Lecture 11 Neurons and Neural signals Flashcards
Functions of the Nervous system
sensation
communication
integration
control
Neurons
functional cells of the nervous system
excitable cells- generate electrical signals (changes in membrane potentials)
communicate information in the form of electrical and chemical signals
Cell body of a neuron
contains the nucleus and most organelles
dendrites
branch from the cell body
receive signals from other cells through synapse
axon
extends from the cell body, conducts action potentials
axon hillock
region where axon joins the cell body
Trigger zone
initial segment adjacent to the axon hillock is the trigger zone for AP
axon terminals
contain vesicles with neurotransmitter, form synapses with other cells
Central Nervous system
brain and spinal cord
where most neurons are located
Peripheral Nervous System
Nerves, ganglia and sensory receptors
Afferent Division
Efferent Division
Afferent Division
Sensory neurons, input to CNS from sensory receptors
somatic sensory- from skin, muscles, bones & joints
Visceral sensory- from internal organs
Special senses- vision, hearing, equilibrium, olfaction, taste
Efferent Division
Motor neurons, output from CNS to effectors
Somatic Motor- to skeletal muscles (voluntary)
Autonomic (ANS) - to heart, smooth muscle, gland, adipose tissue (involuntary)
a.sympathetic
b. parasympathetic
Enteric Nervous System
Nerve network of the GI tract
Functional Types of Neurons
Sensory neurons
Motor neurons
Interneurons
Sensory Neurons
afferent
Input to CNS from sensory receptors; dendrites located at receptors, axon in nerves, cell bodies in ganglia outside the CNS
Motor Neurons
efferent
Output from CNS to effectors
cells bodies and dendrites located in the CNS, axons in nerves
Interneurons
communicate and integrate information within the CNS
located entirely within the CNS
Most common
Astrocytes
CNS
structural and chemical support, blood-brain barrier
Oligodendrocytes
CNS
Myelin in CNS
Microglia
CNS
Phagocytes
Ependymal cells
CNS
produce CSF
Schwann cell
myelin in PNS
satellite cells
in PNS ganglia
Graded Potentials
small, localized changes in membrane potential
formed at the cell body and dendrites
can be depolarization or hyperpolarization
spread passively and weaken with distance
size depends on stimulus strength
seen at cell bodies and sensory receptors
Stimulates action potentials
caused by opening of chemically gated channels
must exceed threshold to start AP
Action Potentials (nerve impulses)
large change in membrane potential
formed along the axon
rapid depolarization followed by repolarization
actively conducted along the axon
all or none- size is not dependent on stimulus strength
Doesn’t weaken with distance
Phase 1 of action potentials
Rising (Depolarization) phase
- initial depolarization stimulus must be above threshold to form an AP
- voltage gated Na+ channels open
- activation gate opens in response to initial depolarization
- > rapid Na+ inflow -> rapid depolarization
Phase 2 of action potentials
Falling (repolarization) Phase -Voltage gated Na+ channels close inactivation gate - closes when depolarization reaches peak -voltage gated K+ channels open ->rapid K+ outflow-> repolarization
Phase 3 of action potentials
Undershoot
voltage gated K+ channels remain open, high K+ permeability results in hyperpolarization
resting states of channels and resting potential restored at end of undershoot phase
-both voltage gates closed only the leak channels open when RP is restored
Name the properties of action potentials
threshold
all or more
regenerative
refractory period
Threshold
stimulus must be greater than a certain strength to evoke an AP
(subthreshold - cant start AP below threshold)
enough activation gates must open
-55mV
“All or None”
once threshold is reached size of the AP is constant regardless of stimulus
regenerative
AP is regenerated and does not decrease in strength along the axon
Refractory period
short delay following an AP before another AP can be formed
absolute refractory period
ARP
period in which another AP can not be formed
relative refractory period
RRP
period in which a larger stimulus is required to form another AP
Main importance of refractory period
ARP sets an upper limit on frequency of AP
During RRP, a stronger stimulus can result in increased frequency of APs
-stimulus intensity is coded by the frequency of APs
Refractory period prevents AP from traveling backward along the axon
Are concentration gradients affected during an AP
No they are not, Na+/K+ pump still maintains gradients
How are stimulus intensity coded
by the frequency of AP
Unmyelinated Axons
AP depolarization spreads a short distance down the axon
depolarization stimulates formation of AP farther down the axon
axons are leaky to Na+ and K+; need to regenerate AP often along the axon -> slow conduction speed
increasing axon diameter increases conduction speed
Myelinated Axons
myelin sheath formed by membrane of schwann cells in PNS and oligodendrocytes in CNS
insulates axon, reduced leakage of Na+ and K+
nodes of ranvier- gaps in myelin sheath are sites of AP regeneration
AP “jumps” from node to node (saltatory conduction)
faster conduction speeds (up to 120 m/s)
