Lecture 6: Nervous System 5 and 6 Flashcards
A synapse may be either excitatory or inhibitory depending on
Permeability changes induced in the post synaptic neuron by the combination of neurotransmitter with receptor sites
Excitatory synapse
Opening of Na channels within the postsynaptic membrane-leads to an increase in permeability to these ions
A large number of Na enters the cell, causing a net movement of positive ions into the cell, which results in a small depolarization of the postsynaptic neuron
The postsynaptic change in an excitatory synapse is called
An excitatory postsynaptic potential (EPSP)
EPSP brings the membrane potential closer to
- threshold
- firing an action potential
Excitatory neurotransmitters include (4)
acetylcholine
norepinephrine
epinephrine
glutamate
Inhibitory synapse
Neurotransmitter-receptor combo causes an increase in permeability of postsynaptic membrane to K or Cl, resulting in ion movement into the cell and a small hyperpolarization of the postsynaptic membrane
Postsynaptic change in an inhibitory synapse
inhibitory postsynaptic potential
ISPS takes the membrane potential away from
- threshold
- farther from firing an action potential
Two types of inhibitory neurotransmitters
acetylcholine
glycine
What is the mechanism responsible for integration of EPSPs, IPSPs, or a combo of the two in a postsynaptic neuron
summation
The postsynaptic neuron can be brought to threshold in one of two ways
Temporal summation
Spatial summation
Temporal summation
The summing of several EPSP’s occurring very close together in time because of successive firing of single presynaptic neuron
Spatial summation
Summation of EPSPs originating simultaneously from several different presynaptic inputs
Not all info received by a postsynaptic neuron is
passed along
When will information be passed along?
Only if an excitatory presynaptic signal is reinforced by other supporting signals
Interacting of postsynaptic potentials provide a way for one set of signals to
offset another set (IPSPs negating EPSPs)
Neurotransmitter
chemical compound released by one neuron to affect the membrane potential of postsynaptic cell
Neuromodulator
A chemical that adjusts the sensitivities of another cell to specific neurotransmitters
Presynaptic inhibition
The amount of neurotransmitter released from the presynaptic terminal decreases
Presynaptic facilitation
The amount of neurotransmitter released from the presynaptic terminal increases
Axoaxonic synapse
- The axon of one neuron synapses with the presynaptic terminal of another
- When action potential reaches the presynaptic terminal, neuromodulators released in the axoaxonic synapse can alter the amount of neurotransmitter released from the presynaptic terminal
- Does not initiate an action potential in the presynaptic terminal
Example of presynaptic inhibition (tetanus)
- Tetanus toxin prevents the release of the inhibitory neurotransmitter, gamma amino butyric acid (GABA) from inhibitory presynaptic inputs terminating on neurons that supply skeletal muscle
- Unchecked excitatory inputs to these neurons result in uncontrolled muscle spasms
Example of presynaptic inhibition (pain prevention)
Enkephalins and endorphins in the CNS produces presynaptic inhibition of neurons transmitting pain sensation. They bind to opiate receptors and terminate pain signals by inhibiting the release of substance P from pre-synaptic neurons
Presynaptic facilitation example
A pre-synaptic neuron releases glutamate, which binds to glutamate receptors on the postsynaptic membrane and stimulates the postsynaptic neuron to produce nitric oxide, which diffuses out of the postsynaptic neuron, crosses the synaptic cleft, diffuses into the presynaptic neuron and stimulates the release of additional glutamate from the presynaptic neuron
Reflexes
rapid, automatic responses to stimuli
How do reflexes preserve homeostasis
by making rapid adjustments in the function of organs or organ systems
The response shows little variability because
activation of a particular reflex usually produces the same motor response
What body functions are controlled through reflexes
movement of respiration and digestion
The basic functional unit of the nervous system
The reflex arc
Reflex arc
The smallest, simplest portion capable of receiving a stimulus and yielding a response
Two types of reflex
Neural
Endocrine
Neural reflex
- Sensory fibers deliver info to the CNS and motor fibers carry motor commands to the peripheral effectors
- Can be spinal or cranial
Endocrine reflex
A stimulus triggers the production of a hormone whose activities lead to a reduction in the magnitude of the stimulus
Two types of neural reflex
Spinal and cranial
Spinal neural reflex
Spinal cord serves as the integrating center
5 steps of a reflex arc
- Arrival of a stimulus and activation of a receptor
- Activation of a sensory neuron
- Information processing in the CNS
- Activation of a motor neuron
- Response by an effector
Negative feedback
corrective mechanism that opposes or negates a variation from normal limits
Reflexes can be classified according to (4)
- Development
- Processing site
- Motor response
- Complexity of circuit
What is the simplest arc
monosynaptic arc
Monosynaptic arc
A sensory neuron synapses directly on a motor neuron, which itself serves as the processing center
Polysynaptic arc
One or more interneurons synapse with the sensory and motor neurons
Can produce more complicated responses because the interneurons can control several groups
5 Steps of the Stretch reflex mechanism
- Stretch muscle stimulates muscle spindle
- Propagation of action potentials along sensory neurons to the spinal cord
- Information processing at the motor neuron
- Activation of motor neurons
- Contraction of muscle
Stretch reflex maintains a _____ _____ ______ length at rest
normal skeletal muscle
Is the stretch reflex mono or polysynaptic reflex?
Monosynaptic
The stretch reflex provides
autonomic regulation of skeletal muscle length
In the stretch reflex, what is the stimulus
increasing muscle length, which activates a sensory neuron which triggers the response
In the stretch reflex, what is the response?
Contraction of the stretched muscle, which counteracts the stimulus
In the stretch reflex, action potentials traveling towards and away from the spinal cord are conducted along
large myelinated Type A Fibers
