Class 19 - Nervous System Flashcards
3 steps of neuron messaging
- Neurons receive stimuli from external environment and transmits messages to the CNS
- CNS processes the info and determines response
- CNS issues commands to muscle and gland cells to carry out response
2 main components of the PNS
- Nerves - Bundle of nerve fibers (axons) wrapped in fibrous connective tissue
- Ganglion - Knot-like swelling in a nerve where neuron bodies of PNS are concentrated
2 functional subdivisions of the PNS
- Sensory (afferent) division - carries signals from receptors to the CNS
- Motor (efferent) division - Carries signals from CNS to effectors (glands/muscles)
2 subdivisions of the sensory division
- Somatic sensory division - Carries signals from receptors in skin, muscles, bones, and joints.
- Visceral sensory division - Carries signals from the viscera (heart, lungs, stomach, etc.)
2 subdivisions of the motor division
- Somatic motor division - Carries signals to skeletal muscles; causes voluntary and involuntary muscle contractions
- Visceral (autonomic) motor division - Carries signals to glands and cardiac/smooth muscle. NO voluntary control (visceral reflexes)
3 subdivisions of visceral motor division
Remember: Visceral motor division = autonomic nervous system
- Sympathetic division - Prepares the body for action
- Parasympathetic division - Has calming effect on the body
- Enteric plexus - Enables coordination/communication within digestive tract
3 universal properties of neurons which allow them to communicate
- Excitability - Respond to stimuli
- Conductivity - Produce electrical signals conducted to other cells
- Secretion - When signal reaches end of axon, neuron secretes neurotransmitters which stimulate next cell
3 functional classes of neurons
- Sensory (afferent) neurons - Detect stimuli and transmit info to CNS. Begin in every organ of body and terminate in CNS.
- Interneurons - Receive signals from other neurons, process it, and make “decisions.” Lie entirely in CNS. Most common functional type.
- Motor (efferent) neurons - Sends signals to muscles and gland cells (effectors)
Components of a neuron (3)
- Cell body - Contains mitochondria, Golgi, inclusion, compartmentalized rough ER, cytoskeleton.
NO centrioles (no mitosis in mature cells)
Neurites - Extensions reaching out to other cells
- Dendrites - Most numerous neurites, receive signals from other neurons
- Axon (nerve fiber) - Long, cylindrical extension; rapid conduction of nerve signals
Components of a neuron axon (5)
- Axon collaterals - Axons unbranched except for these
- Axon hillock - Mound on side of cell body where axon attaches
- Axoplasm and axolemma - Cytoplasm; plasma membrane
- Terminal arborization - Branches at the end of the axon
- Axon terminal - Bulbous end of each branch of arborization; forms a synapse with next cell
Axonal transport + 2 types
Axonal transport - Two-way passage of materials along the axon.
- Anterograde transport - Movement away from cell bod; kinesin
- Retrograde transport - Movement up the axon toward cell body; dynein
Fast vs. slow axonal transport (3)
- Fast anterograde - Transport of organelles, enzymes, synaptic vesicles, small molecules
- Fast retrograde - Transport of recycled materials, pathogens
- Slow anterograde - “Stop-and-go” makes it slower.” Transports enzymes, cytoskeletal components, new axoplasm; damaged neurons regenerate slowly due to requirement of this transport
4 structural classes of neurons
- Multipolar - One axon, multiple dendrites (most common, most CNS neurons).
- Bipolar - One axon, one dendrite (olfactory cells, neurons of retina)
- Unipolar - splits into a peripheral process and central process, both comprise axon. Only endings of peripheral process are dendrites
- Anaxonic - Many dendrites, no axon (brain, retina, adrenal gland)
Functions of glial cells (4)
- Bind neurons together
- Form supportive tissue framework
- In fetus, guide migrating neurons to their destination
- Cover mature neurons (except at synapses) - Prevents neurons from touching, keeps conduction precise
4 types of glial cells in the CNS
- Oligodendrocytes - Form myelin sheath in CNS
- Ependymal cells - Line internal brain cavities. Secrete cerebrospinal fluid.
- Microglia - Macrophages; engulf debris, pathogen defense
- Astrocytes - Most abundant, wide variety of functions (blood-brain barrier, supply neurons w/ lactate, nerve growth factors)
2 types of glial cells in the PNS
- Schwann cells - Form myelin sheath in PNS, assist regeneration of damaged fibers
- Satellite cells - Insulate and regulate internal chemical environments of nerve cell bodies in ganglia of PNS
Where brain tumors typically arise form (3)
NOT from mature neurons—little capacity for mitosis, which runs rampant in tumors
- Meninges - Protective CNS membranes
- Metastasis from non-neural tumors in other organs
- Glial cells—mitotically active throughout their lifetimes. Form gliomas
Blood-brain barrier reduces effectiveness of chemotherapy—typically use radiation or surgery instead
Myelin sheath
Spiral layers of insulation around axon (20% protein, 80% lipid; essentially plasma membrane)
Formed by Schwann cells in PNS; oligodendrocytes in CNS.
Myelination begins in fetal development and completes by late adolescence
Myelination in the PNS vs. CNS + nuerilemma
- PNS - Schwann cell wraps repeatedly around a small section of a single axon. Spirals outward.
Neurilemma - Thick, outermost coil of myelin sheath in PNS; contains Schwann cell nucleus and cytoplasm
- CNS - Oligodendrocyte extends processes that wrap around small portions of many axons in immediate vicinity
Nucleus cannot migrate around axon; myelination spirals inward; no neurilemma
Many Schwann cells and oligodendrocytes needed to myelinate one axon.
Parts of myelin sheath (4)
- Nodes of Ranvier (sheath gaps) - Gaps between segments
- Internodal segments - Myelin-covered segments
- Initial segment - Bare section of axon between axon hillock and first glial cell
- Trigger zone - Axon hillock and initial segment. Help initiate nerve signal
2 diseases of the myelin sheath
- Multiple sclerosis - Oligodendrocytes and myelin sheaths in CNS deteriorate, replaced by hardened scar tissue
- Tay-Sachs disease - Abnormal accumulation of GM in myelin sheath disrupts nerve signal conduction
Unmyelinated axons
Many axons in both CNS and PNS unmyelinated.
In PNS, Schwann cells hold small unmyelinated axons in surface grooves. Membrane folds once around each axon, serving as neurolemma.
2 factors increasing nerve speed
- Diameter - Larger axons have more surface area; conduct signals more rapidly
- Myelin sheath - Presence of sheath speeds conduction
Fast - Large, myelinated
Med - Small, lightly myelinated
Slow - Small, unmyelinated
Nerve regeneration steps (5)
Does NOT occur in CNS.
SLOW (up to 2 years), imperfect, not always possible, cells body must be intact w/ some neurilemma remaining
- Axon distal to injury degenerates, cleaned by macrophages
- Cell body swells, ER breaks up, nucleus moves off-center due to loss of growth factor from target cells
- Axon stump sprouts multiple growth processes
- Schwann cell forms regeneration tube, which guides the regrowth to its old attachment site
- If contact established with original target, cell body shrinks and returns to normal
