9.2 Structure and Function of the Nervous System

Overview of the Nervous System and Neurons

Within the PNS, the autonomic nervous system (ANS) controls involuntary processes and regulates vital functions such as heart rate, digestion, respiration, and glandular secretion, while the somatic nervous system (SNS) consists of motor and sensory pathways that are associated with the voluntary movement of skeletal muscles. The PNS relays information between the CNS and the rest of the body via neurons, the basic units of the nervous system. The brain contains approximately 100 billion neurons (Chen, 2023). Glial cells are also found in nervous tissue and serve to support neurons. While glial cells are unable to generate action potentials, they play a key role in the development, support, and protection of neurons.

Structurally, neurons consist of three main components: the cell body (soma), dendrites, and an axon. The cell body contains the nucleus and other essential cellular components. Dendrites branch out from the cell body, acting as receivers of signals from other neurons. The axon is a long, slender extension that carries electrical impulses away from the cell body toward other neurons or target cells. See Figure 9.3 for a visual representation of a neuron.

Neurons grouped together are bundles of nerve cells that transmit electrochemical information via a process known as neurotransmission. Neurotransmitters, which are chemical messengers, are stored in the axon terminals of presynaptic neurons. These neurotransmitters then bind to receptors on the receiving neuron's dendrites, transmitting the signal across the synapse and allowing the information to be passed on. The two main types of synapses are electrical and chemical. Electrical synapses have direct communication by physical connections. With chemical synapses, when an electrical signal reaches the end of an axon, it triggers the release of one of the chemical messengers (or neurotransmitters) into the chemical synapse, which is the small gap between neurons. If the postsynaptic cell is also a neuron, there may be a subsequent increase or decrease in firing rate. However, if the postsynaptic cell is a muscle cell, muscle contraction or relaxation can result. If the postsynaptic cell is a glandular cell, there will be an increase or decrease in secretion.

The connections formed by neurons create neural networks, which serve as the framework for the brain's ability to process and integrate information. These networks allow for the transmission of sensory input, coordination of motor functions, and the execution of higher cognitive processes such as learning, memory, and decision-making.

Neurons are diverse in terms of their structure, function, and location within the nervous system. Sensory neurons transmit information from sensory organs to the brain, motor neurons carry signals from the brain to muscles and glands, and interneurons facilitate communication between different neurons within the central nervous system.

Figure 9.3 The major parts of the neuron are labeled on a multipolar neuron from the CNS. (credit: modification of work from Anatomy and Physiology 2e. attribution: Copyright Rice University, OpenStax, under CC BY 4.0 license)

Overview of the Brain

The brain has four main components: the cerebrum, cerebellum, brain stem, and limbic system (see Figure 9.4).

Figure 9.4 Basic brain structures include the cerebrum, cerebellum, brain stem, and the limbic system. (attribution: Copyright Rice University, OpenStax, under CC BY 4.0 license)

The cerebrum is divided into two hemispheres that are connected by the corpus callosum. The lobes and structures within the left and right hemispheres are contained in both hemispheres with the exception of the pineal body. The pineal body is an endocrine gland located between the two cerebral hemispheres. Each hemisphere is divided into four lobes: the frontal lobe, parietal lobe, temporal lobe, and the occipital lobe.

The cerebellum, located below the cerebrum, controls coordination of movement and postural adjustment.

The brain stem is composed of the midbrain, pons, and medulla oblongata. The brain stem is responsible for basic involuntary respiratory and cardiovascular functions.

The limbic system is composed of a thalamus, hypothalamus, hippocampus, and amygdala. These structures regulate various bodily functions (e.g., activity, sensation, emotion, temperature, appetite, endocrine function, and sexual drive).

The spinal cord is approximately 18 inches long, encased in the spinal column, and surrounded by cerebrospinal fluid. The spinal cord originates from the brain stem and extends down the back. The spinal cord is protected by bone segments, or vertebrae, supported by discs and ligaments. The vertebrae are identified by their relative position on the body. The spinal cord is divided into four sections (see Figure 9.5):

• Cervical vertebrae (C1–C7) in the neck
• Thoracic vertebrae (T1–T12) in the upper back and attached to the ribcage
• Lumbar vertebrae (L1–L5) in the lower back
• Sacral vertebrae (S1–S5) in the pelvis

Nerves extend from the spinal cord and spread throughout the body. There are 31 pairs of spinal nerves. Spinal nerves are the conduit for communication between the CNS and the PNS.

Peripheral Nervous System

The peripheral nervous system (PNS) delivers signals between the central nervous system (CNS) and the body. The PNS contains both motor neurons and sensory neurons. Motor neurons coordinate signals that correspond to muscle and gland activity, while sensory neurons provide the signals to sensory organs. The somatic nervous system and autonomic nervous system are both types of motor neurons. The somatic nervous system regulates voluntary movements of skeletal muscles. The autonomic nervous system regulates involuntary responses by controlling the cardiac muscle and smooth muscle contractions in addition to gland activity.

Figure 9.5 The spinal cord is divided into four sections, and there are 31 pairs of spinal nerves. (credit: modification of work from Anatomy and Physiology 2e. attribution: Copyright Rice University, OpenStax, under CC BY 4.0 license)

The autonomic nervous system is divided into the sympathetic and parasympathetic nervous systems. The sympathetic nervous system is commonly known as the “fight or flight” system and is activated when the body is under actual or perceived stress to ready the body’s response to a potential threat. Activation results in the following physiological changes:

• Increased heart rate, heart contraction, and blood pressure
• Shunting of blood to skeletal muscle
• Bronchial dilation
• Pupillary dilation
• Decreased salivation and digestion
• Glucose release
• Epinephrine and norepinephrine secretion, resulting in peripheral vasoconstriction
• Bladder relaxation

Conversely, the parasympathetic system is known as the “rest and digest” system. Activation of the parasympathetic nervous system translates into a series of bodily reactions that are, generally but not always, opposite of the reactions seen in the sympathetic nervous system. These changes are:

• Slowed heart rate, heart contraction, and decreased blood pressure
• Bronchial constriction
• Pupillary constriction
• Increased salivation and digestion
• Bladder constriction