Nervous System Flashcards
Nervous System
the master of controlling and communicating system of body via electrical and chemical signals
Two principal parts of nervous system
CNS and PNS
Central Nervous System
-brain and spinal cord of dorsal body cavity
-integration and control center
Peripheral Nervous System
-portion of nervous system outside CNS
-mainly nerves that extend from brain and spinal cord (spinal nerves and cranial nerves)
How many cranial and spinal nerves are in the PNS?
12 pairs of cranial nerves
31 pairs of spinal nerves
afferent
sensory fibers, receive stimuli from the body or the outside environment and transmit impulses to the CNS
efferent
motor fibers, receive stimuli from other neurons and transmit impulses to effector organs (muscles and glands)
Somatic Nervous System
involved in voluntary control of skeletal muscle and involuntary reflex arcs
Autonomic Nervous System
unconsciously regulates the function of internal organs and other involuntary functions (sweating, flushing, goosebumps)
What cells do nervous tissue consist of?
neuroglia and neurons
neuroglia
glial cells; small cells that surround and wrap delicate neurons
neurons
-nerve cells; excitable cells that transmit electrical signals
-structural units of nervous system
-large, highly specialized
-ALL have. cell body and one or more processes
What are the most abundant CNS neuroglia?
astrocytes
Oligodendrocytes…
have processes that form myelin sheaths around CNS nerve fibers
What are the two major neuroglia in PNS?
satellite and Schwann cells (neurolemmocytes)
satellite cells
-surround neuron cell bodies in PNS
-function similar to astrocytes of CNS
Schwann cells
-surround all peripheral nerve fibers and form myelin sheaths in thicker nerve fibers
-similar function as oligodendrocytes
-VITAL to regenerations of damaged peripheral nerve fibers
What are the special characteristics of neurons?
-extreme longevity
-amitotic w/ few exceptions
-high metabolic rate (requires continuous supply of oxygen and glucose
Neuron cell body
-biosynthetic center of neuron (synthesizes proteins, membranes, chemicals; Rough ER)
-contains spherical nucleus w/ nucleolus
Where are neurons mostly located?
In the CNS
nuclei
clusters of neuron cell bodies in CNS
ganglia
clusters of neuron cell bodies in PNS
What are neuron processes?
armlike processes that extend from cell body
-CNS: both neuron cell bodies and their processes
-PNS: chiefly neuron processes
What are the two types of neuron processes?
-dendrites
-axon
Dendrites
-motor neurons can contain 100s of these short, tapering, diffusely branched processes
-receptive region of neuron
-convey incoming messages toward cell body as graded potentials (short distance signals)
-FINER dendrites > highly specialized to collect info
-contain dendritic spines: appendages w/ bulbous or spiky ends
The axon: structure
-each neuron has ONE axon > starts at axon hillock
-can be short, long, or absent
-branch profusely at their end
-distal endings = axon terminals or terminal boutons
The axon: functional characteristics
-conducting region of neuron
-generates nerve impulses and transmits them along AXOLEMMA (neuron cell membrane) to AXON TERMINAL (region that secretes neurotransmitters)
-communicates w/ diff neurons at same time
-axons rely on cell bodies to renew proteins and membranes
-quickly delay if cut or damaged
-molecules and organelles > moved by motor proteins and cytoskeletal elements
-movement occurs in both directions: anterograde and retrograde
Anterograde axon movement
-away from cell body
-Ex: mitochondria, cytoskeletal elements, membrane components, enzymes
Retrograde axon movement
-toward cell body
-Ex: organelles to be degraded, signal molecules, viruses, and bacterial toxins
Myelin sheath
Composed of myelin (whitish, protein-lipid substance) that protects and electrically insulates axon and increases speed of nerve impulse transmission
Myelinated fibers
Segmented sheath surrounds most long or large-dm axons
Nonmyelinated fibers
Do not contain sheath so impulses are conducted more slowly
Myelination in the PNS (steps)
- A Schwann cell envelops an axon
- Schwann cell rotates around axon (jelly roll fashion), wrapping its plasma membrane loosely around it in successive layers > one cell forms one segment of myelin sheath
- Schwann cell cytoplasm is forced from between the membranes. The tight membrane wrappings surrounding the axon for the myelin sheath.
-LESS protein in plasma membrane so good electrical insulators
Myelin sheath gaps
-gaps between adjacent Schwann cells
-sites where axon collaterals can emerge
Myelin sheaths in CNS
-Formed by processes of oligodendrocytes, not whole cells
-Each cell can wrap up to 60 axons at once
-myelin sheath gap = PRESENT
-No outer collar of perinuclear cytoplasm
-thinnest fibers = unmyelinated but covered by long extensions of adjacent neuroglia
White Matter
Regions of brain and spinal cord w/ dense collections of myelinated fibers
Gray Matter
Mostly neuron cell bodies and nonmyelinated fibers
Structural Classification of Neurons
- Multipolar
- Bipolar
- Unipolar
Multipolar Neurons
-Three or more processes (1 axon, others dendrites)
-MOST common and major neuron type in CNS
Bipolar Neurons
-2 processes (one axon, one dendrite)
-RARE: retina and olfactory mucosa
Unipolar Neurons
-1 T-like process (2 axons)
-pseudounipolar
-peripheral process: associated w/ sensory receptor
-proximal process: enters CNS
Functional Classification of Neurons
- Sensory
- Motor
- Interneurons
Sensory neurons
-transmit impulses from sensory receptors toward CNS
-almost ALL are unipolar
-cell bodies are located in ganglia in PNS
Motor neurons
-carry impulses from CNS to effectors
-multipolar
-most cell bodies are located in CNS
Interneurons
-aka association neurons
-lie btwn motor and sensory neurons
-shuttle signals through CNS pathways
-most are entirely within CNS
-99% of body’s neurons are interneurons
voltage
a measure of potential energy generated by separated charge
Current
flow of electrical charge (ions) between two points
resistance
hindrance to charge flow
Ohm’s law
gives relationship of voltage, current, resistance
current (I) = voltage (V)/resistance (R)
Leakage (nongated) channels
channels that are always open
Gated channels
Protein changes shape to open/close the channel
1. chemically (ligand) gated
2. voltage gated
3. mechanically gated
ligand-gated channels
