Chapter 2: Biology and Human Potential
The Nervous and Endocrine Systems
The Nervous System: Connecting Sensation and Movement
As we consider the human genotype, we will start by providing an overview of the nervous system (see Figure 2.7), those structures which transmit information regarding external and internal stimulation and coordinate behavior.
Figure 2.7 Overview of human nervous system
The central nervous system , consisting of the brain and spinal cord, organizes and interprets information received from the peripheral nervous system and initiates responding. The somatic division of the peripheral nervous system responds to sensory information originating outside the body and stimulates the skin, joints, and skeletal muscles. This type of behavior is often considered voluntary. The autonomic nervous system governs the activity of the smooth muscles and glands internal to the body involved in circulation, respiration, and digestion (see Figure 2.8). This type of activity is often considered involuntary. The sympathetic division results in arousal under stressful or dangerous conditions as the body is prepared for “fight or flight.” The parasympathetic division calms the body upon removal of the stress or danger.
Figure 2.8 The autonomic nervous system
Making the Physical Connections: The Neuron
Even very simple animals require some way of connecting environmental input with behavioral output. Specialized nerve cells called neurons are required to respond to external and internal stimulation (i.e., sensory neurons) and carry information to parts of the body capable of responding (i.e., motor neurons). A third type of cell referred to as an interneuron connects nerve cells to each other. Nervous systems consist of these types of specialized neurons and range in size from a few hundred nerve cells in worms to approximately 100 billion nerve cells in humans. Neurons are capable of transmitting information electrically and chemically. Figure 2.9 portrays the major parts of a neuron. Dendrites are small branches which can connect to nearby neurons. A single axon can extend in length up to about a meter in humans and connect to the dendrites of more distant neurons. For example, a neuron could connect the spinal cord to a foot.
Figure 2.9 The neuron
Nerve cells “fire” (i.e., achieve their electrical action potential) according to an all-or-none principle. That is, either the cell is totally activated or not at all. Increasing the intensity of stimulation does not increase the likelihood of a nerve responding. Rather, it increases the nerve’s rate of firing (i.e., frequency over time). For example, as a lamp becomes brighter, this does not increase the likelihood of a receptor cell in your eye firing. Rather, it increases the frequency with which the receptor cell fires. Nerves can fire at rates as high as a thousand times per second.
Making Chemical Connections: Neurotransmitters
The chemical exchange between neurons occurs at synapses, the smal