Week 2 Flashcards
how does the body exert fine control over tissues and organs?
Through a huge diversity of regulatory molecules and receptors. There are millions of receptor proteins, alternative splicing, and posttranslational mods
what determines where a receptor is located?
The type of regulatory molecule. If it is a non polar signal, it can easily diffuse through the membrane so the receptor is intracellular. If it is polar it cannot diffuse through membrane so receptor is on the membrane
*examples of non polar regulatory molecules
steroid hormones
thyroid hormones
nitric oxide gas
intracellular receptors!!
*examples of polar regulatory molecules
epinephrine (hormones)
acetylcholine (amine neurotransmitter)
insulin (hormone)
receptor on plasma membrane!
if a polar molecule binds to the outside of a cell, how does it affect intracellular processes?
Through second messengers. These are intermediates in the cytoplasm that are activated by the receptor and which cause some other response in the cell
Common second messengers
- Ions such as Ca2+ that enter the cell from the extracellular fluid
- Cyclic adenosine monophosphate which is a molecule produced within the cytoplasm in response to the signal binding the receptor
describe cyclic adenosine monophosphate activation pathway and an example
An important second messenger with the following sequence of events: a polar regulatory molecule binds its receptor in the PM. this indirectly activates an enzyme in the PM that produces cAMP from ATP in the cytoplasm. cAMP then activates other enzymes which change cell activities.
Example: epinephrine (adrenaline) uses cAMP as a second messenger in stimulation of the heart
Why are G proteins necessary?
the binding of a polar regulatory molecule to its receptor activates an enzyme protein in the PM indirectly because the receptor and enzyme are in different locations. Something has to travel in the plasma membrane between receptor and enzyme. This is the G protein!
what would you call a receptor that binds a polar molecule and thus acts through a specific intracellular intermediary to produce a second messenger?
G protein COUPLED receptor
describe the process of G protein activation
Regulatory molecule reaches PM and binds receptor. Alpha subunit dissociates from beta-gamma. Alpha dissociation occurs because of GDP release and GTP binding. The alpha then moves through membrane and binds to effector protein (enzyme or ion channel) and activates it. Then alpha hydrolyzed GTP to GDP and Pi, causing the three subunits to reaggregate and move back to the receptor protein.
2 examples of 2 types of effector proteins activated by G proteins
Enzyme: epinephrine and norepinephrine activate cAMP producing enzyme and stimulates heart
Ion channel: acetylcholine causes heart rate to slow by opening channels
two divisions of the nervous system
Central nervous system (CNS): brain and spinal cord
Peripheral nervous system (PNS): cranial nerves arising from the brain and the spinal nerves arising from spinal cord. Everything that is NOT the brain and spinal cord
two principle types of cells in the nervous system
neurons: basic structural and functional unit of the system. They respond to physical and chemical stimuli, conduct electrochemical impulses, and release chemical regulators.
Supporting cells: aid the functions of neurons and are 5x more abundant than neurons. Also commonly called NEUROGLIA or GLIAL CELLS
why are brain tumors in adults usually composed of glial cells?
Neurons cannot divide by mitosis while glial cells (neuroglia, supporting cells) can. So in a cancerous overgrowth situation, glial cells will be the ones dividing too much.
three principle regions of all neurons
Cell body: Enlarged portion that contains the nucleus. nutritional center.
Dendrites: Processes, or extensions from cell body, which are thin and branched. Provides receptive area that transmits impulses TO the cell body
Axon: Longer process that conducts impulses AWAY from the cell body
what are nissl bodies
Dark stained granules in the cell body and larger dendrites (not axons). They are composed of large stacks of rough endoplasmic reticulum needed for synthesis of membrane proteins
terms for clusters of cell bodies in the CNS and PNS
CNS clusters of cell bodies are nuclei (singular nucleus)
PNS clusters of cell bodies are ganglia (singular ganglion)
Parts of the axon
The origin of the axon near the cell body is called the axon hillock, this is where the excitement threshold must be reached. Adjacent to that is the axon initial segment, where first action potentials are generated. Axons can be a millimeter to over meter in length. Towards the ends are lots of branches called axon collaterals, and each of those can divide many more times.
why is axonal transport necessary?
Axons can be very long, over a meter, and special mechanisms are required to transport organelles and proteins from the cell body to the axon terminals. This is accomplished by energy dependent axonal transport
classifications of neurons based on direction in which they conduct impulses
Sensory or Afferent neurons: conduct impulses from sensory receptors INTO the CNS
Motor or Efferent neurons: conduct impulses OUT of the CNS to effector organs
Interneurons: located entirely within CNS and serve integrative functions
Afferent = adding to CNS Efferent = exiting CNS
types of motor neurons
Somatic motor neurons: responsible for reflex and voluntary control of skeletal muscles
Autonomic motor neurons: innervate the involuntary effectors like smooth muscle, cardiac muscle, and glands. Includes the Sympathetic (speeds up, except GI tract) and Parasympathetic (slows down, except GI tract) subdivisions.
classifications of neurons based on structure and examples of each type
Pseudounipolar neurons: single short process that branches like a T to form a pair of longer processes. Includes sensory neurons where one branch receives sensory stimuli and the other delivers impulses to CNS
Bipolar neurons: two process, one at either end. Found in retina of the eye
Multipolar neurons: most common type, have several dendrites and one axon extending from the cell body. Includes motor neurons
What activity is the gray matter of the spinal cord often associated with?
reflexes
Term for a bundle of axons in the PNS and the CNS
Nerve is a bundle of axons in the PNS. Most nerves are composed of motor and sensory fibers and are called mixed nerves. Some cranial nerves are only sensory, such as for sight, hearing, taste, and smell.
Tract is a bundle of axons in the CNS
types of neuroglia in the PNS
- Schwann cells or Neurolemmocytes: form the myelin sheaths around peripheral axons
- Satellite cells: support neuron cell bodies within the ganglia of the PNS by movement of nutrients and waste
types of neuroglia in the CNS
- Oligodendrocytes: form myelin sheaths around multiple nearby axons of the CNS (like octopus)
- Microglia: migrate through CNS and phagocytose foreign and degenerated material. “Sculpts” the CNS
- Astrocytes: regulate the external environment of neurons in the CNS and wrap blood capillaries to create the Blood Brain Barrier
- Ependymal cells: epithelial cells that line the ventricles of the brain and central canal of spinal cord. They have cilia which move cerebrospinal fluid and maintain it’s homeostasis
what makes microglia unique among CNS neuroglial cells
They are derived from cells that were produced in the embryonic yolk sac and migrated into the developing neural tube. They are thus related to macrophages found elsewhere in the body (but are replaced only by replication of microglial cells not like macrophages made from blood monocytes)
describe the function of microglia
They have many fine processes that constantly wave and survey the environment to maintain healthy neuronal and synaptic function. Microglial Activation results from infection or trauma. The cells become amoeboid in shape and are transformed into phagocytic, motile cells that sense damage by ATP receptors. They kill exogenous pathogens and remove damaged debris in the CNS. They also shape neural circuits by pruning axons
All PNS cells (myelinated or not!) are surrounded by a continuous living ______ which the CNS lacks
sheath of Schwann cells or neurilemma.
CNS lack a neurilemma because Schwann cells are found only in the PNS (CNS has oligodendrocytes)
What forms the myelin sheath and on which cells?
Some axons (not all) in the PNS have a myelin sheath formed by successive wrappings of the cell membrane of Schwann cells.
In the CNS, some axons have a sheath formed by oligodendrocytes.
Axons smaller than 2 micrometers in diameter are usually unmyelinated and those that are larger are likely to be myelinated.
In myelinated PNS axons, is the entire axon covered by a neurilemma?
Yes, but not all parts of the axon are myelinated. The myelination is composed of Schwann cells that wrap multiple times around only a millimeter of axon, so there are gaps of unmyelinated (but still has neurilemma/schwann cell!) axon between adjacent Schwann cells called Nodes of Ranvier.
Unmyelinated axons also have neurilemma but lack the multiple wrappings that compose the myelin sheath.
how is myelination accomplished in the CNS
oligodendrocytes have multiple extensions that form myelin sheaths around several axons. They also provide lactate for energy needs as well as rapid conduction.
explain white and gray matter
White matter is area in the CNS with a high concentration of myelinated axons. It is located in the inner brain and outer spinal cord
Gray matter is area in the CNS with a high concentration of cell bodies and dendrites which lack myelin sheaths. It is located in the outer brain and inner spinal cord
*How are PNS axons regenerated?
After the cut, the distal portion of the axon is degenerated and phagocytose by Schwann cells. These Schwann cells, and the basement membrane, form a regeneration tube. They secrete chemicals that attract the growing axon tip and guide the regenerating axon to its proper destination.
*describe regeneration in CNS axons
Central axons have a much more limited ability to regenerate. Injury stimulates growth of axon collaterals but regeneration is prevented by inhibitory proteins (Nogo) in the membranes of myelin sheaths. Regeneration may also be physically blocked by a glial scar (astrocytes) or the scar might aid, we don’t know.
Describe the proteins that prevent axon regeneration
In the CNS, Nogo proteins are produced by oligodendrocytes and have been shown in rodents to inhibit axon regeneration.
In the PNS, Schwann cells also produce myelin proteins that can inhibit axon regeneration. But, after axon injury these are removed and Schwann cells stop producing the inhibitory proteins, creating an environment conducive to axon regeneration in PNS
What features of the brain capillaries create the blood-brain barrier
Brain capillaries do not have pores between adjacent endothelial cells, all are joined by tight junctions. Astrocytes wrap around the capillaries to regulate permeability. Therefore, the brain cannot obtain molecules from the blood plasma by nonspecific filtering process. Molecules must be moved through the endothelial cells by diffusion and active transport, also endocytosis and exocytosis. This is the blood-brain barrier
Some molecules that can pass through the blood-brain barrier
Nonpolar O2 and CO2 and some organic molecules like alcohol and barbiturates.
How does glucose get into the brain?
GLUT1 is a specialized glucose carrier that is always present in the brain
How is the degree of “tightness” and selectivity of the blood-brain barrier regulated?
Astrocytes release regulatory molecules that stimulate the capillary endothelial cells to produce the proteins of the tight junctions. These regulatory molecules can also stimulate production of carrier proteins, channels, etc. that are required for rapid transport into CNS. The CNS capillaries are also periodically surrounded by cells called pericytes which are tightly associated and produce important transporters.
Communication between astrocytes and CNS capillaries (with pericytes) controls “tightness”
