The absence of astrocytes would cause derangement of brain development because they produce growth factors that stimulate the growth of neurons and formation of synapses, and because they constitute a supportive framework that guides the growth and distribution of nervous tissue.
Such an arrangement would enable synapses to transmit signals in both directions. The result would be a less precise direction of nerve signaling throughout the brain and the rest of the body. Nerve signals would spread randomly throughout the nervous system, and effectors everywhere could respond to a sufficiently strong stimulus received anywhere. This would result in relatively uncoordinated and poorly localized responses. (Such bidirectional synaptic transmission and relatively crude motor responses are seen in some simple animals such as hydras. Unidirectional synaptic transmission was an important step in the evolution of more complex animals.)
The lack of centrioles prevents neurons from undergoing mitosis. New neurons therefore cannot be produced by the division of preexisting neurons.
Neurons and muscle fibers share the properties of excitability and conductivity. Excitability is necessary for neurons to respond to stimuli and for muscles to be activated to contract. Conductivity is necessary for the spread of excitation along the length of the cell, to reach more remote cells in the case of neurons and to activate contraction of the entire fiber in the case of muscle. Neurons also have the property of secretion, whereas skeletal muscle fibers do not. Neurons must secrete chemical neurotransmitters to stimulate other cells, whereas skeletal muscle fibers do not chemically stimulate each other.
Pupillary constriction: the autonomic nervous system (specifically the parasympathetic division, but that information is not provided until chapter 16). Handwriting movements: the somatic motor division, since hand movements are achieved by voluntary skeletal muscles. Stomach ache: the visceral sensory division, which carries sensory signals from the internal organs of the body cavities. Blinking: the somatic motor division, since blinking is achieved by voluntary skeletal muscles. Awareness of hand position: the somatic sensory division, which carries sensory signals from skeletal muscles and joints.
