Nervous System Flashcards
What is the nervous system?
The master controlling & communicating system of the body
What is the function of the nervous system?
Sensory input – monitoring stimuli occurring inside & outside body
Integration – interpretation of sensory input
Motor output – response to stimuli by activating effector organs
What are oligodendrocytes?
Branched cells that wrap CNS nerve fibers
How are nerve fibres classified?
Diameter
Degree of myelination
Speed of conduction
What is a synapse?
A junction that mediates information transfer from one neuron to:
- Another neuron
- -An effector cell
Name the different types of synapse
- Axodendritic
- Axosomatic
- Axoaxonic
- Dendrodenritic
- Dendrosomatic
What type of synapse is an axodendritic synapse?
A synapse between the axon of one neuron and a dendrite of another
What type of synapse is an axosomatic synapse?
A synapse between the axon of one neuron and the soma of another
What type of synapse is an axoaxonic synapse?
A synapse between the axon of one neuron and the axon of another
What type of synapse is a dendrodendritic synapse?
A synapse between a dendrite of one axon and a dendrite of another
What type of synapse is a dendrosomatic synapse?
A synapse between a dendrite of one neuron and the soma of another
Electrical synapses:
- Are less common than chemical synapses
- Correspond to gap junctions found in other cell types
Why are electrical synapses important in the CNS?
They are important for:
- Arousal from sleep
- Mental attention
- Emotions and memory
- Ion and water homeostasis
What are the two parts of a chemical synapse?
- Axonal terminal of presynaptic neuron, which contains synaptic vesicles
- Receptor region on the dendrite(s) or soma of the postsynaptic neuron
What is the synaptic cleft?
Fluid filled space separating the pre-synaptic and post-synaptic neurons
What is the function of the synaptic cleft?
Prevents nerve impulses from directly passing from one neuron to the next
Help ensure one way transmission of impulse
Transmission across synaptic cleft:
Is a chemical event (as opposed to an electrical one)
Ensures unidirectional communication between neurons
How is information transferred across a synapse?
- Nerve impulses reach axonal terminal of presynaptic neuron & open Ca2+ channels
- Neurotransmitter is released into synaptic cleft via exocytosis in response to synaptotagmin
- Neurotransmitter crosses synaptic cleft & binds to receptors on postsynaptic neuron
- Postsynaptic membrane permeability changes, causing an excitatory or inhibitory effect
What happens when a neurotransmitter binds to a postsynaptic neuron?
- Produces continuous postsynaptic effect
- Blocks reception of additional “messages”
- Must be removed from its receptor
How are neurotransmitters removed from postsynaptic neuron?
- Degraded by enzymes
- Reabsorbed by astrocytes or presynaptic terminals
- Diffuse away from synaptic cleft
What is synaptic delay?
The time it takes for neurotransmitter to be released, diffuse across the synapse and bind to the receptors
0.3 - 5.0 ms
Synaptic delay is the rate limiting step of neural transmission
Neurotransmitter receptors mediate changes in membrane potential according to:
- Amount of neurotransmitter released
- Amount of time the neurotransmitter is bound to receptors
What are the two types of postsynaptic potentials?
- EPSP – excitatory postsynaptic potentials
- IPSP – inhibitory postsynaptic potentials
What is an Excitatory Postsynaptic Potential?
EPSP
Graded potentials that can initiate an action potential in an axon
Use only chemically gated channels
Na+ & K+ flow in opposite directions at the same time
True or False:
Postsynaptic membranes generate action potentials?
False
Neurotransmitter binding to a receptor at
inhibitory synapses:
- Causes membrane to become more
permeable to potassium & chloride ions - Leaves the charge on the inner surface
- Negative
- Reduces the postsynaptic neuron’s ability to
produce an action potential
Why must EPSP’s summate?
A single EPSP cannot induce an action
potential, therfore EPSPs must summate temporally or spatially to induce an action potential
What is temporal summation?
When presynaptic neurons transmit impulses in
rapid-fire order
What is spatial summation?
When a postsynaptic neuron is stimulated by a large
number of terminals at the same time
What happens when IPSPs and EPSPs with each other?
The inhibitory and exitatory potentials cancel each other out
What are neurotransmitters?
Chemicals used for neuronal communication with body & brain
50 different neurotransmitters identified
They are classified chemically & functionally
Name the different types of chemical neurotransmitters
- Acetylcholine (ACh)
- Biogenic amines
- Amino acids
- Peptides
- Novel messengers: ATP & dissolved gases NO & CO
What are the different neurotransmitter receptor mechanisms?
Direct: neurotransmitters that open ion channels
Promote rapid responses
Indirect: neurotransmitters that act through second messengers
Promote long-lasting effects
Channel-Linked Receptors:
- Are composed of integral membrane protein
- Mediate direct neurotransmitter action
- Their action is immediate, brief, simple, & highly localized
- Ligand binds receptor, & ions enter the cells
- Excitatory receptors depolarize membranes
- Inhibitory receptors hyperpolarize membranes
G Protein-Linked Receptors:
- Their responses are indirect, slow, complex, prolonged, and often diffuse
- These receptors are transmembrane protein complexes
Give examples of G Protein-Linked Receptors
- Muscarinic ACh receptors
- Neuropeptide receptors
- Miogenic amine receptors
What is the mechanism of G Protein-Linked Receptors?
- Neurotransmitter binds to G protein-linked receptor
- G protein is activated and GTP is hydrolyzed to GDP
- The activated G protein complex activates adenylate cyclase
- Adenylate cyclase catalyzes the formation of cAMP from ATP
- cAMP, a second messenger, brings about various cellular responses
What is the role of the Sodium-Potassium Pump?
Repolarization:
- Restores the resting electrical conditions of the neuron
- Does not restore the resting ionic conditions
Ionic redistribution back to resting conditions is restored by the sodium-potassium pump
What are the stages of an action potential?
1 – Resting state
2 – Depolarization phase
3 – Repolarization phase
4 – Hyperpolarization
What happens when an action potential is first generated?
At time = 0ms
- Na+ influx causes a patch of axonal membrane to depolarize
- Positive ions in axoplasm move toward polarized (negative) portion of membrane
- Sodium gates are shown as closing, open, or closed
What happens when an action potential is at time = 1ms?
- Ions of extracellular fluid move toward area of greatest negative charge
- A current is created that depolarizes adjacent membrane in a forward direction
- The impulse propagates away from its point of origin
What happens when an action potential is at time = 2ms?
- The action potential moves away from the stimulus
- Where sodium gates are closing, potassium gates are open and create a current flow
Threshold Potential:
membrane is depolarized by 15 to 20 mV
- Established by total amount of current flowing through membrane
- Weak (sub-threshold) stimuli are not relayed into action potentials
- Strong (threshold) stimuli are relayed into action potentials
- All-or-none phenomenon – action potentials either happen completely, or not at all
Coding for action potential stimulus intensity:
- All action potentials are alike & are independent of stimulus intensity
- Strong stimuli can generate an action potential more often than weaker stimuli
- The CNS determines stimulus intensity by the frequency of impulse transmission
What is the absolute refractory period?
Time from the opening of the Na+ activation gates until the closing of inactivation gates
What is the function of the absolute refractory period?
- Prevents the neuron from generating an action potential
- Ensures that each action potential is separate
- Enforces one-way transmission of nerve impulses
What is the relative refractory period?
The interval following the absolute refractory period when:
- Sodium gates are closed
- Potassium gates are open
- Repolarization is occurring
How is the rate of impulse propagation in neurons determined?
- Axon diameter – the larger the diameter, the faster the impulse
- Presence of a myelin sheath – myelination dramatically increases impulse speed
Saltatory Conduction:
- Current passes through a myelinated axon only at nodes of Ranvier
- Voltage-gated Na+ channels are concentrated nodes
- Action potentials triggered only at nodes and jump from one node to the next
- Much faster than conduction along un-myelinated axons
What is Multiple Sclerosis?
- An autoimmune disease that mainly affects young adults
- Symptoms include visual disturbances, weakness, loss of muscular control & urinary incontinence
What causes the symptoms of MS?
- Nerve fibers are severed & myelin sheaths in the CNS become non-functional scleroses
- Shunting & short-circuiting of nerve impulses occurs
What is the treatment for MS, and what does it do?
The advent of disease-modifying drugs including interferon beta-1a and -1b, Avonex, Betaseran & Copazone:
- Hold symptoms at bay
- Reduce complications
- Reduce disability
What is the structure of axons?
- Slender processes of uniform diameter arising from the hillock
- Usually there is only one unbranched axon per neuron
- Rare branches, if present, are called axon collaterals
- Axonal terminal – branched terminus of an axon
- Long axons are called nerve fibers
What is the function of an axon?
- Generate & transmit action potentials
- Secrete neurotransmitters from the axonal terminals
How does movement along axons occur?
Movement along axons occurs in two ways:
- Anterograde — toward axonal terminal
- Retrograde — away from axonal terminal
What is the myelin sheath?
Whitish, fatty (protein-lipoid), segmented sheath around most long axons
What is the function of the myelin sheath?
- Protection of the axon
- Electrically insulate fibers from one another
- Increase the speed of nerve impulse transmission
What is the function of a Schwann cell?
- Envelopes an axon in a trough
- Encloses the axon with its plasma membrane
- Has concentric layers of membrane that make up the myelin sheath
What is the Neurilemma?
Remaining nucleus & cytoplasm of a Schwann cell when it wraps around an axon to form the myelin sheath
Where do you find Schwann cells?
PNS
What are the nodes of Ranvier?
Gaps in myelin sheath between adjacent Schwann cells
They are sites where axon collaterals can emerge
Unmyelinated Axons:
A Schwann cell surrounds nerve fibers but coiling does not take place
Schwann cells partially enclose 15 or more axons
Axon of the CNS:
- Both myelinated and unmyelinated fibers are present
- Myelin sheaths are formed by oligodendrocytes
- Nodes of Ranvier are widely spaced
- There is no neurilemma
How are neurons classified?
- Structurally
- Functionally
- Neurochemically
What are the different structural neuron classifications?
- Multipolar — three or more processes
- Bipolar — two processes (axon and dendrite)
- Unipolar — single, short process
What are the different functional neuron classifications?
- Sensory (afferent) — transmit impulses toward the CNS
- Motor (efferent) — carry impulses away from the CNS
- Interneurons (association neurons) — shuttle signals through CNS pathways
Neurons are highly irritable.
Action potentials, or nerve impulses, are:
Electrical impulses carried along the length of axons
Always the same regardless of stimulus
The underlying functional feature of the nervous system
Define Voltage (V)
Measure of potential energy generated by separated charge
Define Potential Difference
Voltage measured between two points
Define Current (I)
The flow of electrical charge between two points
Define Resistance (R)
Hindrance to charge flow
What is an insulator?
Substance with high electrical resistance
What is a conductor?
Substance with low electrical resistance
When is there a potential difference on either side of a membrane?
When:
- The number of ions is different across the membrane
- The membrane provides a resistance to ion flow
What are the different types of Ion channel?
- Passive (leak) channels
- Chemically gated channels
- Voltage gated channels
- Mechanically gated channels
When do the different types of ion channel open?
Passive - Always open
Chemically gated - Open with binding of a specific neurotransmitter
Voltage gated - Open an close in response to membrane potential
Mechanically gates - Open and close in response to physical deformation of receptors
How does a chemically gated channel operate?
e.g. Na+ - K+ gated channel
Closed when a neurotransmitter is not bound to the extracellular receptor:
(Na+ cannot enter the cell & K+ cannot exit the cell)
Open when a neurotransmitter is attached to the receptor:
(Na+ enters the cell & K+ exits the cell)
How does a Voltage gated channel operate?
e.g. Na+ channel
Closed when the intracellular environment is negative:
(Na+ cannot enter the cell)
Open when the intracellular environment is positive:
(Na+ can enter the cell)
What happens when gated channels are open?
- Ions move quickly across the membrane
- Movement is along their electrochemical gradients
- An electrical current is created
- Voltage changes across the membrane
How do ions move along gradients?
Ions flow along their chemical gradient when they move from an area of high concentration to an area of low concentration
Ions flow along their electrical gradient when they move toward an area of opposite charge
What is an electrochemical gradient?
The electrical and chemical gradients taken together
What is the Resting Membrane Potential (Vm)?
RMP is the potential difference (–70 mV) across the membrane of a resting neuron
How is Resting Membrane Potential generated?
Generated by different concentrations of Na+, K+, Cl, & protein anions (A)
What are ionic difference the consequence of?
Differential permeability of the neurilemma to Na+ & K+
Operation of the sodium-potassium pump
What is the function of a membrane potential?
Used to integrate, send, & receive information
How are membrane potential changes produced?
Changes in membrane permeability to ions
Alterations of ion concentrations across the membrane
Types of signals – graded potentials and action potentials
What are the events that cause a change in membrane potential?
Depolarization – the inside of the membrane becomes less negative
Repolarization – the membrane returns to its resting membrane potential
Hyperpolarization – the inside of the membrane becomes more negative than the resting potential
Describe a graded potential
Short-lived, local changes in membrane potential
Decrease in intensity with distance
Their magnitude varies directly with the strength of the stimulus
Sufficiently strong graded potentials can initiate action potentials
Graded Potentials:
Voltage changes in graded potentials are decremental
Current is quickly dissipated due to the leaky plasma membrane
Can only travel over short distances
Describe an action potential
A brief reversal of membrane potential with a total amplitude of 100 mV
Action potentials are only generated by muscle cells & neurons
They do not decrease in strength over distance
They are the principal means of neural communication
An action potential in the axon of a neuron is a nerve impulse
Describe the resting state of an action potential
Na+ & K+ channels are closed
Leakage accounts for small movements of Na+ and K+
Activation gates – closed in the resting state
Inactivation gates – open in the resting state
Describe the depolarization phase of an action potential
Na+ permeability increases: membrane potential reverses
Na+ gates are opened, K+ gates are closed
Threshold – a critical level of depolarization (-55 to -50 mV)
At threshold, depolarization becomes self-generating
Describe the repolarization phase of an action potential
Sodium inactivation gates close
Membrane permeability to Na+ declines to resting levels
As sodium gates close, voltage-sensitive K+ gates open
K+ exits the cell & internal negativity of the resting neuron is restored
Describe the hyperpolarization phase of an action potential
Potassium gates remain open, causing an excessive efflux of K+
This efflux causes hyper-polarization of the membrane (undershoot)
The neuron is insensitive to stimulus & depolarization during this time
What are the structures of the hindbrain?
- Medulla Oblongata
- Pons
What are the functions of the hindbrain?
- Basic survival functions
- Breathing
- Blood Pressure
- Sleeping
What are the structures of the midbrain?
- Thalamus
- Hypothalamus
- Pituitary Gland
What are the functions of the Midbrain?
- Basic awareness
- Endocrine control
- Thirst
- Hunger
What are the structures of the cortex?
- Neocortex
- Cerebellum
What are the functions of the cortex?
- Conciousness
- Thinking
- Memory
- Motor skills
- Special senses
What are the functions of the Parietal lobe?
- Sensation
- Hearing
- Taste
What are the functions of the frontal lobe?
- Cognition
- Motor
What are the functions of the temporal lobe?
- Memory
- Emotion
- Smell
What are the functions of the cerebellum?
- Learned motor
What are the functions of the occipital lobe?
- Vision
What sensations are somatic?
Special senses
- Touch, vibration, pressure, temperature, pain
- Vision
- Hearing and balance
- Gustation (Taste)
- Olfaction (Smell)
Proprioception
Which sensations are automonic?
- Blood Pressure
- Temperature
- Osmoregularity
- Glucose
- O2, CO2
- pH
- Gastric Stretch
If all Action Potentials are the same size, how do you generate stronger or weaker response, e.g. Muscle contractions
Higher frequency of action potentials
What is proprioception?
Knowing what joints and muscles are doing.
Involved in standing and running
What is signal transduction?
The process of converting stimuli into action potentials in an afferent nerve
