Nervous System
What causes membrane potential changes?
Concentrations of ions across the membrane change Permeability of membrane to ions changes
How are Myelin Sheaths in the CNS different from those in the PNS?
Formed by processes of oligodendrocytes, not the whole cells Nodes of Ranvier are present No neurilemma Thinnest fibers are unmyelinated
What is the function of axon terminals?
- Secretory regions of neuron - Release neurotransmitters to excite or inhibit other cells
List types of neuroglia and in what systems they are found
Astrocytes (CNS) Microglia (CNS) Ependymal cells (CNS) Oligodendrocytes (CNS) Satellite cells (PNS) Schwann cells (PNS)
Describe a Neuron Cell Body
Biosynthetic center of a neuron Spherical nucleus with nucleolus Well-developed Golgi apparatus Rough ER = Nissl bodies (chromatophilic substance) Network of neurofibrils (neurofilaments) Axon hillock—cone-shaped area from which axon arises
Amitotic
Cells do not replace themselves.
What is another name for the terminal branches of an Axon?
Telodendria
Two functional subdivisions of Autonomic (involuntary) nervous system (ANS)
Sympathetic: FIGHT/FLIGHT Parasympathetic: REST/DIGEST
Describe the structure and function of Astrocyte cells
*SUPPORT / COMMUNICATION* Most abundant, versatile, and highly branched glial cells Cling to neurons, synaptic endings, and capillaries Support and brace neurons Help determine capillary permeability Guide migration of young neurons Control the chemical environment Participate in information processing in the brain Communication between capillary & neurons A = A: astrocytes attach to arteries
Two main divisions of Nervous System
1. Central nervous system (CNS): - Brain and spinal cord - Integration and command center 2. Peripheral nervous system (PNS) - Paired spinal and cranial nerves carry messages to and from the CNS
What are the three types of gated membrane ion channels in a neuron?
1. Chemically gated (ligand-gated) channels—open with binding of a specific neurotransmitter 2. Voltage-gated channels—open and close in response to changes in membrane potential 3. Mechanically gated channels—open and close in response to physical deformation of receptors
What are the three structural classifications of Neurons?
1. Multipolar—1 axon and several dendrites (most abundant; found in both motor neurons and interneurons) 2. Bipolar—1 axon and 1 dendrite (rare, e.g., retinal neurons) 3. Unipolar (pseudounipolar)—single, short process that has two branches
Two principal cell types of Nervous Tissue
1. Neurons—excitable cells that transmit electrical signals 2. Neuroglia (glial cells): supporting cells
What are the two branches of unipolar neurons?
1. Peripheral process—more distal branch, often associated with a sensory receptor 2. Central process—branch entering the CNS
Two functional divisions of Peripheral Nervous System (PNS)
1. Sensory (afferent) division: - Somatic afferent fibers—convey impulses from skin, skeletal muscles, and joints - Visceral afferent fibers—convey impulses from visceral organs 2. Motor (efferent) division - Transmits impulses from the CNS to effector organs
What are the three functional classifications of Neurons?
1. Sensory (afferent): transmit impulses from sensory receptors toward the CNS 2. Motor (efferent): carry impulses from the CNS to effectors 3. Interneurons (association neurons): shuttle signals through CNS pathways; most are entirely within the CNS
Three main functions of the Nervous System
1. Sensory input: Information gathered by sensory receptors about internal and external changes [afferent] 2. Integration: Interpretation of sensory input 3. Motor output: Activation of effector organs (muscles and glands) produces a response [efferent]
Two divisions of PNS
1. Somatic (voluntary) nervous system: conscious control of skeletal muscles 2. Autonomic (involuntary) nervous system (ANS): - Visceral motor nerve fibers - Regulates smooth muscle, cardiac muscle, and glands
Define Synapse
A junction that mediates information transfer from one neuron: - To another neuron, or - To an effector cell
Describe Depolarization
A reduction in membrane potential (toward zero) Inside of the membrane becomes less negative than the resting potential Increases the probability of producing a nerve impulse
Describe information transfer in a chemical synapse
AP arrives at axon terminal of the presynaptic neuron and opens voltage-gated Ca2+ channels Synaptotagmin protein binds Ca2+ and promotes fusion of synaptic vesicles with axon membrane Exocytosis of neurotransmitter occurs Neurotransmitter diffuses and binds to receptors (often chemically gated ion channels) on the postsynaptic neuron Ion channels are opened, causing an excitatory or inhibitory event (graded potential) See p. 409 in textbook
Are there more neurons or neuroglia cells in the CNS?
Abundantly more neuroglia than neurons in CNS
What are some places an axon terminal can connect?
Another nerve cell (dendrites) A muscle or gland
List and describe different types of synapses
Axodendritic—between the axon of one neuron and the dendrite of another Axosomatic—between the axon of one neuron and the soma of another Less common types: - Axoaxonic (axon to axon) - Dendrodendritic (dendrite to dendrite) - Dendrosomatic (dendrite to soma)
What two parts typically make up a chemical synapse?
Axon terminal of the presynaptic neuron, which contains synaptic vesicles Receptor region on the postsynaptic neuron Gap between the two is the SYNAPTIC CLEFT
What is another name for the synaptic knobs of axon terminals?
Boutons
Describe the structure and function of Oligodendrocytes
Branched cells Processes wrap CNS nerve fibers [jelly roll], forming insulating myelin sheaths (once wrapped) Function: insulation
What are bundles of neural processes called in the CNS? In the PNS?
Bundles of processes are called: Tracts in the CNS Nerves in the PNS
What is a Perikaryon or Soma?
Cell Body of a Neuron
What are clusters of cell bodies called in the CNS? In the PNS?
Clusters of cell bodies are called: 1. Nuclei in the CNS (not to be confused with organelle) 2. Ganglia in the PNS
What is the general function of an axon?
Conducting region of a neuron Generates and transmits nerve impulses (action potentials) away from the cell body [efferent] Molecules and organelles are moved along axon by motor molecules both toward the axon terminal and toward the cell body
What are three ways to terminate a neurostransmitter effect?
Degradation by enzymes Reuptake by astrocytes or axon terminal Diffusion away from the synaptic cleft
What are the two kinds of processes found in a neuron?
Dendrites and axons
What are the two types of membrane potential changes that act as signals:
Graded potentials: incoming short-distance signals (depolarizations or hyperpolarizations) Action potentials: long-distance signals of axons (occurs in muscle cells and axons of neurons)
What are the differences in ionic makeup between ICF and ECF in a "resting" neuron?
ICF has lower concentration of Na+ and Cl- than ECF ICF has higher concentration of K+ and negatively charged proteins (A-) than ECF
What is the difference between white and gray matter?
White matter: Dense collections of myelinated fibers Gray matter: Mostly neuron cell bodies and unmyelinated fibers
Define POSTsynaptic neuron
Postsynaptic neuron: transmits impulses away from the synapse
Define PREsynaptic neuron
Presynaptic neuron: conducts impulses toward the synapse
Describe the structure and function of Ependymal Cells
Range in shape from squamous to columnar; may be ciliated Line the central cavities of the brain and spinal column Separate the CNS interstitial fluid from the cerebrospinal fluid in the cavities (barrier)
What two types of cells make up myelin sheaths?
Schwann cells (neurolemmocytes) in the PNS Oligodendrocytes in the CNS
Describe the structure and function of Myelin Sheaths
Segmented protein-lipoid sheath around most long or large-diameter axons In the PNS, Schwann cells wrap many times around the axon (look like little sausage links) It functions to: - Protect and electrically insulate the axon - Increase speed of nerve impulse transmission
Describe the structure and function of Dendrites
Short, tapering, and diffusely branched Receptive region of a neuron [afferent] - receive sensory input Convey electrical signals toward the cell body as graded potentials
Describe the structure and function of Microglial cells
Small, ovoid cells with thorny processes Migrate toward injured neurons Phagocytize microorganisms and neuronal debris M = M: microglia macrophage
Describe the structure and function of Satellite cells
Surround (wrap around) neuron cell BODIES in the PNS Similar to astrocytes in CNA - serve communication function
Describe the structure and function of Schwann cells (neurolemmocytes)
Surround peripheral nerve fibers and form myelin sheaths (whole cell, not just processes) Cytoplasm squeezed out is called neurilemma Vital to regeneration of damaged peripheral nerve fibers
What are chemical synapses?
Synapses specialized for the release and reception of neurotransmitters
Describe chemical concentrations of K+ and Na+ inside and outside a neuron.
The cell membrane is much more permeable/leaky to K+ ions Na+ / K+ pumps help to maintain resting gradients
Describe Unmyelinated Axons
Thin nerve fibers are unmyelinated One Schwann cell may incompletely enclose 15 or more unmyelinated axons
Describe the structure of an Axon
One axon per cell arising from the axon hillock Long axons (nerve fibers) Occasional branches (axon collaterals) Numerous terminal branches (telodendria) Knoblike axon terminals (synaptic knobs or boutons)
What are the two main types of membrane ion channels in a neuron?
1. Leakage (nongated) channels—always open 2. Gated channels (three types)
What is the electrical "resting" state of a neuron?
Inside the cell is more negative; outside the cell is more positive.
List some characteristics of Neurons (Nerve Cells)
Long-lived (100 years or more) Amitotic (with few exceptions) High metabolic rate: depend on continuous supply of O2 / glucose Plasma membrane functions in: - Electrical signaling - Cell-to-cell interactions during development
Describe the threshold stage of action potential
Membrane is depolarized by 15 to 20 mV Na+ permeability increases Na influx exceeds K+ efflux The positive feedback cycle begins
Describe Multiple Sclerosis
Myelin sheaths around axons wear away and axons are scleroted Signal is slowed or even stopped Pg. 405 in textbook
What is the peripheral bulge of the Schwann cell cytoplasm called?
Neurilemma [where the jelly roll bulges out]
Describe ELECTRICAL synapses
Neurons are electrically coupled (joined by gap junctions) Communication is very rapid, and may be unidirectional or bidirectional Are important in: - Embryonic nervous tissue - Some brain regions, eyes [Less common than chemical synapses]
What are the Myelin sheath gaps between adjacent Schwann cells called?
Nodes of Ranvier [These are sites where axon collaterals can emerge]
What are some examples of molecules and organelles moved along an axon toward its TERMINAL end?
mitochondria, membrane components, enzymes
What are some examples of molecules and organelles moved along an axon toward its cell body?
organelles to be degraded, signal molecules, viruses, and bacterial toxins