Lecture 1: Nervous System – Structure and Function Flashcards
How can we prove that the brain/nervous system is actually important?
damage to something in head and/or brain (and nowhere else) can result in problems with using other body parts
What is the vertebrate CNS entirely encased in?
protective bony armour
except retina
What are the protective structures of the central nervous system? (5)
- skull
- vertebral column
- meninges (membranes)
- cerebrospinal fluid (CSF)
- blood brain barrier (BBB)
How does the skull protect the CNS?
cranium – casing made of 8 bones cemented together with sutures (special joints) that supports and protects brain
What sort of injuries does the skull protect the brain from?
blunt injuries
What is the spinal/vertebral column?
group of bones (vertebrae) that support spinal cord
each vertebrae has hole that contains the running spinal cord
What are the components of the central nervous system?
- brain
- spinal cord
(everything inside bony shells)
What sort of injuries does the vertebral column protect the brain from?
puncturing injuries
What sort of injuries does the vertebral column NOT protect the brain from?
blunt injuries (there are so many processes that can be cracked)
What are meninges?
three layers of protective membranes that lie between skull and brain
What are the 3 meninges?
- dura mater
- arachnoid mater
- pia mater
What do meninges primarily consist of?
elastic collagen fibres
What is the dura mater?
outer layer – thickest and toughest
What is the arachnoid mater?
middle layer – has extra structures to separate from third layer
What is the pia mater?
inner layer – thinnest and softest
Between which meninges can CSF be found?
in the space between arachnoid and pia layers
What does CSF do?
cushions brain from skull (and vertebrae)
What sort of injuries would meninges protect the brain from?
- friction – layers of membranes allow organs to slide around
- puncturing injuries
What are the contents of CSF?
- carefully balanced ionic concentrations
- variety of neurotrophic factors, other proteins, and nutrients
What is CSF produced by?
secreted by cells in choroid plexus within brain’s ventricular system
choroid plexus is associated with blood vessels
Where does CSF circulate/flow?
- flows within subarachnoid space, through ventricles, and drains back into bloodstream at sagittal sinus
- percolates through brain tissue via glymphatic system
- some CSF flows into spinal cord, but most flows out and surrounds outside
What is the glymphatic system?
system of blood vessels that forces CSF into brain tissue
lined by glial cells that modify rate of flow
(system is also under circadian control – highest during sleep)
What sort of protection could be provided by CSF?
acts as cushion – when soft brain tissue slams against hard bony shells
What extra protection would come from forcing CSF to flow directly through brain tissue (glymphatic system)?
remove metabolic waste – brain does not have lymphatic system
Can CSF production be altered?
yes – it is a dynamic, responsive process
can depend on the state of the animal (ie. awake vs. asleep, mutation vs. wild type)
TRADE-OFF: What’s the disadvantage in having a rigid skull encasing fluid-producing structures?
in adults, bones of skull are fused, therefore blockages to CSF (or bleeding in brain) cannot increase volume, so intracranial pressures rise
What is hydrocephalus?
complication seen in very young children (before skull sutures close) when blockages prevent CSF circulation or draining, causing overly large head growth and neurological symptoms
What is trephination/trepanation?
drilling of holes in skull – used to release intracranial pressure
Are brain capillaries less permeable than other capillaries in the body (ie. heart capillaries)?
yes – HRP diffuses from capillary lumen into tissue beyond endothelial cells in heart, but does not reach brain tissue or even perivascular space in brain tissue
What kind of anatomical specializations do capillaries in the brain have?
specializations that control and seal them tightly
What is the blood brain barrier (BBB) created by?
tight junctions between capillary endothelial cells that prevent water and solute movement
What is the function of the blood brain barrier (BBB)? (2)
- prevents entry into brain of circulating cells in blood
- limits diffusion of many molecules which circulate in blood into CNS
What type of barrier is the blood brain barrier (BBB)?
selective barrier – some molecules can cross it and enter the brain
- free diffusion across endothelial cell membranes
- facilitated diffusion across capillary cell membranes
What is the blood brain barrier (BBB) created by?
tight junctions between capillary endothelial cells that prevent water and solute movement
What are astroglia?
support cells within brain that surround brain-side of capillary
What are the functions of astroglia? (2)
- contribute to formation and maintenance of BBB
- can locally control capillary diameter
What sort of protection could be provided by the BBB?
- protect from microbiological hazards
- protect from toxins
- keep out certain medications
- protect from messenger molecules that may be circulating the blood that could influence what goes on in brain
What does the brain need to do in order to keep its metabolism going?
needs molecules carried in blood to get across BBB
What affects molecules’ ability to diffuse across cell membranes?
their chemical properties
TRADE-OFF: What does the brain do for metabolism if the BBB keeps polar substances out?
- free diffusion (across endothelial cell membranes)
- facilitated diffusion (across capillary cell membranes)
What molecules use free diffusion (across endothelial cell membranes)?
small and hydrophobic molecules
What molecules use facilitated diffusion: (across capillary cell membranes)?
polar and/or large key nutrient and signalling molecules that cannot diffuse on their own
What is required for facilitated diffusion?
endothelial cells must express specific transporter proteins for each molecule that needs to cross BBB
BBB is a key factor that affects…
efficacy of drugs in the brain
Imagine you are working for Big Pharma: You need to create a drug that can be swallowed or injected, but will still have effects on the brain. What properties should it have? Why?
soluble in water – needs to be able to go through blood
- blood can be the transport system into brain
soluble in lipids – this is the only way it will get across BBB
- there is no transporter since it is an invented drug
How does BBB look different when looking at live vs. preserved tissue? Why?
in vivo: perivascular space between astrocyte end feet and capillary wall is clearly visible
post mortem (fixed) tissue: perivascular space is greatly reduced or fully absent
this is because process of fixing tissue (with formalin or alcohol) involves some level of dehydration and tissue shrinkage
Where are the semi-impermeable barriers?
there are separate semi-impermeable barriers between blood, choroid, CSF and brain
How does each semi-impermeable barrier differ?
each barrier has slightly different gross anatomy and cell biology (ie. different levels of tight junction and transporter proteins), therefore limit ion and fluid flow differently
What are the 4 semi-impermeable barriers?
- blood brain barrier (BBB)
- blood-CSF barrier
- brain-CSF barrier
- CSF-brain barrier
Where is the blood brain barrier (BBB)?
What is it the source of?
occurs between blood and brain tissue
source of O2 and glucose into brain
Where is the blood-CSF barrier?
What is it the source of?
occurs between blood and choroid plexus (CSF secreting cells)
source of water into brain
Where is the brain-CSF barrier?
What is its function?
occurs between subarachnoid space and brain surface
keeps wastes from getting back into brain
Where is the CSF-brain barrier?
occurs between ventricles and brain tissue
glymphatic system
What is the permissibility of the CSF-brain barrier?
fairly permissive in most adults, but can vary
Can gross structure reveal the function of the nervous system?
yes, it is possible to learn about nervous system’s role from its gross anatomy – if you are careful
How is the PNS connected to CNS?
PNS radiates from CNS in hub-and-spoke organization, which originate and/or terminate in many locations throughout the rest of the body
PNS vs. CNS
PNS: outside the bones – everything that is nervous tissue connected to CNS
CNS: inside the bones
What does afferent mean?
relating to nerves that run from periphery into CNS
What function do all afferent nerves have?
sensory function
What does efferent mean?
relating to nerves that run from CNS to peripheral structures – including (but not only) muscles
What is the somatic nervous system?
portion of nervous system that innervates muscles and integument (skin, tendons, etc.)
What is the autonomic/visceral nervous system?
portion of nervous system that innervates viscera and other non-somatic organs – organs that you control, that is not skeletal muscle
What does this view of the nervous system as a whole suggest to you about its function?
nervous system’s function is coordination of the rest of the body – the rapid and specific integration and transfer of information between different body systems and different regions
- constantly collecting information from afferents about what is going on in the body
- can target what it’s saying to various places
- to understand how it does this, need to take a closer look at brain and spinal cord
What type of protection do the protective structures and adaptations of the CNS have?
both physical and chemical protection and isolation
What is the BBB?
complex and dynamic ‘fence’ between CNS and rest of body
What does the BBB do?
- controls access of blood born molecules into CNS
- locally regulates blood flow to different regions of CNS
Is the BBB highly regulated?
yes, by specialized cells
What does the BBB have profound consequences for?
practical manipulation of vertebrate brain (ie. by metabolic fuel, drugs)
When are relationships in gross anatomy meaningful?
CNS and PNS are a connected whole – it is only when considering the diverse and ubiquitous innervations by the PNS that structure-function relationships in gross anatomy are meaningful
function of brain cannot be considered in isolation
What effects might breakdown of the BBB have on the CNS? (4)
- immune cells enter CNS and disrupt its function
- chemicals from blood enter CNS and disrupt its function
- ionic composition of CNS ECF is disrupted by mixing with plasma
- neurotrophic factors from CNS leach out through the BBB
What are some reasons why it would be dangerous for an animal if the flow of CSF through this system is disrupted by a blockage?
- increased pressure can cause seizures/neurological dysfunction
- failure to remove waste products from neurons
- failure to keep brain supplied with neurotrophic factors
- less shock absorption
