Movement, Cycles And Rhythms

1. Movement, Cycles, and Rhythms

The control of movement is often divided into mechanisms of reflexes and of voluntary movement.

What is a reflex? It is a stereotyped, automatic movement evoked by a specific stimulus. It is uniform across members of a species. Some examples in humans are the patellar or knee-jerk reflex, the salivary reflex, the orienting reflex, and the pupillary reflex.

In reviewing the reflex arc, pay special attention to proprioception, muscle spindles, the stretch reflex, and primary motor cortex. (Prof. Suzuki also discusses the basal ganglia and the cerebellum, but in another lecture that I have not assigned.)

Further, let’s think about the cycles in our behavior. They are ancient and varied. We certainly inherited them from species that evolved earlier than we did.

We harbor a number of rhythms. Generally, we have cellular cycles that augment the day-night rhythm to regulate our behavioral cycles. There are possibly very long cycles that govern our evolution. Men and women both run on cycles, though with differences. But do our cycles still make biological sense?

Questions (answer one only)

1) Can you point to any movements that do not involve reflexes–or some reflexes that do not involve movement?

2) Sleep is tied to the recovery of normal function and memory formation. Why does it make sense, then, to stop for sleep just because the sun sets? Why not sleep just whenever we have a backlog of memories to form or we’re under stress or sick? Does the wide variation in our daily experience require such an extreme regularity of sleep regulation? Since we don’t learn or exercise the same amount each day, why do we nevertheless sleep the same amount? A number of possible answers are given in the first 10 minutes of this fascinating podcast but offer your own insights as well.

3) Furthermore, women show more pronounced cycles in sex hormones than men do, including menstrual cycles and a rather imprecise biological clock with a different origin. Is this an evolutionary leftover, no longer necessary or helpful for humans? Would you want the equivalent of a birth control pill for all of our cycles?

2. Visual System

It’s easy to get swamped in detail about the visual system. (This animation may help.)

Videos 10 and 11 discuss the visual pathways that originate in the retina and “ascend” to the visual cortex via the thalamus.

In this week’s resources the retina is revealed as more than a screen for images. Light falls on different classes of photoreceptors, rods and cones, which organize light stimulation according to their pigments by intensity and wavelength. From the duplex nature of the retina there arises a tradeoff between greater acuity in the center and greater sensitivity in the periphery. The output of the retina is further organized into receptive fields, and sideways connections between the photoreceptors and horizontal cells sustain lateral inhibition that sharpens contrasts. A retinotopic map is preserved by the visual pathways all the way to the visual cortex; on the other hand, the image still requires a lot of interpretation by the brain.

Yet when we look at the world we notice only the rich panorama of detail that relayed to the brain via the fovea.

Questions (answer one only)

1) How is the retina different from a movie projection screen? Try to be specific.

2) How on earth did we end up with eyes that are mostly peripheral retina and

only tiny foveas? Should a sensible retina have more foveas—or larger ones, perhaps? Or might peripheral retina be more than leftover real estate?

3. Auditory System 

Choose one part of the human ear (or ears in other animals or the auditory pathways) and describe its contribution to hearing.

Try not to duplicate what another class member has chosen. You have many choices:

pinna

ear canal

eardrum (tympanic membrane)

ossicles (malleus, incus, stapes)

middle ear muscles (tensor tympani, stapedius)

oval window

round window

basilar membrane

tectorial membrane

auditory nerve

Eustachian tube

inner hair cells

outer hair cells

differences in animal ears (whales, birds, bats, desert animals)

earwax (cerumen)

Hensen’s stripe (obscure)

cochlear nuclei

inferior colliculi

medial geniculate nucleus

primary auditory cortex