1. Cell structure and function Flashcards
What is the average thickness of a cell membrane
7.5 - 10 nm
What are the cell membrane constituents (in decreasing order of importance)?
Proteins 55%
Phospholipids 25%
Cholesterol 13%
Other lipids 4%
Carbohydrates 3%
What are the 3 types of lipids in a cell membrane?
Phospholipids
Sphingolipids
Cholesterol
What are the two structural portions of a membrane phospholipid?
- Phosphate end: hydrophilic, in contact with extra- and intra-cellular water
- 2 Fatty acid chains: hydrophobic, forming the middle layer, impermeable to water-soluble substances (ions, glucose, urea) and permeable to fat-soluble substances (O2, CO2, alcohol)
Which cells are most rich in sphingolipids?
Neurons.
Note: Sphingolipids have a role in signal transmission, adhesion site for extracellular proteins, and protect from harmful environmental factors.
What are the two main roles of membrane cholesterol?
Controls the degree of…
1) Membrane permeability
2) Membrane fluidity
What are the two types of membrane proteins?
Integral & surface proteins
What are the three forms of membrane carbohydrates, and where are they usually found?
Attached to proteins or lipids, as:
- Glycoproteins
- Glycolipids
- Proteoglycans
Usually attached to integral proteins and form protrude on the outside, forming the glycocalyx
What are the 4 roles of the glycocalyx
1) provides a NEGATIVE charge to the outer cell membrane
2) ATTACHMENT site for other cells’ glycocalyx
3) RECEPTOR for hormone bindings (e.g. insulin)
4) involved in IMMUNE reactions
2 types of endocytosis
1) PINOcytosis (small molecules)
2) PHAGOcytosis (large molecules)
Name the types of transport across cell membranes
1) DIFFUSION - simple vs facilitated (along electrochemical gradient)
2) ACTIVE - primary vs secondary (against electrochemical gradient)
What is the difference between simple vs facilitated diffusion?
SIMPLE diffusion:
- Diffusion through protein pores or channels
- No interaction with carrier proteins
- Rate of diffusion is linearly related to substrate concentration
FACILITATED diffusion:
- Interaction with carrier protein
- Saturation: Rate of diffusion is limited (Vmax) by rate of protein conformational changes
- Competition
What is the difference between primary vs secondary active transport?
PRIMARY:
- Energy source: high-energy phosphate (e.g. ATP)
- E.g. Na/K ATPase pump (3 Na out, 2 K in)
SECONDARY:
- Energy source: stored energy created by active transport
- Co-transport vs counter-transport
- One solute (usually Na) is transported downhill while other is transported uphill
- E.g. Na-K-2Cl or Na-Glucose cotransporter or Na-H countertransporter
What determines the resting membrane potential?
1) equilibrium potential of individual ions (RMP is closest to the equilibrium potential of K+, being the most permeable at rest)
2) relative membrane permeability to individual ions
3) electrochemical gradients
4) fixed, negatively charged organic ions
(…)
What is the RMP of
1) Smooth muscle cell
2) Striated muscle
3) Resting neurons
1) Smooth muscle cell: -50mV
2) Striated muscle: -95mV
3) Resting neurons: -65-90 mV
Understand the phases of an action potential
1) Membrane depolarizes to threshold potential (-55 mV)
2) Activation of v-gated Na and K channels
3) Na flows in faster than K flows out -> cell depolarizes (rising phase) to +20-30 mV (peak membrane potential).
4) Inward flow of Na spreads to adjacent v-gated channels causing spread to adjacent areas of membrane
5) Nav channels inactivate while K+ continues to flow out -> cell membrane reverses and hyperpolarizes
What is the absolute vs relative refractory period
ABSOLUTE:
- Time for Nav channels to de-inactivate (1 ms)
- NO stimulus can generate AP. Prevents overlap of APs.
RELATIVE
- follows refractory period
- Period of hyperpolarization
- Time for K+ channels to close
- STRONGER stimulus needed to generate AP
Other name for cell soma
Perikaryon
What is the Nissl substance
Rough ER + polysomes
Name given to a cluster of perikarya in the CNS vs PNS
CNS: nucleus
PNS: ganglion (synaptic sites for ANS)
Each neuron contains __ axon(s) and __dendrite(s)
Single axon
Variable dendrites
3 types of neurons (know how to recognize them)
UNIPOLAR (single stem that bifurcates into two)
BIPOLAR (2 stem processes)
MULTIPOLAR (1 axon, multiple dendrites)
Which is the most common neuron type
Multipolar
Example(s) of monopolar neurons
Primary sensory neurons
Example(s) of bipolar neurons
Found in
- Retina
- Olfactory epithelium
- Spiral ganglion (cochlea)
- Vestibular ganglion
Example(s) of multipolar neurons
Sensory
Motor (α,γ𝛼,𝛾 - to skeletal muscle)
Autonomic, interneurons
Name the neuroglia (glial cells) of the PNS vs CNS
CNS:
- Microglia (immune/macriphages)
- Oligodendrocytes (myelin)
- Ependymal cells (CSF producing)
- Astrocytes (nutrition, BBB…)
PNS:
- Schwann cells (myelin)
Unlike neurons, glial cells..
Continue to divide
Oligodendrocytes
Each contains cell processes that wrap around MULTIPLE axons
Schwann cells
Concentrically wrap their plasma membrane around ONE axon, forming Nodes of Ranvier -> saltatory conduction
Astrocytes
- SEAL outer and inner surfaces of CNS (outer and inner glial limiting membranes)
- Release neurotropic factors (e.g. NGF)
- Perivascular end-feet form BBB
- Reulate extracellular env
- Repair processes following injury
- Glutamate metabolism
Which 7 areas lack a BBB (slide 35)
= Circumventricular organs, CVO
Ependymal CVO
- Choroid plexus
- Subcomissural organ
Paraependymal CVO
- Subfornical organ
- Median eminence/ neurohypophyseal (hypophysis) complex
- Pineal gland
- Area postrema
- Vascular organ of the lamina terminalis
Classification of nerve fibers
Know table slide 38
Where can an axon terminal synapse?
- Perikarya
- Dendrites
- Axons
2 types of synapses
Chemical vs electrical
Describe an electrical synapse
Ion channels connect the cytoplasm of the pre- and postsynaptic cells at a gap junction
Describe a chemical synapse
NT released in a synaptic cleft between pre- and post-synaptic cell
What is a neuromuscular synapse
Between the terminal end of a MOTOR ENDPLATE (specialized effferent endings of an axon terminating on a striated muscle) and a MUSCLE FIBER
What is a motor unit
1 motor neuron forming NM synapses with a group of skeletal muscles
Understand the sequence of events in a neuromuscular synapse
1.Presynaptic neuron AP
2.Opening V-gated Ca2+ channels
3.Fusion of ACh vesicle on pre-synaptic membrane
4. Release of ACh
5. Opening of nAChRs, permeable to Na and K
6. Generation of EPP (not AP)
7. Opening of V-gated Na channels on post-synaptic membrane
8. If summation of EPP is large enought – > AP –> propagate along muscle fiber –> contraction
Name the neurotransmitters of:
- PNS
- CNS (excitatory vs inhibitory)
PNS:
- ACh, NE, Epi
CNS
- Excitatory: glutamate, aspartate
- Inhibitory: GABA, glycine
2 mechanisms of NT clearance from synaptic cleft (p 53)
- Enzymatic (AChE)
- Diffusion (glutamate)
Skeletal muscles - define
- Fasciculi
- Epimysium
- Perimysium
- Endomysisum
- Fasciculi: group of myofibers
- Epimysium: surrounds 1 muscle
- Perimysium: surrounds 1 fasciulus
- Endomysisum: surrounds 1 myofiber
What are the two types of myofilaments? What are they made of ?
THICK
- Myosin
- Has binding sites for ATP and actin
THIN
- Actin, troponin, tropomyosin
- Actin has binding sites for myosin during contraction
- Troponin + tropomyosin block these sites during relaxation
What is the smallest contractile unit?
Sarcomere
- Repeating units between 2 Z lines
- Microscopically: Alternating dark and light bands –> striated appearance
Know the organization of sarcomeres
- A band
- I band
- Z-disk
- A band: superposed myosin + actin filaments
- I (light) bands: only actin
-Z disk: ends of actin filaments
Name the steps of excitation-contraction coupling (EMG slide 13)
1) ACh released from motoneuron binds nAChRs at the motor end plate
2) Na+ & K+ influx –> membrane depolarization –> summation of EPP –> AP –> travels across the sarcolemma
3) AP travels across T-tubules –> activation of ryanodine receptors (RyRs) –> releases Ca2+ from SER into the sarcoplasm (where myofibrils are)
4) Ca2+ binds troponin –> exposes actin-binding site on myosin heads –> cross-bridge formation between thin & thick filaments
Name the steps of the sliding filament model (EMG slide 14)
5) Phosphate released from myosin head –> pivots towards the center of sarcomere = sliding motion (power stroke) –> shortens sarcomere –> CONTRACTION
6) A new ATP binds myosin –> myosin detaches from actin–> RELAXATION
7) ATP hydrolyzed to ADP + Pi –> myosin returns to neutral position, ready for a next cycle
Smooth muscles
slides 70-75:)
2 types of somatic EFFERENT neurons in the ventral horn?
1) SKELETO-MOTOR (Aα) neurons
- Motorneurons
- Innervate EXTRAfusal muscle fibers
2) FUSI-MOTOR (Aγ) neurons
- Innervate INTRAfusal muscle fibers (part of the muscle spindle)
What are the two constituents of MUSCLE SPINDLES (muscle proprioceptors)?
(slide 78)
1) INTRAFUSAL (4-20) muscle fiber
- Small striated muscle fiber
- Innervated by fusimotor neurons (Aγ)
- Have two contractile regions (a, c) and one central stretchable nuclear region (b)
2) ANNULOSPINAL receptor
- Endings of primary afferent (Ia) neuron, wrapping around the stretchable region of intrafusal muscle fibers
- Activated by stretch of intrafusal muscle fibers → Detecting muscle length
What are the 2 types of infrafusal fibers?
Based on the distribution of nuclei in the central (nuclear) region
1) nuclear BAG fiber
- Cluster of nuclei
2) nuclear CHAIN fiber
- Single row of nuclei
How are muscle spindles activated
Receptor is silent at rest, but activated by STRETCHING of the nuclear region, either by:
- Tapping on a tendon/ muscle
- Stimulating fusimotor neurons directly
Describe the myotatic (stretch) reflex
Describe a GOLGI TENDON ORGAN (GTO)
- Encapsulated sensory receptor with branched and unmyeliated nerve endings, embedded at the muscle-tendon junction
- Meausures muscle TENSION (not lenght)
- Activates an INHIBITORY interneuron to α the activated motoneurons
- Prevents muscle overloading
Slide 85: spindle vs GTO
MONOSYNAPTIC (patellar/myotatic) reflex
(slide 86)
Synaptic contacts are made between sensory & motor neurons that form a reflex arc
Tapping patellar tendon → stretches extrafusal and intrafusal muscles of the muscle spindle → activate annulospinal receptors (Ia) → excite α motor neurons in the ventral horn → skeletal muscle contracts → Stimulates GTO → feedback inhibition of α motor neurons
Mediated by spinal cord segments L4-L6
POLYSYNAPTIC reflex
Involves sensory, motor and interneurons
Interneurons =
- Between sensory and motor neurons
- Within gray matter of SC
- Synapse on motor neurons or to other interneurons
- Can cross midline of spinal cord to terminate on contralateral motor neurons
- Excitatory or inhibitory
Understand WITHDRAWAL (flexor) reflex
Slide 87
Undersand PERINEAL reflex
Tests the functional integrity of sacral cord segments S1-S3, caudal & pudendal and caudal nerves
contraction of anal sphincter and tail flexion in response to tactile stimulation of perineum
Understand CROSS-EXTENSOR reflex
Flexing 1 limb -> extension of contralateral limb
- Required to support increasing weight on a limb
Not present in lateral recumbency
UMN (e.g. lateral funiculus lx):
- Opposing tracts in ventral funiculus exert powerful excitatory influence on α and 𝛾 motor neurons supplying extensor muscles → stimulus of flexing limb by pushing pad is sufficient to excite motor neurons in contralateral side of spinal cord → crossed extensor reflex in lateral recumbency
Understand CUTANEOUS TRUNCI reflex
Stimulus applied to skin → dorsal horn → axons of projection neurons ascent in the fasciculus proprius to reach C8 & T1 → synapse with LMN of the lateral thoracic nerve → panniculus muscle → reflex present bilaterally (stronger on ipsilateral side)
