The Myelin Sheath

The Myelin Sheath
Boron & Boulpaep, fig 7-21
1
Continuous and saltatory propagation
z
z
z
Unmyelinated nerve. Na+ and K+ channels are
uniformly distributed. The inward current of the
ap flows laterally and depolarizes adjacent
membrane areas. This leads to continuous ap
propagation at ~1 m/s.
Myelinated nerve. Na+ channels are confined to
nodes of Ranvier. The lateral current spreads
passed the myelinated segment and triggers an
ap at the next node of Ranvier at ~100 m/s.
In both cases ap propgation is unidirectional
because of Na+ channel refractory period.
2
The Myelin Sheath
Germann & Stanfield, fig 8.5
3
The Myelin Sheath
Boron & Boulpaep, fig 16-14
z
Found throughout the entire
CNS and PNS
z
Myelin
z Surrounds and insulates
axons
z Provides 50-fold increase in
conduction velocity
z Created and maintained by
two cell types:
z
CNS: Oligodendrocyte
z provides multiple myelin
segments for multiple axons
z
PNS: Schwann cell
z Provides single myelin
segment to a single axon
4
5
Nerves & Neurons
Boron & Boulpaep, fig 10-12
6
White & Grey matter
Sherwood 6th ed, Ch5
7
The Nervous System
Germann & Stanfield, fig 8.1
8
The Nervous System
Sherwood 6th ed, Ch5
9
Somatic
Nervous System
Germann & Stanfield, fig 10.8
10
Boron & Boulpaep, tab 15-8
11
Summation
Germann & Stanfield, fig 9.8
12
Summation
Sherwood 6th ed, Ch4
13
z
z
z
z
EPSP (excitatory postsynaptic potential) due to
inflow of Na+ through an ionotropic receptor.
IPSP (inhibitory psotsynaptic potential) due to
Cl- influx.
Summation: EPSPs (or IPSPs) can summate
spatially (from different synapses) or
temporally (repetitively from 1 synapse).
Usually several EPSPs need to summate to
activate a postsynaptic ap (except NMJ).
14
CNS: cell body with many synapses
15
Muscle motor end plate
16
Motor end plate
z
Specialised site on a muscle cell, where an
α-motoneuron forms a synapse called:
z The neuromuscular junction
z Only 1 per muscle cell
17
Germann & Stanfield, fig 12.15
Neuromuscular transmission
18
Neuromuscular transmission & the
motor end plate
19
Neuromuscular transmission
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
AP arrives at terminal of α-motoneuron
Terminal α-motoneuron voltage-gated Ca2+ channels open
Ca2+ influx into terminal of α-motoneuron
Ca2+ activates migration of ACh vesicles toward synapse
Exocytosis releases ACh into synaptic cleft
ACh diffuses across synaptic cleft
ACh binds nicotinic receptor on sarcolemma in motor end plate
Nicotinic AChR’s generate EPP via small influx of Na+ into
muscle fibre at motor end plate
EPP migrates away from motor end plate
EPP triggers voltage-gated Na+ channels to open
AP generated in muscle cell
20