Nervous system III


All animals groups above the level of a sponge have some form of a nervous system. (see figure 26.4)



Simple stereotype movement in response to sensory stimuli. One input, no alternative pathways, the behavioral response involves no coordination of effector cells, no central neural control. Examples the knee-jerk reflex, hand withdrawal response to heat, can involve single cells, 2 cells or 3 cells, sensory neuron, interneuron and motor neuron. ( see fig. 34.11 your text book and figure 1)

B. Divergence:

In sensory systems the receptor neurons at the input stage branch out and make multiple connections with neurons that represent the second stage of information processing. (see figure 2)

C. Convergence:

The target cell receives the sum of information from many presynaptic cells. ( see figure 2)

D. Interneurons:

Collects information from many cells and processes the information to a target cell. (see figure 2)


A. Simplest nervous systems Nerve nets see fig. 35.3

B. How to get a head in this world :

Nervous systems evolved into bilaterally symmetric systems in the flatworm. It's no accident that brains are usually locate in the head. The anterior end usually first encounters new stimuli as the animal moves forward. ( see fig 35.4)


A. What is special about the vertebrate system:

The spinal cord is single cord and located dorsally.
The brain is dominant.

B. Evolution of vertebrate nervous system

Took place in the sea in a fish like organism. A bony segmented hollow column took the place of the notochord, a nerve cord. Evolve into spinal cord and three specialized areas of the brain.( figure 35.5)
Evolution of vertebrae segmented hard elements.
The vertebral column was the foundation for fast-moving predators.
The vertebral column provided a strong internal skeleton for muscles to work against.
A second trend was the elevation of a complex brain at the end of the nerve cord.

At first simple reflex pathways predominated.
Then expansion of the brain occurred after jaws evolved. At first the ability to process sensory information evolved and then the brain began better able to process complex information. The trend toward a complex brain began in fish and continued in land vertebrate.
(see figure 27.8)

C. Human Nervous system ( see figure 35.6)

Central nervous system and peripheral nervous system


Somatic: skeletal, voluntary motor system

Autonomic: Involuntary, unconscious controlling smooth muscle
heart muscle, endocrine glands

Sympathetic: fight /flight system
Parasympathetic system: rest and digest
( see figure 35.7)

2) CNS

a) Spinal cord is the expressway for signals from the periphery.
It receives sensory information form skin, joints and muscles and sends messages involved in voluntary and reflex movement. It is segmented and has 31 pairs of spinal nerves.

b) Vertebrate brain Organized into three regions that develop from three portions of the neural tube, hindbrain, midbrain, and forebrain. ( see fig 35.9) in the brain neurons are precisely arranged on the basis of function. Pathways connect different regions of the brain. Right and left brain and the cortex is divided into functional lobes.

c) Methods for studying the brain. Autopsy, CAT scans and PET scans.