Neurons = Nerve cells
Neuron anatomy
• Cell Body = site of protein synthesis Signal
integration
• Dendrites = receive input from other
neurons
• Axon = conducts signal as electrical
impulse (action potential)
• Myelin sheath = insulates axon, produced
by Schwann cells
• Nerve terminal = site of synapses with
target cell
Ligand- or
signal-gated Voltage-
Na+ channel gated Na+ Voltage- Na+/K+
channel gated K+ pump
channel
-70 mV
K+ Hi
Na+ Hi
Action Potentials
• Resting potential = -70 mV
• Signal opens some signal-gated Na+ channels in
dendrites
• Some Na+ rushes in & raises potential =
depolarization
• If raised to ~ -60 mV, no response
• If raised to -50 mV or higher, opens lots of
voltage-gated Na+ channels – triggers an action
potential.
– -50 mV is the threshold. All-or-none response.
Action Potentials II
• In action potential, depolarizes to +40 mV
• Extreme depolarization opens voltage-gated K+
channels
– voltage-gated K+ channels require + voltage to open
• K+ rushes out down electrochemical gradient
• Membrane hyperpolarizes (repolarizes) to -80 mV
• Na+/K+ pump maintains the electrochemical
potential over long haul
Action potentials jump between
nodes of Ranvier. Speeds up signal.
= saltatory conduction
Decision to fire is based on
summation of excitatory and
inhibitory inputs
Muscle
organization
M Myosin thick
filament
Neuromuscular Transverse (T)
junction tubule
Sarcoplasmic
reticulum
Excitation/contraction coupling
• Nerve impulse releases neurotransmitters
• Bind to receptors on muscle which generate
an action potential (AP)
• AP spreads down T tubule (part of plasma
membrane)
• T tubule close proximity to sarcoplasmic
reticulum opens Ca2+ channels in SR
• Ca2+ rushes into cytoplasm
Muscle cell types
• Striated = skeletal
• Cardiac (heart): also striated
– intercalated discs = gap junctions between cells
to directly link them electrically
• Smooth = non-striated, short myosin thick
filaments throughout
– Dense bodies: Actin filaments contact plasma
membrane, cell-cell connections
– Myosin light chain kinase regulation
– Slow, graded contractions
Cumulus
layer
Cortical
granule
Fertilization (mammalian)
• Sperm forces thru’ cumulus layer
1. Sperm P.M. binds to zona pellucida
2. Acrosome reaction releases digestive
enzymes
3. Sperm P.M. binds oocyte P.M.
4. Membrane fusion of sperm & oocyte
5. Cortical granule release causes block to
polyspermy
• Pronuclear migration to form diploid nucleus
Cleavage rounds to form blastula
blastocoel
blastula
Gastrulation
• Cells on outer top (AP)
migrate around. At dorsal
lip, cells migrate inward
into blastocoel.
• Germ layers established as
archenteron forms
• Future ectoderm migrates
to cover whole surface
Embryonic germ layers
• Ectoderm: epidermis, epithelial lining of mouth
& rectum, cornea & lens, nervous system
• Endoderm: Epithelium of digestive tract &
respiratory system, liver, pancreas, thyroid, &
lining of urethra, bladder, & reproductive system
• Mesoderm: Notochord, skeleton, muscles,
excretory system, circulatory & lymphatic
systems, dermis of skin, reproductive system
except lining & germ cells, lining of body cavity
Neurulation
Determination & Differentiation
• Embryonic precursor or stem cell
• Determination: master control gene is
expressed – determines cell fate
• Differentiation: master control gene turns
on transcription of cell-type-specific
functional genes
– Cell adopts form and function
Homeobox (Hox) genes
• Contain a DNA-binding homeodomain.
• Act as master control genes to regulate
transcription of sets of genes during
development
Soluble signal
molecule
IMP signal
molecule
Different signals bind different receptors to
turn on transcription (Txn) of different genes
Txn. B on Txn. B on
Txn. A on
Apoptosis
• Programmed cell death
• Extraneous cells are signaled to commit
suicide during development
– Excess neurons
– Tissue between fingers & toes
– Autoimmune cells
• Carefully controlled, neat, & specific