Nervous tissue Flashcards
1
Q
that are the 3 functions of the nervous system?
A
- Senses changes in the environment, both internal and external
- Integrates and interprets the sensory input for understanding
- Responds by initiating muscular contractions or glandular secretions
2
Q
How does the nervous system accomplish its homeostatic role?
A
through reactions, carried out by electrical messages called nerve impulses (action potentials), allow for second-to-second adjustments in homeostasis
3
Q
How does the role of the endocrine system compare with that of the nervous system?
A
The endocrine system, using blood-borne chemical messengers called hormones, controls long-term homeostasis. Rather than making second-to-second adjustments, the endocrine system controls processes over days, weeks, months, and years.
4
Q
Name the two principal divisions of the nervous system.
A
central nervous system (CNS)
peripheral nervous system (PNS)
5
Q
Describe the central nervous system
A
The CNS consists of the brain and the spinal cord, within which incoming sensory information is processed, thoughts and emotions are generated, and memories are stored. Most nerve impulses that stimulate muscle contraction and glandular secretion originate in the CNS.
6
Q
describe the peripheral nervous system
A
The PNS consists of 12 pairs of cranial nerves associated with the brain and 31 pairs of spinal nerves associated with the spinal cord.
7
Q
what is the relationship between the peripheral and central nervous system
A
Cranial and spinal nerves of the PNS carry sensory information from the CNS to effectors (muscles and glands) in the periphery.
8
Q
what are sensory neurones
A
The input component of the PNS consists of nerve cells called sensory (afferent) neurons that conduct nerve impulses from sensory receptors to the CNS and end within the CNS.
9
Q
What are motor neurons?
A
The output component of the PNS consist of nerve cells called motor (efferent) neurons that originate in the CNS and conduct nerve impulses away from the CNS to the effectors.
10
Q
Somatic nervous system
A
concerned with sensory information from the skin, skeletal muscles, and special senses, and motor information to the skeletal muscle only.
11
Q
Autonomic nervous system
A
carries sensory information from the viscera to the CNS and motor information from the CNS to cardiac muscle, smooth muscles, and glands.
12
Q
Name the autonomic subdivisions.
A
The motor portion of the autonomic nervous system is divided into two portions: the sympathetic nervous system and the parasympathetic nervous system.
13
Q
What are neuroglial cells
A
Neuroglial cells are the supportive, nurturing, and protective cells for the neurons. They occupy only half of the CNS, are much smaller than neurons and outnumber them. They remain mitotic throughout life and tend to fill in spaces of injured neurons after disease and injury.
14
Q
Astrocytes
A
attach blood vessels to neurons,
helping to form the blood-brain barrier. They help maintain the proper balance of K+ for the neurons and participate in the metabolism of neurotransmitters. They are responsible for forming scars in the CNS after injury.
15
Q
Oligodendrocytes
A
give support to
neurons of the CNS. They produce the myelin sheath found around axons of the CNS. Each oligodendrocyte uses its processes to wrap several axons.
16
Q
Microglia
A
small phagocytic cells that engulf
and destroy microbes and cellular debris in the CNS. They migrate to areas of injured nervous tissue and help to clean the area.
17
Q
Ependyma
A
form a continuous epithelial
lining for the ventricles of the brain and the central canal of the spinal cord. They probably assist in the circulation of cerebrospinal fluid, but their role is mostly unknown.
18
Q
Schwann cells
A
also known as neuro-
lemmocytes, produce the myelin sheath around the axons of motor neurons and the dendrites of sensory neurons in the PNS.
19
Q
Satellite cells
A
support neurons found in
ganglia in the PNS; function is obscure.
20
Q
What is the myelin sheath?
A
Most nerve fibers are surrounded by a multilayered lipoprotein produced by the neuroglia (Schwann cells in the PNS and oligodendrocytes in the CNS) called the myelin sheath.
21
Q
what is the function of the myelin sheath
A
electrically insulates the nerve fiber, greatly increasing the speed of nerve impulse conduction.
Nerve processes with such a covering are said to be myelinated while those without are unmyelinated. Therefore, there are neurons with different speeds of transmission.
22
Q
How is the myelin sheath formed in the PNS?
A
Schwann cells form the myelin sheath around motor axons and sensory dendrites during fetal life and the first postnatal year. In this process, Schwann cells line up along the length of the nerve fiber, attach to it, then begin to spiral around it, leaving behind multiple layers of glial cell membrane.
23
Q
What is the neurilemma?
A
As the multiple layers of membrane are formed, the cytoplasm and organelles of the Schwann cells are pushed to the outside. This portion of the Schwann cell is known as the neurilemma. It is found only around neurons of the PNS; oligodendrocytes do not form a neurilemma.
24
Q
What are the nodes of Ranvier?
A
At intervals along the length of a nerve process, between the individual Schwann cells (PNS) or pieces of oligodendrocytes (CNS), are gaps in the myelin sheath called the nodes of Ranvier (neurofibral nodes).
25
What is the function of neurons?
Nerve cells, called neurons, are responsible for conducting impulses from one part of the body to another and are therefore the structural and functional units of the nervous system.
26
what is the describe the neuron cell body
The cell body (perikaryon, soma) contains
typical cellular organelles surrounded by cytoplasm. There is a large nucleus with a very prominent nucleolus and neurofibrils, elements of the cytoskeleon that give the neuron structure and shape
27
Nissl substance
Scattered throughout the cytoplasm of the neuron
cell body are structures called Nissl bodies (chromatophilic substance), orderly arrays of rough endoplasmic reticulum used for protein synthesis.
28
Dendrite
The dendrite is usually a short, tapering, and
highly branched process extending from the cell body; usually unmyelinated (sensory dendrites are the exception). It is ALWAYS a process that carries the nervous message towards the cell body.
29
Axon
The axon is a single, long, thin, cylindrical projection
from the cell body that moves toward an effector or another neuron. It ALWAYS carries the nervous message away from the cell body.
30
Axon hillock
The axon hillock, a cone-shaped elevation, is
| that region of the neuronal cell body from which the axon arises.
31
Trigger zone
Just distal to the axon hillock is an area called the trigger zone where nerve impulses arise for propagation along the axon. This area of membrane is rich with voltage-gated sodium channels (to be described later).
32
Axon collateral
Along the axon’s length, side branches
| called axon collaterals may depart from the main axon to innervate other structures.
33
Telodendrion
At their terminations with effectors, the axon
| and axon collaterals end by dividing into many fine processes called axon terminals or telodendria
34
End bulbs
The tips of the axon terminals swell into bulb-
shaped synaptic end-bulbs that contain synaptic vesicles filled with a chemical known as a neurotransmitter. Neurons utilize a single type of neurotransmitter.
35
what is a nerve fiber
a general term for any nerve process (sensory dendrite or motor axon).
36
what is a nerve
a bundle of many nerve fibers, both sensory and motor, that course along the same path in the PNS
37
what is a tract
a bundle of related nerve fibers in the CNS that connects different areas of the CNS.
38
Multipola neuron
have several
| dendrites and a single axon; most neurons are of this type.
39
Bipolar neruon
have a single dendrite and a
| single axon extending from the cell body; associated with the special senses.
40
Unipolar neuron
sensory only) have a single
process extending from the cell body called the central process that divides into two parts. The axonal portion carries an impulse away from the cell body; the dendritic portion is attached to a receptor distally and carries an impulse to the cell body.
41
Gray matter
In a section of fresh brain and spinal
cord, some regions appear white and glistening while others are grayish. The gray areas are the gray matter of the CNS, areas of nerve cell bodies, dendrites, axon terminals, and/or unmyelinated axons, and neuroglia.
42
White matter
The whitish areas are the white matter of the CNS; this refers to aggregations of the myelinated processes of many neurons, arranged into tracts.
43
nucleus
A nucleus is a collection of similar neuronal
cell bodies and dendrites within the gray matter of the CNS that perform a specific function. These collections of neurons are often called centers.
44
Ganglion
A ganglion is a collection of similar neuronal cell bodies outside the CNS, lying in the periphery. Ganglia contain the cell bodies of sensory neurons or the second neuronal cell body of an autonomic pathway.
45
Communication by neurons depends upon two basic properties of their cell membranes. List them.
1. There is an electrical voltage, called the resting membrane potential
(RMP), across the cell membrane.
2.Their cell membranes contain a variety of ion channels (pores) that
may be open or closed.
46
What is a membrane channel?
A membrane channel (pore) allows a specific substance to move through
a water-filled passageway to either enter or leave the cell. In neuronal membranes, sodium and potassium channels are of utmost importance
47
Assuming that there is a diffusion gradient, what happens when a channel opens?
When the channels are open, specific ions in the intracellular fluid or the extracellular fluid flow through them, according to their diffusion gradients
48
Describe the concept of the channel as a gate or door.
Part of the integral protein that forms such a channel may act as a gate or door, opening and closing on demand, to alter the flow of ions along their diffusion gradient.
Depending on the types of channels that are present, a portion of a neuron may be able to produce either a gradient potential or and action potential (nerve impulse).
49
What is the resting membrane potential?
The resting membrane potential (RMP) occurs because there is a small build up of negative charges just inside the cell membrane of the neuron and an equal build up of positive charges outside.
Such a separation of charges by the membrane is a form of potential energy that is measured in milliVolts (mV).
The greater the difference in charge across the membrane, the larger the membrane potential (voltage).
50
how is membrane potential related to the formation of a polarized membrane
the buildup in charges is very close to the cell membrane; elsewhere, there are equal numbers of positive and negative charges.
In neurons the RMP ranges from -40 to -90 mV with a typical value of -70 mV. The minus sign indicates that the inside of the neuron is negative by 70 mV relative to the outside.
A cell that exhibits a membrane potential is called a polarized cell or has a polarized membrane.
51
what is current
The resting membrane potential (RMP) serves as a type of battery. Flow of the electrical charges
In living cells, this current is created when the ions flow across the membrane.
52
What are the paths for ion flow through the neuronal membrane?
Since the lipid bilayer is a good insulator, the main paths for current flow across the membrane are the ion channels.
Thus, when the ion channels are open in the membrane, current flows and this changes the membrane potential.
53
Describe the two main factors related to ions that contribute to the resting membrane potential.
1. Distribution of ions across the cell membrane.
--extracellular fluid is rich in Na+ and Cl-
--intracellular fluid is rich in K+ and anions such as
organophosphates and proteins
2. Relative permeability of the cell membrane to Na+ and K+
--membrane is moderately permeable to K+ and Cl-; slightly
permeable to Na+; impermeable to intracellular anions.
54
Why does the inside of the neuronal cell membrane become negatively-charged?
Since the membrane is moderately permeable to potassium, there is always a slow diffusion of positively-charged potassium ions out of the cell. The intracellular fluid just next to the inner surface of the neuronal membrane becomes more and more negatively-charged as potassium diffuse out.
55
Why does the outside of the neuronal cell membrane become positively-charged?
Since the neuronal membrane is only slightly permeable to sodium, the sodium ions accumulate outside the cell. They are attracted to the anions within the cell and because of the large diffusion gradient for Na+ into the cell, the extracellular fluid just next to the outer surface of the neuronal membrane becomes more and more positively-charged.
56
What is the net result of this ion distribution?
The net result is that neuronal membrane at rest tends to have positive charges lined up along its outer surface and negative charges lined up along its inner surface.
57
It is important that the membrane potential be maintained and that the ions remain in their respective resting positions. With such great diffusion gradients, how is this accomplished
In the membrane are Na/K-ATPase pumps that expel 3 Na+ for 2 K+ imported. Such pumps are said to be electrogenic since they contribute to the negativity of the resting membrane potential
58
Describe the different types of ion channels and how they work?
An ion channel (pore) is a specific membrane protein structure that allows only a specific substance to pass through the membrane while excluding others
59
what are the two basic types of ion channels?
Leakage (non-gated) channels are always open
(glucose channels, for example)
2. Gated channels open and close in response to
some sort of stimulus: voltage, chemical, mechanical, light.
60
How do voltage gated channels work>
Voltage-gated (or regulated) channels open in response to a direct change in the membrane potential (voltage).
The presence of these channels in nerve and muscle cell membranes makes them excitable (irritable); that is, the ability to respond to certain types of stimuli by producing impulses.
61
what is the trigger zone for a neuron
The trigger zone for a particular neuron is the place where the voltage-gated ion channels are clustered most densely.
62
Describe Chemically-gated channel
Chemically-gated ion channels open
| and close in response to specific chemical stimuli, such as neurotransmitters and hormones.
63
Describe Mechanically-gated channel
-- Mechanically-gated ion channels
| open or close in response to mechanical stimuli such as touch, pressure, or vibration (sensory receptors).
64
Describe light gated channels
Light-gated ion channels close in response
| to light energy (found only in the photoreceptors of the eyes).
65
Describe graded respones
The presence of chemical-,mechanical-, or
light-gated channels allows for a graded response, an electrical response that varies in size, depending on how many channels are opened and for how long.
66
in general describe an action potential through depolarization and repolarization
During an action potential (nerve impulse), two types of voltage-gated ion channels open and then close, first the channels for Na+, then those for K+.
Rapid opening of the voltage-gated Na+ channels results in depolarization, the loss and then reversal of the membrane polarization by the neuron.
The slower opening voltage-gated K+ channels and the closing of the previously open Na+ channels result in repolarization, the recovery of the resting membrane potential.
67
Describe the specifics of the process of depolarization
If a graded potential causes the membrane to depolarize to a
critical level, called the threshold (about -55 mV), the voltage-gated Na+ channels open.
Na+ ions rush through the channel, driven by both the
electrical and concentration gradients formed during rest, and the membrane potential changes from -70 mV towards 0 mV, then up to +30 mV.
Throughout depolarization, Na+ ions continue to diffuse into
the neuron until the membrane potential reverses so that the inside of the cell becomes +30 mV.
Each voltage-gated Na+ channel has two separate gates, an activation gate and an inactivation gate.
In a resting neuron, the inactivation gate is open and the activation gate is closed. As a result, Na+ ions cannot diffuse into the cell.
At threshold, many voltage-gated Na+ channels suddenly change from the resting state to the activated state, allowing Na+ ions to now diffuse into the cell.
68
why is depolarization a positive feedback mechanism
As more channels open, more Na+ moves into the cell and the membrane depolarizes further. This positive feedback mechanism allows current created by Na+ influx at one channel to activate adjacent Na+ channels.
Voltage-gated Na+ channels are open only a few 10/1000ths of a second, so that only about 20,000 Na+ ions move into the cell. Since this is only a small fraction of the total Na+, the Na-K pump can move them back out.
69
Describe the specifics of the process of repolarization.
A threshold depolarization not only opens the voltage-gated Na+ channels, but it also opens the voltage-gated K+ channels, initiating repolarization.
The K+ channels open more slowly than the Na+ channels, however, so that their opening coincides with the closing of the Na+ channels.
Na+ channel inactivation slows the influx of the Na+ ions into the neuron and is coupled with the efflux of K+ as it flows down its concentration gradient to the outside. The membrane potential moves back towards -70 mV.
Repolarization, therefore, restores the resting membrane potential and allows the Na+ channels to return to their inactivated state.
While the K+ channels are open, outflow of the K+ is so great that there is hyperpolarization of the membrane, meaning that the membrane becomes even more negative than at rest (-90 mV).
As the voltage-gated K+ channels close, however, the membrane potential returns to the normal resting levels as the Na-K pumps continue to move ions.
70
Describe the refractory period of a neuronal membrane.
The period of time during which an excitable cell (muscle or neuron) cannot generate another action potential is called the refractory period.
The absolute refractory period refers to the time period during which a second stimulus cannot initiate a second action potential. It coincides with Na+ channel activation and inactivation.
Since inactivated Na+ channels cannot reopen, they must first return to the resting state before they can be activated again.
The relative refractory period is the period of time when a second action potential can be initiated, but only by a suprathreshold stimulus.
The relative refractory period coincides with the period of time when voltage-gated K+ channels are still open after the inactivated Na+ channels have returned to their resting state.
71
Condition in AP propagation
A stimulus or stress disrupts membrane
| homeostasis by causing a threshold depolarization
72
receptor is AP Propagation
The receptors in this case are voltage-gated
| Na+ channels in their resting state.
73
Control Center in AP propagation
The shape of the voltage-gated Na+
| channel depends on membrane voltage
74
Effector in AP propagation
Voltage-gated Na+ channels are also the
| effectors. Threshold depolarization causes shape changes in the channel.
75
Response in AP propagation
Opening of the voltage-gated Na+ channels
| causes depolarization in adjacent membrane, opening more voltage-gated Na+ channels. This is positive feedback.
76
What is the all-or-none principle of neuron function?
A single neuron, like a single muscle fiber, generates an action potential according to the all-or-none principle.
If depolarization reached threshold, voltage-gated Na+ channels open, the positive feedback mechanism is initiated, and an action potential arises.
Each time an action potential is formed, it has a constant and maximum strength for the existing conditions.
77
Describe continuous conduction.
Nerve impulses communicate from one part of the body to another. To do this, they must travel from there they arise, at a trigger zone, to axon terminals at a synapse.
The special mode of impulse travel across a neuron is called propagation (conduction) and is dependent upon positive feedback.
As Na+ flows into the neuron, depolarization occurs and adjacent Na+ channels are opened. In this way, the nerve impulse self-propagates along the membrane.
Also, since the membrane is refractory just behind the leading edge of the impulse, an impulse normally travels in one direction only from where it arises (the trigger zone).
This type of conduction, in which each piece of neuronal membrane becomes depolarized during propagation, is called continuous conduction. It is common in muscle membranes and unmyelinated neuronal membranes.
78
Describe salutatory conduction.
In myelinated membranes conduction is somewhat different because the myelin sheath acts as an electrical insulator to block ionic currents across the membranes.
At the nodes of Ranvier, however, the myelin sheath is interrupted and the membrane has a high density of Na+ channels. It is here that depolarization occurs and current is carried into the neuron.
In this matter, ionic current flows into the nerve fiber. The Na+ ions then follow their diffusion gradient, moving along the inside of the nerve fiber to the next node of Ranvier. This movement produces just enough voltage at the next node to cause voltage-gated Na+ channels to open there, thus self-propagating the message.
As a result, the nerve impulse appears to “jump” from node to node as each area depolarizes and so conducts the impulse as it arises anew at each node (saltare = to leap). This is saltatory conduction.
Since the impulse “jumps” long intervals of membrane with-out depolarizing it, the impulse travels much faster than by continuous conduction (0.5 m/sec vs 130 m/sec).
In addition, only small regions of the membrane must be repolarized, so that there is much less work involved by the Na-K pumps and therefore energy is conserved.
79
What is a synapse?
The point of communication between cells, either neuron-neuron or neuron-effector (“synapsis” = connection)
80
Why are synapses essential for homeostasis?
Because they allow information to be integrated and filtered; some signals are transmitted while others are inhibited.
81
Distinguish between presynaptic and postsynaptic neurons.
At a synapse, the neuron sending the signal is the presynaptic neuron, and the neuron receiving the message is the postsynaptic neuron.
82
Describe conduction across the synapse.
Presynaptic and postsynaptic neurons or effectors (muscle and glands) do not touch. They are separated by the synaptic cleft (20-50 nm), a space filled with extracellular fluid.
Within the axon terminals the presynaptic neurons are synaptic vesicles filled with a particular neurotransmitter.
Arrival of the action potential in the membrane of he axon terminal opens Ca++ channels in the membrane. This, in turn, opens calcium channels, allowing extracellular calcium to diffuse into the axon terminal.
Elevated intracellular Ca++ causes some of the synaptic vesicles to fuse with the membrane of the axon terminal and release into the synaptic cleft the stored neurotransmitter.
Neurotransmitter then diffuses across the synaptic cleft through the extracellular fluid to the adjacent membrane of the postsynaptic neuron or effector.
Interaction between neurotransmitter and its specific receptor protein in the membrane of the postsynaptic neuron initiates the formation of a postsynaptic potential.
The time required for this process, called the synaptic delay, is about 0.5 msec.
At the chemical synapse there can only be one-way information transfer, from presynaptic neuron to postsynaptic neuron or effector.
As a result, graded potentials and action potentials must move forward over their pathways. Action potentials cannot back up into another presynaptic neuron.
83
Excitatory neurotransmitter
If the neurotransmitter
released at a synapse causes depolarization, it is excitatory because it brings the membrane of the postsynaptic neuron closer to threshold (more positive). Such an event is called the excitatory postsynaptic potential (EPSP).
84
Facilitation and summation
During the time that the
postsynaptic membrane is brought closer to its threshold, it is said to be facilitated. If a series of facilitations brings the membrane to threshold, then the effect is called summation and an action potential is generated.
85
Inhibitory neurotransmitter
If the neurotransmitter causes
hyperpolarization of the postsynaptic membrane, it is said to be inhibitory because it moves the membrane potential further from threshold (more negative). Such an event is called the inhibitory postsynaptic potential (IPSP) and the neuron is inhibited.
86
Spatial summation
When the summation is the result of
| several presynaptic neurons releasing their neurotransmitter into the trigger zone within the same time frame,
87
Temporal summation
When summation results from
| buildup of neurotransmitter released by a single presynaptic neuron firing several times in rapid succession
88
The sum of all the effects on a postsynaptic neuron, both excitability and inhibitory, determines the action taken by the neuron. Therefore, one of three events must occur. Name them
1. If the excitatory effect is greater than the inhibitory
effect, but less than threshold, the result is an EPSP and the process is facilitation.
2. If the excitatory effect is greater than the inhibitory
effect and reaches threshold, the result is summation and an action potential in the postsynaptic neuron or effector.
3. If the inhibitory effect is greater than the excitatory
effect, the result is membrane hyperpolarization and inhibition of the postsynaptic neuron.
89
diffusion as a means of NT removal
The neurotransmitter may diffuse through the
| extracellular fluid and away from the synapse.
90
Enzymatic degradation as a means of NT removal
There may be specific enzymes
| present in the synaptic cleft or on the postsynaptic membrane to degrade the neurotransmitter
91
NT Uptake into the cell as a means of removal
In some cases the neurotransmitter is
| actively transported back into the presynaptic neuron and reused.
92
What is a neuronal pool?
The CNS contains billions of neurons organized into complex patterns called neuronal pools, each with its own role in homeostasis. Neuronal pools are arranged into patterns called circuits over which nerve impulses are conducted. There are five common types of these circuits
93
Simple series Circuits
The simplest circuit is the simple series circuit in
which a single presynaptic neuron synapses with a single postsynaptic neuron, which stimulates another, which stimulates another, etc.
94
Diverging Circuits
a single presynaptic neuron
synapses with more than one postsynaptic neuron, in a process called divergence (Ex: a sensory signal is relayed to several different parts of the brain.)
95
Parallel after-discharge Circuit
have a
single synaptic neuron that diverges, then each neuron in the pathway synapses on a common postsynaptic neuron (Ex: precise mental activities such as math.)
96
Reverberating Circuits
branches from neurons
later in a pathway synapse with neurons found earlier in the pathway (Ex: breathing, sleep-wake cycles, short-term memory.)
97
Converging Circuits
several presynaptic neurons
with a single postsynaptic neuron, in a process known as convergence (Ex: a single motor neuron receives motor information from several sources.)
