CNS basic physiology Flashcards
Astroglia
integrate/modulate signals
part of BBB
part of synaptic transmission
Microglia
blood-borne macrophages
resident immune cells of the brain
AP propagation in the brain
1) NaV open, AP generated
2) passive current flows to the next NaV
3) passive current opens next NaV, another AP generated
Resistance (AP)
lower in larger axons
Capacitance (AP)
larger in larger axons - want to minimize this for faster current propagation
role of myelin sheath
Saltatory conduction
AP is not jumping; tunneling under myelin sheath
Synapse events
1) AP opens CaV channels
2) Ca influx causes NT-filled vesicles to fuse with the cell membrane
3) NT released into synaptic cleft; binds to receptors, causes opening of ion channels
4) postsynaptic cell depolarizes, AP generated
EPSP channels
Na channels
IPSP channels
Cl- channels
Amino acid NTs
Glutamine - ex
GABA - in
Glycine - in
Biogenic amine NTs
Dopamine - D1 ex, D2 in
Norepi, epi, histamine - ex
Serotonin - in or ex
Purine NTs
ATP - ex
Neuropeptide NTs
Substance P - ex
Met-enkephalin - in
Opioids - in
Adrenocorticotropin - ex
Synesthesia
Neurological phenomenon in which stimulation of one sensory pathway –> automatic, involuntary experiences in a second sensory pathway
e.g. Grapheme
Pressure receptor types (general)
Exteroceptors
Interoceptors and proprioceptors
Meissner’s corpuscles
light touch receptors
Palcinian corpuscle
deep pressure and vibration receptors
20-60 concentric lamellae composed of fibrous CT separated by gelatinous material
Centre: inner bulb, a fluid-filled cavity with a single afferent unmyelinated nerve ending
Ruffini’s corpuscle
sensitive to skin stretch sense of position and movement
Merkel’s disc
sensitive to vibrations at low frequencies
Temporal summation
frequency coding
more impulses along a single fiber
Spatial summation
population coding
increasing number of parallel fibers that transmit information
Adaptation
prolonged presence of a stimulus –> decreased perceived intensity
all sensory receptors adapt to constant stimulation, but rate of adaptation varies
2-point discrimination
Helps to assess nerve damage
Depends on receptor density and size of receptive fields
Lateral inhibition
Blocks lateral spread of excitatory signals –> increase degree of contrast in sensory pattern
Alpha motor neuron
skeletomotor
innervate extrafusal fibers
Gamma motor neuron
fusimotor
innervate intrafusal fibers
2 main types
Small motor units
small number (e.g. 10) of fibers per motor neuron
fine movements
e.g. eye muscles
Large motor units
large number (1000s) of fibers per motor neuron
gross movements
e.g. leg muscles
Muscle spindles
stretch-sensitive mechanoreceptors
found in virtually all skeletal muscles
particularly dense in muscles connected with fine, manipulative tasks (intrinsic hand muscles, highest density in neck muscles for direction)
small, elongated structure - scattered among and parallel to contractile extrafusal fibers
CT sheath surrounding intrafusal muscle fibers
Intrafusal fibers
modified muscle fibers lacking myofibrils in the centre
1/3 length of extrafusal fibers
Provide information about muscle length and velocity of contraction to the CNS
Bag2 intrafusal fibers
largest
most important?
no striations in the middle region and swells to enclose nuclei
“bag”
Bag1 intrafusal fibers
smaller than Bag2
Bag intrafusal fibers
extend beyond capsule
based upon contraction speed and motor innervation
Chain fiber3
half as long as bag, smaller diameter
row of nuclei in the middle
“chain”
Typical muscle spindle
1 Bag1, 1 Bag2, 4 chain fibers
Muscle spindle - sensory innervation
Large diameter group Ia afferent
- enter capsules and branches
- unmyelinated terminals wrap around fiber
- annulospiral ending on nucleated Bag1, Bag2 and chain intrafusal fibers
- All 3 types of intrafusal fibers receive innervation from Goup Ia afferents
Smaller group II afferents
- enter with Ia afferents; unmyelinated spray terminals on one end of Bag 2 and chain fibers
- no Bag1
Muscle spindle - motor innervation
Gamma or fusimotor neurons
2 types of gamma:
1) static - Bag2 and chain
2) dynamic - Bag1 ONLY
Muscle spindle function
Sensory neurons fire when centre intrafusal fiber stretches
Opening of spiral endings and initiation of impulses: stretch-operated cation channels; membrane depolarization and action potential firing
Viscoelastic properties affect activation
Ends of intrafusal fibers contract when stimulated by gamma motor neurons –> affect centre of intrafusal fibers, not overall muscle tension
Group Ia: muscle length and velocity info
Group II: length only
Gamma dynamics: increases velocity sensitivity
Gamma static: increases length sensitivity
Muscle spindle sensory neurons
tonically active
Fire AP when muscle is at resting length
Signal to alpha motor neurons in ventral horn
tonic excitation –> muscle contraction in extrafusal fibers = resting muscle tension
Firing rates of gamma MN can change in different situations
Stretch reflex
Intrafusal fibers stretched –> sensory neurons fire more rapidly
Reflex contraction of muscle to relieve stretch –> stimulus removed (negative FB)
Monosynaptic and polysynaptic excitatory connections between Group Ia and II afferents and alpha motor neurons
Myotatic unit
collection of nervous pathways controlling a single joint
Reciprocal inhibition
Relaxation of antagonist muscle during agonist contraction
Divergent pathways in spinal cord and inhibitory interneurons
Hoffman reflex
electrically evoked analogue of the stretch reflex
Ia inhibitory interneuron
receives convergent input from CST and other descending pathways
