Neurobiology and Behavior

Stimulus, response and reflex

Reaction time link above

external image image005.jpg

Nerve and perceptions

The Eye and Retina

When a doctor looks at the back of your eye through an ophthalmoscope, here's the view:

How are Neurons Classified?
Neurons come in a large variety of shapes, sizes and forms. They are often classified according to the number of processes that extend from the cell body. Because these processes essentially "pull" on the cell itself, cell bodies of these different types of axons also exhibit different shapes.
Bipolar cells are rare- these cells consist of a single axon and a single dendrite. These cells are found in the retina of the eye and in several of the cranial nerves.

The rods and cones are photoreceptors; these cells contain pigments and are sensitive to light, in response to which action potentials are generated and transmitted to other retinal cells – bipolar cells, then ganglionic cells leading to the optic nerve.

Cones contain 3 types of photopigments, called rhodopsins, which are sensitive to different wavelengths of light; cones are involved in colour vision. On the other hand, rods contain only one type of photopigment and are therefore only used for night vision. A rod is sensitive enough to respond to a single photon of light.
The 3 photopigments present in cones confer trichromatic vision. The wavelengths of visible light to which the 3 types of cones have peak sensitivity are 440, 544 and 580 nanometres, corresponding to blue, green and red light respectively. The combined activation of the 3 types of cones to differing extents enables perception of all the colors in the visible light spectru

Intensity of photoreceptor response to light wavelengths by rods, "red" cones, "green" cones, and "blue" cones.

Rods and Cones

Why are there two types of photoreceptor cell? The rods and cones serve two different functions as shown in this table:
  • Outer segment is rod shaped
· Outer segment is cone shaped
· 109 cells per eye, distributed throughout the retina, so used for peripheral vision.
· 106 cells per eye, found mainly in the fovea, so can only detect images in centre of retina.
· Good sensitivity – can detect a single photon of light, so are used for night vision.
· Poor sensitivity – need bright light, so only work in the day.
· Only 1 type, so only monochromatic vision.
· 3 types (red green and blue), so are responsible for colour vision.
· Many rods usually connected to one bipolar cell, so poor acuity (i.e. rods are not good at resolving fine detail).
· Each cone usually connected to one bipolar cell, so good acuity (i.e. cones are used for resolving fine detail such as reading).

Vitamin A and carrots myth link
Quick summary of action potential and photoreceptors

Light hits retinal photoreceptor cells stimulating and action potential to the bipolar cells and then to the ganglion cells, which converge to form the form the optic nerve.

Retinal Bipolar Cells
Retina. Bipolar cells are shown in red.
Convey gradients between photoreceptor cells to retinal ganglion cells

Presynaptic connections
Rods , cones and Horizontal Cells
Postsynaptic connections
Retinal ganglion cells

Very good review link


Rod vision is acute but coarse.

Rods do not provide a sharp image for several reasons.
  • several nearby rods often share a single circuit to one ganglion cell
  • a single rod can send signals to several different ganglion cells.
So if only a single rod is stimulated, the brain has no way of determining exactly where on the retina it was.
However, rods are extremely sensitive to light. A single photon (the minimum unit of light) absorbed by a small cluster of adjacent rods is sufficient to send a signal to the brain. So although rods provide us with a relatively grainy, colorless image, they permit us to detect light that is over a billion times dimmer than what we see on a bright sunny day.


Although cones operate only in relatively bright light, they provide us with our sharpest images and enable us to see colors. Most of the 3 million cones in each retina are confined to a small region just opposite the lens called the fovea. So our sharpest and colorful images are limited to a small area of view. Because we can quickly direct our eyes to anything in view that interests us, we tend not to be aware of just how poor our peripheral vision is.
The three types of cones provide us the basis of color vision. Cones are "tuned" to different portions of the visible spectrum
· red absorbing cones; those that absorb best at the relatively long wavelengths peaking at 565 nm
· green absorbing cones with a peak absorption at 535 nm
· blue absorbing cones with a peak absorption at 440 nm.
Retinal is the prosthetic group for each pigment. Differences in the amino acid sequence of their opsins accounts for the differences in absorption.

The response of cones is not all-or-none. Light of a given wavelength (color), say 500 nm (green), stimulates all three types of cones, but the green-absorbing cones will be stimulated most strongly. Like rods, the absorption of light does not trigger action potentials but modulates the membrane potential of the cones.


Color Blindnes s

The term color blindness is something of a misnomer. Very few (~1 in 105) people cannot distinguish colors at all. Most "color-blind" people actually have abnormal color vision such as confusing the red and green of traffic lights. As high as 8% of the males in some populations have an inherited defect in their ability to discriminate reds and greens. These defects are found almost exclusively in males because the genes that encode the red-absorbing and green-absorbing opsins are on the X chromosome. [Discussion of X-linkage]
The X chromosome normally carries a cluster of from 2 to 9 opsin genes. The minimum basis for normal red-green vision is one gene that absorbs efficiently in the red and one that absorbs well in the green (chromosome 1 in the figure). Multiple copies of these genes are also fine (2 and 3). Males with either a "green gene" or "red gene" missing are severely color blind (4 and 5). However, if all the red genes carry mutations (this seldom seems to be the case for the green genes — nobody knows why), then they may have red-green color blindness that ranges from mild to severe depending on the particular mutations involved (6). The rule seems to be that the more the mutations shift the pigment towards green, the more serious the defect. However, a large number of mutations doesn't always produce serious defects. Multiple mutations in a single gene may offset each other producing only mild defects. And as long as one normal copy of each gene is present, the presence of additional mutated genes seldom produce a serious problem

Neuroscience of Touch: Touch and the Brain

How Cells "Feel" Mechanical Tension and Osmotic Stress (May 2001)
MscL channel
MscL channel

image size: 247.9KB
made with VMD
"How do you feel?" Biologists now have an answer that may surprise you. Our sense of touch relies upon the fact that cells in our fingertips can sense the pressure from a tabletop and transmit a signal to the brain. But until recently, the molecular mechanism for turning the stretching of a cell membrane into a cellular signal was unknown. An important step in understanding this process was the discovery of a protein known as a the mechanosensitive channel of large conductance, or MscL. Though this protein has been studied primarily in bacteria, homologues exist in all major kingdoms of life. Researchers in the Theoretical Biophysics Group have used molecular dynamics simulations to study, at the atomic level, how MscL opens in response to pressure changes. Models of MscL will give us new insight, not only into how we feel, but also how our hearts beat and how we keep our balance. Feel better now? ( more, publication )

Have you ever felt the touch of someone’s hand on your shoulder and found yourself letting go of tension you didn’t even know you were holding? We can probably all remember the impact a touch has had on us, whether casually or in a therapeutic setting. As practitioners, we have daily experiences of the effects of touch on our clients. Current research in neuroscience is looking directly at the brain’s response to touch. Lucy Brown, a neuroscientist interested in studying alternative therapies, presented recent findings at a talk at Harvard Medical School. What follows is primarily my summary of her talk with a few comments from the point of view of a practitioner.
Conscious and Unconscious
Touch appears to affect multiple brain regions at conscious and unconscious levels. FMRI scans offer a way to see a response even when the subject does not report having consciously experienced an effect. This is significant since until now we have had to depend on verbal reports to assess the impact of touch. As practitioners we know that perceiving and verbalizing sensation are skills that many patients may not have highly developed, so it is helpful to have an external, objective measure in addition to felt-experience
Functional MRI
Current research tracks the impact of touch on the brain with functional MRI (fMRI). FMRI can detect activity in different brain areas by measuring the increase in blood flow that is correlated with an increase in neuronal activity. While this research is still in its infancy, it holds some promise for elucidating what we intuit from our experience as practitioners and recipients of therapies based on touch. The studies are part of a new trend: to look for integration of response rather than investigating each part/function separately. The fMRI studies show that touch has a wide impact on the brain, influencing our sensations, our movements, our thought processes and our capacity to learn new movements.
external image FMRIscan.jpg

Image Source:

Conscious and Unconscious
Touch appears to affect multiple brain regions at conscious and unconscious levels. FMRI scans offer a way to see a response even when the subject does not report having consciously experienced an effect. This is significant since until now we have had to depend on verbal reports to assess the impact of touch. As practitioners we know that perceiving and verbalizing sensation are skills that many patients may not have highly developed, so it is helpful to have an external, objective measure in addition to felt-experience.

The Homunculus
On the surface of the brain is an area known as the somato-sensory cortex. It is the brain’s map of the body, known familiarly as the “homunculus.” As might be expected, touch increases activity here (Young et al 2004). When someone touches you, receptors in the skin and or in the muscles transmit a signal via the spinal cord and medulla to this area of your brain; this corresponds to an increase in the activity of the neurons in this area. Touch receptors in skin are distinct from those in muscle. There may be measurable differences in brain response to different depths and duration of touch. This is an as yet unexplored area.
Previous research on the sensory cortex shows that it varies somewhat based on individual experiences, that it is plastic (subject to change) and that it is influenced by learning. In other words, this is not a fixed map. Hopefully future studies will begin to shed light on how touch may influence changes in the body map, our “sense of self.”
external image homunculus.jpg

external image brain.jpg


Sensory and Motor
Perhaps a little less anticipated is the responsiveness of motor areas (motor cortex, supplementary motor cortex) to touch. In the past, these two functions, motor and sensory, may not have been so obviously correlated. The fMRI scans reveal an increase in activity in the motor areas as well as the sensory areas of the brain in response to touch.
Most of us take the relationship between our proprioceptive (proprio=self) sense and our ability to move for granted. Jonathan Cole (1995) writes about an unusual case of a man who lost his capacity for proprioceptive sensation and appeared to be paralyzed. This patient has been able to learn to move painstakingly, by depending on his visual system alone. As long as he can see his limbs, he can control their movement.
Once we think about it, it seems straightforward: for most of us in order to move, we depend on our ability to feel ourselves. However, traditional exercise does very little to maintain the sensory system as an aspect of our movement capacity. We think of the muscles as moving independently from our feeling them. Just go to any gym and see how people are mostly tuning out of their sensory experience as they exercise while reading the paper or watching TV. This is in contrast to more ancientAnother study showed that the response to touch in the brain was greater when it was connected to a meaningful task, i.e. when the subject had to pay attention to the sensations; (Porro et al 2004) (Nelson et al 2004) Ida P. Rolf had a famous directive to practitioners: “Put it where it belongs and call for movement,” she would insist. Intuitively, she knew that the client’s active participation had an important impact. While it may not be necessary to touch deeply to have an effect, the patient’s attention/active participation may be a significant aspect to consider in our work.
external image limbic.jpg
Touch and Emotion
Touch also has been shown to have a positive emotional impact. In one study a group of women were told they were to receive a shock. The effect of hand-holding by their husbands and by a technician was measured. In both cases, the effect of the touch was to lessen the threat response that registers in the limbic system, the parts of the brain associated with emotion. (Coan 2006)

Basal Ganglia
The basal ganglia, part of the brain active in learning/establishing new motor patterns, were also affected by touch. Studies show the basal ganglia are very active while you are learning a video game like Tetrus, and keep some activity even after you have mastered the game. The basal ganglia are also implicated in certain motor disorders like Parkinson’s as well as obsessive/compulsive disorder (OCD). Along with the motor and sensory systems, and parts of the limbic system, touch also stimulated this area of the brain.
external image basalganglia-2.jpg

Image Source:

From what can be seen so far, touch impacts many parts of the brain and multiple functions. Our thinking, feeling, sensory and motor systems are all affected by touch as well as parts of the brain involved in learning new movements. Future research is planned that will correlate some of these measured changes with therapeutic effects. Eventually we may be able to use the results of fMRI studies to get insight into the effects of different kinds of touch as well as individual differences in response. Studies such as these will contribute to our understanding of what we do as practitioners as well as help support the value of alternative therapies in the mainstream view.

Cole J (1995) Pride and a Daily Marathon. Cambridge: MIT Press.
Nelson AJ, Staines WR, Graham SJ, McIlroy WE (2004) Activation in SI and SII: the influence of vibrotactile amplitude during passive and task-relevant stimulation. Brain Res Cogn Brain Res 19:174-184.
Porro CA, Lui F, Facchin P, Maieron M, Baraldi P (2004) Percept-related activity in the human somatosensory system: functional magnetic resonance imaging studies. Magn Reson Imaging 22:1539-1548.

© Aline Newton 2008
® The words Rolfing and Rolfer are service marks of the Rolf Institute of Structural Integration

Why We Can Enjoy a van Gogh Painting (Aug 2001)
Spectral Tuning in Sensory Rhodopsin II
Spectral Tuning in Sensory Rhodopsin II

image size: 331.7KB
Perception of light and color permits humans to enjoy art, though the sense evolved more likely to better find ripe fruits. The recognition, centuries ago, that the wondrous sense of color comes from three visual receptors to which is added in the eye a fourth black & white one is one of the major achievements in the history of science. The visual receptors of all animals rely on one molecule for light absorption, retinal. How do the receptors tune then their spectral sensitivity? Exploiting a similarity of visual receptors to proteins in an archaebacterium, Natronobacterium pharaonis, researchers have finally been given an opportunity to answer this question quantitatively. In the bacterium, two structurally almost identical proteins absorb maximally light of 497 nm and 568 nm wavelength. X-ray crystallography and advanced quantum chemical studies could explain the difference and pinpoint to the protein side groups (see figure) that actually tune the spectra.

[E3] Innate and Learned Behavior

Innate behavior is inborn behavior and develops independently of environmental context. For example, a spider spins a web correctly the first time (Damon, p 348). Innate behavior is controlled by genes and inherited from parents. It is usually necessary to survival of the species.

Learned behavior is not genetically programmed (although the ability to learn a particular task may be a genetic input). Learned behavior can be defined as the process of gaining new knowledge or skills or modifying existing knowledge or skills. For example, learning to read at 6 years old and modifying this ability at an older age.
We can measure learning by performance. (Damon pp 348 -349)

KNOW tables on pp349 - 350

INNATE behavior involving MOVEMENT:
Taxis is a movement in a directed response to a stimulus.
A movement toward a stimulus is a positive response, while a movement away from a stimulus is a negative response.
Each response is describe by its stimulus; for example, phototaxis (stimulus is light); chemotaxis, gravitaxis, rheotaxis (water flow) and thigmotaxis (touch). For example, euglena requires light for photosynthesis and is, therefore, positively phototaxis. (Damon p 350)

Kinesis is a movement in response to a nondirectional stimulus.
The amount of movement depends on the intensity of a stimulus, but the direction of the movement is random. Thus, when condtions are favorable the organism tends to stay in this space due to small movement; however, when conditons are not favorable, the organism moves more and tends to move out of that space. For example, Isopods move randomly in dry conditions, since they require moisture (humidity) for gill-breathing, and slow down when they sense humid conditions. (Damon p. 351)

Design experiments and analyze data to show effects of innvertebrate behavior on survival and reproduction - see Damon pp 352 - 356.

LEARNED behavior involving effects on survival, conditioned-type learning and the role of inheritance and learning:

Learning occurs most easily when it improves the chance of an animal's survival. Some special learning ability which improve survival are imprinting, food hoarding and bird song.
For example, baby duck's follow their "mother" at an early age and learn how to search for food (Korenz); squirrels learn to hoard nuts in the fall ready for the winter; birds learn to sing the song they hear within the first days from birth.

Conditoned learning - Pavlov's dog:

Inheritance and learning in the development of birdsong in young birds: