- Phasic Receptors: These are fast-adapting. They fire a burst of activity when a stimulus is first applied but quickly decrease their firing rate if the stimulus remains constant. This allows for sensory adaptation, such as when you stop noticing a background smell or the feeling of clothes against your skin.
- Tonic Receptors: These are slow-adapting. They maintain a high, continuous firing rate as long as the stimulus is present. This ensures the brain remains aware of persistent, often vital, conditions like pain.
Exam Tip: If a question asks which receptor type is responsible for "detecting change" versus "monitoring status," remember: Phasic = Change; Tonic = Status.
Sensory Pathways: Hearing and Vision
The Auditory Pathway
Hearing involves a complex mechanical-to-electrical transduction process. The journey of a sound wave begins in the outer ear, where the pinna (auricle) collects sound waves and directs them through the external auditory meatus to strike the tympanic membrane (eardrum).
The vibrations from the tympanic membrane are then transferred through the middle ear ossicles: the malleus (hammer), the incus (anvil), and the stapes (stirrup). The stapes pushes against the oval window, creating fluid pressure waves in the cochlea. These waves move the hair cells in the Organ of Corti, causing their stereocilia to bend and open potassium channels, which ultimately generates a neural impulse via the vestibulocochlear nerve.
The Visual Pathway and Photoreception
Vision relies on the retina's ability to convert light into electrical signals. There are two primary types of photoreceptors:
- Rods: These facilitate scotopic vision (low-light/black-and-white). They are highly sensitive due to high spatial summation (many rods converging on one ganglion cell), but they provide low visual resolution.
- Cones: These facilitate photopic vision (bright light/color). Concentrated in the fovea, they provide high-acuity color vision because they have a 1:1 ratio with ganglion cells.
The Bleaching Mechanism
The conversion of light to a nerve impulse involves a process called photoreceptor bleaching. In the dark, the pigment rhodopsin consists of an opsin protein bound to a bent molecule called 11-cis-retinal. When light strikes the retina:
- The 11-cis-retinal straightens into trans-retinal.
- The trans-retinal separates from the opsin, causing the pigment to change color and appear "bleached".
- This dissociation triggers the change in membrane potential that sends a signal to the brain.
Endocrine System Fundamentals
While the nervous system uses neurotransmitters for rapid, localized communication, the endocrine system uses hormones—chemical messengers secreted into the bloodstream to influence distant target cells. This results in effects that are slower to start but much longer-lasting (seconds to weeks) and more widespread.
Hormone Operation and Target Cell Receptors
A hormone cannot affect every cell it encounters. Its action is governed by two fundamental principles:
- Specificity: A hormone will only affect a cell if that cell expresses a specific receptor designed to bind to that hormone.
- Saturation: There is a limit to how much a hormone can do. Once every available receptor on a target cell is occupied, the physiological response reaches its maximum, a state known as saturation.
Target cell receptors can be located in different parts of the cell depending on the hormone's chemical nature:
- Cell membrane: For water-soluble hormones.
- Cytoplasm or Nucleus: For lipid-soluble hormones that can diffuse through the membrane.
Comparison Summary: Nervous vs. Endocrine
| Feature | Nervous System | Endocrine System |
|---|
| Messenger | Neurotransmitters | Hormones |
| Speed of Response | Milliseconds | Seconds to Days |
| Duration of Effect | Short-lived | Long-lasting |
| Scope of Effect | Localized | Widespread |
Feedback Mechanisms
To maintain homeostasis, the endocrine system relies heavily on negative feedback. In this mechanism, the rising level of a hormone (or its effect) circulates back to the hypothalamus or pituitary gland to inhibit further secretion, keeping levels within a narrow range.
Conversely, positive feedback amplifies a change to push a process to completion, such as the release of oxytocin during childbirth, where uterine contractions trigger more oxytocin, leading to even stronger contractions.
