Chapter 12 Flashcards
The nervous system is the foundation of what?
Our conscious experience, personality, and behavior
_______ combines the behavioral and life sciences
Neurobiology
What two systems maintain internal coordination?
The endocrine and nervous systems
Describe the endocrine system
Communicates by means of chemical messengers (hormones) secreted into to the blood
Describe the nervous system
Employs electrical and chemical means to send messages from cell to cell
The nervous system carries out tasks in 3 basic steps; what are they?
1) Receive information about changes in the body and external environment
2) Process this information, relates it to past experiences, and determines appropriate response
3) Issue commands to muscles and glands cells to carry out such a response
Describe the basic pathway of the nervous system
1) Sensory receptor detects a stimulus
2) Sensory (afferent) neuron
3) Integrating center (CNS)
4) Motor (efferent) neuron
5) Effector responds (muscle or gland)
The central nervous system (CNS) is composed of what?
The brain and spinal cord
What is the CNS enclosed by?
Enclosed by cranium and vertebral as well as the meninges
The peripheral nervous system is composed of what two primary things?
Composed of nerves & ganglia
What is a part of the PNS?
The rest of the nervous system, excluding the brain and spinal cord
Describe nerves
A bundle of nerve fibers (axons) wrapped in fibrous connective tissue; spinal versus cranial nerves
Describe a ganglion
A knot-like swelling in a nerve where neuron cell bodies are concentrated
What are the two main components of the PNS?
Somatic and visceral fibers
What are the two subdivisions of the PNS?
Sensory (afferent) and motor (efferent)
Define sensory and motor neurons
Sensory (afferent neurons): to the CNS
Motor (efferent neurons): away from the CNS
Describe the somatic and visceral fibers of the sensory (afferent) PNS
1) Somatic fibers from the skin, skeletal muscle & joints
2) Visceral fibers from internal organs (heart, lungs, stomach, and urinary bladder)
Describe the somatic and visceral fibers of the motor (efferent) PNS
1) Somatic fibers to skeletal muscles: Somatic Nervous system
2) Visceral fibers to smooth muscles, cardiac muscle or glands: Autonomic nervous system
True or false: the somatic and autonomic nervous systems are a part of the motor (efferent) PNS
True
ALS (Amyotrophic lateral sclerosis or Lou Gehrig’s Disease) affects the motor neurons of the somatic nervous system (they die). What affect will this do to skeletal muscles?
Atrophy
Describe the somatic (voluntary) nervous system of the PNS
- Motor neurons to skeletal muscle tissue
- Only 1 motor neuron is used
- Somatic reflexes: involuntary muscle contractions
Describe the autonomic (involuntary) nervous system of the PNS
- Motor neurons to smooth & cardiac muscle, endocrine glands, & exocrine glands
- 2 motor neurons used
- Autonomic/visceral reflexes: involuntary responses
The visceral motor division is also called what?
The autonomic nervous system
What are the two parts of the autonomic nervous system?
Sympathetic and parasympathetic
Describe the sympathetic division of the autonomic nervous system
- Tends to arouse body for action
- Motor neurons originate from thoracolumbar region
- “Fight or flight” responses; “E” responses (excitement, emergency, exercise)
Describe the parasympathetic division of the autonomic nervous system
- Tends to have calming effect
- Motor neurons originate from craniosacral region
- “Resting and digesting” responses; SLUDD (salivation, lacrimation, urination, digestion, defecation)
The two principal cell types of the nervous system are what?
Neurons and neuroglia
Describe neurons and neuroglia
Neurons: Excitable cells that transmit electrical signals; functional units of the nervous system
Neuroglia: supporting cells
What are the three universal properties of neurons?
Excitability (irritability), conductivity, and secretion
Describe the neuron property of excitability
Respond to environmental changes called stimuli
Produce an electrical signal
Describe the neuron property of conductivity
Conduct the electrical signal to other cells
Describe the neuron property of secretion
When an electrical signal reaches the end of nerve fiber, the cell secretes a chemical neurotransmitter that influences the next cell
What are the three functional classes of neurons?
1) Sensory (afferent) neurons
2) Interneurons (association neurons)
3) Motor (efferent) neurons
Describe the functions of interneurons (association neurons)
- Lie entirely within CNS
- Connects motor and sensory pathways
- Makes decisions (integrating center)
- About 90% of all neurons
True or false: motor neurons are the effectors
True
Describe the soma of a neuron
- Control center of neuron
- Also called neurosoma or cell body
- Nucleus with one nucleolus
- Cytoplasm contains: mitochondria, lysosomes, Golgi complex, inclusions, extensive rough ER and cytoskeleton, nissl bodies
- No centrioles or mitosis
- Extreme longevity
Describe the cytoskeleton of a neuron’s soma
Cytoskeleton has dense mesh of microtubules and neurofibrils (bundles of actin filaments) that compartmentalizes rough ER into dark-staining
Describe the inclusions found in a neuron’s soma
Glycogen, lipid droplets, melanin, and lipofuscin pigment (produced when lysosomes digest old organelles)
Describe the dendrites of a neuron
Dendrites: branches that come off the soma
- Receives signals from other neurons
- The more dendrites the neuron has, the more information it can receive
Describe the axon of a neuron
Axon (nerve fiber): originates from axon hillock and transmits signals away from soma
- Only one (or none)
- Mostly unbranched except for axon collaterals
- Axolemma may be enclosed by a myelin sheath
The more ____ the neuron has, the more information it can receive
dendrites
Describe the synaptic knob of a neuron
Synaptic knob (terminal button) contains synaptic vesicles full of neurotransmitter
List the different types of neurons
1) Multipolar
2) Bipolar
3) Unipolar
4) Anaxonic
Describe multipolar neurons
- One axon and multiple dendrites
- Most common; describes most neurons in CNS
Describe bipolar neurons
One axon and one dendrite
Describe unipolar neurons and give examples
- Single process leading away from soma
- Sensory cells from skin and organs to spinal cord (somas in dorsal root ganglia)
Describe anaxonic neurons
Many dendrites but no axon
Describe the movement of proteins in a neuron
Proteins made in the soma are transported to the axon & axon terminal to repair the axolemma, to transport organelles
Differentiate between retrograde and anterograde transport in a neuron
- Anterograde transport: movement down the axon away from soma
- Retrograde transport: movement up the axon toward the soma
Describe how proteins move around a neuron
Motor proteins (kinesin and dynein) carry materials “on their backs” while they “crawl” along microtubule
What are some of the functions of proteins transported around neurons?
They serve as gated ion channels, enzymes, neurotransmitters
About 1 _____ neurons in the nervous system; neuroglia outnumber neurons by at least ___ to 1
trillion; 10
List the four primary functions of neuroglia (glial cells)
1) Protect neurons and help them function
2) Bind neurons together and form framework for nervous tissue
3) If mature neuron is not in synaptic contact with another neuron, it is covered by glial cells
4) Prevents neurons from touching each other and gives precision to conduction pathway
List the 4 types of neuroglia (glial cells) found in the CNS
1) Oligodendrocytes
2) Ependymal cells
3) Microglia
4) Astrocytes
Describe oligodendrocytes of the CNS
Form myelin sheaths in CNS that speed signal conduction using arm-like processes
Describe ependymal cells of the CNS
Line internal cavities of the brain; secrete and circulate cerebrospinal fluid (CSF)
Describe microglia of the CNS
Wander through CNS looking for debris and damage; get rid of debris with exocytosis
The most abundant glial cells of the CNS are ______
astrocytes
Describe astrocytes of the CNS
- Most abundant glial cell in CNS
- Covers brain surface and most nonsynaptic regions of neurons in the gray matter (framework)
- Forms blood-brain barrier using perivascular feet
- Absorbs excess neurotransmitters and ions
- Astrocytosis or sclerosis: when neuron is damaged, astrocytes form hardened scar tissue and fill in space
What are the two types of neuroglia (glial cells) in the PNS?
Schwann cells and satellite cells
Describe Schwann cells of the PNS
- Produce a myelin sheath around axons similar to the ones produced by oligodendrocytes in CNS
- Assist in regeneration of damaged fibers
Describe satellite cells of the PNS
- Surround the somas in ganglia of the PNS
- Provide electrical insulation around the soma
- Regulate the chemical environment of the neurons
Satellite cells of the PNS are similar in function to the _____ of the CNS.
astrocytes
Define the myelin sheath and describe what forms it
- Defined as insulation around a nerve fiber (axon)
- Formed by oligodendrocytes in CNS and Schwann cells in PNS
Describe what composes the myelin sheath
Consists of the plasma membrane of glial cells; 20% protein and 80% lipid
Define myelination and describe this process
- Defined as the production of the myelin sheath
- Begins at week 14 of fetal development
- Proceeds rapidly during infancy
- Completed in late adolescence
- Dietary fat is important to CNS development
True or false: there is no cytoplasm between the membranes of a Schwann cell
True
Describe the structure of Schwann cells
-Entire Schwann cell spirals repeatedly around a single nerve fiber
-As many as one hundred layers of membrane
-No cytoplasm between the membranes
-Neurilemma: outermost coil of myelin sheath
Contains nucleus and most of its cytoplasm
Describe the neurilemma of Schwann cells
The outermost coil of myelin sheath.
Contains nucleus and most of its cytoplasm
External to neurilemma is basal lamina and a thin layer of fibrous connective tissue (endoneurium)
What myelinates neurons in the CNS?
Oligodendrocytes
Describe how oligodendrocytes myelinate neurons in the CNS
- An oligodendrocyte myelinates several nerve fibers in its immediate vicinity
- Cannot migrate around any one of them like Schwann cells
- Must push newer layers of myelin under the older ones; so myelination spirals inward toward nerve fiber
- No neurilemma or endoneurium
True or false: Many Schwann cells or oligodendrocytes are needed to cover one nerve fiber
True
Define the nodes of ranvier of the myelin sheath
Gap between segments
Define the internodes of the myelin sheath
Myelin-covered segments
Define the trigger zone of the myelin sheath
The axon hillock & the initial segment; play an important role in initiating a nerve signal
Tumors are masses of rapidly dividing cells from what three things?
1) Meninges (protective membranes of CNS)
2) Metastasis from other tumors in other organs
3) Glial cells (mitotically active throughout life)
Describe gliomas
- Gliomas grow rapidly and are highly malignant
- Blood–brain barrier decreases effectiveness of chemotherapy
- Treatment consists of radiation or surgery
True or false: Neurons often become cancerous
False; mature neurons have little mitosis so not cause of brain tumors
Name a degenerative disorder of the myelin sheath
Multiple sclerosis
Describe multiple sclerosis
- Oligodendrocytes and myelin sheaths in the CNS deteriorate
- Myelin is replaced by hardened scar tissue
- Nerve conduction disrupted (double vision, tremors, numbness, speech defects)
- Onset between 20 and 40 years
- Cause may be autoimmune triggered by virus
Describe Tay-Sachs disease
- A hereditary disorder of infants of Eastern European Jewish ancestry
- Abnormal accumulation of glycolipid called GM_2 (ganglioside) in the myelin sheath due to dysfunctional lysosomes disrupts conduction of nerve signals
- Blindness, loss of coordination, and dementia
- Fatal before age 4
What organelle is dysfunctional in Tay-Sachs disease?
Lysosomes
The speed at which a nerve signal travels along surface of nerve fiber depends on what two factors?
1) Diameter of fiber
- Larger fibers have more surface area and conduct signals more rapidly
2) Presence or absence of myelin
- Myelin further speeds signal conduction
Describe the conduction speed of nerve fibers
- Slow signals sent to the gastrointestinal tract where speed is less of an issue
- Fast signals sent to skeletal muscles where speed improves balance and coordinated body movement
What kinds of nerves can regenerate, and under what conditions?
- Only peripheral nerve fibers can, and only if:
1) soma is intact
2) some neurilemma remains
Describe the 3 steps of regeneration of PNS nerves
1) Fiber distal to the injury degenerates
2) Axon stump sprouts multiple growth processes
3) Schwann cells, basal lamina and neurilemma form a regeneration tube
When atrophied muscles from lack of movement due to nerve damage recover?
Once contact reestablished, atrophied muscle fibers regrow
What are the problems with nerve regeneration?
- Regeneration not fast, perfect, or always possible
- Slow regrowth means may take 2 years
- Some nerve fibers connect with the wrong muscle fibers; some die
- Regeneration of damaged nerve fibers in the CNS cannot occur at all
Define electric potential
A difference in concentration of charged particles between one point and another
Define electrical currents
A flow of charged particles from one point to another
Describe electrical currents in the context of the human body
- In the body, currents are movements of ions, such as Na^+ or K^+, through channels in the plasma membrane
- Living cells are polarized and have a resting membrane potential
- Gated channels are opened or closed by various stimuli
- Enables cell to turn electrical currents on and off
Resting membrane potential: the cell is ___ outside the cell membrane and ____ inside
positive; negative
The resting membrane potential of nerve fibers at rest is _____
-70 mV
Why does resting potential exist?
High concentration of sodium outside and high concentration of potassium inside
Resting membrane potential results from the combined effect of what three factors?
1) Ions diffuse down their concentration gradient through the membrane
2) Plasma membrane is selectively permeable and allows some ions to pass easier than others
3) Electrical attraction of cations and anions to each other
Name the most common cations in the ICF and ECF
Na+ is concentrated outside of cell (ECF)
K+ is concentrated inside cell (ICF)
True or false: some potassium ions pass through cell membranes easier than sodium
True
What has the greatest influence on resting membrane potential?
Potassium ions
Explain how potassium ions affect resting membrane potential
- Plasma membrane is more permeable to K^+ than any other ion
- Leaks out until electrical charge of cytoplasmic anions attracts it back in and equilibrium is reached (no more net movement of K^+)
- K^+ is more concentrated in the ICF than in the ECF
What kind of ions cannot escape the cell?
Cytoplasmic anions cannot escape
Describe how sodium slightly impacts resting membrane potential
- Na^+ is more concentrated in the ECF than in the ICF
- Some Na^+ leaks into the cell, diffusing down its concentration and electrical gradients
- This Na^+ leakage makes RMP slightly less negative than it would be if RMP were determined solely by K^+
Describe the sodium-potassium pump
- 〖𝐍𝐚〗^+/𝐊^+ pump moves 3 Na^+ out for every 2 K^+ it brings in
- Exchange of 3 positive charges for only 2 positive charges contributes about −3 mV to the cell’s resting membrane potential of −70 mV
- Works continuously to compensate for Na^+ and K^+ leakage
What 3 events can cause changes in resting membrane potential?
Depolarization, repolarization, and hyperpolarization
Define depolarization, repolarization, and hyperpolarization
1) Depolarization: the inside of the membrane becomes less negative. Will reverse polarity.
2) Repolarization: the membrane returns to its resting membrane potential
3) Hyperpolarization: the inside of the membrane becomes more negative than the resting potential
Does hyperpolarization inhibit or lead to an electrical impulse?
Inhibit
Name the 2 types of signals that can cause changes in membrane potential
Local (graded) potentials & action potentials
Describe local (graded) potentials
- Changes in membrane potential of a neuron occurring at & nearby the part of the cell that is stimulated
- Short distances; die out quickly
- On dendrites and cell bodies
- Sodium gates open in response to chemicals, light, heat or mechanical stimulation
- Size of signal depends on stimulus strength
- Membrane will either depolarize or hyperpolarize
Describe action potentials
- Action potential: change in membrane polarity produced by voltage-gated ion channels
- Long distances; does not decrease in strength
- On axon of nerve cells and in muscle cells
- Sodium gates open in response to electrical stimulation (change in membrane potential); called voltage-gated
- All-or-nothing threshold
Where are action potentials generated?
In the trigger zone
Describe the 7 steps of an action potential
1) Arrival of current at axon hillock depolarizes membrane
2) Depolarization must reach threshold: critical voltage (about -55 mV) required to open voltage-regulated gates
3) Voltage-gated Na^+ channels open, Na^+ enters and depolarizes cell, which opens more channels resulting in a rapid positive feedback cycle as voltage rises
4) As membrane potential rises above 0 mV, Na^+ channels are inactivated and close; voltage peaks at about +35 mV
5) Slow K^+ channels open and outflow of K^+ repolarizes the cell
6) K^+ channels remain open for a time so that membrane is briefly hyperpolarized (more negative than RMP)
7) RMP is restored as Na^+ leaks in and extracellular K^+ is removed by astrocytes
Describe what action potentials affect and their speed
1) Only a thin layer of the cytoplasm next to the cell membrane is affected; in reality, very few ions are involved
2) Action potential is often called a spike, as it happens so fast
Describe the characteristics action potentials have but local potentials don’t
1) Follows an all-or-none law; if threshold is reached, neuron fires at its maximum voltage. If threshold is not reached, it does not fire
2) Does not get weaker with distance
3) Irreversible: once started, goes to completion and cannot be stopped
Define absolute and relative refractory periods
- Absolute: no stimulus of any strength will trigger an action potential
- Relative: really strong stimulus (super stimulus) will trigger an action potential
Describe how novocain and lidocaine work
Nerve impulses cannot pass the anesthetized region because the drugs prevent voltage gated Na+ channels from opening in the region.
Since all nerve action potentials are identical in strength, how do you get different intensities?
1) Frequency of stimulus
2) Number of neurons stimulated
Describe the differences between nerve and muscle action potentials
Nerve: occurs only on the axon, -70mV
Muscle: entire sarcolemma (cell membrane); skeletal and cardiac close to -90mV
Describe the differences in speed of action potentials in nerves, skeletal muscles, and cardiac and smooth muscles. What do the differences in speed mean?
Nerve impulse: ½ to 2 msec
Skeletal muscle: 1-5 msec
Cardiac and smooth muscle: 10-300 msec
The longer the duration, the less often they can produce impulses
The fastest nerve impulse is ___ times faster than a muscle fiber
18
Describe continuous (unmyelinated) conduction
- Unmyelinated fibers have voltage-gated channels along their entire length
- Produce action potential the entire length of the axon
- Chain reaction continues until the nerve signal reaches the end of the axon
What ensures an action potential only goes in one direction?
Refractory period
Describe saltatory conduction
- Myelinated fibers conduct signals with saltatory conduction; signal seems to jump from node of Ranvier to node of Ranvier
- Very fast
Describe what happens when a nerve signal reaches the end of an axon
- A nerve signal can go no further when it reaches the end of the axon
- Triggers the release of a neurotransmitter
- Stimulates a new wave of electrical activity in the next cell across the synapse
Describe synapses between two neurons
- First neuron in the signal path is the presynaptic neuron; releases neurotransmitter
- Second neuron is postsynaptic neuron; responds to neurotransmitter
A spinal motor neuron is covered by about _______ axon terminals from other neurons
8,000 ending on its dendrites; ______ on its neurosoma
10,000; 2,000
In the _______ of brain, one neuron can have as many as 100,000 synapses
cerebellum
What are the two types of synapses?
Electrical and chemical
Describe the characteristics of electrical synapses
- Spreads through gap junctions
- Faster
- Two-way transmission
- Can’t make decisions
- Arousal from sleep, mental attention, emotions and memory, ion and water homeostasis
Describe the characteristics of chemical synapses
- One-way transmission
- From a presynaptic neuron to a postsynaptic neuron
- Uses neurotransmitters
Describe the process of chemical synapses
1) Nerve impulses reach the axonal terminal of presynaptic neuron and open Ca2+ channels
2) Neurotransmitter is released into synaptic cleft via exocytosis
3) Neurotransmitter crosses synaptic cleft and binds to receptors on postsynaptic neuron (only bound for about 1ms)
4) Postsynaptic membrane permeability changes, causing an excitatory (EPSP) or inhibitory (IPSP) effect
Describe the differences between presynaptic and postsynaptic neurons in chemical synapses
Presynaptic neurons have synaptic vesicles with neurotransmitter
Postsynaptic neurons have receptors and ligand-regulated ion channels
There are more than 100 neurotransmitters, but there are 6 chemical categories; what are they?
Acetylcholine, amino acids, monoamines, purines, neuropeptides, and gases.
Describe monamine neurotransmitters
- Include the catecholamines:
- Epinephrine, norepinephrine, dopamine
- Also includes serotonin
Describe the removal of a neurotransmitter
1) Diffusion: move down concentration gradient away from synapse
2) Enzymatic degradation (Ex: acetylcholinesterase and monoamine oxidase)
3) Uptake by neurons or glia cells
neurotransmitter transporters
What do MAOI medications do?
Breaks down dopamine, serotonin, norepinephrine, epinephrine
What category of neurotransmitters does serotonin belong to?
Monamines
Define neural integration
The ability to process, store, and recall information and use it to make decisions
Describe how chemical synapses allow for decision making
- Chemical synapses allow for decision making; brain cells are incredibly well connected allowing for complex integration
- The neurons of cerebral cortex have about 40,000 contacts with other neurons
Describe the tradeoff of chemical synapses
Chemical transmission involves a synaptic delay that makes information travel slower than it would be if there was no synapse
What differentiates IPSP from EPSP signals?
Different neurotransmitters cause different types of postsynaptic potentials in the cells they bind to, so some produce IPSPs and others produce EPSPs
The same neurotransmitter might excite some cells and inhibit others, depending on the type of receptors the postsynaptic cells have; give an example of this
Acetylcholine (ACh) and norepinephrine work this way; ACh excites skeletal muscle but inhibits cardiac muscle because of different ACh receptors
True or false: One neuron can receive input from thousands of other neurons
True
Define summation and describe where it occurs
- The process of adding up postsynaptic potentials and responding to their net effect
- Occurs in the trigger zone
True or false: The balance between EPSPs and IPSPs enables the nervous system to make decisions
True
Define temporal summation
Occurs when a single synapse generates EPSPs so quickly that each is generated before the previous one fades
Define spatial summation
Occurs when EPSPs from several different synapses add up to threshold at an axon hillock
What is spatial summation an example of?
Facilitation, which is a process in which one neuron enhances the effect of another
Describe strychnine poisoning
- In spinal cord, Renshaw cells normally release an inhibitory neurotransmitter (glycine) onto motor neurons preventing excessive muscle contraction
- Strychnine binds to and blocks glycine receptors in the spinal cord
- Causes tetanus or tetanic muscle spasms, uncontrolled convulsions, and respiratory arrest.
Describe alzheimer’s disease
- 6th leading cause of death in United States
- Memory loss for recent events, moody, combative, lose ability to talk, walk, and eat
- Show deficiencies of acetylcholine
- Diagnosis confirmed at autopsy
- Atrophy of gyri (folds) and formation of abnormal proteins
- Blocks normal synaptic function
- Treatment: No cure or true prevention exists, but trying to find ways to clear or halt the production of abnormal proteins
Describe the SHIELD prevention method for alzheimer’s
Sleep, handle stress, interact with others, exercise, learn new things, diet (mind diet)
