chapter 47 Flashcards
Types of sensory receptors?
There are 5 main types:
1. Mechanoreceptors – respond to mechanical pressure or stretching, such as touch, vibration, or being pressed/pulled.
2. Thermoreceptors – detect temperature changes (some for cold, some for heat).
3. Nociceptors – sense pain from physical (e.g., cuts, strong pressure) or chemical damage (e.g., acid).
4. Electromagnetic receptors – located in the retina, respond to light, and allow vision.
5. Chemoreceptors – detect chemical changes like taste, smell, blood oxygen levels, osmolality, and CO2 levels.
Why do different receptors detect different things?
Each receptor is sensitive to a specific stimulus and ignores others. For example: rods and cones detect light, not heat or pressure; osmoreceptors respond to changes in fluid concentration, not sound; nociceptors respond to damage, not normal touch. This is called differential sensitivity—each receptor is built for its own signal type.
What is a modality of sensation?
A modality of sensation is a specific kind of feeling, like touch, pain, sound, or vision. Even though all nerves send the same electrical signal, the sensation depends on which brain area receives the signal.
What is the labeled line principle?
The labeled line principle states that the sensation type depends on where the signal ends in the brain, not how it was triggered. For example, activating a pain nerve always feels like pain to the brain, no matter how it’s activated (pressure, heat, electricity).
How do receptors turn stimuli into signals?
Receptors generate a receptor potential by changing their electrical state in response to a stimulus.
Mechanisms include:
1. Mechanical deformation opening ion channels.
2. Chemicals binding and opening channels.
3. Temperature changes altering membrane permeability.
4. Light altering membrane properties (especially in the eye). All cause ion flow, changing the receptor’s membrane potential.
What is a receptor potential?
A receptor potential is an electrical change in a receptor’s membrane due to ion movement triggered by a stimulus. It is the first step in converting a stimulus into a nerve signal.
How strong can receptor potentials get?
Receptor potentials can reach up to ~100 mV with strong stimuli, similar in size to a full action potential.
How do receptor potentials lead to action potentials?
If the receptor potential exceeds a threshold, it triggers action potentials in the connected nerve. Stronger stimuli raise the receptor potential more, causing a higher frequency of action potentials. The brain reads stimulus strength by counting these action potentials.
What is the Pacinian corpuscle?
It is a mechanoreceptor that responds to pressure. It has a central unmyelinated nerve tip inside onion-like connective layers. Outside the capsule, the fiber becomes myelinated. It converts mechanical pressure into electrical signals.
How does the Pacinian corpuscle create a receptor potential?
When pressure is applied, the connective layers compress the nerve tip, opening ion channels. Sodium ions flow in, creating a receptor potential. This causes local currents that travel to the first node of Ranvier and trigger action potentials.
What happens when stimulus strength increases?
As stimulus (e.g., pressure) increases, the receptor potential increases, causing more action potentials. However, this increase plateaus at a point—stronger stimuli eventually stop producing more signals. This prevents overstimulation while still allowing light-touch sensitivity.
Is this stimulus-to-signal behavior true for all receptors?
Yes. Most receptors in the body follow this pattern: stronger stimulus → stronger receptor potential → higher action potential frequency, until a saturation point is reached.
What is adaptation in sensory receptors?
Adaptation is the process by which sensory receptors reduce their response over time when a stimulus remains constant. They start by responding strongly, then slow down or stop entirely.
Which receptors adapt quickly?
Rapidly adapting receptors include Pacinian corpuscles (stop in <1 sec), hair receptors (stop in ~1 sec), and some joint and muscle receptors (take longer but still adapt).
Which receptors adapt slowly or not at all?
Slow or non-adapting receptors include baroreceptors (blood pressure sensors), pain receptors, and chemoreceptors (e.g., those detecting oxygen or carbon dioxide). Baroreceptors may never fully adapt.
How does the Pacinian corpuscle physically adapt?
The Pacinian corpuscle is viscoelastic. When pressure is applied, its layers move and transmit the force to the nerve fiber, creating a receptor potential. Then, fluid within the layers redistributes the force, reducing pressure on the nerve tip and stopping the response—even if pressure remains.
How does the Pacinian corpuscle electrically adapt?
Over time, the nerve fiber inside the Pacinian corpuscle undergoes accommodation. Sodium channels start to close with prolonged stimulation, preventing new signals from being generated. This is an internal electrical mechanism of adaptation.
What are the two ways the Pacinian corpuscle adapts?
- Physical adaptation through viscoelastic spread of pressure in connective layers. 2. Electrical adaptation via accommodation where sodium channels stop working due to prolonged stimulation.
What are tonic receptors?
Tonic receptors are slowly adapting receptors that keep firing as long as the stimulus is present. They are important for continuous monitoring of the body’s state and are used in sensing muscle tension, body position, blood pressure, and chemical levels.
Examples of tonic receptors?
Examples of tonic (slowly adapting) receptors include: muscle spindles, Golgi tendon organs, pain receptors, baroreceptors, and chemoreceptors (oxygen/CO₂ detectors).
What are phasic receptors?
Phasic receptors are rapidly adapting receptors that respond only to changes in stimulus. They fire when a stimulus starts or ends, and then stop. They are specialized for detecting movement, vibration, or changes.
Other names for phasic receptors?
Phasic receptors are also called: rate receptors, movement receptors, or rapidly adapting receptors.
Why are phasic receptors useful?
Phasic receptors are great for detecting changes or motion. For example, Pacinian corpuscles respond when pressure starts or stops but ignore constant pressure. They help detect quick changes in the environment.
Why are phasic receptors important for prediction?
Phasic receptors, like those in the vestibular system and joints, detect movement speed. This helps the brain predict body position during activities like running, allowing coordinated muscle action and balance.
How does the vestibular system help with prediction?
The vestibular system detects head movement speed, which helps the brain predict future position and adjust muscle movement accordingly—critical for activities like turning while running.
Why do some nerve signals need to travel faster than others?
Some signals (like those for movement and balance) must travel quickly to allow immediate reactions and coordination—e.g., knowing where your legs are while running. Others, like dull pain, are less urgent and can travel slower.
How does the diameter of a nerve fiber affect signal speed?
Larger diameter fibers conduct signals faster than smaller ones. Speed ranges from 0.5 m/s (smallest fibers) to 120 m/s (largest).
What is the range of diameters for nerve fibers?
Nerve fiber diameters range from 0.5 to 20 micrometers.
What is the range of conduction speeds for nerve fibers?
Nerve conduction speeds range from about 0.5 m/s to 120 m/s.
How long would a fast signal take to travel a football field?
A fast nerve signal (~120 m/s) could cross a football field (~100 meters) in less than 1 second.
How long could a slow signal take from toe to spine?
A slow signal (0.5 m/s) from the toe to the spine (about 1 meter) could take approximately 2 seconds.
What are Type A nerve fibers?
Type A fibers are myelinated and fast. They’re further divided by size and speed into: Aα (fastest), Aβ, Aγ, and Aδ (slowest among A).
What are Type C nerve fibers?
Type C fibers are unmyelinated, very small, and slow. They make up most sensory fibers and all postganglionic autonomic fibers.
Which fiber types carry autonomic signals?
Type C fibers carry postganglionic autonomic signals (e.g., for heart rate, digestion, etc.).
What are the subtypes of Type A fibers (from fastest to slowest)?
Aα > Aβ > Aγ > Aδ.
What is the sensory physiologist classification of fibers?
Instead of Aα, Aβ, etc., sensory physiologists use Group I–IV to describe fibers, focusing on origin and function.
What are Group Ia fibers?
Group Ia fibers: from muscle spindles (annulospiral endings), ~17 μm thick, very fast, same as Aα; detect muscle stretch and help with proprioception.
What are Group Ib fibers?
Group Ib fibers: from Golgi tendon organs, very fast like Ia, same as Aα; detect muscle tension.
What are Group II fibers?
Group II fibers: from skin touch receptors and muscle spindle flower-spray endings, ~8 μm, same as Aβ/Aγ; transmit fine touch and proprioception.
What are Group III fibers?
Group III fibers: ~3 μm, small and myelinated, same as Aδ; carry cold, sharp pain, and crude touch.
What are Group IV fibers?
Group IV fibers: unmyelinated, 0.5–2 μm, same as Type C; carry dull pain, heat, itch, and crude touch.
How does the nervous system increase signal strength?
Two main ways: Spatial Summation (recruiting more fibers) and Temporal Summation (increasing firing rate in the same fiber).
What is spatial summation?
Spatial summation = more fibers are activated as stimulus strength increases. A stronger stimulus activates more nearby receptors in the same region.
What is a receptor field?
A receptor field is the area of skin (or organ) that one sensory neuron and its branches cover. More endings are in the center than at the edges.
What is temporal summation?
Temporal summation = the same fiber fires more frequently over time. Closely timed impulses add up, creating a stronger signal (like clapping faster).
What is a neuronal pool?
A neuronal pool is a group of connected neurons that work together. They vary in size—some are small, and others like the cerebral cortex are huge. Each pool processes signals uniquely but also follows common rules.
What happens when a nerve fiber enters a neuronal pool?
It branches multiple times and connects with many neurons. The area it connects to is its stimulatory field. Neurons closest to the incoming fiber get more connections and stronger input; those farther away get fewer connections and weaker input.
What is excitation in neuronal pools?
Excitation happens when a neuron receives enough input to reach its threshold and fire. This is called a suprathreshold stimulus.
What is facilitation in neuronal pools?
Facilitation occurs when a neuron receives subthreshold input—it’s not enough to make it fire but makes it more ready to fire if additional input comes.
What is the discharge zone?
The discharge zone (also called excited zone or liminal zone) is the center area in a neuronal pool where the input is strong enough to make neurons fire.
What is the facilitated zone?
The facilitated zone (also known as the subliminal zone) is the outer area in a neuronal pool where neurons are almost excited but do not reach threshold.
What is the inhibitory zone?
The inhibitory zone is the area where input fibers reduce the chance of neurons firing. It’s strongest in the center and weaker at the edges.
What is divergence in signal processing?
Divergence is when one signal spreads out to affect many neurons. There are two types: 1) Amplifying divergence—one signal activates more and more neurons, like a motor cortex neuron controlling thousands of muscle fibers. 2) Divergence into multiple tracts—one signal splits and travels to multiple places, e.g., spinal cord signal going to both the cerebellum and cerebral cortex.
What is convergence in signal processing?
Convergence is when many inputs come together to influence one neuron. There are two types: 1) From one source—one fiber branches to connect multiple times to a single neuron. 2) From multiple sources—signals from different areas (spinal cord, brain) converge on a single neuron. This allows signal summation to reach threshold.
Why are divergence and convergence important?
They allow the nervous system to: 1) adjust message strength, 2) combine information from different areas, 3) sharpen or block signals, 4) amplify weak signals, 5) send one signal to multiple areas, and 6) coordinate complex actions like movement, pain, and touch.
What is a reciprocal inhibition circuit?
It’s a type of neuronal circuit that sends both excitatory and inhibitory signals. For example, during leg movement, excitatory signals go to front leg muscles, while inhibitory signals prevent back leg muscles from resisting. The inhibitory signal travels via an intermediate inhibitory neuron activated by the same input.
What do reciprocal inhibition circuits help with?
They allow smooth, controlled movement, especially when antagonistic muscles (like biceps and triceps) are involved. They also help prevent overactivity in the brain.
What is afterdischarge?
Afterdischarge is when a signal continues after the original stimulus is gone. It can occur due to synaptic afterdischarge or reverberatory circuits.
What is synaptic afterdischarge?
It occurs when a neurotransmitter causes a prolonged postsynaptic potential. If the neurotransmitter acts slowly, the signal can last for milliseconds even after the stimulus ends.
What is a reverberatory (oscillatory) circuit?
It’s a feedback loop where neurons send signals forward and also back to themselves or others in the same circuit, allowing the signal to continue after the initial stimulus.
What is a simple reverberatory loop?
It’s a basic loop where a neuron sends a branch back to itself. It’s rare but can prolong the signal briefly.
What is a delayed reverberatory loop?
The signal travels through several neurons before looping back. This delay lets the signal last longer as it continues around the loop.
What is a controlled reverberatory loop?
It includes facilitatory fibers (which enhance the loop) and inhibitory fibers (which stop it). This allows control over the signal’s duration and strength.
What is a wide reverberatory loop?
Most real reverberatory circuits are wide, involving many neurons and parallel fibers. They spread widely and connect at various points, with signal strength depending on the number of active fibers.
Why do reverberatory signals eventually stop?
Due to synaptic fatigue. Neurons get tired after prolonged firing, reducing synaptic effectiveness. Feedback becomes too weak to sustain the loop, and it eventually stops.
What influences how long a reverberatory signal lasts?
Other brain regions can lengthen or shorten reverberation. This controls timed outputs, like maintaining muscle contraction or attention after a stimulus.
What is intrinsic neuronal excitability?
Intrinsic neuronal excitability refers to the built-in ability of some neurons (e.g., in the cerebellum and spinal cord) to continuously fire signals due to a naturally high internal electrical charge. These neurons don’t need external input to fire but can be modulated—sped up by excitatory signals or slowed/stopped by inhibitory signals.
How do reverberating circuits create continuous output?
Reverberating circuits create continuous output through feedback loops where signals keep bouncing around and returning to the starting point. Even without continuous input, the loop sustains signal output. Excitatory input increases output; inhibitory input reduces or stops it. They act like radio transmitters, regulating signal strength rather than creating it.
What body functions are controlled by continuous neuronal output?
Continuous neuronal output helps control vital functions like blood vessel tone, gut tone, heart rate, and pupil size. These outputs are modulated by excitatory and inhibitory signals, allowing fine control over body functions.
What generates rhythmical signal output in the nervous system?
Rhythmical signal output is generated by reverberating circuits in the brainstem (e.g., medulla and pons), controlling breathing and rhythmic movements like walking or scratching. These circuits send repeated signals in loops and can be modulated in amplitude and frequency by external signals like oxygen levels.
What causes instability in neuronal circuits?
Instability occurs when interconnected neurons keep exciting each other in a loop, potentially causing uncontrolled, non-informative signals like those seen in epileptic seizures. The brain prevents this using inhibitory circuits and synaptic fatigue.
How do inhibitory circuits maintain stability in the brain?
Inhibitory circuits prevent overexcitation by sending signals that suppress activity. There are two types: (1) Inhibitory feedback circuits that calm overactive neurons, common in sensory pathways. (2) Large inhibitory circuits, like those in the basal ganglia, that send widespread inhibition to control movement and prevent overactivity.
What is synaptic fatigue and how does it help?
Synaptic fatigue is when neurons fire too much and their synapses weaken, reducing the signal strength. It prevents continuous, uncontrolled activity by weakening overly active pathways, thus maintaining balance in neuronal function.
What is automatic short-term adjustment in neurons?
Automatic short-term adjustment refers to the process where neurons adjust their sensitivity based on usage: fatigue reduces sensitivity in overused neurons, while underused neurons become more sensitive. This helps maintain neural balance and proper function.
How do neurons make long-term adjustments in sensitivity?
Neurons make long-term sensitivity adjustments by changing the number of receptors at their synapses. Underused circuits add more receptors to increase sensitivity, while overused circuits reduce receptors to decrease sensitivity, allowing self-regulation.
