Homeostasis, Feedback Loops, and Nervous System Physiology

Homeostasis

The process by which the body maintains a stable internal environment despite external changes.

Feedback loops

A process in which the response influences the original stimulus.

Stimulus

A change in a regulated condition.

Receptor

Detects the change and sends information to the control center.

Control Center

Processes the information received from the receptor and determines the appropriate response.

Effector

Carries out the response to restore homeostasis (e.g., muscles, glands, organs).

Result

The resulting change in the regulated condition.

Negative Feedback Loops

The response counteracts the original change, helping maintain homeostasis.

Purpose of Negative Feedback

The response opposes the original change to stabilize conditions.

Example of Negative Feedback

Body temperature regulation, blood sugar regulation, blood pressure control.

Blood Sugar Regulation - Stimulus

Blood sugar rises after eating.

Blood Sugar Regulation - Receptor

Pancreas detects high blood sugar levels.

Blood Sugar Regulation - Control Center

Pancreas releases insulin into the bloodstream.

Blood Sugar Regulation - Effector

Insulin signals cells to absorb glucose, lowering blood sugar levels.

Blood Sugar Regulation - Result

Once blood sugar returns to normal, insulin release stops.

Positive Feedback Loops

The response amplifies the original change until a specific endpoint is reached.

Purpose of Positive Feedback

To intensify a process until a specific event is completed.

Example of Positive Feedback

Blood clotting, childbirth, lactation.

Childbirth - Stimulus

Baby pushes against the cervix.

Childbirth - Receptor

Stretch receptors in the cervix send signals to the brain.

Childbirth - Control Center

Brain releases oxytocin from the pituitary gland.

Childbirth - Effector

Uterus contracts more forcefully.

Childbirth - Result

Contractions continue to intensify until the baby is born, at which point the loop stops.

Diffusion

The net movement of particles from an area of higher concentration to an area of lower concentration.

Osmosis

The net movement of water across a selectively permeable membrane toward the side with a higher concentration of nonpenetrating solutes.

Selective permeability

A property of a membrane that allows some substances to cross more readily than others.

Tonicity

The ability of a solution to change a cell's volume by causing water to enter or leave the cell.

Hypertonic solution

Has a higher concentration of nonpenetrating solutes than the cell.

Hypotonic solution

Has a lower concentration of nonpenetrating solutes than the cell.

Isotonic solution

Has an equal concentration of nonpenetrating solutes compared with the cell.

Electromyography (EMG)

The recording of skeletal muscle electrical activity.

Motor unit

One motor neuron and all the muscle fibers it innervates.

Motor unit recruitment

The activation of additional motor units to increase muscle force.

Motor unit rotation

During some sustained contractions, motor units alternate activity, allowing some muscle fibers to rest while others maintain force.

Muscle fatigue

A reduced ability to generate or maintain muscle force during sustained or repeated activity.

Muscle twitch

A brief contraction and relaxation of a muscle fiber in response to a single action potential.

Grip Strength Activity

Stronger squeezes generally produced greater EMG signal amplitude.

Muscle Fatigue Activity

Muscle fatigue is a decreased ability to maintain muscle force during continued activity.

Fatigue

Occurs as repeated activity changes energy availability and the muscle's ability to activate and contract effectively.

Electroencephalography

The recording of brain electrical activity using electrodes placed on the scalp.

Electroencephalogram (EEG)

The resulting recording from electroencephalography.

Amplitude

The height of an EEG wave, representing the voltage of the recorded signal.

Frequency

The number of wave cycles per second.

Alpha waves

8-13 Hz. Common during relaxed wakefulness with eyes closed, especially over the occipital and parietal regions.

Beta waves

14-30 Hz for this lab. Common during alertness, attention, and mental activity.

Alpha block

A decrease in alpha activity when the eyes open or during mental activity, such as calculations.

Amplitude and frequency relationship

Slower rhythms often have higher amplitudes, while faster rhythms often have lower amplitudes.

Expected lab findings

Relaxing with eyes closed generally increases alpha activity.

Membrane potential

The difference in electrical charge across a cell membrane, measured in millivolts (mV).

Resting membrane potential

The membrane voltage of a neuron at rest, typically approximately −70 mV.

Action potential

A rapid change in membrane potential that travels along a neuron's axon.

Threshold potential

The membrane voltage that must be reached to trigger an action potential, approximately −55 mV.

All or nothing principle

If threshold is reached, a full action potential occurs; if not, no action potential occurs.

Depolarization

If threshold potential is reached, the membrane potential becomes less negative.

Repolarization

The membrane potential returns toward its resting negative value as K⁺ leaves the neuron.

Hyperpolarization

The membrane potential becomes more negative than the resting membrane potential.

Refractory period

A period following an action potential when another action potential is either impossible or requires a stronger stimulus.

Excitatory postsynaptic potential (EPSP)

A graded change that moves the membrane potential closer to threshold, making an action potential more likely.

Inhibitory postsynaptic potential (IPSP)

A graded change that makes an action potential less likely, often by hyperpolarizing the membrane.

EEG

A tool that helps evaluate conditions such as epilepsy and sleep disorders.

Sodium-potassium pump

Maintains ion concentration differences, allowing potassium to leave more readily than sodium can enter.

Sodium (Na⁺)

Ion that enters during the rising phase of an action potential, making the membrane potential more positive.

Potassium (K⁺)

Ion that leaves during repolarization, making the membrane potential more negative.

EPSPs

Excitatory postsynaptic potentials that move the neuron closer to threshold, making an action potential more likely.

IPSPs

Inhibitory postsynaptic potentials that make an action potential less likely, often by hyperpolarizing the membrane.

Reflex

A predictable, involuntary response to a stimulus.

Reflex arc

The neural pathway that produces a reflex, consisting of five components: receptor, sensory neuron, integration center, motor neuron, and effector.

Babinski sign

Extension of the great toe upward when the sole of the foot is stroked; normal in infants but may indicate damage in adults.

Cutaneous receptors

Sensory receptors in the skin that detect stimuli such as touch, pressure, vibration, temperature, and pain.

Receptive field

The area of skin in which stimulation changes the activity of a particular sensory neuron.

Two point threshold

The minimum distance between two points of contact at which they are perceived as two separate touches.

Emmetropia

Normal vision in which light focuses on the retina when accommodation is relaxed.

Myopia

Nearsightedness; light focuses in front of the retina, causing distant objects to appear blurry.

Hyperopia

Farsightedness; without accommodation, light would focus behind the retina, making near viewing difficult.

Astigmatism

Uneven curvature of the cornea or lens causes light to focus at different positions for different orientations.

Visual acuity

The ability to distinguish fine visual detail.

Snellen test

Measures distance visual acuity using letters of different sizes.

Ophthalmoscope

An instrument used to examine the inside of the eye, including the retina and optic disc.

Air conduction

Sound travels through the external and middle ear to reach the inner ear.

Bone conduction

Vibrations travel through the skull directly to the inner ear, bypassing the external and middle ear.

Rinne test

Uses a tuning fork to compare air conduction and bone conduction in one ear.

Weber test

Uses a tuning fork placed at the midline of the head to compare sound perception between the two ears.