Dr. Gerald S. Hecht

PSYC 488 HISTORY AND SYSTEMS EXAM 2 STUDYGUIDE

BELL-MAGENDIE LAW OF SPINAL NERVE FUNCTION

Sir Charles Bell

Bell-1811 Published Idea for a New Anatomy of the Brain Published for Friends in which he speculated about the functional significance of parts of the brain AND described some experiments on the spinal cords of “stunned rabbits”

Stimulating the POSTERIOR spinal nerves = NO MUSCLE MOVEMENT

Stimulating the ANTERIOR spinal nerves = YES MUSCLE MOVEMENT

Bell speculated that there was a FUNCTIONAL DIVISION of the SPINAL NERVE ROOTS:

ANTERIOR ROOTS send messages to muscles “telling” them to move

POSTERIOR ROOTS carry messages about sensations into the cord

Francois Magendie –1822

Used Conscious subjects

ANTERIOR ROOT severed: NO limb movement when the subjects limb was pinched – YES vocalizing in response to the pinch

POSTERIOR ROOT severed: YES limb movement when pinched – NO vocalizing to the pinch.

Magendie concluded that:

SENSORY NERVES enter through the POSTERIOR ROOTS

MOTOR NERVES exit through the ANTERIOR ROOTS.

Big Brouhaha ensues--- Bell’s brother-in-law John Shaw challenges the priority of Magendie’s discovery—yelling all over Europe about Bell’s pamphlet published for his friends… Magendie claims to have never seen it (probably really hadn’t) … Controversy (and international incident) settled by experts agreeing that Bell’s use of “STUNNED” rabbits precluded proof of VOLUNTARY motor responses. In the end BOTH were given credit and the BELL-MAGENDIE LAW of spinal reflexes still stands today.

Bell-Magendie Law provided:
- PHYSIOLOGY with specific knowledge of the anatomical and functional specificity of spinal root function and
- PSYCHOLOGY with one of its first and longest lasting paradigms: The idea that sensations precede behavior -- THE STIMULUS AND RESPONSE (S-R).

SENSORY PHYSIOLOGY

17th century: Luigi Galvani attached a wire from the roof of his lab into the “frog room” and attached the other end of the wire to a frog’s leg… the muscles would contract when the atmospheric conditions (weather, etc.) changed. Galvani believed that electricity was generated by the brain and distributed throughout the body by the “wires” that are the nervous system. he was wrong about the last part (the brain as a powerplant)

Alessandro Volta challenged Galvani’s theory: He said that the muscle contractions observed by Galvani were due to a potential difference (today known as voltage… get it?) across the membranes of nerve cells. Volta said that Galvani’s frog’s nervous systems had not generated electricity, they had alternately stored and conducted it…. BASICALLY STATED THAT NERVES WERE BATTERIES! (turns out to be true!!)

1850- Emil du Bois-Reymond

developed a very sensitive amp/volt meter… attached one of the electrical leads into a nerve and the other to the outside membrane of the nerve. When the muscle was stimulated.. he was able to DETECT the spread of current down the nerve…It took a “moment” for the meter to register… so Du Bois Reymond suggested that it could be POSSIBLE TO MEASURE THE SPEED OF NEURAL IMPULSE AFTER ALL.

THE ROLE OF THE GERMAN UNIVERSITY SYSTEM IN NEURAL/SENSORY PHYSIOLOGY AND ACCEPTANCE OF PSYCHOLOGY AS A SEPARATE SCIENCE--Germany was more conductive to this type of research than other countries because
England and U.S. Religious beliefs would not have the “HUMAN SOUL MEASURED BY SCALES AND ELECTRODES”
England, France and the U.S. Practiced the manner of the so called “GENTLEMAN SCIENTIST”… living on an independent income.
Wave of reform over German Universities encouraged research, outstanding academic freedom, reputation for scientific excellence and academic rigour… the “publish or perish” ethic, fellowships, scholarships, graduate assistantships, postdoctoral research positions, grants and tenure based on ability rather than socioeconomic or family background.

Hermann von Helmholtz

Product of the aforementioned German University system
Improving upon Du Bois-Reymond’s method measured the time from electrical stimulation of a nerve until the occurrence of a muscle contraction. The time from stim to contrac in a 60mm section of nerve was 0.0015 seconds… so the speed of the neural impulse was around 50-90 meters per second
This is MUCH slower than the speed of light or speed of electrical current in a wire which is around 300 MILLION METERS/SECOND.
The implications of this in detail are the subject of another course… but suffice it to say… it showed that whatever the nervous system was doing, it was slow enough to study.
Helmholtz opened up an AMAZING DOOR… The interval between THOUGHT and ACTION was MEASURABLE. The machinery of the “mind” could be understood…. A science of psychology was possible!

Johannes Mueller
Doctrine of Specific Nerve Energies

The nerves must 1) differ for each sensory system or 2) must project to different parts of the brain and it is the brain area which imposes the specificity.
Mueller thought that it was impossible to know if 1 or 2 was the correct answer BUT… today we know that it is the latter—Different sensory projection areas of the brain impose the specific quality.
The reason that Mueller thought it impossible was because the “electricity” traveling down the nerves was moving faster than could be measured (it was naturally assumed be like electrical current traveling close to the speed of light… physics hadn’t reached that point yet).

Gustav Fechner and Psychophysics

The division between mind and body (the mental realm and the physical realm) was an illusion caused by the nervous system refracting or distorting physical properties of the outside world.

Discovered the exact quantitative relationships between the magnitudes of physical stimuli and magnitudes of the resulting mental sensations.

The relationship fits a logarithmic function for all of the senses S=k log R

S = change in sensation (JND),

R = the magnitude of the physical stimulus

k = the constant derived from Weber’s law :

The effects of stimulus intensities are relative to baseline amounts of sensation (e.g., the perceived brightness of dashboard lights are dependent upon how bright it is outside even though the amount of “physical light” emitted by the lights does not change).

Fechner proposed two ways to measure sensation: 1) the presence or absence of a stimulus (for the physical side of things) and 2) the absolute threshold of perception OF the stimulus (the mental side of the equation).

In brief, Fechner provided the final prerequisites for a science of psychology with precise and elegant measurement.