APPP 03: Gross Anatomy of the CNS – Spinal Cord and CNS Protection Flashcards
What are the 2 structures of the central nervous system?
- brain
- spinal cord
Describe the structure of the spinal cord.
- continuous with the medulla of the brainstem
- segments – 7 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal
- grey matter – shape of an H with a dorsal-ventral orientation
- white matter (myelinated axons) – surrounding area
What does the spinal cord do?
large-diameter and small-diameter sensory neuron fibres enter through the dorsal horn and feed into axons of motor neurons involved in rapid, involuntary movements (reflex arc), and signal to cortex
What is the reflex arc?
- incoming sensory signals from the body (afferent sensory nerve fibres) synapse at the posterior/dorsal horn with interneurons found in the intermediate grey
- interneurons signal to efferent motor nerve fibres in the anterior horn to stimulate muscle movement
Describe the pathway of afferent nerves.
propagates information toward the CNS (coming in)
- activation of pain receptor
- AP travels through dorsal root ganglion nerve fibre
- synapse at posterior horn/dorsal horn (PH)
- action on afferent nerves leading to actions at specific sites in the brain
Describe the pathway of efferent nerves.
propagate information away from CNS to a muscle or gland (relay signals going out)
- efferent nerves from cortex signal to anterior horn
- motor movements
How does the spinal cord link to the autonomic system?
intermediate grey matter contains autonomic neurons with axons that leave through ventral roots
Describe the structure of the skull.
- 22 bones
- cranial bones enclose and protect the brain – frontal, temporal, occipital, parietal, sphenoid, and ethmoid
What is the spine protected by?
vertebrae
What are meninges?
form a major part of the mechanical suspension of the CNS – necessary to keep it from self-destructing
What are the 3 layers of meninges?
- dura mater
- arachnoid
- pia mater
What is the dura mater?
thick connective tissue membrane
- below skull
What is the arachnoid?
thinner collagenous membrane
- cellular layer
What is the pia mater?
thinner collagenous membrane
How do the three layers of meninges connect to each other?
- dura connects the skull to the arachnoid layer
- arachnoid connects to the pia mater cell layer by delicate strands of connective tissue (arachnoid trabeculae) – suspends the CNS in its bath of cerebrospinal fluid
What stabilizes the CNS during head movement?
partial flotation of the CNS in subarachnoid CSF, in combination with mechanical suspension
What is cerebral spinal fluid (CSF)?
- contains nutrients
- formed by filtration of blood
- enters the venous circulation through arachnoid villi
- around 150 ml coats the brain
What are arachnoid villi?
outpouchings that poke through holes in the walls of the venous system
What do arachnoid villi do?
act like valves
- when CSF pressure is higher than venous, CSF moves into the venous system
- in reverse, villi snap shut and venous fluid does not enter subarachnoid space – this imbalance can lead to intracranial pressure
What is an epidural hematoma?
bleeding occurs between the dura mater and the skull
How do epidural hematomas occur?
skull fracture causes a blood vessel (that runs through dura mater) to rupture, resulting in epidural bleeding
What can epidural hematomas lead to?
increased pressure on, and damage to, brain tissues
What are signs and symptoms of epidural hematomas?
- severe headache
- nausea and vomiting
- slurred speech
How does a stroke occur?
spontaneous rupture of blood vessel, resulting in subarachnoid bleeding
- buildup of blood can cause pressure on the brain
What are the symptoms of a stroke?
similar to an epidural hematoma
- severe headache
- nausea and vomiting
- slurred speech
What is the blood brain barrier (BBB)?
protective functional separation of the circulating blood from the CNS
What is the role of the BBB?
limit the penetration of substances
- keep toxins and pathogens out – including drugs
- facilitates the select transport of molecules
Describe arachnoid mater.
impermeable barrier to the CNS
- cell are joined by tight junctions
Describe pia mater meninges.
impermeable and joined by tight junctions
What are astrocytes?
brain glial cell type that lines the surface of the brain
What are the 4 layers of the BBB?
- endothelial cell layer
- pericyte layer
- protecting basement membrane
- glial barrier formed by astrocyte endfeet
What is the pericyte layer?
cell layer that lines around 80% of the capillary layer
What is the role of the pericyte layer of the BBB?
- physical role is to stabilize the endothelial layer
- regulation of BBB permeability
What is the role of the glial barrier formed by astrocyte endfeet of the BBB?
(astrocytes interact with blood vessels with their endfeet)
- regulate dilation and constriction of microvessels to control blood flow
What do both the pericyte layer and the glial barrier formed by astrocyte endfeet contribute to?
contributes to the maintenance and regulation of endothelial cell tight junctions (length, width, and complexity)
What is the role of the endothelial cell layer of the BBB?
has specific transport systems that permit the supply of nutrients, ions, and bioactive molecules
- diffusion of CO2 and O2 and lipid-soluble (small nonpolar) molecules (transcellular) or small water soluble (paracellular)
- active transport – ie. glucose
- receptor mediated (ie. insulin) and adsorptive transcytosis (albumin)
What is the key to proper BBB function?
maintenance and regulation of endothelial cell tight junctions
What does transport of drugs into the BBB often require?
requires the drug to engage specific transport mechanisms
ie. L-DOPA (precursor to dopamine) uses an amino acid transporter
How do we reduce uptake of drugs in the brain?
create more polar compounds
- ie. 2nd generation (modified) antihistamines cause less drowsiness
What are the 3 types of glia/support cells in the CNS?
- astrocyte – blood flow, K+ house keeper
- oligodendrocyte – myelinate
- microglia – phagocyte
What do astrocytes do?
support or protection
- role in BBB
- provide nutrients to nerve cells
- control the chemical composition of fluids around nerve cells, enabling them to thrive
What do oligodendrocytes do?
make myelin
What is myelin?
a fatty substance that insulates nerve axons and speeds the conduction of impulses along nerve fibres
What do microglia do?
help protect the brain against infection and help remove debris from dead cells
- ‘macrophages’ of the brain
What has microglia activation been implicated in?
in the inflammatory alterations observed in severe neurodegenerative disease
- Alzheimer’s disease (AD)
- Parkinson’s disease (PD)
- Huntington disease (HD)
- multiple sclerosis (MS)
- amyotrophic lateral sclerosis (ALS)
What do these neurodegenerative diseases have in common?
share pathological characteristics
- abnormal protein aggregation
- failure in protein degradation pathways
- impaired axonal transport
