Chapter 11 Nervous System & Nervous Tissue Flashcards
Nervous system
the body system consisting of the brain, spinal cord, & nerves; one of the chief homeostatic systems in the body
Central Nervous System (CNS)
Brain: made up of billions of nerve cells or neurons; protected by bones of skull
Spinal Cord: Made up of millions of neurons; much fewer than brain; Spinal cord begins at foramen magnum and continues through vertebral foramina of first cervical to first or second lumbar vertebra
•Connects the brain w/ the peripheral nervous system
•Main function is to integrate information
Peripheral Nervous System (PNS)
Nerves: carry signals to & from the central nervous system; Nerves consist of axons of neurons bundled together with blood vessels and connective tissue; carry signals to and from CNS; classified based on origin or destination
Comprised of:
12 pairs of cranial nerves; nerves originating from or traveling to the brain
31 pairs of spinal nerves; nerves originating from or traveling to the spinal cord
Functional Divisions of the Nervous System
Sensory Functions- gathers information about the internal & external environment of the body
Integrative Functions- analyzes & interprets the detected sensory stimuli & determines an appropriate response; 99% of input is “filtered out”
Motor Functions- actions performed in response to integration; carries impulses from CNS to responsive parts, such as muscles or glands (autonomic & somatic)
Sensory Functions
gathers information about the internal & external environment of the body
PNS Sensory Division: sensory stimuli are first detected by structures of the PNS
Somatic Sensory Division: (special sensory division) consist of neurons that carry signals from skeletal muscles, bones, joints, & skin
Visceral Sensory Division: consist of neurons that transmit signals form
Integrative Functions
analyzes & interprets the detected sensory stimuli & determines an appropriate response; 99% of input is “filtered out”
Motor Functions
actions performed in response to integration; carries impulses from CNS to responsive parts, such as muscles or glands (autonomic & somatic)
PNS Motor Division: consist of motor neurons that carry out motor functions of the nervous system
Somatic Motor Division: (voluntary motor division) consist of neurons that transmits signals to skeletal muscles
Visceral Motor Division: (autonomic nervous system (ANS)/ involuntary motor division) consist of neurons that carry signals primarily to thoracic & abdominal viscera
Neurons
the excitable cell type responsible for sending & receiving signals in the form of action potentials.
•Conduct electrical impulses
•React to chemical and physical changes in their environment
Neuroglia cells
(neuroglia) does NOT transmit signals but serves a variety of supportive functions
•Supporting cells of the CNS
•Similar function to connective tissues
Structural Classification of Neurons:
Multipolar Neurons: a neuron with one axon & two or more dendrites; widest variability in shape & size. Most common type of neuron
Bipolar Neurons: a neuron with one axon & one dendrite; sensory neurons, located in the retina of the eye & the olfactory epithelium of the nasal cavity
Unipolar Neurons: a neuron with two axons- a peripheral process that brings input to the cell body & a central process that brings input to a target cell; detect stimuli such as touch, pressure, & pain
Functional Classification of Neuron
Sensory Neurons: (afferent) a neuron that detects changes in the environment & carry signals towards the CNS
Interneurons: (association) neurons between sensory & motor neurons that perform integrative functions; relay messages within the CNS
Motor Neurons: (efferent) a neuron that transmits motor impulses away from the CNS to muscles & glands
CNS Neuroglia
Astrocytes: a neuroglia cell of the CNS that facilitates information of the blood brain barrier, regulates extracellular environment of the brain, anchors neurons & blood vessels in place, & repair damaged brain tissue
Oligodendrocytes: a neuroglia cells of the CNS that myelinates certain axons
Microglia: act as phagocytes
Ependymal Cells: a ciliated neuroglia cell of the CNS that lines the hollow cavities of the brain & spinal cord; cilia beat to circulate cerebrospinal fluid
PNS Neuroglia
Schwaan Cells: a neuroglia cell of the PNS that myelinates the axons of certain neurons; repair damaged axons in PNS. Interneuron/association neurons
Satellite Cells: a neuroglia cell of the PNS that surrounds & supports cell bodies of neurons
The Myelin Sheath
Myelin: a fatty substance that envelops & insulates the axon of certain neurons, increasing the speed of action potential conduction; formed from the plasma membranes of oligodendrocytes & Schwann cells
•Phospholipids, other lipids such as cholesterol, & proteins
•Myelinated axon: white matter; faster action potentials
•Unmyelinated axon: grey matter; are made up primarily of cell bodies & dendrites
Myelination: the process of myelin sheath formation
Importance: protection & insulation of axons
Differences in myelination of PNS & CNS
Presence or Absence of a Neurolemma- outer surface of a myelinated axon in PNS; composed of Schwann cell nucleus, organelles, & cytoplasm; not present in CNS
Number Axons Myelinated by a single glia cell- Oligodendrocytes have multiple process that can provide myelination for multiple axons in CNS while a Schwann cell only provides myelination for ONE axon in PNS
Timing of Myelination- myelination begins early in fetal development in PNS and much later in the CNS; very little myelin present in brain of newborn
Regeneration of Nervous Tisssue
Regeneration: the process by which a damaged tissue is replaced w/ the same tissue during healing
Regeneration in CNS- damaged axons & dendritesnearly nonexistent
•Lacks growth factors that trigger mitosis are absent in CNS
•Growth of astrocytes creates space-filled scar tissue that prohibits regeneration
•Lost function may be regained by retraining the remaining neurons
Regeneration in PNS- limited; can only occur if cell body remains intact
If regeneration occurs, its often imperfect. The axon will contact the wrong target cell or contact between the cells will not be re-established
The electrical changes across a neuron’s plasma membrane comes in two forms
1) Local potentials–>travel only short distances
2) Action potentials–>travel the entire length of an axon
Importance of Ion Channels, Pumps & Membrane Polarization:
•The resting membrane of the neuron is polarized
o Na+ and K+ ions are uneven distributed across the membrane; more Na+ outside & more K+ inside results in charge difference which give “potential energy”
- Channels that could allow the ions to move are “gated” and closed under resting conditions
- The Na+/K+ pump maintains the gradient
- Na+/K+ pump brings 2 potassium ions into the cytosol as it moves 3 sodium ions into the extracellular fluid
- When you measure the membrane potential of a neuron at rest, or the resting membrane potential, it measures about -70mV (polarized)
Leak Channel
Leak Channel: always open
Location: all over neuron
Ligand-gated Channel
Ligand-gated Channel: opens in response to a certain chemical (neurotransmitter), called a ligand binding to the channel or to a receptor associated with the channel
Location: Dendrites & cell body
Voltage-gated Channel
Voltage-gated Channel: opens or closes in response to changes in the cell’s membrane potential (electrical signal)
Location: mostly on axons; axolemma of a neuron
Mechanically-gated Channel
Mechanically Gated Channel: opens or closes in response to mechanical stimulation such as stretch, pressure, & vibration
Location: dendrites
Voltage numbers of States of Membrane Polarity:
inside is negative w/ respect to outside
•Polarization: -70mV
•Threshold: -55 to -50mV
•Depolarization: voltage shoots to + values (+30mV)
•Hyperpolarization: greater than -70mV
Local Potential
(graded potentials) a small change in the membrane potential in a specific region of a cell’s plasma membrane, when a neuron is stimulated just once
Action Potential
a quick, temporary rapid depolarization & repolarization of the membrane potential of a cell
•Signals are sent through an axon to another neuron, a muscle fiber, or a gland
•Axons can generate action potentials; dendrites & cell bodies generate LOCAL potentials only
•Action potentials START at the axon hillock & END at the axon terminal
States of Voltage-Gated Channels
Two types of voltage-gated channels are involved in depolarization & repolarization of an action potential one for sodium ions & one for potassium ions
Found in the axolemma of a neuron, which is why only axons have action potential
Events of an Action Potential
1) A local potential depolarizes the axolemma of the trigger zone to threshold
2) Voltage-gated Na+ channels activate, Na+ enter, & the axon section depolarizes
3) Na+ channels inactivate & voltage-gated K+ channels activate, & repolarization begins
4) Na+ channels return to the resting state & repolarization continues
5) The axolemma may hyperpolarize before K+ channels return to the resting state; after this, the axolemma returns to the resting membrane potential
Refractory Period & Two Phases
Refractory Period: for a brief time after a neuron has produced an action potential, the membrane cannot be stimulated to fire another one. Neurons are limited in how often they can fire action potentials.
Two Phases: Absolute refractory period & Relative Refractory period
Absolute refractory period: no additional stimulus, no matter how strong, is able to produce an additional action potential
Relative refractory period: Only a strong stimulus will produce an action potential
Local Potentials
Due to signal at the dendrites/cell body
Decremental (strength decreases over short distances)
May be inhibitory or excitatory
Uses ligand gated channels (cell body/dendrites)
Reversible
Only travels a short distance
Action Potentials
Due to signal summation at the axon hillock/trigger zone
Non-decremental (strength does NOT dimmish)
Always causes a depolarization (excitatory)
Uses voltage gated channels (axon)
Irreversible
Travels all the way down the axon
Classification of Axons by Conduction Speed
axons are classified according to conduction speed; the diameter of the axon & the presence or absence of myelin sheath
Type A fiber: largest-diameter axons & thick myelinated; fastest conduction speed. Found in CNS where rapid communication occurs such as certain sensory axons from joints & muscle fibers, as well as motor axons to skeletal muscles
Type B fiber: medium in diameter & lightly myelinated; intermediate speeds. Found in certain efferent fibers of the autonomic nervous system & certain sensory axons coming from organs
Type C fiber: smallest fibers & unmyelinated; slowest conduction speed. Found in efferent fibers of the autonomic nervous system & certain sensory axons that transmit pan, temperature, & certain pressure sensations
Synapse
the location where a presynaptic neuron communicates with its target cell
Neuronal synapses
synapses that occur between two neurons; electrical & chemical
Pre-synaptic Neuron
the neuron that is sending the message from its axon terminal
Post-synaptic Neuron
the neuron that is receiving the message from its dendrite, cell body, or axon
Synaptic transmission
the transfer of chemical or electrical signals between neurons at a synapse; allow voluntary movement, cognition, sensation, emotion, etc.
Synaptic cleft
the small space between the axon terminal of a presynaptic neuron & its target cell; filled w/ extracellular fluid & proteins such as enzymes
Chemical Synapses
a presynaptic neuron releases neurotransmitter to trigger a change in a postsynaptic neuron; more common b/c they’re more efficient
Events of a Local Potential (Chemical Synapse)
1) An action potential in the presynaptic neuron triggers Ca2+ channels in the axon terminal to open
2) Influx of Ca2+ causes synaptic vesicles to release neurotransmitters into the synaptic cleft
3) Neurotransmitters bind to receptors n the postsynaptic neuron
4) Ion channels open, leading to a local potential & possibly an action potential
Post-synaptic potentials
a positive or negative charge in the membrane potential of a neuron as a result of a synaptic transmission. It can move the membrane potential at the rigger zone either closer or farther way from the threshold.
Excitatory Postsynaptic potential (EPSP):
a small local depolarization; becomes less negative due to the influx of positive ions
•The membrane potential of the postsynaptic neuron moves closer to threshold
•Ligand-gated channels (sodium or calcium) open & these positively charged ions enter the postsynaptic neurons & will cause a local depolarization
Inhibitory postsynaptic potential (IPSP):
a small, local hyperpolarization; becomes more negative due to the outflow of positive ions or the influx of negative ions
•The membrane potential of the post-synaptic neuron moves away from threshold
•Ligand-gated potassium ions channels open causes the cytosol to lose + charge & makes the membrane potential become more negative
•Ligand-gated chloride ion channels open these anions enter the neuron & makes the membrane potential more negative
Neural Integration
The process by which a neuron integrates all of the postsynaptic potentials from multiple presynaptic neurons
Summation
adding the input from several postsynaptic potentials to affect the membrane potential at the trigger zone/axon hillock
- Temporal Summation: the additive effect of a excitatory postsynaptic triggered by a SINGLE presynaptic neuron that fires action potential in rapid succession
- Spatial Summation: The additive effect of excitatory postsynaptic potentials triggered by MULTIPLE presynaptic neurons firing action potentials simultaneously
Summation of IPSP
the postsynaptic neuron becomes less & less likely to fire an action potential
Summation of IPSPs & EPSP
results depend in the individual strength of IPSP & EPSP—if the IPSP is stronger, the membrane potential will hyperpolarize slightly, & if the EPSP is stronger, the membrane potential will depolarize slightly
Synaptic Transmission Termination
it’s how neurotransmitters are removed from the synaptic cleft. When neurotransmitter molecules have been removed from the synaptic cleft, synaptic transmission is complete
•Neurotransmitters usually remain in the synaptic left for a short time
Diffusion & Absorption: neurotransmitters diffuse away from the synaptic left into the surrounding extra-cellular matrix
Degradation: neurotransmitters are degraded by enzymes in the synaptic cleft
Example: AChE digest ACh
Reuptake: neurotransmitters are taken back into the presynaptic neuron by proteins in its axolemma; they’re either repackaged into synaptic vesicles or degraded by enzymes.
Classes of neurotransmitter receptors:
Ionotropic receptors: rapid, but short-lived effects on the membrane potential of the postsynaptic neuron; receptors that are part of ligand-gated ion channels
Metabotropic receptors: elicit much slower changes in the membrane potential of the postsynaptic neuron, but the effects are typically longer lasting & more varied than those of the ionotropic receptors
Acetylcholine
ACh: in PNS & CNS how association neurons “talk to each other (nicotine)
Norepinephrine & Epinephrine
Norepi= neurotransmitter Epi= hormone adrenaline
Dopamine
pleasure & reward system
Serotonin
Mood/depression-target of many antidepressants
Glutamate
High stimulatory- learning & memory
GABA
Major depressant- but is critical for modulating how neurons talk to each other (increased w/ alcohol)
Endorphins
Natural pain killers
Adenosine
Lowers brain activity=sleepy
caffeine suppresses this activity
Neuronal Pool
a network of neurons that perform a common function; they have a specific patterns of organization called neural circuits
Neural Circuits
pattern of connection between neuronal pools
Diverging circuit
One or more input neurons that contact an increasing number of postsynaptic neurons
Converging circuit
the signals from multiple neurons converge into one or more final postsynaptic neuron. Axon terminals from multiple input neurons join onto a single postsynaptic neuron.
