Lesson 1, Topic 1
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Physiology of the Nervous System

April 11, 2024

Physiology of the Nervous System

Learning Objective: Examine the physiology of the nervous system.

The nervous system is a complex system that plays a major role in homeostasis. The nervous system works in partnership with the endocrine system to help the body respond to its internal and external environments. This effort is responsible for communication and control throughout the body. There are three main functions of the nervous system:

• Collecting information about the external and internal environments (sensing)
• Processing this information and making decisions about action (interpreting)
• Directing the body to put into action the decisions made (acting)

For example, the sensory function begins with a stimulus (e.g., the uncomfortable pinch of a tight shoe). That information travels to the brain, where it is interpreted. The return message is sent to react to the stimulus (e.g., remove the shoe).
The basic flow of information in the nervous system includes the following:

• Afferent or sensory neurons, which collect stimuli received from receptors. Receptors are found throughout the body (e.g., skin, eyes, nose, tongue, and internal organs). Pain receptors are also found throughout the body. The afferent neurons carry the stimuli from the receptors to the CNS (e.g., brain and spinal cord).
• The brain and spinal cord process the information and direct the necessary response. The central nervous system contains interneurons, nerve cells, that connect sensory and motor neurons.
• The response from the CNS is sent via the efferent or motor neurons. The efferent neurons carry the response to an organ, gland, or muscle. The efferent neurons that direct the skeletal muscles are part of the somatic nervous system. The efferent neurons that direct internal organs to contract and secrete are part of the autonomic nervous system.

Peripheral Nervous System

Learning Objective: Describe the role of the somatic and autonomic nervous systems.

The peripheral nervous system consists of voluntary and involuntary nerves. The PNS can be divided into the somatic nervous system (voluntary nervous system) and the autonomic nervous system (involuntary nervous system). The autonomic nervous system can be further divided into the sympathetic nervous system and the parasympathetic nervous system.

Somatic Nervous System

The somatic nervous system is the part of the peripheral nervous system that sends motor impulses to the skeletal muscles. The somatic nervous system transmits signals from the CNS to the skeletal muscles and the sense receptors (vision, hearing, and touch). It is associated with the voluntary control of body movements and is also involved with involuntary reflex arcs.

Reflex Arc

A reflex is an involuntary, almost instantaneous movement in response to a specific stimulus. A reflex arc is the route followed by nerve impulses in the production of a reflex act. The stimulus is picked up by the receptors. The impulse is carried by the afferent neuron to the central nervous system (brain or spinal cord). The efferent neuron carries the message from the central nervous system to the effector organ, where the response occurs.
A three-neuron arc consists of sensory neurons, interneurons, and motor neurons (FIGURE 22.6). A two-neuron arc is the simplest reflex arc. It does not require the brain to cause a response. The two-neuron arc consists of just sensory neurons and motor neurons. The knee-jerk reflex, also called the patellar reflex, is an example of the two-neuron arc. A sharp tap on the patellar tendon below the patella results in a sudden kicking movement of the lower leg. The tap on the tendon (the stimulus) is picked up by receptors. The impulse is carried by the afferent neuron (part of the femoral nerve) to the spinal cord. The response from the spinal cord is sent back by the efferent neuron to the thigh muscles. The muscles contract and the kick occurs.

Autonomic Nervous System

The autonomic nervous system consists of nerves that conduct impulses from the brainstem or spinal cord to cardiac and smooth muscle tissue and glands. The ANS controls the muscles in blood vessel walls, organs, and glands. It regulates involuntary functions such as breathing, heart rate, sweating, circulation, and digestion.

FIGURE 22.6  Three-neuron arc. From Niedzwiecki B: Kinn’s medical assisting fundamentals: administrative and clinical competencies with anatomy & physiology, ed 2, St. Louis, 2022, Elsevier.

The motor portion of this system is further divided into the sympathetic and parasympathetic nervous systems. These two opposing systems help maintain homeostasis throughout the body (FIGURE 22.7).

Sympathetic Nervous System

The sympathetic nervous system can produce a fight-or-flight response. This part of the nervous system helps the individual respond to perceived stress. It speeds up the heart rate, raises blood glucose levels, raises blood pressure, slows the digestive system, and widens the bronchioles, allowing more oxygen to enter the body quickly. It also stimulates the adrenal glands to increase their secretions.

Parasympathetic Nervous System

The parasympathetic nervous system does the opposite of the sympathetic nervous system. It slows the heart rate, lowers blood pressure, increases digestive functions, and reduces adrenal and sweat gland activity.

Action Potential

Neurons carry information from one cell to another by creating and spreading electronic impulses. An action potential is a self-propagating wave of electrical impulse that travels along the surface of a neuron membrane. An action potential proceeds in the following steps

1. A stimulus (e.g., pressure, temperature, sound wave) starts an impulse.
2. The resting nerve cell has a slightly positive charge (due to sodium ions) on the outside of the cell membrane and a negative charge inside the cell. This state is called polarization.
3. When a section of the membrane is stimulated, the positively charged ions on the outside of the cell enter the nerve cell. This changes the outside charge to a negative charge. This state is called depolarization.
4. Almost immediately after the impulse passes, the positively charged ions move outside the nerve cells again. This returns the outside charge to a positive charge. This state is called repolarization.

FIGURE 22.7  Structure and function of the autonomic nervous system. From Applegate E: The anatomy and physiology learning system, ed 4, St. Louis, 2011, Saunders.

5. Once everything changes back, the cell is at rest again, and the cycle continues.

The transfer of the action potential begins as the electrical impulse travels down an axon. It becomes a chemical impulse while moving across the synapse, the gap between the neurons. The transfer of impulses from the end of one neuron to the dendrites of another is enhanced by chemical neurotransmitters found in the synapse (FIGURE 22.8). Examples of neurotransmitters include epinephrine, norepinephrine, dopamine, serotonin, endorphins, and enkephalins.
Neurotransmitters bind to specific receptor sites on the dendrites of the next neuron or the target tissue. Messages move throughout the entire nervous system in this manner. Impulses in the neuron are electrical; the impulses become chemical as a neurotransmitter is released at each synapse. The action potentials become electrical again as they are picked up by the next dendrites of another neuron or by the target tissue. Changes in the number of available neurotransmitters can cause different conditions.


Critical Thinking Application

It is important to know the differences between the sympathetic and parasympathetic nervous systems. List three characteristics of each, and compare your list with that of a classmate. Do your lists agree?

Life Span Changes in the Neurologic System

Infants are born with an immature nervous system. Over the first few years of life, the process of myelination continues, causing an increase in the child’s brain size. As the child matures, learning and thinking processes become more complex. During adulthood, brain function is stable.
As a person ages, the nervous system changes. The brain and spinal cord lose nerve cells. Nerve cells break down in the brain, and waste products accumulate, which can cause plaques and neurofibrillary tangles to form. Nerve impulses become slower. This causes a slower reaction time, and tasks may take longer to perform. Reflexes can also be reduced. Slowing of thought, thinking, and memory is a normal part of aging.