BIOL1019 Chap.2 Receptors and Nervous System Pathways
Receptors and Nervous System Pathways
Sensation begins with receptors that convert a stimulus into action potentials, and the properties of those receptors decide what reaches the brain: their sensitivity, the size of their receptive field, the labelled line they feed and how quickly they adapt.
This chapter follows conscious sensation along three-neuron pathways in which the second-order axon crosses the midline and the thalamus filters the signal on its way to the cortex.
It compares the spinothalamic, posterior column and spinocerebellar routes, explains fast and slow pain and the analgesic pathway, introduces the corticospinal motor pathway, and finishes with the meninges, cerebrospinal fluid and blood-brain barrier that protect the CNS.
A full pathway answer earns its marks by naming each neuron in order, saying where the signal crosses and where it ends, so the chapter practises exactly that sequence, along with the receptor properties that explain why some sensations fade while pain persists.
What this chapter covers
- 01
Receptor classes by stimulus
- 02
Receptive fields and two-point discrimination
- 03
Tonic and phasic adaptation
- 04
Three-neuron somatosensory pathways
- 05
Spinothalamic, posterior column and spinocerebellar routes
- 06
Substance P and the analgesic pathway
- 07
Meninges, CSF and the blood-brain barrier
- 08
Primary and association areas of the cortex
Worked example · free
Which pathway carries which sensation
- +1Vibration is precise touch information carried by the posterior column pathway to the thalamus and cortex.
- +1Heat and pain from the cup travel in the lateral spinothalamic tract of the spinothalamic pathway.
- +1Conscious leg position uses the posterior column pathway, while the spinocerebellar pathway carries position to the cerebellum without awareness.
Key terms
- Receptive field
- The area within which a single receptor can detect a stimulus.
- Labelled line
- A dedicated route that carries one sensory modality from its receptor to a specific cortical region.
- Phasic receptor
- A receptor that adapts rapidly when a stimulus continues, such as a thermoreceptor.
- Blood-brain barrier
- Tight junctions between CNS capillary cells that restrict what passes from blood into brain tissue.
- Nociceptor
- A receptor that responds to tissue-damaging stimuli and starts the pain pathway.
Receptors and Nervous System Pathways FAQ
Why do we stop noticing a smell after a few minutes?
Smell fades through central adaptation: sensory nuclei along the pathway inside the CNS are inhibited, so less information reaches awareness even though the receptors are still being stimulated. This differs from peripheral adaptation, where the receptor itself becomes less sensitive, as phasic thermoreceptors do when you enter a cool room.
What is the difference between fast and slow pain?
The lectures attribute fast pain to A-delta fibres and slow pain to C fibres. In the substance P pathway, pain fibres activate excitatory interneurons in the dorsal horn, which relay to the thalamus and cortex for perception, to the reticular formation for alertness, and to the hypothalamus and limbic system for the emotional response.
What does cerebrospinal fluid actually do?
Cerebrospinal fluid is produced at the choroid plexuses and surrounds every exposed surface of the brain and spinal cord. It cushions and supports the brain, exchanges with interstitial fluid, and transports nutrients, chemical messengers and wastes, which makes it part of the physical protection of the CNS along with bone and the meninges.
How does the corticospinal pathway control movement?
The corticospinal pathway carries commands for skeletal muscle and uses at least two motor neurons, an upper motor neuron in the cortex and a lower motor neuron that reaches the muscle. Its anterior and lateral tracts cross at the decussation of the pyramids, so each side of the motor cortex controls muscles on the opposite side of the body.
Exam move
Build a three-row table for the ascending pathways with columns for sensation carried, tracts, crossing and destination, and test yourself by covering one column. Practise writing a full somatosensory pathway answer in under five minutes, naming the first-, second- and third-order neurons in order. Then list the meninges from outside in and attach one function of CSF to each line you write.
Add the pain pathway to the same table, with A-delta and C fibres and the three destinations of the dorsal horn signal, and rehearse the corticospinal pathway as a two-neuron chain that crosses at the pyramids.
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