Lecture 9: CNS Tissue Flashcards
Cranial nerves
Extend from the brain
Spinal nerves
Extend from the spinal cord
Afferent
Sensory - to the CNS
Efferent
Motor - away from the CNS
Somatic body region
External to the ventral body cavity (i.e., outer tube and appendicular bones and muscles including skin, skeletal musculature, axial bodies)
Visceral body region
Viscera within the ventral body cavity (i.e., inner tube as well as smooth muscle and glands throughout the body including digestive tube, lungs, heart, spleen, bladder etc)
Integration
Processes and interprets sensory input and makes decisions on what should be done
Neurons
Excitatory cells
Neuroglial cells
Non-conducting cells that wrap around, nourish, insulate, and protect delicate neurons
Neuron structure
Cell body, processes (dendrites and axons)
Nerve impulses
Action potentials
Dendrite
Receives messages; transmit toward cell body
Axon
Transmits messages; transmit impulses away from the cell body; uniform diameter; axon hillock
Cell body
5-140 um; single nucleus; chromatophilic bodies (Nissl bodies); neurofibrils
Chromatophilic bodies (Nissl bodies)
Clusters of rER and free ribosomes
Neurofibrils
Bundles of intermediate filaments that prevent cell from being pulled apart
Axon structural support
Neurofilaments, actin microfilaments, and microtubules; aids in axonal transport
Axon length
Long or short; can be 3-4 feet long
Axon diameter
Large diameter = resistance to passage of electrical current decreases = faster impulses
Axon branching
Branch less frequently than dendrites; about 90 degree angles; terminal branches with axon terminals (aka end bulbs or boutons)
Nerve impulse
Generated from axon hillock; releases neurotransmitters into synaptic cleft
Synapses
Site at which neurons communicate
Presynaptic neuron
Conducts signal toward a synapse
Postsynaptic neuron
Transmits signals away from a synapse
Synaptic vesicles
Membrane-bound sacs containing neurotransmitter chemicals
Multipolar neuron (structure)
> 2 processes; multiple dendrites; one axon; >99% of neurons; most interneurons within CNS
Bipolar neuron (structure)
Two processes; one dendrite; one axon; rare; special sensory organs (inner ear, olfactory epithelium of nose, retina)
Unipolar (pseudounipolar) (structure)
One short single process; sensory ganglia found mainly in PNS; one branch (axon) extends to CNS (central process); one branch (dendritic) extends peripherally to receptors (peripheral process); start as bipolar
Sensory neurons
Afferent; virtually all unipolar
Motor neurons
Efferent; most multipolar; within CNS
Interneurons (association neurons)
Confined to CNS; most multipolar; between motor and sensory neurons; 99.98% of neurons
Characteristics of neurons
Extreme longevity; no mitotic division; high metabolic rate
Neuroglial (glial) cells
Support cells; insulate neurons; smaller than neurons; outnumber neurons 10:1; make up half of the mass of the brain; can divide in mitotic division
Astrocytes
Most common CNS type; radiating processes which cling to neurons and/or capillaries; extract blood sugar for energy; sense neuron release of neurotransmitters; signal increased blood flow; take up and release ions; help synapses form in developing neural tissue; produce molecules necessary for neural development; propagate calcium signals that may be involved in memory; unmyelinated
Oligodendrocyte
Form myelin sheaths around CNS; have fewer branches; wrap around thicker axons; produce myelin sheaths, may have multiple processes that coil around and myelinate several different axons; Nodes of Ranvier
Microglial
Defensive types; smallest and least abundant; many pointed projections; phagocytes-macrophages of CNS; engulf dead neuron; originate from monocyte blood cells, not nervous tissue
Ependymal
Line cerebral fluid filled cavities; bathes cells of CNS; bear cilia to circulate CSF
Satellite cells
Surround neuron cell bodies within ganglia
Schwann cells
Form myelin sheaths and surround axons; concentric layers; Neurilemma (sheath of Schwann)-outermost nucleated cytoplasmic layer of Schwann cells external to myelin sheath layers; develop during fetal period and in first year of post-natal life
Multiple sclerosis
Immune system attacks myelin around axons in CNS; more women than men affected; more devastating and quick in men
Myelin sheaths
Produced by oligodendrocytes in CNS and Schwann cells in PNS; surround thicker axons; composed of lipoprotein myelin; concentric layers; prevents leakage of electrical current; increase speed of impulse conduction
Unmyelinated axons in the PNS
Thin axons; conduct impulses more slowly; surround thin axons but do not wrap in concentric rings and do NOT form myelin sheath; single Schwann cell can surround multiple unmyelinated axons; found in portions of autonomic nervous system and in some sensory fibers
Nerve fascicles
Groups of axons bound into bundles by connective tissue wrapping called perineurium
Endoneurium
Layer of delicate connective tissue covering Schwann cells surrounding the axons
Epineurium
Touch fibrous sheath which surrounds nerve
Gray matter in spinal CNS
Surrounds hollow central cavities; butterfly-shaped; where neuron cell bodies cluster; dorsal half: interneurons; ventral half: motor neurons; primarily composed of neuronal cell bodies, dendrites, short unmyelinated axons of interneurons, and neuroglia
White matter in CNS
Myelinated (white); tracts: bundles of axons passing between specific regions of CNS; axons either ascend from spinal cord to brain or descend from brain to spinal cord; connect various gray matter areas
Cortex
In cerebrum and cerebellum; additional thin layer of gray matter external to white matter
Gray matter interneurons
Process and receive sensory information; direct information to specific CNS regions; initiate appropriate motor responses
White matter interneurons
Transport information (sensory and motor) from one area of the CNS to another
Reflex arcs
Simple chains of neurons; explain simplest reflex behaviors; responsible for rapid involuntary automatic motor responses to stimuli; can be somatic (skeletal muscle) or visceral (smooth muscle, cardiac muscle, or glands)
Essential components of reflex arc
Receptor, sensory neuron (afferent impulses to CNS), integration center (gray matter), motor neuron (efferent impulses to effector), effector
Monosynaptic reflexes
Simplest of all reflexes; one sensory neuron and a motor neuron; no interneuron; stretch reflexes include the “knee-jerk” reflex and reflexes which help maintain equilibrium and upright posture; fastest of all body reflexes
Polysynaptic reflexes
More common; one or more interneuron; in most simple polysynaptic reflex arcs such as withdrawal reflex in response to pricked finger: single interneuron between sensory and motor neurons (thus have two synapses and three neurons)
Diverging circuit
One presynaptic neuron synapses with multiple other neurons (diverges); stretch of muscle stimulates numerous sensory neurons; some motor neurons directly innervate stretch muscle and stimulate contraction; others are interneurons that act to inhibit the activity of the antagonistic muscle group; still others are interneurons that project sensory information to the brain
Converging circuit
Many neurons synapse on a single postsynaptic neuron (convergence) and a single motor neuron may receive both excitatory and inhibitory messages
Reverberating circuit
One neuron in the circuit receives feedback from another neuron in the same circuit; a branch off the axon of neuron circles back and synapses with a previous neuron in the circuit; involved in control of rhythmic activities (e.g., breathing)
Serial processing
Input processing; serial processing: neurons pass a signal to a specific destination in sequence (in series) along a single pathway from one neuron to the next (e.g., reflex arc or long chain of interneurons)
Parallel processing
Withdrawal reflex: painful stimulus triggers nerve impulses in a sensory neuron, which initiates withdrawal reflex; Parallel processing: simultaneously, nerve impulses travel in an axon branch that extends into the spinal white matter and extends up to the brain as an ascending pathway
Example of parallel processing
Integration in gray matter: multiple interneurons process nerve impulses to localize the stimulus, identify its source, and plan a response; Voluntary motor response: a voluntary (non-reflexive) motor response is initiated in cerebral gray matter and is transmitted down a descending fiber in white matter to stimulate somatic motor neurons
Input processing: parallel processing
Single sensory stimulus results in multiple perceptions; single neuron sent along two or more parallel pathways allowing a single sensory stimulus to result in multiple perceptions; allows brain to rapidly evaluate stimuli and enables information to be processed and integrated along multiple pathways
