Unit 4: Ch 12 (Nervous Tissue) Flashcards
Action potential conduction types
- Continuous
- Saltatory
Action potentials
- Characteristics
- Effects
-
Characteristics
- All or none: occurs at the same intensity. Occurs or does not occur
- Irreversible: goes to completion once it begins
- Nondecremental: signal maintains same strength regardless of distance
-
Effects
- Depolarization
- Repolarization
- Hyperpolarization
Anatomical classes of neurons
-
Unipolar neurons
- Have only a single process leading away from the soma
- Examples: carry signals to the spinal cord for touch and pain
-
Bipolar neurons (related to the head)
- 1 axon and 1 dendrite
- Examples: cells of nose, eyes, & ear
-
Multipolar neurons (most common)
- 1 axon and multiple dendrites
- Examples: most neurons of the brain and spinal cord
-
Anaxonic neurons (rare)
- Multiple dendrites but no axon
- Communicate locally through their dendrites and do NOT produce action potentials
- Examples: brain, retina, and adrenal medulla
Anatomical subdivisions of the NS
-
Central Nervous System (CNS)
- Brain & spinal cord
-
Peripheral Nervous System (PNS)
- Cranial & spinal nerves
- Sends & receives information to/from the CNS
CNS-specific components
- Astrocytes: maintain the chemical environment of neurons
- Ependymal cells: secrete and circulate cerebrospinal fluid; line cavities of brain and spinal cord
- Microglia: phagocytize and destroy foreign matter; helps with immunity
- Nucleus
- Oligodendrocytes: form myelin
- Tract: axon
Continuous conduction
- Second way of nerve impulse transmission
- Occurs in unmyelinated axons
- Action potential is generated along the entire length of the axon. Hence, it takes time to generate and transmit action potential
Depolarization
- Any voltage shift to a less negative value
Excitatory adrenergic synapse
- Description
- Steps
-
Description
- Neurotransmitter is norepinephrine (NE) & monoamines, and neuropeptides
- Acts through second-messenger systems
- Receptor is associated with a G protein on the inner face of the membrane
-
Steps
- Unstimulated NE receptor is bound to a G protein
- Binding of NE to the receptor causes the G protein to dissociate from it
- G protein binds to adenylate cyclase, converting ATP to cAMP
Excitatory cholinergic synapse
- Description
- Steps
-
Description
- Ligand specific
- Neurotransmitter is ACh
-
Steps
- Voltage-gated Ca channels open with nerve signal
- Ca enters the terminal and triggers exocytosis of the synaptic vesicles, releasing ACh
- Empty vesicles drop back into the cytoplasm to be refilled with ACh, while synaptic vesicles in the reserve pool move to the active sites and release their ACh
- ACh diffuses across the synaptic cleft and binds to ligand-gated channels on the postsynaptic neuron
- As Na enters, it spreads out along the inside of the plasma membrane and depolarizes it, producing a local voltage shift called the post-synaptic potential
Key structures of a neuron
-
Axon
- Axon hillock: A mound on one side of the neurosoma
- Axoplasm: cytoplasm
- Axolemma: phospholipid bilayer (membrane)
- 2 forms
- Myelin sheath
- Synaptic knobs
-
Dendrites
- Primary site for receiving signals from neurons
-
Soma/cell center/neurosoma/perikayron/cell body
- Central nucleus with large nucleolus
- Cytoplasm: mitochondria, lysosomes, Golgi complex, inclusions, rER, & cytoskeleton
- Cytoskeleton: microtubules & neurofibrils
- Lacks centrioles
Local potentials
- Characteristics
- Effects
-
Characteristics
- Graded: proportional to stimulus strength
- Decremental: signal grows weaker with distance
- Reversible: returns to RMP if stimulation ceases before the threshold is reached
-
Effects
- Excitation: cause a more positive or less negative change in charge on the cell membrane (depolarizing)
- Inhibition: cause a more negative voltage change (hyperpolarizing)
Myelin description
- Segmented (discontinuous)
- Myelin-covered segments from each node to the next are internodal segments
- Each gap between segments is a myelin sheath gap or node of Ranvier
- Myelin is found in the internodes
- Myelination is the production of the myelin sheath
Nerve fibers conduction speed
- Description
- Factors impacting speed
- Fiber classifications
-
Description
- The speed at which a nerve signal travels along a nerve fiber
-
Factors
- Diameter of nerve fiber (larger is faster)
- Presence or absence of myelin
- Temperature
-
Classification system
- A fibers: fastest, largest diameter, myelinated
- B fibers: intermediate, myelinated
- C fibers: slowest, smallest diameter, unmyelinated
Neural circuits
- Description
- Intensity variables
- Types
- Neural circuits interconnect to one another to form large-scale brain networks
- Intensity variables
- Population
- Frequency
- Types
- Converging circuit: many neurons condensed into 1 output
- Diverging circuit: 1 neuron provides instruction to multiple neurons
- Parallel after-discharge circuit: neurons run in parallel. They do not synapse with each other
- Reverberating circuit: allows for the signal to multiple neurons that returns a signal to the original neuron
- Simple circuit: contains presynaptic neuron that provides to 1 postsynaptic neuron
- Notes
- Important for regulation of the heart and regions of the respiratory system
Neuroglia
- Helps to maintain homeostasis of the NS
- Unable to create instructions
Neuron
- Functional cell of the NS
- Provide electrical and chemical signals to other cells to be able to produce a response
Neuron classes
-
Sensory (afferent) neurons
- The receptor
- Detect stimuli such as light, heat, pressure, & chemicals
- Transmit info to the CNS
-
Interneurons
- The integrating center
- Limited to the CNS
- Process, store, and retrieve information and “make decisions” that determine how the body responds to stimuli
-
Motor (efferent) neurons
- The effector
- Send signals to muscle and gland cells
- Called motor neurons because most of them lead to muscle cells
Neurotransmitter classifications
- Classification name
- Categories
-
Acetylcholine
- Formed from acetate & choline
-
Amino acids
- Aspartate
- GABA (inhibitory)
- Glutamate (excitatory)
- Glycine
-
Monoamines
- Catecholamines
- Histamine
- Serotonin
-
Neuropeptides (Help with pain)
- Endorphin
- Enkephalin
- Cholecystokinin
- SO4
- Substance P
Neurotransmitters
- Function
- Effects
-
Function
- Transmit a message in the NS
- Excitatory & inhibitory
-
Effects
- Ionotropic: receptors/quick response; only 1 effect (ligand-regulated ion gates)
- Metabotropic: receptors/longer response; can create multiple effects (2nd messenger system)
NS functions
- Receive information about changes in the body and the external environment and transmit messages to the CNS
- CNS processes this information and determines what response, if any, is appropriate
- CNS issues instruction, carried out by the motor neurons
Peripheral NS (PNS) divisions
-
Sensory (afferent) division
- Visceral sensory division: viscera/organs (ie stretch)
- Somatic sensory division: general senses (ie touch, pain)
-
Motor (efferent) division
- Effectors
-
Visceral motor division (ANS)
- Autonomic reflexes
- Sympathetic division
- Parasympathetic division
-
Somatic motor division
- Somatic reflexes
Physiological subdivisions of the NS
-
Enteric NS (ENS)
- System of the gut
-
Somatic NS (SNS)
- Neuromuscular junction
- Deals with skeletal muscle control
-
Autonomic NS (ANS)
- Sympathetic NS: fight or flight
- Parasympathetic NS: rest & digest
PNS-specific components
- Ganglion: The cell body of peripheral neurons
- Nerve: a bundle of nerve fibers; axon
- Satellite cells: electrical insulation and maintain the chemical environment of PNS
- Schwann cells: creates myelin
Postsynaptic potentials
- Description
- Methods
- Potentials with the function to initiate or inhibit action potentials
- 2 methods
-
EPSP
- Help produce depolarization (less negative)
- Closer to threshold
- Helps to produce an action potential
-
IPSP
- Induces hyperpolarization
- Less likely to produce an action potential
- Important for pain & anesthesia
-
EPSP
Potential
- Indicates an electrical change or a different gradient between the sides of a cell
Refractory period
- Description
- Periods
-
Description
- The time immediately following a stimulus and when a muscle is contracting
- A period when it cannot respond to a second stimulus
- Since this is occurring at the same time as the contraction, it does not appear on the myogram as a separate event
-
Periods
-
Absolute refractory period
- occurs during the depolarization phase of action potential
- cannot recreate an action potential during this time due to K channels needing to be closed and reset before being reopened
-
Relative refractory period
- lasts until hyperpolarization ends
- possible to create an action potential; available if a suprathreshold stimulus is available
- needs suprathreshold stimulus to reopen
-
Absolute refractory period
Resting Membrane Potential (RMP)
- RMP exists because electrolytes are unequally distributed between the ECF & ICF
- Combined effect of 3 factors:
- Diffusion of ions down their concentration gradients through the membrane
- Selective permeability of the membrane, allowing some ions to pass more easily than others
- The electrical attraction of cations and anions to each other
- Net intracellular charge that is negative due to
- K-Na pump moving in unequal numbers (net -1 charge)
- Large anions in the cell that cannot escape
- Leaky channels; more K channels that are leaky inside the cell membrane, therefore K is more permeable to the cell
Reuptake
- Reabsorption of a neurotransmitter
Saltatory conduction
- Occur on myelinated fibers/axons
- Faster than unmyelinated action potentials
- Steps
- Na inflow at node generates an action potential
- Positive charge flows rapidly along the axon and depolarizes membrane; signal grows weaker with distance
- Depolarization of membrane at next node opens Na channels, triggering a new action potential
Sensory (afferent) division
- Carries signals away from various receptors to the CNS
- Informs CNS of stimuli within and around the body
- Subdivisions
- Somatic sensory division: carries signals from receptors in the skin, muscles, bones, & joints
- Visceral sensory division: carries signals from the viscera of the thoracic and abdominal cavities, such as the heart, lungs, stomach, and bladder
Action potential cessation
- Methods to prevent continual instructions/signals
- Diffusion
- Reuptake
- Degradation in the synaptic cleft (via enzyme)
- AChE
- COMT & MAO
Na+ & K+ channel process during an action potential
-
Resting membrane potential
- Na+ and K+ channels closed
-
Depolarization begins
- Na+ channels open
- Na+ enters cell
- K+ channels beginning to open
-
Depolarization ends, hyperpolarization begins
- Na+ channels closed
- K+ channels fully open
- K+ leaves cell
-
Repolarization complete
- Na+ channels closed
- K+ channels closing
Notes
- Na opens at a faster rate than K, so Na move down the concentration gradient faster than K
Spatial summation
- Occurs when EPSPs from several synapses add up to threshold at the axon hillock
- Any one synapse may generate only a weak signal, but several synapses acting together can bring the hillock to the threshold
- The presynaptic neurons collaborate to induce the postsynaptic neuron to fire
Summation
- The process of adding up postsynaptic potentials and responding to their net effect
- A balance between EPSPs and IPSPs
- 2 ways in which EPSPs can add do this; may occur simultaneously
- Temporal summation
- Spatial summation
Synapses
- Description
- Parts to every synapse
- Types
-
Description
- Physical space that separates a cell
- Stimulates neurons
-
Parts
- Presynapse: before the cleft; releases the chemical
- Postsynapse: after the cleft; receives the chemical
-
Types
- Axosyomatic
- Axoaconic
- Axodendritic
Temporal summation
- Occurs when a single synapse generates EPSPs so quickly that each is generated before the previous one fades
- Deals with the variable of time or frequency (1:1 ratio)
Universal properties of neurons
-
Secretion
- When the signal reaches the end of a nerve fiber, the neuron secretes a neurotransmitter that crosses the gap and stimulates the next cell
-
Excitability (irritability)
- Respond to environment changes/stimuli
-
Conductivity
- Neurons respond to stimuli by producing electrical signals that are conducted to other cells at distant locations