•  Regulates homeostasis (temperature, blood volume and pressure, ion concentration, pH, O2 and glucose)
•  Controls pituitary which is on the bottom of the hypothalamus (master endocrine gland; regulates hormones)

Question 14

Give some features of the brainstem?

Answer 14

Brainstem:
•  Controls:
- Facial muscles
- Sensation from face and head
- Cardiorespiratory control; drives breathing and helps the hypothalamus to regulate BP
- Arousal, sleep/wake cycle; damage to brainstem can cause permanent coma
•  Cranial Nerves run from the brainstem

Question 15

Give some features of the cranial nerves?

Answer 15

Cranial Nerves:
•  Sensory and motor supply to/from face and deeper structures via ten of the twelve cranial nerves
•  Cranial nerves can be purely motor, purely sensory or mixed

Question 16

Give some features of the spinal cord?

Answer 16

Spinal Cord
•  Part of CNS
•  Connects to medulla (most distal part of brainstem)
•  Enclosed in vertebrae
• Can be divided into grey and white matter
- Core of grey matter
- Surrounded by white matter (axons that run up and down the spinal cord connecting different parts (local circuits) or all the way into the brain or from the brain into spinal cord (motor elements))
–> Dorsal (posterior) horns, ventral (anterior) horns, intermediate zone
•  Spinal cord is divided in relation to the vertebrae
- Comprises cervical, thoracic, lumbar and sacral divisions
•  Varies in shape and size over the length of the spinal cord
- Enlarged at lower cervical and lumbar regions (for limbs); more grey matter due to the muscles in the limbs

Question 17

Give some features of spinal nerves?

Answer 17

Spinal Nerves:
•  Ends around L1/L2; spinal cord is shorter than the vertebral column
•  For each vertebra, there is corresponding spinal segment
•  Each spinal segment gives rise to a pair of spinal nerves; this includes that region beyond L1/L2 and so the bottom part of the vertebral canal is not filled with spinal cord but spinal roots instead

Question 18

What is the difference between dorsal and ventral roots?

Answer 18

Dorsal And Ventral Roots:
•  Each spinal nerve formed by dorsal and ventral roots
- Dorsal root = sensory axons (not part of the central nervous system and are instead a part of the PNS and lie within the dorsal root ganglia)
- Ventral root = ventral horn motor axons (control skeletal muscle of body) (Efferent Neurons = brain –> periphery)
•  Dorsal and Ventral Roots connect to form the spinal nerves

Question 19

What are dorsal root ganglia?

Answer 19

Dorsal Root Ganglia:
•  Dorsal root contains neuronal somata forming a ganglion (dorsal root ganglion, DRG)
•  Hidden between vertebrae; between the vertebral joints
•  Sensory neurons; also called primary afferent (periphery –> brain) neurons

Question 20

Give some features of the dorsal root ganglion?

Answer 20

Dorsal Root Ganglia Neurons/Primary Afferent Neuron/Sensory Neuron:
•  Branching axon; splits immediately after it leaves the cell body; one goes into the body to sense things and one goes into the CNS in the spinal cord)
•  One branch in peripheral nerve – detects stimulus
•  One branch enters dorsal horn via dorsal root
•  Some cranial nerves have cranial nerve ganglia (equivalent in function to DRGs)

Question 21

Give some features of peripheral nerves?

Answer 21

Peripheral Nerves:
•  Most peripheral nerves are mixed (both sensory and motor axons)
•  Because the dorsal and ventral fibres mix and so damage to a nerve will cause both motor and sensory problems

Question 22

What are dermatomes?

Answer 22

Dermatomes:

•  The region of body innervated by a bilateral pair of dorsal root ganglia is a dermatome

Question 23

What are ventricles in the brain?

Answer 23

Ventricles:
•  Hollow centre of brain forms ventricles; due to the development of the brain as an embryo
•  Complex 3-d shape
•  2 x lateral ventricles, 1 x third and 1 x fourth ventricle, cerebral aqueduct
•  Full of cerebrospinal fluid (CSF)
•  Landmark in MRI and CT; easy to visualise

Question 24

What is CSF? What is its purpose?

Answer 24

Cerebrospinal Fluid (CSF):
•  Vascular choroid plexus forms CSF - ultrafiltrate of plasma ( CSF is has different ion concentration such as low levels of potassium etc)
•  Choroid plexus present in ventricles which produces the CSF to fill the ventricles
•  CSF flows out of ventricles at fourth ventricle
•  Brain floats in CSF, acts as shock absorber

Question 25

What are the meninges?

Answer 25

Meninges:
•  CNS is protected by bone of skull and vertebrae
•  Second layer of defence are meninges; mechanical protection
•  Three layers of thick connective tissue
1.  Dura (thickest, outermost)
2.  Arachnoid (fibrous, middle)
3.  Pia (thinnest, innermost)

Question 26

Give some features of the sub-arachnoid space?

Answer 26

Sub-arachnoid Space:
•  CSF fills open subarachnoid space
•  CSF resorbed by major veins at arachnoid granulations

Question 27

What are the falx and tetorium in the brain?

Answer 27

Falx and Tentorium:
•  Falx divides cerebral hemispheres
•  Tentorium separates cerebellum from occipital lobe
•  The meninges extend as far as possible into the grooves of the brain and forms the falx and tentorium.
•  These take the open space in the skull and divides it so that the brain cannot move; means that if the brain swells it has nowhere to go and thus compresses the brain

Question 28

What are some features of the BBB?

Answer 28

Blood Brain Barrier:
•  CSF is different to plasma
•  Neurons are sensitive to fluctuations in ion concentration
•  Extracellular fluid in brain is protected by blood brain barrier
•  Formed by endothelial cells of capillaries
•  Lack pinocytotic transfer, have very tight tight junctions
•  Most transport is active