CH11 Flashcards
3 Functions of the Nervous System
**1. Sensory input **
2. Integration
- *3. Motor output
- *
• Information gathered by sensory receptors about internal and external changes
- Sensory input
• Interpretation of sensory input
Integration
• Activation of effector organs (muscles and glands) produces a response
Motor output
Divisions of the Nervous System
1 Central nervous system (CNS)
• Brain and spinal cord
• Integration and command center
2 Peripheral nervous system (PNS)
• Paired spinal and cranial nerves carry messages to and from the CNS
Peripheral Nervous System (PNS)
• Two functional divisions
- Sensory (afferent) division
• Somatic afferent fibers—convey impulses from skin, skeletal muscles, and joints
• Visceral afferent fibers—convey impulses from visceral organs - Motor (efferent) division
• Transmits impulses from the CNS to effector organs
Motor Division of PNS
has what 2 types of nervous systems
- Somatic (voluntary) nervous system
• Conscious control of skeletal muscles
- Autonomic (involuntary) nervous system (ANS)
• Visceral motor nerve fibers
• Regulates smooth muscle, cardiac muscle, and glands
• Two functional subdivisions
• Sympathetic
• Parasympathetic
Histology of Nervous Tissue
• Two principal cell types
- Neurons—excitable cells that transmit electrical signals
- Neuroglia (glial cells)—supporting cells:
• Astrocytes (CNS)
• Microglia (CNS)
• Ependymal cells (CNS)
• Oligodendrocytes (CNS)
• Satellite cells (PNS)
• Schwann cells (PNS)
MOSSEA
This supporting cell has=
• Most abundant, versatile, and highly branched glial cells
• Cling to neurons, synaptic endings, and capillaries
• Support and brace neurons
• Help determine capillary permeability
• Guide migration of young neurons
• Control the chemical environment
• Participate in information processing in the brain
Astrocytes
* This Supporting cell has=*
• Small, ovoid cells with thorny processes
• Migrate toward injured neurons
• Phagocytize microorganisms and neuronal debris
Microglia
This Supporting Cell has=
- Range in shape from squamous to columnar
- May be ciliated
- Line the central cavities of the brain and spinal column
- Separate the CNS interstitial fluid from the cerebrospinal fluid in the cavities
Ependymal Cells
_ This Supporting Cell has=_
• Branched cells
• Processes wrap CNS nerve fibers, forming insulating myelin sheaths
Oligodendrocytes
• Surround neuron cell bodies in the PNS
• Satellite cells
- Surround peripheral nerve fibers and form myelin sheaths
- Vital to regeneration of damaged peripheral nerve fibers
• Schwann cells (neurolemmocytes)
Neurons (Nerve Cells)
• Special characteristics:
- Long-lived (→ 100 years or more)
- Amitotic—with few exceptions
- High metabolic rate—depends on continuous supply of oxygen and glucose
- Plasma membrane functions in:
- Electrical signaling
- Cell-to-cell interactions during development
What is
• Biosynthetic center of a neuron
• Spherical nucleus with nucleolus
• Well-developed Golgi apparatus
• Rough ER called Nissl bodies (chromatophilic substance)
• Network of neurofibrils (neurofilaments)
• Axon hillock—cone-shaped area from which axon arises
• Clusters of cell bodies are called nuclei in the CNS, ganglia in the PNS
Cell Body (Perikaryon or Soma)
• Rough ER are called what
Nissl bodies (chromatophilic substance)
• Network of neurofibrils are also know as
(neurofilaments)
• Axon hillock are
cone-shaped area from which axon arises
• Clusters of cell bodies are called
* nuclei* in the CNS,
ganglia in the PNS
Processes
are
• Dendrites and axons
• Bundles of processes are called
• Tracts** in the **CNS
• Nerves in the PNS
- Short, tapering, and diffusely branched
- Receptive (input) region of a neuron
- Convey electrical signals toward the cell body as graded potentials
Dendrites
The Axon
How many axon per cell arising from the axon hillock
one
The Axon
• Long axons are known as
(nerve fibers)
The Axon
• Occasional branches are known as
(axon collaterals)
The Axon
• Knoblike axon terminals are known as
(synaptic knobs or boutons)
The Axon
• Secretory region of neuron do what
• Release neurotransmitters to excite or inhibit other cells
Axons Functions are
- Conducting region of a neuron
- Generates and transmits nerve impulses (action potentials) away from the cell body
Axons: Function
• Molecules and organelles are moved along axons by motor molecules in two directions:
_ • Anterograde_—toward axonal terminal
• Examples: mitochondria, membrane components, enzymes
• Retrograde—toward the cell body
• Examples: organelles to be degraded, signal molecules, viruses, and bacterial toxins
• Segmented protein-lipoid sheath around most long or large-diameter axons are what
Myelin Sheath
Myelin Sheaths
functions is to:
- Protect and electrically insulate the axon
- Increase speed of nerve impulse transmission
• Schwann cells wraps many times around
• Schwann cells wraps many times around _the axon
_
• Myelin sheath are
•
• Myelin sheath—_concentric layers of Schwann cell membrane
_
peripheral bulge of Schwann cell cytoplasm
are known as
• Neurilemma
- Myelin sheath gaps between adjacent Schwann cells
- Sites where axon collaterals can emerge
• Nodes of Ranvier
• Thin nerve fibers that are unmyelinated
& One Schwann cell may incompletely enclose 15 or more ____________?
Unmyelinated Axons
- Formed by processes of oligodendrocytes, not the whole cells
- Nodes of Ranvier are present
- No neurilemma
- Thinnest fibers are unmyelinated
Myelin Sheaths in the CNS
• Dense collections of myelinated fibers
• White matter
• Mostly neuron cell bodies and unmyelinated fibers
• Gray matter
Structural Classification of Neurons
• Three types:
- Multipolar—
- Bipolar—
- Unipolar (pseudounipolar)—
1 axon and several dendrites
• Most abundant
• Motor neurons and interneurons
- Multipolar—
1 axon and 1 dendrite
• Rare, e.g., retinal neurons
Bipolar—
—single, short process that has two branches:
• Peripheral process—more distal branch, often associated with a sensory receptor
• Central process—branch entering the CNS
Unipolar (pseudounipolar)—
Functional Classification of Neurons
• Three types:
- Sensory (afferent)
- Motor (efferent)
- Interneurons (association neurons)
• Transmit impulses from sensory receptors toward the CNS
- Sensory (afferent)
• Carry impulses from the CNS to effectors
- Motor (efferent)
• Shuttle signals through CNS pathways; most are entirely within the CNS
Interneurons (association neurons)
- Respond to adequate stimulus by generating an action potential (nerve impulse)
- Impulse is always the same regardless of stimulus
- Neurons are highly irritable
Neuron Function
• Energy is required to separate opposite charges across a membrane
- Energy is liberated when the charges move toward one another
- If opposite charges are separated, the system has potential energy
• Opposite charges attract each other
Principles of Electricity
measure of potential energy generated by separated charge
• Voltage (V)
voltage measured between two points
• Potential difference:
the flow of electrical charge (ions) between two points
• Current (I):
hindrance to charge flow (provided by the plasma membrane)
• Resistance (R):
substance with high electrical resistance
• Insulator:
substance with low electrical resistance
• Conductor:
• serve as membrane ion channels
•_ Proteins_
• Two main types of ion channels
- Leakage (nongated) channels—always open
-
Gated channels (three types):
• Chemically gated (ligand-gated) channels—open with binding of a specific neurotransmitter
- Voltage-gated channels—open and close in response to changes in membrane potential
- Mechanically gated channels—open and close in response to physical deformation of receptors
Gated Channels
• When gated channels are open:
- Ions diffuse quickly across the membrane along their electrochemical gradients
- Along chemical concentration gradients from higher concentration to lower concentration
- Along electrical gradients toward opposite electrical charge
- Ion flow creates an electrical current and voltage changes across the membrane
- Potential difference across the membrane of a resting cell
- Approximately –70 mV in neurons (cytoplasmic side of membrane is negatively charged relative to outside)
Resting Membrane Potential (Vr)
Resting Membrane Potential (Vr)
• Generated by:
- Differences in ionic makeup of ICF and ECF
- Differential permeability of the plasma membrane
• Differences in ionic makeup in a
Resting Membrane Potential** **
• ICF has lower concentration of Na+ and Cl– than ECF
• ICF has higher concentration of K+ and negatively charged proteins (A–) than ECF
• Differential permeability of membrane
in a
Resting Membrane Potential
• Impermeable to A–
• Slightly permeable to Na+ (through leakage channels)
• 75 times more permeable to K+ (more leakage channels)
• Freely permeable to Cl–
• Negative interior of the cell is due to what?
- much greater diffusion of K+ out of the cell than Na+ diffusion into the cell
- *
**• Sodium-potassium pump stabilizes the **
resting membrane potential by maintaining the concentration gradients for Na+ and K+
- *• Membrane potential changes when:
- *
• Concentrations of ions across the membrane change
- *• Permeability of membrane to ions changes
- *
**• Changes in membrane potential are **
** signals used to receive, integrate and send information**
Membrane Potentials That Act as Signals
• Two types of signals
• Graded potentials
Incoming short-distance signals
• Action potentials
Long-distance signals of axons
Changes in Membrane Potential thats
- A reduction in membrane potential (toward zero)
- Inside of the membrane becomes less negative than the resting potential
- Increases the probability of producing a nerve impulse
• Depolarization
Changes in Membrane Potential that has
- An increase in membrane potential (away from zero)
- Inside of the membrane becomes more negative than the resting potential
- Reduces the probability of producing a nerve impulse
• Hyperpolarization
** • Occur when a stimulus causes gated ion channels to open
• E.g., receptor potentials, generator potentials, postsynaptic potentials
• Magnitude varies directly (graded) with stimulus strength
• Decrease in magnitude with distance as ions flow and diffuse through leakage channels
• Short-distance signals**
• Short-lived, localized changes in membrane potential
• Depolarizations or hyperpolarizations
- *• Graded potential spreads as local currents change the membrane potential of adjacent regions
- *
- *Graded Potentials
- *
- 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
Action Potential (AP)
Generation of an Action Potential
- Only leakage channels for Na+ and K+ are open
- All gated Na+ and K+ channels are closed
• Resting state
• Properties of gated channels
- *• Each Na+ channel has two voltage-sensitive gates
- *
• Activation gates
Closed at rest; open with depolarization
• Inactivation gates
Open at rest; block channel once it is open
Opens slowly with depolarization
• Some K+ channels remain open, allowing excessive K+ efflux
• This causes after-hyperpolarization of the membrane (undershoot)
• Hyperpolarization
- Membrane is depolarized by 15 to 20 mV
- Na+ permeability increases
- Na influx exceeds K+ efflux
- The positive feedback cycle begins
• At threshold:
weak local depolarization that does not reach threshold
• Subthreshold stimulus—
strong enough to push the membrane potential toward and beyond threshold
• Threshold stimulus—
action potentials either happen completely, or not at all
• AP is an all-or-none phenomenon
Coding for Stimulus Intensity
- All action potentials are alike and are independent of stimulus intensity
- How does the CNS tell the difference between a weak stimulus and a strong one?
• Strong stimuli can generate action potentials more often than weaker stimuli
• The CNS determines stimulus intensity by the frequency of impulses
**
• Larger diameter fibers have less resistance to local current flow and have faster impulse conduction
**
- *• Effect of axon diameter
- *
- *• Continuous conduction in unmyelinated axons is slower than saltatory conduction in myelinated axons
- *
• Effects of myelination
- Myelin sheaths insulate and prevent leakage of charge
- Saltatory conduction in myelinated axons is about 30 times faster
- Voltage-gated Na+ channels are located at the nodes
- APs appear to jump rapidly from node to node
• Effect of myelination
• Nerve fibers are classified according to:
**
• Diameter
• Degree of myelination
• Speed of conduction**
• Large diameter, myelinated somatic sensory and motor fibers
• Group A Nerve fibers
• Intermediate diameter, lightly myelinated ANS fibers
• Group B Nerve fibers
Smallest diameter, unmyelinated ANS fibers
• Group C Nerve fibers
- A junction that mediates information transfer from one neuron:
- To another neuron, or
- To an effector cell
- *The Synapse
- *
conducts impulses toward the synapse
• Presynaptic neuron—
transmits impulses away from the synapse
• Postsynaptic neuron—
Types of Synapses
—between the axon of one neuron and the dendrite of another
**• Axodendritic **
Types of Synapses
• between the axon of one neuron and the soma of another
• Axosomatic
Types of Synapses
• Less common types:
- Axoaxonic (axon to axon)
- Dendrodendritic (dendrite to dendrite)
- Dendrosomatic (dendrite to soma)
- Less common than chemical synapses
- Neurons are electrically coupled (joined by gap junctions)
- Communication is very rapid, and may be unidirectional or bidirectional
Electrical Synapses
Electrical Synapses
• Are important in:
• Embryonic nervous tissue
• Some brain regions
• Specialized for the release and reception of neurotransmitters
Chemical Synapses
Chemical Synapses
• Typically composed of two parts
• Axon terminal of the presynaptic neuron, which contains synaptic vesicles
• Receptor region on the postsynaptic neuron
- Fluid-filled space separating the presynaptic and postsynaptic neurons
- Prevents nerve impulses from directly passing from one neuron to the next
Synaptic Cleft
Synaptic Cleft
• Transmission across the synaptic cleft:
• Is a chemical event (as opposed to an electrical one)
• Involves release, diffusion, and binding of neurotransmitters
• Ensures unidirectional communication between neurons
• What happens 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
2. IPSP—inhibitory postsynaptic potentials
• One or more presynaptic neurons transmit impulses in rapid-fire order
• Temporal summation
• Postsynaptic neuron is stimulated by a large number of terminals at the same time
• Spatial summation
- Released at neuromuscular junctions and some ANS neurons
- Synthesized by enzyme choline acetyltransferase
- Degraded by the enzyme acetylcholinesterase (AChE)
- *• Acetylcholine (Ach)
- *
Chemical Classes of Neurotransmitters
• Peptides (neuropeptides) include:
• Substance P
Mediator of pain signals
• Endorphins
Act as natural opiates; reduce pain perception
• Gut-brain peptides
Somatostatin and cholecystokinin
- Act in both the CNS and PNS
- Produce fast or slow responses
- Induce Ca2+ influx in astrocytes
- Provoke pain sensation
- *• Purines such as ATP:
- *
- Synthesized on demand
- Activates the intracellular receptor guanylyl cyclase to cyclic GMP
- Involved in learning and memory
• Nitric oxide (NO)
is a regulator of cGMP in the brain
**• Carbon monoxide (CO) **
- Lipid soluble; synthesized on demand from membrane lipids
- Bind with G protein–coupled receptors in the brain
- Involved in learning and memory
• Endocannabinoids
