Chapter 11 Nervous System Flashcards
3 main functions of the nervous system
sensory input, integration and motor output
Sensory input
monitoring stimuli/changes inside and outside the body
Integration
interpretation of sensory input
Motor output
response to stimuli
Organization of the nervous system
Central Nervous System (CNS) and Peripheral Nervous System (PNS)
What makes up the CNS?
*Brain and spinal cord
*integration and command center
What makes up the PNS?
*Paired spinal and cranial nerves
*spinal nerves: impulses to/from the spinal cord
*cranial nerves: impulses to/from the brain
**carries messages to and from the spinal cord and brain
2 functional divisions of the PNS
*sensory (afferent) division
*motor (efferent) division
Sensory afferent fibers-
carry impulses from skin, skeletal muscles, and joints to the brain (CNS)
Visceral afferent fibers-
transmit impulses from visceral organs to the brain (CNS)
Motor (efferent) division-
transmits impulses f_rom CNS to effector organs, muscles and glands_
Two main parts of the motor division are
Somatic Nervous System-Voluntary
Autonomic Nervous System (ANS)-Involuntary
Somatic Nervous System is
voluntary
Autonomic Nervous System is
involuntary
What does ANS (autonomic Nervous System) regulate?
smooth muscle and cardiac muscle
What are the divisions of ANS?
sympathetic and parasympathetic -one stimulates and the other will inhibit
2 principle cell types of the nervous system
Neurons and supporting cells
Neurons are
excitable cells that transmit electrical signals
Supporting cells are
cells that surround and wrap neurons
Other names for supporting cells: neuroglia or glial cells
provide a supportive scaffolding for neurons
Other names for supporting cells: astrocytes
*most abundant, versatile and highly branched glial cells *barrier between neurons and capillaries
Other names for supporting cells: microglia
small ovoid cells with spiny processes
Other names for supporting cells: ependymal cells
range in shape from squamous to columnar *line cavities of CNS (brain &spinal cord)
Other names for supporting cells: oligodendrocytes
*branched cells that wrap CNS nerve fibers *form myelin sheaths
Other names for supporting cells: Schwann cells
*surround fibers of PNS *form myelin sheaths
Other names for supporting cells: satellite cells
surround neuron cell bodies with ganglia, in the PNS
What type of division occurs in neurons?
long, lived, amitotic (a simple division of the nucleus with replication of chromosomes)
The function of a neurons plasma membrane is
electrical signaling
other names for a nerve cell body
Perikaryon or Soma
The structure of a nerve cell body
*contains the nucleus and a nucleolus *has no centrioles *has well developed Nissl bodies (rough ER) *contains an axon hillock
what are the processes?
axons and dendrites
Dendrites
*short branched processes *receptive *create enormous surface area for receptions of signal from other neurons *conduct impulse toward cell body
Axons-structure
*slender processes arising from the hillock *may be short or very long *long axons are called nerve fibers *usually there is only one *rare branches, if present, are called axon collaterals *larger diameter causes faster conduction
Axons-Function
*generates and transmits action potential
*carries impulses away from cell body
*secrete neurotransmitters from the axon terminals
*movement:
–anterograde: toward axonal terminal
–retrograde: away from axonal terminal
Function of myelin sheaths
*protect the axon
*electrically insulate fibers from one another
*increase the speed of nerve impulse transmission
Nodes of Ranvier are
gaps in myelin sheath between adjacent Schwann cells
Bothe myelinated and unmyelinated fibers are
present in CNS
Myelin sheaths are formed bu
oligodendrocytes in the CNS
White mater is
dense collections of myelinated fibers
White matter is in
regions of the brain and spinal cord that have myelinated fibers
Gray matter is
mostly soma and unmyelinated fibers
Gray matter is mostly in
nerve cell body-unmyelinated
Structural classifications of neurons
*multipolar-three or more process (most common)
*Bipolar-two processes (axon & dendrite)-rare
*Unipolar-single, short process in PNS ganglia
Functional classifications of neurons
*Sensory (afferent)-transmit impulses toward the CNS
*Motor (efferent)-carry impulses away from the CNS
*Interneurons (association neurons)-shuttle signals through CNS pathways-99% of neurons in the body
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
There is potential on either side of membranes when:
1) the number of ions is different across the membrane
2) the membrane provides a resistance to ion flow
Chemically gated channels open with binding of a
specific neurotransmitter
Voltage-gated channels open and close in
response to membrane potential
what is the resting membrane potential?
*the potential difference across the membrane of a resting neuron
*the polarity is more negative on the inside
What generates the resting membrane potential?
It is generated by different concentrations of Na+, K+, Cl-, and protein (A-)
Ion concentrations
*differential permeability of the neurilemma to Na+ and K+
*Operation of the sodium-potassium pump
Changes in membrane potential are caused by
Depolarization, repolarization and hyperpolarization
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
graded potentials are:
- short-lived
- decrease in intensity with distance
- magnitude varies directly with the strength of the stimulus
- sufficiently strong graded potentials can initiate action potentials
Action potentials
- a brief reversal of membrane potential
- only generated by muscle cells & neurons
- do not decrease in strength over distance
- an action potential in the axon of a neuron is a nerve impulse
- they are all or none
Describe the resting state of an action potential
- Na+ and K+ channels are closed
- leakage accounts for small movements of Na+ and K+
- Each Na+ channel has two voltage-regulated gates
- 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+ (activation) gates are opened; K+ gates are closed
- Threshold-a critical level of depolarization
(-55 to -50 mV)
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 and 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 hyperpolarization of the membrane (undershoot)
- the neuron is insensitive to stimulus and depolarization during this time
Describe the role of the sodium-potassium pump in relation to an action potentials
- repolarization
1) restores the resting electrical conditions of the neuron
2) Does not restore the resting ionic conditions - ionic redistribution back to resting conditions is restored by the sodium-potassium pump
What are the phases of the action potential?
1-resting state
2-depolarization phase
3-repolarization phase
4-hyperpolarization
What is threshold? Describe all or none phenomenon.
- threshold-membrane is depolarized by 15-20 mV
- established by the total amount of current flowing through the membrane
- weak (subthreshold) stimuli are not relayed into action potentials
- strong (threshold) stimuli are relayed into action potentials
- All-or-nothing phenomenon-action potentials either happen completely or not at all
Describe the absolute refractory period
Time from the opening of the Na+ activation gates until the closing of the inactivation gates
the absolute refractory period:
- prevents the neuron from generating an action potential (can’t respond to another stimulus)
- ensures that each action potential is separate
- enforces one-way transmission of nerve impulses
Describe the relative refractory period
The interval following the absolute refractory period when:
*sodium gates are closed
*potassium gates are open
*repolarization is occurring
- the threshold level is elevated, allowing strong stimuli to increase the frequency of action potential events
- during the ‘after-hyperpolarization’ stage-another stimulus could open the Na gates, but only a strong stimulus (Shari)
Rate of impulse propagation is determined by:
axon diameter-the larger the diameter, the faster the impulse
presence of a myelin sheath-myelination dramatically increases impulse speed
Describe Saltatory Conduction
*current passes through a myelinated axon only at the nodes of Ranvier
*action potentials are triggered only at the nodes and jump from one node to the next
Explain multiple sclerosis
*it’s an autoimmune disease that mainly affects young adults
-the immune system attacks myelin proteins
-axons are not damaged
*symptoms: visual disturbances, weakness, loss of muscular control and urinary incontinence
*nerve fibers are severed and myelin sheaths in the CNS become nonfunctional sclerosis (when sheaths are reduced to hard lesions)
presynaptic neuron
conducts impulses toward the synapse
Postsynaptic neuron
transmits impulses away from the synapse
Describe two types of postsynaptic potentials
*EPSP-excitatory postsynaptic potentials
*IPSP-inhibitory postsynaptic potentials
what is temporal summation
presynaptic neurons transmit impulses in rapid-fire order
what is spatial summation
a postsynaptic neuron is stimulated by a large number of terminals at the same time
Neurotransmitters
*chemical used for neuronal communication with the body and the brain
*50 different neurotransmitters have been identified
*classified chemically and functionally
Neurotransmitters and diseases
*GABA-Gamma(y)-aminobutyric acid
-the most prevalent neurotransmitter in the brain
-inhibitory
-Huntington’s-lack of GABA releasing neurons
*Acetylcholine-refer to next card
*Norepinephrine-feel good, reuptake blocked by cocaine
*Dopamine-feel good, regulation of skeletal movement
-Parkinson’s-substantia nigra degenerates-decreased dopamine
*serotonin-inhibitory, regulates mood, Prozac block reuptake
*endorphins-inhibit pain, morphine, heroin mimic
Neurotransmitters : Acetylcholine
*first neurotransmitter identified, and best understood
*released at the neuromuscular junction
*synthesized and enclosed in synaptic vesicles
*degraded by the enzyme acetylcholinesterase (AChE)
*released by:
-all neurons that stimulate skeletal muscle
-some neurons in the autonomic nervous system
Two classifications of Neurotransmitters
excitatory and inhibitory
*excitatory neurotransmitters cause depolarizations (e.g., glutamate)
*_Inhibitory neurotransmitters cause hyper_polarizations (e.g., GABA and glycine)
Can neurotransmitters have both effects? Give an example.
*some neurotransmitters have both excitatory and inhibitory effects
*determined by the receptor type of the postsynaptic neuron
*example: acetylcholine
- Excitatory at neuromuscular junctions with skeletal muscle
- inhibitory in cardiac muscle
