Chapter 11 (A&P 1)
How are nerves classified?
according to their origin or destination (cranial nerves= originating from or traveling to the brain TWELVE PAIRS) (spinal nerves= originating from or traveling to the spinal cord THIRTY ONE PAIRS)
Where does the brain merge with the spina cord?
at the foramen magnum (the spinal cord passes through the vertebral foramen of the first cervical vertebra, and continues inferiorly to the first or second lumbar vertebrae) (spinal cord contains fewer cells than the brain)
What extends from some axons typically at right angles to the axon?
branches, known as axon collaterals (both the axon and its collaterals split near their ends to produce multiple fine branches known as TELODENDRIA, which terminates in axon terminals or synaptic knobs)
Gray matter are made up primarily of......... ?
cell bodies, and dendrites (which are never myelinated)
What is the most metabolically active part of the neuron?
cell body (responsible for maintaining the huge cytoplasmic volume of the neuron) (manufacturing all the proteins the neuron needs) (has free ribosomes, and rough er: protein synthesis "dark staining clusters, Nissl bodies" "Golgi, nucleoli") (mitochondria: high metabolic demands)
Unlike neurons, neuroglia maintain their ability to........ ?
divide (neurons are amitotic) (SIX DIFF TYPES OF NEUROGLIA: 4 in CNS, 2 in PNS)
Organs that carry out the effects of the nervous system are often called......... ?
effectors
Oligodendrocytes?
fewer in number, and smaller than astrocytes flattened ends of some of these processes wrap around part of the axons of other neurons to form myelin (has several of these processes that wrap around multiple axons)
The segments of an axon that are covered by neuroglia are called.....?
internodes: between each internode is a gap known as a node of Ranvier, where no myelin is found (unmyelinated is a short region from the axon hillock to the first neuroglial cell, which is known as the initial segment)
Microglia?
least numerous activated by injury in the brain, and become wandering phagocytes (can also secrete chemicals that stimulate inflammation)
What is the most abundant cell type in the CNS?
neuroglia (10 times more abundant than neurons, and make up about half the mass of the brain)
Myelin is composed of....... ?
repeating layers of the plasma membrane of the neuroglial cell (in unmyelinated axons, the axon has to be continually regenerated) (high lipid content of myelin makes it an excellent insulator, which results in a increase of the speed of conduction in action potentials)
The characteristic shape of the cell body is maintained by....... ?
the neuronal cytoskeleton, which is composed largely of intermediate filaments that bundle together to form neurofibrils
Conduction speed?
the rate at which propagation occurs, and it also determines how rapidly signaling can occur within the nervous system SALARORY: myelin sheath is present ("leaping" action potential) CONTINUOUS: myelin sheath is absent
Axons of certain neurons can carry a signal both......... ?
toward and away from the cell body, therefore axons are processes that can generate and conduct action potentials
Each axon splits into how many axon terminals?
1000 or more
A nerve consists of a bundle of long neuron "arms" known as........ ?
AXONS that are packaged together with blood vessels and surrounded by connective tissue sheaths
Structural divisions of the central nervous system?
Central nervous system (brain and spinal cord) Peripheral nervous system (nerves: most numerous organs of the nervous system)
Ependymal?
Ciliated cells that circulate the cerebrospinal fluid (some play a role in the formation of CSF, and others are thought to monitor its composition)
What are the divisions of the PNS motor division?
Somatic motor: neurons that transmit signals to skeletal muscles (AKA; voluntary motor division) Visceral motor (Autonomic Nervous System): neurons that carry signals primarily to thoracic and abdominal viscera (regulates secretion from certain glands, contraction of smooth/cardiac muscle, important for internal homeostasis) (Involuntary motor division)
What are the divisions of the PNS sensory division?
Somatic sensory: neurons that carry signals from skeletal muscles, bones, joints, and skin (also includes neurons that carry signals of special senses) Visceral sensory: neurons that transmit signals from organs (sensory input from both divisions is carried from sensory receptors to the spinal cord and/or brain by cranial and spinal nerves of the PNS)
Most neurons are........ ?
amitotic; meaning that at a certain point in development, they lose their centrioles and after that lack the ability to undergo mitosis
Plasma membrane that envelops the axon is called......... ?
the axolemma (and its cytoplasm is known as axoplasm)
Refractory period?
the brief time after a neuron has produced an action potential, when the membrane cannot be stimulated to fire another one (two types: RELATIVE & ABSOLUTE= limit the frequency of action potential production)
Short axons in the CNS and PNS are nearly always........ ?
unmyelinated (however, in the PNS, even axons that lack myelin associate with schwann cells; don't wrap themselves around axons, but envelope axons like a hot dog in a bun)
What are the differences in Myelin in the CNS and PNS?
-shwann cell wraps outward, away from the axon, oligodendrocyte wraps inward, toward the axon -no outer neurolemma is found in the CNS because the nucleus and cytoplasm of oligodendrocytes remain in a centralized location; nucleus and bulk of the schwann cells cytoplasm and organelles on the outer surface of the myelinated axon -schwann can only encircle a portion of a single axon, oligodendrocytes can can encircle several -PNS starts earlier (in early fetal period)
What are the 4 types of neuroglial cells in the CNS?
1. ASTROCYTES 2. OLIGODENDROCYTES 3. MICROGLIA 4. EPENDYMAL
What are the 3 structural classifications of neurons?
1. MULTIPOLAR: makes up over 99% of neurons in the human body. single axon and multiple highly branched dendrites. widest variability in terms of shape and size 2. BIPOLAR: has only two processes, one axon and one dendrite. majority are sensory neurons, located in places like the retina of the eye, and olfactory epithelium of the nasal cavity. 3. PSEUDOUNIPOLAR: formerly known as unipolar neurons. began developmentally as bipolar neurons, but the two processes fuse to give rise to a single axon. no dendrites. as the axon extends from the cell body, it splits into two processes, one that brings stimuli from sensory receptors to the cell body called the peripheral process of the axon, one that travels to the spinal cord away from the cell body called the central process of the axon. sensory neurons that detect stimuli such as touch, pressure, and pain.
What are the three functional regions of a neuron?
1. RECEPTIVE REGION: dendrites and cell body (dendrites can receive signals that are connected in the cell body, or monitor internal/external environments via sensory receptor) 2. CONDUCTING REGION: axon 3. SECRETORY REGION: secrete chemicals that trigger changes in their target cells
What are the 2 types of neuroglial cells in the CNS?
1. SCHWANN CELL -creates myelin (unmyelinated axons are also encased in schwann cells) -sausage shaped -big role in repair of damages axons in the PNS 2. SATELLITE CELL -flat cells that surround and support the cell bodies of neurons in the PNS -regulate extracellular environment around neuronal cell body (similar to astrocytes)
What are the 3 functional classifications of neurons?
1. SENSORY (AFFERENT) NEURONS: carry signals toward the central nervous system. electrical changes are transmitted to their cell bodies in the PNS, then down there axons to the brain or spinal cord. generally pseudounipolar or bipolar in structure because they receive stimuli from only one area. detect internal and external environments. facilitate motor coordination. 2. INTERNEURONS (association neurons): relay messages within the CNS, primarily between sensory and motor neurons. location of most information processing. vast majority of neurons are interneurons. multipolar in structure. 3. MOTOR (EFFERENT) NEURONS: carry stimuli away from their cell bodies in the CNS to muscles and glands. most are multipolar, one from spinal cord, one from hippocampus, one from the cerebellum.
What are the 3 different types of nervous system tasks/functions known as?
1. SENSORY FUNCTION: involve gathering information about the internal and external environments of the body (performed by the sensory/afferent division) 2. INTEGRATIVE FUNCTION: analyze and interpret the detected sensory stimuli and determine an appropriate response (performed entirely by the CNS, mostly the brain) 3. MOTOR FUNCTION: actions performed in response to integration (performed by motor/efferent division)
Most neurons consist of what three parts?
1. central cell body: where majority of the biosynthetic processes of the cell occur 2. one or more dendrites: carry electrical signals to the cell body 3. one axon: carries electrical signals away from the cell body
What are the classes of neuronal cell membrane channels?
Leak channel Ligand gated channel Voltage gated channel Mechanically gated channel
Do dendrites (processes) generate/conduct action potentials?
NO (branches of the dendritic tree are change throughout an individuals lifetime, as they grow and are "pruned" as a person develops and matures & as functional demands on the nervous system change)
Astrocytes?
a star-shaped glial cell of the central nervous system. most numerous and largest of the neuroglia in the CNS each astrocyte has a central portion and numerous processes, all of which terminate in structures called end-feet FUNCTIONS -ANCHORING NEURONS AND BLOOD VESSELS IN PLACE: help form 3-dimensional structure of the brain by using their end-feet to anchor neurons and blood vessels in place. facilitate the transport of nutrients and gases from the blood vessels to neurons. -REGULATION THE EXTRACELLULAR ENVIRONMENT OF THE BRAIN: astrocytes are connected by gap junctions that allow them to communicate with one another about the local extracellular environment within the brain. "clean up crew" removing excess extracellular potassium ions as well as chemicals known as neurotransmitters. (although neurons use neurotransmitters to send signals, their extracellular accumulation can lead to toxicity) -ASSISTING IN THE FORMATION OF THE BLOOD BRAIN BARRIER: astrocytes facilitate the formation of the blood brain barrier by ensheathing capillaries and inducing their cells to form tight junctions. (only things that can cross are nonpolar and lipid-soluble, etc) -REPAIRING DAMAGED BRAIN TISSUE: astrocytes divide rapidly which stabilizes damaged tissue, but may also impede complete healing (recent research has demonstrated that excess astrocyte activity actually inhibits the regrowth of neurons, leading to more permanent defects)
About 80% of nervous tissue consists of........... ?
cells (neuron: the excitable cell type responsible for sending and receiving signals in the form of action potentials) (neuroglia: supportive functions "smaller and more prevalent that neurons")
Absolute refractory period?
no additional stimulus, no matter how strong, is able to produce an additional action potential (this period coincides with the voltage gated sodium ion channels being in their activated and inactivated states) (sodium channels may not be activated until they return to their resting states, and their activation gates closed, and their inactivation gates open)
Relative refractory period?
only a strong stimulus will produce an action potential (follows the absolute refractory period) (marked by a return of voltage gated sodium ion channels to their resting state, while some potassium ion channels remain activated) (its difficult to depolarize the membrane to threshold during this period because the potassium ion channels are activated and the membrane is repolarizing/hyperpolarizing) (if a greater than normal stimulus is applied, the membrane may depolarize to threshold, and the axon may fire off another action potential)