Unit 2 Flashcards
1
Q
how did the vertebrate central nervous system develop
A
-neural plate bends
- is joined together at the neural fold
- epidermis forms on top
- neural crest cells migrate through body to form peripheral nervous system
- neural tube is formed which is the precursor of the CNS
2
Q
development of the human CNS
A
4 weeks:
- anterior end of neural tube specialized into three regions (forebrain, midbrain, hindbrain)
6 weeks:
- neural tube differentiated into major brain regions present at birth (Medulla oblongata, Cerebellum and Pons, midbrain, Diencephalon, Cerebrum
11 weeks:
- growth of cerebrum much more rapid than that of other regions
Birth:
- cerebrum covers most of other brain regions; convoluted surface due to rapid growth in confined space
3
Q
how is the CNS protected and supported
A
- surrounded by bony cage – cranium, vertebrae
- three layers of connective tissue - meninges
- fluid between layers – cerebrospinal fluid
4
Q
what are the 3 meninges
A
- dura mater
- arachnoid mater
- pia mater
5
Q
what are the fluid filled ventricles in the brain
A
- ventricles within brain, hollow central canal within spinal cord
- two lateral ventricles, two descending ventricles that extend through in brain stem
- CSF in ventricles continuous with fluid in central canal of spinal cord
6
Q
what is choroid plexus
A
where CSF is created in each ventricle
7
Q
what are ependymal cells
A
cells that line the choroid plexus and determine the composition of CSF
8
Q
what is interstitial fluid
A
surrounds neurons and glial cells
9
Q
what is plasma
A
within cerebral blood vessels
10
Q
what is CSF
A
- within ventricular system
- bathes external surfaces of brain, between meninges
11
Q
compared to plasma, CSF has
A
- lower K+, Ca2+, HCO3- , glucose, pH similar Na+
- very low protein, no blood cells
- increase presence of blood cells or elevated protein in CSF collected via lumbar puncture (sampling of fluid from subarachnoid space between vertebrae) suggests infection
12
Q
how much CSF is removed daily
A
- removed and replaced about 4 times daily
- produce about 500ml of CSF daily
13
Q
how is CSF removed
A
- flows through arachnoid villi back into venous blood
14
Q
circulation of CSF
A
lateral ventricles <–> Third ventricle <–> fourth ventricle –> subarachnoid space –> arachnoid villi –> superior sagittal sinus –> venous return to heart
15
Q
special features of cerebral vasculature
A
- very tight junctions
- not many things get through
16
Q
blood brain barrier
A
- lipid soluble molecules cross readily
- hydrophilic substances (ions, amino acids, peptides) will only cross if specific transporters / carriers are present on endothelial cells of capillaries within CNS
- considerations for drugs that are and are not wanted to reach the
CNS:
– antihistamines
– treating diseases of the CNS
17
Q
metabolic needs of neural tissue
A
- oxygen requirement:
- neurons are ‘obligate aerobes’
– unable to switch to anaerobic metabolism
– O2 readily crosses blood-brain barrier - glucose requirement:
- capillaries of CNS express high levels of glucose transporters to provide adequate levels of glucose
- brain responsible for approximately half of body’s glucose consumption
vasculature to deliver oxygen and glucose:
- approximately 15% of cardiac output received by brain
– critically dependent on adequate O2, glucose (and therefore blood flow)
- hypoglycemia = confusion, loss of consciousness, death
18
Q
spinal cord
A
- major path for information flow between CNS and skin, joints, muscles
- contains neural networks involved in locomotion
- divided into four regions (cervical, thoracic, lumbar, sacral), each of which is divided into segments
- each segment gives rise to pair of spinal nerveswh
19
Q
what are the ascending tracts for
A
- dorsal columns : touch/pressure, proprioception
- spinocerebellar: proprioception (posture, coordination)
- spinothalamic: pain, temp
20
Q
what are the descending tracts for
A
- corticospinal tracts: voluntary movement
21
Q
brainstem
A
- oldest and most primitive part of brain
- contains structures derived from
embryonic hind and midbrain - organized much like spinal cord
- most (10 of 12) cranial nerves
originate from here - carry sensory and motor info for
head/neck - cranial nerve X = vagus
- contains nuclei associated with reticular formation
- diffuse network of neurons involves
in processes such as arousal/sleep, muscle tone, coordination of breathing, blood pressure, et al
22
Q
functions of brain stem structures
A
- midbrain:
- coordination of eye movement, visual and auditory reflexes - pons:
- relay station between cerebrum and cerebellum
– works with medulla to regulate breathing - medulla:
- gray matter involved in control of many involuntary functions – blood
pressure, breathing, swallowing, vomiting
– white matter – ascending somatosensory tracts, descending corticospinal tracts
– site of decussation (crossing over) for most neurons in corticospinal tract
23
Q
diencephelon
A
- between brain stem and cerebrum
1. thalamus: - relays and integrates sensory info from lower parts of CNS, ears, eyes, motor info from cerebellum
2. hypothalamus: - tiny region of brain yet major centre for homeostasis
- contains centres that drive behaviour related to hunger, satiety, thirst
- influences autonomic responses, endocrine systems
3. pituitary gland: - regulated by hypothalamus (more later in course)
4. pineal gland: - secretes hormone melatonin - involved in circadian and seasonal rhythms
24
Q
Cerebrum
A
- site of ‘higher’ brain functions
– largest and most distinctive part of brain in higher primates - each cerebral hemisphere divided into four lobes: frontal, occipital, parietal, temporal
- furrow or groove = sulcus (pl sulci)
- convolution = gyrus
25
organization of cerebrum
- three regions of cerebral gray matter:
1. ‘basal ganglia’ (more correctly - basal nuclei)
- coordination of movement
2. limbic system
- linking emotion/fear with higher cognitive functions
3. cerebral cortex
26
functional areas of cerebral cortex
1. Sensory areas:
– sensory input translated into perception (awareness)
2. Motor areas:
– control skeletal muscles
3. Association areas:
– integrate information from sensory and motor areas
27
Primary Motor and Somatosensory contexes
1. primary motor cortex
– on ridge just anterior to central sulcus
- aka precentral gyrus
– cell bodies of descending ‘upper’ or ‘first order’ motor neurons
2. primary somatosensory cortex
– on ridges just posterior to central sulcus
- aka postcentral gyrus
– terminals of ascending sensory pathways from skin,
musculoskeletal system, viscera
- information about touch/pressure, pain, temperature, body
position
3. ‘special senses’ have devoted regions
– visual cortex, auditory cortex, olfactory cortex, gustatory cortex
4. neural pathways extend from sensory areas to association areas, which integrate stimuli into perception
28
usually conscious stimulus processing
- touch
- temperature
- pain
- itch
- proprioception
29
usually unconscious stimulus processing
- blood pressure
- distension of gastrointestinal tract
- blood glucose concentration
- internal body temperature
- osmolarity of body fluids
- lung inflation
- pH of cerebrospinal fluid
- pH and oxygen content of blood
30
sensory pathway
Stimulus -- (acts on receptor) --> receptor transduces stimulus into intracellular signal (typically Δ in membrane potential, MP) -- (if Δ in MP reaches threshold) --> action potentials (APs) travel along afferent neuron -- (decoding: frequency of APs, pattern, travelling on which fibre) --> information reaches subcortical integrating / relay centres
(e.g. thalamus, medulla, ...) -- (subconscious processing) --> information reaches appropriate regions in cortex -- (conscious processing)
31
types of receptors
- Chemoreceptors
- mechanoreceptors
- photoreceptors
- thermoreceptors
32
how does signal transduction get turned into a graded potential
- each sensory receptor has an adequate stimulus (type of energy to which it responds best)
– thermoreceptors- respond best to increase in temp (vs pressure)
– mechanoreceptors- deformations of membrane that open ion channels
– photoreceptors of eye- light
- stimulus opens or closes ion channels in receptor cell membrane
- directly or via second messenger systems
- mostly: open cation channels --> influx of Na or Ca --> depolarization
sometimes: efflux of K --> hyperpolarization
- Δ in membrane potential (graded potential) = receptor potential
33
what are receptive fields
- somatosensory neurons and visual neurons are activated by stimuli that fall within a certain physical area
– cutaneous receptors – patch of skin
– photoreceptors – light falling on area of retina
- at least two afferent neurons in pathway to brain:
– first order (primary) sensory neuron
- directly associated with stimuli
– second order (secondary) neuron
- relays information from first neuron
- receptive field often defined by neurons further up the pathway
– sensory input can then be gathered from more than one primary sensory neuron
34
overlapping of primary sensory neurons
- several primary neurons converge onto a secondary neuron
- convergence allows summation of multiple stimuli
- creates larger receptive fields
35
convergence of sensory receptors
- The receptive fields of three primary sensory neurons overlap to form one large secondary receptive field
- Convergence of primary neurons allows simultaneous subthreshold stimuli to sum at the secondary sensory neuron and initiate AP
- two stimuli fall within same secondary receptive field --> only one signal goes to brain --> perceived as a single point
- no two point discrimination
36
smaller receptive fields
- The two stimuli activate separate pathways to the brain
- The two points are perceived as distinct stimuli
- two point discrimination
37
sensory pathway to CNS
- somatic senses, hearing, vision, taste: to appropriate cortex after processing in thalamus
- olfactory: direct to brain (olfactory bulb --> olfactory cortex)
- equilibrium: mostly to cerebellum, minor input to thalamus
- What about visceral sensory information?
- mostly integrated in brain stem and spinal cord
- does not usually reach conscious perception
- completely subconscious --> blood pressure
- can reach consciousness --> ‘fullness’ (pressure), pain
38
what happens to stimuli
- all stimuli converted to graded potentials --> APs
- all APs are identical
39
So how are different sensations distinguished?
- CNS must be able to decode:
– type of stimulus --> modality
– location
– intensity
– duration
