CH12: Nervous Tissue Flashcards
(36 cards)
What are the divisions of the nervous system
Nervous system is divided into CNS which is the brain and spinal cord and the PNS which includes all nervous tissue outside the nervous system
What is a nerve
A nerve is a bundle of hundred to thousands of axons plus associated connective tissue and blood vessels that lie outside the brain and spinal cord
What are the two divisions of the PNS and their functions
The PNS is divided into the sensory (afferent), which sends input from sensory receptors to the nervous system and the motor (efferent) division which sends input from the CNS to muscles and glands, known as effectors
What are the divisions of the motor division and their functions? Are there further divisions?
The motor division of the PNS is divided into the somatic, autonomic and enteric plexuses divisions.
The somatic division of the PNS conveys information from the CNS to skeletal muscles under conscious control, and is therefore the voluntary division of the PNS.
The autonomic division is responsible for sending information from the CNS to cardiac muscle, smooth muscle and glands. The majority of the ANS is under involuntary control. The autonomic division can be divided into the sympathetic and parasympathetic divisions. The parasympathetic division is associated with rest and digest activities while the sympathetic division is associated with fight-or-flight responses
What are the functions of the nervous system?
The nervous system carries out an array of tasks which can be grouped into three basic functions: sensory (input), integration (processing), and motor (output).
In sensory functions, the sensory receptors detect changes to controlled environments due to internal and external stimuli and relay the information to the brain via cranial and spinal nerves.
In integrative functions, the nervous system analyzes inputs from sensory receptors, store memories if applicable and respond accordingly to the stimuli, an activity known as integration
In motor functions, once the sensory information is integrated, the nervous system may elicit a response by sending impulses through cranial and spinal nerves towards effectors, causing muscles to contract and/or glands to secrete
What is the third hardly mentioned division of the motor division of the PNS?
The enteric plexuses are the third division of the M division of the PNS besides the somatic and autonomic division. They are a network of over 100 million neurons confined to the walls of the digestive canal and are responsible for helping regulate smooth muscle activity and gland activity of the digestive canal
What are the two types of cells in the nervous system?
Neurons and neuroglia are the two types of cells. Neurons play very unique functions, having functions in both sensory and motor functions as well as integrative functions, and complex stuff like thinking, memorizing, emotions etc. Neuroglia support and nourish neurons, and outnumber them by around 25 times, and are smaller.
Neurons generally do not divide in a persons lifetime, but neuroglia do. Both neurons and neuroglia differ in structure depending on the region of the nervous system they’re located in, corresponding to the different functions of those regions
What is a nerve impulse?
A nerve impulse, also commonly known as an action potential, is an electrical signal that travels along the surface membrane of a neuron due to changes in concentration in the interstitial fluid and the inside of neurons. This occurs through specific ion channels on the plasma membrane.
What are the three parts of most neurons
and their components
Neurons generally contain three parts: dendrites, axons and a cell body
The cell body contains a nucleus surrounded by a cytoplasm which contains normal organelles like mitochondria, lysozymes and golgi complexes.
Neuronal cell bodies also contain free ribosomes and prominent clusters of rough endoplasmic reticulum, termed Nissl bodies. The ribosomes are the sites of protein synthesis. Newly synthesized proteins produced by Nissl bodies are used to replace cellular components, as material for growth of neurons, and to regenerate damaged axons in the PNS
A nerve fiber is the term describing the two processes of neurons, dendrites and axons.
Dendrites are the receiving portions of neurons and contain chemical receptors on their surface to interact with stimuli from neurons or external stimuli. The plasma membrane of dendrites, like the cell body, contains numerous receptor sites for binding chemical messengers from other neurons called dendritic spines.
Dendrites usually are short, tapering, and highly branched. In many neurons the dendrites form a tree-shaped array of processes extending from the cell body.
The single axon of a neuron propagates nerve impulses toward another neuron, muscle fiber, or gland cell. The cytoplasm of an axon, called axoplasm, is surrounded by a plasma membrane known as the axolemma. The axon and its collaterals end by dividing into many fine processes called axon terminals
What is the trigger zone?
The trigger zone an area near the axon hillock (the place where an axon leaves the cell body) that opens voltage gated channels to propagate action potential along the axolemma
How are neurons structurally classified? Name these classifications
Neurons are classified based upon the number of processes extending away from the cell body.
Multipolar neurons have several dendrites and one axon. Most neurons of the CNS are multipolar, and so are all motor neurons.
Bipolar axons have 1 main dendrite and 1 axon. Examples of this include neurons of the retina and inner ear
Pseudounipolar neurons have the dendrites and junction fused to form one process emerging from the cell body. The dendrites of most pseudounipolar neurons function as sensory receptors that detect a sensory stimulus such as touch, pressure, pain, or thermal stimuli.
How are neurons functionally classified? Describe these classifications
Neurons are also classified by the direction the nerve impulses are conveyed with respect to the CNS
Sensory neurons convey information toward the CNS through cranial and spinal nerves after being stimulated.
Motor neurons are neurons that convey impulses from CNS to effectors
Interneurons are located primarily in the CNS between sensory and motor neurons. These neurons integrate sensory information and then elicit motor responses by activating motor neurons
What are the various types of glia and where are they found and their functions
Astrocytes, oligodendrocytes, microglia, and ependymal cells are present in the CNS while the PNS contains Schwann cells and satellite cells
Astrocytes are the most numerous glia and function to provide strength for the neuron, protect it from harmful substances in the blood and maintain optimal composition for impulse generation.
Oligodendrocytes are similar in shape to astrocytes but function to maintain and form myelin sheath of axons
Microglial cells act like tissue macrophages, removing cellular debris formed during normal development of the nervous system and phagocytize microbes and damaged nervous tissue
Ependymal cells form the blood-brain barrier and form cerebrospinal fluid to nourish the brain and spinal cord.
Schwann cells are like oligodendrocytes in that they form the myelin sheaths around axons
Satellite cells regulate the exchanges of materials between neuronal cell bodies and interstitial fluid and provide structural support
What are myelin sheaths and their function?
Myelin sheaths are layers lipids and proteins surrounding axons that are responsible for speeding up electrical impulses by electrically insulating the axons
What are the two means of electrical signals in the nervous system?
There are two methods of conducting signals in the nervous system. Graded potentials and action potentials.
Graded potentials are meant for short distance communication. They can be summed up at the axon hillock and they determine whether an action potential occurs or not. Action potentials serve as the long distance communication method, allowing transmission from different parts of the nervous system
What are the two features of excitable cells allowing for the production of action and graded potentials
The formation of GPs and Aps are dependent upon the resting membrane potential of the cell and ion channels of the cell
The resting membrane potential is the potential (voltage) difference across the axolemma. It is the flow of ions (not charged subatomic particles) across the membrane that generates the current.
Ion channels are protein molecules that span across both sides of the membrane that allow ions to cross from either side out. There are different types of ion channels that respond to different stimuli. The membranes of cells are good electrical insulators, so the flow of ions usually take place through these channels. Present in these molecules are gates, parts of the channel protein that can seal the channel pore or move aside to allow the a specific molecule to enter.
What are the various ion channels?
There are 4 ion channels
Leak channels randomly open and close their gates. There is a larger concentration of potassium ions than sodium ions in the cell, and potassium ions are much more leaky, so the membrane is much more permeable to potassium than sodium. These are found in nearly all types of neurons, cell bodies, axons and dendrites
Ligand-gated channels open and close in response to binding of a ligand stimulus . A wide variety of chemical ligands, including neurotransmitters, hormones, and particular ions, can open or close ligand-gated channels. These are present in sensory neurons, like nociceptors
Mechanical-gated channels open and close in response to mechanical stimuli, like pressure, soundwaves or stretching of tissue. The force distorts the channel from its resting position, opening the gate. These are also present in sensory neurons, like auditory receptors and pressure receptors.
Voltage gated channels open and close in response to a change in membrane potential. These are present in the axons of all types of neurons
What causes the resting membrane potential?
There are three factors that give rise to the resting membrane potential
The first is an unequal distribution of ions in the ECF. The ECF is rich in Na+ and Cl- while the cell’s main cation is K+ and anion source is phosphate having ATP and amino acids. Because more potassium leak channels exist than sodium channels, the number of potassium ions moving down their concentration gradient to exit the cell is greater than the number of sodium ions moving down their concentration gradient into the cell, making the cell negatively charged overall because there is a net movement of positive ions out of the cell.
The second is the inability of anions to leave the cell. Most anions cannot leave the cell because they belong to non-diffusable molecules like ATP and large proteins
Electrogenic nature of Na-K Atpases. Any small leaks of sodium ions in and potassium ions out is offset by sodium-potassium pumps, also known as sodium-potassium atpases. These pumps help maintain the resting membrane potential by pumping out Na+ as fast as it leaks in and at the same time, bringing in K+.
They do this by expelling three sodium ions for ever two potassium ions imported.
Since these pumps remove more positive charges from the cell then they bring into the cell, they are electrogenic, meaning they produce a change in the electrical potential of the cell, in this case, they make the cell more negative relative to the outside, keeping a negative resting membrane potential.
Where do GPs take place, and wh are they called graded?
Graded potentials occur when mechanically gated or ligand gated channels open in response to a stimulus. These occur in the dendrites and cell body of a neuron.
Graded potentials are graded because they vary in amplitude. Their amplitude depends on how many ion channels are open and how long they are open. The signal is localized, meaning it spreads to adjacent regions, but eventually dies out as the ions leak out from leak channels. This localized signal transfer is known as decremental conduction.
However, GPs can be summated in a process called summation, where the amplitude of one GP can be added on to another performed quickly. If a GP is opposite in signal (hyperpolarization vs depolarizing), they can cancel out and no net GP is sent out. Their amplitudes can get stronger and stronger if they are of the same effect.
What are the two phases of an action potential?
In an action potential, there is a polarizing and depolarizing phase.
The depolarizing phase refers to the phase that depolarizes the membrane potential, taking it to 0mv and eventually making it positive.
After the depolarizing phase, the repolarizing phase takes the resting membrane potential back to 0 and eventually makes it negative
How does the intensity of an impulse affect nerve impulse generation?
Nerve impulses will not respond to subthreshold stimuli. A threshold stimulus will cause a nerve impulse to occur, and a suprathreshold stimulus will cause several impulses to occur. All stimuli above the threshold will reach the same membrane potential at the amplitude, so the amplitude of a nerve impulse is always the same and does not depend on stimulus intensity. Instead, the greater the stimulus strength above the threshold, the greater the frequency of the nerve impulse until a maximum frequency is reached as determined by the absolute refractory period
Describe the 4 stages of a nerve impulse
Resting state: During the resting state, all sodium and potassium voltage gated channels are closed, and there is an equal build up of negative ions in the surface of the membrane as there is outside the surface of the membrane
Depolarization phase: During depolarization, a graded potential or some other stimulus causes the depolarization of the membrane potential to threshold.
It should be noted here that the sodium voltage gated channels have two gates. An activation and inactivation gate. When at rest, the i gate is open but the activation gate is closed so the ions cannot move in. Depolarization to threshold causes the activation gate to open and sodium ions to come charging in, depolarizing the cell from the threshold of -55mv to 30mv.
Repolarizing phase: During the repolarizing phase, the sodium voltage gated channels have their inactivation gate close, and because they open slowly, the voltage gated potassium channels being to open. Because potassium ions are slowly decreasing their immigration into the cell and potassium ions are now leaving the cell, the cell is becoming repolarized. Repolarization brings the membrane potential back to -70 mv, or lower, and allows the sodium gated channels to revert to their resting state.
Hyper-polarizing phase: During the polarizing phase, the outflow of potassium may be enough to polarize the membrane past the resting membrane potential, as the potassium VGC also closes slowly. Unlike sodium VGC, the potassium VGC has no inactivation or activation gates, it just has 1 normal gate alternating it between opened and closed
What is a refractory period and what are the two types
A refractory period is the period of time after a nerve impulse has begun where an excitable cell cannot generate a second impulse in response to a threshold stimulus, the two types are:
Absolute refractory period: A stimulus of any strength cannot generate a nerve impulse because the voltage gated sodium channels are in their inactivation state, preventing impulse from propagating backwards and from occuring too quickly
Relative refractory period: A stimulus can cause a nerve impulse, but it will require a greater stimulus than before, as the membrane is in the hyperpolarizing phase, and the voltage gated sodium channels are back to rest and the voltage gated potassium’ channels are still open
What are the two types or propagation:
In continuous propagation, the nerve impulse is regenerated at adjacent segments due to the opening of one sodium VGC to causing the opening of an adjacent one. This occurs in unmyelinated neurons and muscle fibers.
In saltatory conduction, the nerve impulse is regenerated at segments between myelin sheaths known as nodes of Ranvier. The myelin sheath gaps/nodes of ranvier contain much more voltage gated sodium channels than in the myelinated axon parts. The action potential travels down the axon, and at every gap, the axon is depolarized and action potential continues down the axon. The myelin sheath prevents loss of conduction.
This means that saltatory conduction is faster than an unmyelinated continues conduction with a same diameter neuron because rather than regenerating the impulse at every adjacent segment, it can simply travel down the axon and be generated at the gaps
Another advantage is that a smaller number of channels only at the gaps, rather than many in each adjacent segment of membrane, represents a more energy-efficient mode of conduction, because only small regions of the membrane depolarize and repolarize, minimal inflow of Na+ and outflow of K+ occurs each time a nerve impulse passes by, thus less ATP is used by sodium-potassium pumps to maintain low intracellular concentration of sodium ions and the low extracellular concentration of K
