Chapter 11 Flashcards
Functions of the Nervous System
- Sensory input: (afferent neuron)
- > info gathered by sensory receptors about internal and external changes - Integration: (interneuron)
- > interpretation of sensory input - Motor output: (efferent neuron)
- > activation of effector organs (muscles & glands) produces a response
Divisions of the Nervous system
CNS:
- > Brain & spinal cord
- > command center
PNS:
- > paired spinal and cranial nerves carry messages to and from the CNS
- > 2 directions
PNS
Two functional divisions:
- Sensory (afferent) division (to the brain)
- >SOMATIC afferent fibers- convey impulses from skin, skeletal muscles, and joints
-> VISCERAL afferent fibers-convey impulses from visceral organs
- Motor (efferent) division (away from brain)
- >Transmits impulses from the CNS to effector organs (muscles & glands)
Motor division of PNS
- Somatic (voluntary) nervous system
- > conscious control of skeletal muscles - Autonomic (involuntary) nervous system (ANS)
- > regulates smooth muscle, cardiac muscle, and glands
- > 2 functional subdivisions: Sympathetic & parasympathetic
Nervous tissue
2 principle cell types:
1. Neurons- exitable cells that transmit electrical signals
- Neuroglia (glial cells)-supporting cells:
- > Astrocytes (CNS)
- > Microglial (CNS)
- > Ependymal (CNS)
- > Oligodendrocytes (CNS)
- > Schwann cells (PNS)
Neurons
- > have extreme longevity. Given good nutrition can last a lifetime
- > Amitotic
- > High metabolic rate & require continous and abundant supplies of oxygen
- > the plasma membrane of the cell body acts as part of the receptive region that receives info from other neurons.
- > cell body is the major biosynthetic center and metabolic center of a neuron
- > includes mitochondria, golgi app, rough ER (also called the chromatophilic substance), free ribosomes.
Most neuron cell bodies are located in the CNS, where they are protected by the bones of the skull and vertebral column. Clusters of cell bodies in the CNS are called NUCLEI/ NUCLEUS
TRUE!
Cell bodies that lie along the nerves in the PNS are called ganglia
The PNS consists chiefly of neuron processes (whose cell bodies are in the CNS)
TRUE!
2 types of neuron processes
dendrites and axons
Dendrites
of motor neurons are short, tapering, diffusely branching extensions.
- > the main receptive or input regions, provide an enormous surface area for receiving signals from other neurons
- > dendritic spines: thorny appendages with bulbous or spiky ends- which represent points of close contact (synapses) w/ other neurons
- > convey incoming messages toward the cell body. These electrical signals are usually not action potentials (nerve impulses) but are short-distance signals called GRADED POTENTIALS
The axon
- > neuron never has more than 1
- > arises from a cone shaped area of the cell body caled the axon hillock
- > any long axon is also called a nerve fiber
Tract
a bundle of axons in the CNS
Nerve
A bundle of axons in the PNS
The axon: functional characteristics
- > The conducting region of the neuron
- > generates nerve impulses and transmits them, typically away from the cell body, along the plasma membrane, or axolemma
- > when the impulse reaches the axon terminals it causes neurotransmitters-signaling chemicals- to be released in the extracellular space
- > neurotransmitters either excite or inhibit neurons (muscle or gland cells) with which the axon is in close contact.
- > neuron receives and sends signals to other neurons, carrying on “conversations” with many different neurons at the same time
- > axon LACKS rough ER and golgi apparatus- the structures involved with protein synthesis and packaging
What does an axon depend on?
- > Its cell body to renew the necessary proteins and membrane components
- > efficient transport mechanisms to distribute them
- > axons quickly decay if cut or damaged bad
Axonal transport
- > Anterograde movement: movement away from the cell body.
- > Retrograde movement: Movement toward the cell body. Important means of intracellular communication, allows the cell body to be advised of conditions at the axon terminals. Also delivers vesicles to the cell body containing signal molecules (such as nerve growth factor)
- > Bidirectional transport mechanism is responsible for axonal transport. Uses different ATP-dependent “motor” proteins (kinesin or dynein), depending on direction
Myelin sheath
- > whitish, fatty (protein-lipoid) segmented myelin sheath
- > protects and electrically insulates fibers, and it increases the transmission speed of nerve impulses
- > myelin sheaths are only associated with axons.
- > dendrites are NONmyelinated
Myelination in the PNS
-> formed by schwann cells
-
>indent to recieve an axon and then wrap themselves around it in a jelly roll fashion
- A schwann cell envelops an axon
- The schwann cell then rotates around the axon, wrapping its plasma membrane loosely around it in successive layers
- the schwann cell cytoplasm is forced between the membranes. The tight membrane wrappings surrounding the axon form the myelin sheath
- >plasma membranes of mylinating cells contain much less protein than those of most body cells.
- > adjacent schwann cells do not touch one another so there are gaps in the sheath. These myelin sheath gaps or nodes of Ranvier occur along a myelinated axon. Axon collaterals can emerge at these gaps
Schwann cells (PNS)
- > Surround peripheral nerve fibers and form myelin sheaths
- >Vital to regeneration of damaged peripheral nerve fibers
Myelination in the CNS
- > oligodendrocytes form myelin sheath
- > myelin sheaths in the CNS lack an outer collar of perinuclear cytoplasm bc cell extensions do the coiling and the squeezed out cytoplasm is forced back toward the centrally located nucleus instead of peripherally
Multipolar neurons
have 3 or more processes- one axon and the rest dendrites. major type in CNS
Bipolar neurons
have 2 processes- an axon and a dendrite-that extend from opposite sides of the cell body. rare neurons. found in some of the special sense organs such as in the retina of the eye and in the olfactory mucosa
very rare
Unipolar neurons
have a single short process that emerges from the cell body and divides T-like into proximal and distal branches. The distal peripheral process is often associated with a sensory receptor. The central process enters the CNS. Unipolar neurons are more accurately called pseudounipolar neurons because they originate as bipolar neurons
- > found mainly in the PNS. common only in dorsal root ganglia of the spinal cord and sensory ganglia of cranial nerves
- > in place of dendrites unipolar neurons have “receptive endings” (sensory terminals) at the end of the peripheral process
Sensory or afferent neurons
transmit impulses from sensory receptors in the skin or internal organs toward or into the CNS
-> all sensory neurons are unipolar, and their cell bodies are located in sensory ganglia outside the CNS
Motor or efferent neurons
carry impulses away from the CNS to the effector organs (muscles and glands) of the body.
->multipolar
Interneurons or association neurons
lie between motor and sensory neurons in neural pathways and shuttle signals through the CNS pathways where integration occurs
->almost all are multipolar
White matter
dense collections of myelinated fibers (tracts)
Gray matter
mostly neuron cell bodies and unmyelinated fibers
voltage
the measure of potential energy generated by seperated electrical charges
potential difference
voltage measured between 2 points
current
the flow of electrical charge from one point to another is a current and it can be used to do work
the amount of charge that moves between 2 points depends on 2 factors: voltage and resistance
- > substances with high electrical resistance are insulators
- >substances with low resistance are conductors
3 main types of gated channels
- > chemically gated channels
- > voltage-gated channels
- > mechanically gated channels
chemically gated channels
-> also known as ligand-gated channels, open when the appropriate chemical (in this case neurotransmitter) binds
voltage-gated channels
open and close in response to changes in the membrane potential
mechanically gated channels
open in response to physical deformation of the receptor (as in sensory receptors for touch and pressure)
2 factors generate the resting membrane potential
- differences in the ionic composition of the intracellular and extracellular fluids
- differences in the plasma membranes permeability to those ions
Resting membrane potential
Potential difference across the membrane of a resting cell
->Approximately –70 mV in neurons (inside of membrane is negatively charged relative to outside)
Generated by:
- > Differences in ionic makeup of ICF and ECF
- > Differential permeability of the plasma membrane
Differences in ionic makeup
- > Outside the cell has higher concentrations of Na+
- >Inside the cell has higher concentration of K+ and negatively charged proteins (A–)
at resting membrane potential, the negative interior of the cell is due to a much greater ability for K+ to diffuse out of the cell than for Na+ to diffuse into the cell
true
sodium-potassium pump
- > pump first ejects 3 Na+ from the cell and then transports 2 K+ back into the cell
- > this pump stabilizes the resting membrane potential by maintaining the concentration gradients for sodium and potassium
changes in membrane potential can produce 2 types of signals
- > graded potentials: usually incoming signals operating over short distances that have variable (graded) strength. dendrites
- > action potentials: long distance signals of axons that always have the same strength, axons.
Depolarization
a decrease in membrane potential: the inside of the membrane becomes less negative (moves closer to 0) than the resting potential
Hyperpolarization
an increase in membrane potential: the inside of the membrane becomes more negative (moves farter from 0) that the resting potential. For ex: -70 mV-> -75
graded potentials
Short-lived, localized changes in membrane potential
Depolarizations (excite) or hyperpolarizations (inhibit)
Graded potential spreads as local currents
small and localized
Action potential
Brief reversal of membrane potential with a total amplitude of ~100 mV
- > Occurs in muscle cells and axons of neurons
- > Does not decrease in magnitude over distance
- > Principal means of long-distance neural communication
- > all or none
slide 25 in ppw
diagram of action potential
more Na+ ions outside the neuron than inside
true
more K+ inside the neuron than outside
true
leak channels allow Na and K ions to diffuse down their concentration gradients (more K leak channels)
true
Graded potentials
short-lived, localized changes in membrane potential, usually in dendrites or the cell body
- > changes cause current flows that decrease in magnitude with distance
- > magnitude varies directly with stimulus strength - the stronger the stimulus, the more the voltage changes and the farther the current flows
- > triggered by some change (a stimulus) in the neurons env. that opens gated ion channels
- > decays with distance
- > essential in initiating action potentials
Different types of graded potentials
- > receptor potential or a generator potential: produced when a sensory receptor is excited by its stimulus (light, pressure, chemicals)
- > postsynaptic potential: produced when the stimulus is a neurotransmitter released by another neuron. Here, the neurotransmitter is released into a fluid-filled gap called a synapse and influences the neuron beyond the synapse.
Action potentials
- > Only cells with excitable membranes- neurons and muscle cells- can generate action potentials
- > depolarization is followed by repolarization and often a short period of hyperpolarization
- > do not decay with distance
- > action potential also called a nerve impulse (typically only generated in axons)
- > stimulus: changes the permeability of the neuron’s membrane by opening specific voltage-gated channels on the axon
Generation of an action potential
- Resting-state:
- > Only leakage channels for Na+ and K+ are open
- > All gated Na+ and K+ channels are closed
2. Depolarization: voltage-gated Na+ channels open.
3. Repolarization: Na+ channels are inactivating, and voltage-gated K+ channels open
4. Hyperpolarization: Some K+ channels remain open, and Na+ channels reset
Threshold and the All-or-none phenomenon
- > Not all local depolarization events produce APs. The depolarization must reach threshold values if an axon is to “fire”
- > Threshold is the membrane potential at which the outward current created by K+ movement is exactly equal to the inward current created by Na+ movement
- > Na+ permeability rises to such an extent that entering sodium ions “swamp” (exceed) the outward movement of K+, establishing the positive feedback cycle and generating an AP
- > critical factor= the total amount of current that flows through the membrane during a stimulus
Coding for stimulus intensity (action potential)
- > CNS determines stimulus intensity by the frequency of impulses– higher frequency means stronger stimulus
- > strong stimuli cause action potentials to occur more frequently
Absolute & relative refractory period
- > Absolute refractory period: Time from the opening of the Na+ channels and ends when the Na+ channels begin to reset to their original resting state.
- > ensures that each AP is a separate, all-or-none event
- > enforces one-way transmission of the AP
- > Another AP cannot fire during the period
- > Relative refractory period: Most Na+ channels have returned to their resting state, some K+ channels ae still open, and Repolarization is occurring
- > Threshold for AP generation is elevated
- > Exceptionally strong stimulus may generate an AP
The synapse
A junction that mediates information transfer from one neuron:
- > To another neuron, or to an effector cell (NMJ)
- > Release of a neurotransmitter
- > Has a presynaptic neuron and postsynaptic neuron
Chemical Synapses
->Specialized for the release and reception of neurotransmitters
- > Typically composed of two parts:
- Axon terminal of the presynaptic neuron, which contains synaptic vesicles
-Receptor region on the postsynaptic neuron
Termination of neurotransmitter effects
- > Within a few milliseconds, the neurotransmitter effect is terminated:
- Degradation by enzymes
- Reuptake by astrocytes or axon terminal
- Diffusion away from the synaptic cleft
Postsynaptic Potentials
- Graded potentials
- Strength determined by:
- > Amount of neurotransmitter released
- > Time the neurotransmitter is in the area
- Types of postsynaptic potentials
1. EPSP—excitatory postsynaptic potentials
- IPSP—inhibitory postsynaptic potentials
an action potential occurs if the combination of graded potentials exceeds a threshold
true
Temporal summation
one neuron fires many times
Spatial summation
Several neurons firing all at once
Neurotransmitters
-> Most neurons make 2 or more neurotransmitters
Biogenic amines include:
-> Catecholamines-
dopamine, norepinephrine (NE), and epinephrine
-> indolamines: serotonin and histamine
Acetylcholine (Ach):
- > Released at neuromuscular junctions and some ANS neurons
- > Major neurotransmitter that controls muscle action
norepinephrine (NE)
alertness
concentration
energy
Serotonin
obsessions
compulsions
memory
Dopamine
pleasure
reward
motivation/drive
overlap between norepinephrine and dopamine
attention
Overlap between norepinephrine and serotonin
anxiety
impulse
irritability
Overlap between dopamine and serotonin
apetite
sex
aggression
Overlap between norepinephrine and serotonin and dopamine
Mood
cognitive function
Dopamine- Pleasure/reward
-> Our brains are wired to reward us with pleasure when we engage in behavior necessary for our species survival (food/reproduction)
Drugs of abuse:
Cocaine/Crack
-> Blocks the re update of dopamine
->Allows body to feel its effects over a prolonged period
->With continued use, body stops making dopamine
->This produces depression and cravings for the drug
The basis for differentiation between gray matter and white matter in the CNS is the presence of _______ in white matter.
myelinated fibers
Information transfer across chemical synapses
- Action potential arrives at axon terminal
- Voltage gated Ca2+ channels open and Ca2+ enters the axon terminal
- Ca2+ entry causes synaptic vesicles to release neurotransmitter by exocytosis
- Neurotransmitter diffuses across the synaptic cleft and binds to specific receptors on the postsynaptic membrane
- Binding of neurotransmitter opens ion channels, creating graded potentials
- neurotransmitter effects are terminated
What division of the nervous system is most specifically responsible for voluntary motor control?
- > Somatic nervous system
- > The somatic nervous system is composed of somatic motor nerve fibers that conduct impulses from the central nervous system to skeletal muscles. It is often referred to as the voluntary nervous system because it allows us to consciously control our skeletal muscles.
Destruction of which of the neuroglial cell types leads to the disease multiple scleroses (MS)?
The defects in nerve transmission associated with MS are caused by a loss of myelin within the CNS.
Graded potentials originating in the dendrites and cell body are integrated (summated) at the axon hillock (C). Membrane potentials above threshold at the hillock will open voltage-gated Na+ channels found in the “trigger zone,” producing an action potential that proceeds down the axon.
TRUE
Cold sores on the skin of the mouth occur when herpes simplex viruses that are dormant in neural ganglia become active and travel to the skin of the mouth. Which of the following is the mechanism by which these viruses travel from the ganglia (located within the head) to the skin of the mouth?
Anterograde transport
->This transport mechanism defines movement of material from the cell body (soma) of a neuron toward the axon terminals (synaptic knobs).
When a neurotransmitter like acetylcholine is acting in an excitatory manner which of the following is likely a result of the acetylcholine acting on the postsynaptic cell?
chemically gated sodium channels will open
nerve cell body
- biosynthetic center of a neuron
- rough ER: makes neurotransmitters
- axon hillock: cone-shaped area from which axon arises
-clusters of cell bodies are called:
nuclei in CNS
ganglia in PNS
functional classification of neurons
sensory:
- transmit impulses from sensory receptors toward CNS
- almost all are unipolar (1 projection)
motor:
- carry impulses from CNS to effectors
- multipolar
interneurons (association neurons):
- lie between motor and sensory neurons
- shuttle signal through CNS pathways; most are entirely within CNS
- 99% of body’s neurons
- multipolar
