23.1: Foundations of Neuromuscular Functioning

Learning Objectives

By the end of this section, you will be able to:

• Identify structures and functions of the neurological system
• Recognize structures and functions of the musculoskeletal system
• Describe primary functions of the neuromuscular system

The neuromuscular system is composed of both the neurological system and the musculoskeletal system. These systems work effectively together to respond to internal and external stimuli, allowing the body to remain healthy and safe from injury. This section describes the structure and function of the different parts of the neuromuscular system, so that you will have a solid foundation of knowledge about normal anatomy and physiology before discussing alterations in function and specific disease states that affect these systems.

Structures and Functions of the Neurological System

The nervous system includes the brain (the central organ of the nervous system responsible for controlling bodily functions, processing sensory information, and enabling cognitive processes such as learning and memory) and the spinal cord (a long, thin, tube-shaped bundle of nerves that extends from the base of the brain through the vertebral column). Within the brain, a variety of regions are responsible for many different and separate functions.

Central Nervous System

The nervous system can be divided into two major regions: the central and peripheral nervous systems. The central nervous system (CNS) is the brain contained within the cranial cavity of the skull and the spinal cord contained within the vertebral cavity of the vertebral column. The peripheral nervous system (PNS) is everything else; although, some elements of the PNS are located within the cranial or vertebral columns (Figure 23.2). The CNS plays a crucial role in the neuromuscular system by coordinating and regulating voluntary and involuntary movements. It receives sensory input from peripheral nerves, processes this information, and sends motor commands back to muscles via motor neurons, allowing for coordinated muscle contractions and movement control. Additionally, the CNS facilitates reflex actions that protect the body from harm by quickly responding to stimuli without conscious effort. The PNS is so named because it is on the periphery—meaning beyond the brain and spinal cord. Depending on different aspects of the nervous system, the dividing line between central and peripheral is not necessarily universal. The PNS is discussed later in this section.

Figure 23.2 The CNS is composed of the brain and spinal cord, whereas the PNS contains peripheral structures including ganglia and nerves. (credit: modification of work from Anatomy and Physiology 2e. attribution: Copyright Rice University, OpenStax, under CC BY 4.0 license)

Brain

The adult brain has four major regions: cerebrum, diencephalon, brain stem, and cerebellum. The iconic gray matter of the human brain, which appears to make up most of the brain mass, is the cerebrum (Figure 23.3). Many of the higher neurological functions (e.g., memory, emotion, consciousness, voluntary muscle movement) are the result of cerebral function. The gyri (folds) and sulci (grooves) formed by convolutions in the surface of the brain give the cerebral cortex, the outer covering of the brain, a wrinkled appearance. The rest of the cerebrum is beneath that outer covering. The large separation between the two sides of the cerebrum is called the longitudinal fissure. It separates the cerebrum into two distinct halves, a right and left cerebral hemisphere. Deep within the cerebrum, the corpus callosum provides the major pathway for communication between the two hemispheres of the cerebral cortex.

Figure 23.3 The cerebrum is a large component of the human CNS. Its most obvious aspect is the folded surface called the cerebral cortex. (credit: modification of work from Anatomy and Physiology 2e. attribution: Copyright Rice University, OpenStax, under CC BY 4.0 license)

The diencephalon is the region of the adult brain that connects the cerebrum to the rest of the nervous system (Figure 23.4). The rest of the brain, spinal cord, and peripheral nervous system (PNS) all send information to the cerebrum through the diencephalon. Output, including motor commands, language expression, and emotional responses, from the cerebrum also passes through this structure. The two major regions of the diencephalon are the thalamus and the hypothalamus.

Figure 23.4 The diencephalon is composed primarily of the thalamus and hypothalamus. The hypothalamus is inferior and anterior to the thalamus, culminating in a sharp angle to which the pituitary gland is attached. (credit: modification of work from Anatomy and Physiology 2e. attribution: Copyright Rice University, OpenStax, under CC BY 4.0 license)

The thalamus is a collection of nuclei that relay sensory and motor information between the cerebral cortex and the brain stem, spinal cord, or periphery. All sensory signals, except for the sense of smell, and motor signals pass through the thalamus before being processed by the cortex. The thalamus is also responsible for regulating sleep, wakefulness, and consciousness and also emotion and memory. The thalami are two elongated, ovoid structures on either side of the midline that make contact in the middle. The hypothalamus is a collection of nuclei that are largely involved in regulating homeostasis. It is the executive region in charge of the autonomic nervous system and the endocrine system through its regulation of the anterior pituitary gland. Other parts of the hypothalamus are involved in memory and emotion as part of the limbic system.

The brain stem emerges from the ventral surface of the forebrain as a tapering cone that connects the brain to the spinal cord (Figure 23.5). The midbrain and hindbrain (composed of the pons and the medulla oblongata) are collectively referred to as the brain stem. The pons (connection point between the medulla and the thalamus) and the medulla (connection point between the brain stem and spinal cord) regulate several crucial functions, including the cardiovascular and respiratory systems.

Figure 23.5 The brain stem comprises three regions: the midbrain, the pons, and the medulla. (credit: modification of work from Anatomy and Physiology 2e. attribution: Copyright Rice University, OpenStax, under CC BY 4.0 license)

The cerebellum is the “little brain.” It is covered in gyri and sulci like the cerebrum and looks like a miniature version of that part of the brain (Figure 23.6). The cerebellum is largely responsible for comparing information from the cerebrum with sensory feedback from the periphery through the spinal cord. It accounts for approximately 10 percent of the mass of the brain (Van Essen et al., 2018).

Figure 23.6 The cerebellum is situated on the posterior surface of the brain stem. (credit: modification of work from Anatomy and Physiology 2e. attribution: Copyright Rice University, OpenStax, under CC BY 4.0 license)

Life-Stage Context

Neurological Changes in Older Adults

Several neurological changes occur with aging that affect the brain’s functioning. These changes include the following:

• shrinking of certain areas of the brain, which can result in decreased ability to learn new things or perform difficult mental activities
• decreased communication between neurons within the brain
• decreased blood flow to the brain
• atrophy of mass and loss of nerve cells in the brain and spinal cord
• slower neuron message processing, resulting in slower muscle reactions
• accumulation of waste products or chemicals in brain tissues after cellular breakdown

Spinal Cord

The spinal cord is the second major organ of the CNS. The length of the spinal cord is divided into regions that correspond to the regions of the vertebral column. Immediately adjacent to the brain stem is the cervical region, followed by the thoracic, then the lumbar, and finally the sacral region (Figure 23.7). The spinal cord is not the full length of the vertebral column because the spinal cord does not grow significantly longer after the first or second year of life, but the skeleton does continue to grow. The nerves that emerge from the spinal cord pass through the intervertebral foramina at the respective levels.

Figure 23.7 There are four regions of the spinal column including the cervical, thoracic, lumbar, and sacral regions. (CC BY 4.0; Rice University & OpenStax)

As the vertebral column grows, these nerves grow with it and result in a long bundle of nerves that resembles a horse’s tail. This is named the cauda equina (Figure 23.8).

Figure 23.8 The bundle of nerve roots located at the end of the spinal cord is called the cauda equina because it resembles a horse’s tail. (credit: “Cauda Equina” by Daniel Donnelly/Flickr, CCBY 4.0)

Neural Pathways

The gray matter of the spinal cord, a butterfly-shaped structure, is subdivided into regions that are referred to as horns (Figure 23.9). Gray matter consists of neuronal cell bodies and is involved in processing information in the brain. The posterior horn (or dorsal horn) is responsible for sensory processing. The anterior horn (or ventral horn) sends out motor signals to the skeletal muscles. The lateral horn, which is only found in the thoracic, upper lumbar, and sacral regions, contains motor neuron cell bodies of the autonomic nervous system. The myelinated nerve fibers that facilitate communication between different regions of the brain are called white matter. Just as the gray matter is separated into horns, the white matter of the spinal cord is separated into columns. A column of nervous system fibers called an ascending tract carries sensory information up to the brain, whereas a descending tract carries motor commands from the brain.

Figure 23.9 The thoracic spinal cord segment shows the posterior (dorsal), anterior (ventral), and lateral horns of gray matter and columns of white matter. (credit: modification of work from Anatomy and Physiology 2e. attribution: Copyright Rice University, OpenStax, under CC BY 4.0 license)

Peripheral Nervous System

As mentioned previously, the peripheral nervous system (PNS) is the part of the nervous system that lies outside of the brain and spinal cord. The PNS is responsible for transmitting sensory information from the body to the CNS and for sending motor commands from the CNS to the muscles and glands. The PNS contains cranial and spinal nerves, both of which are important for optimal neuromuscular functioning.

Cranial Nerves

A nerve attached to the brain is called a cranial nerve, which is primarily responsible for the sensory and motor functions of the head and neck. There are twelve cranial nerves, which are designated CNI through CNXII, using Roman numerals for 1 through 12 (Figure 23.10).

Figure 23.10 The anatomical arrangement of the roots of the cranial nerves can be observed from an inferior view of the brain. (credit: modification of work from Anatomy and Physiology 2e. attribution: Copyright Rice University, OpenStax, under CC BY 4.0 license)

Cranial nerves can be classified as sensory nerves, motor nerves, or a combination of both (mixed). Sensory axons enter the brain to synapse in a nucleus. Motor axons connect to the skeletal muscles of the head or neck. Nurses should be able to name the cranial nerves and provide a brief description of their function and their source (sensory ganglion, motor nucleus, or mixed) (Table 23.1).

Number/Name	Function	Source
I. Olfactory	Smell	Sensory
II. Optic	Vision	Sensory
III. Oculomotor	Eye movement and pupil reflexes	Motor
IV. Trochlear	Eye movement	Motor
V. Trigeminal	Sensory/motor—face, chewing	Mixed
VI. Abducens	Eye movement	Motor
VII. Facial	Face movement, taste	Mixed
VIII. Vestibulocochlear (Auditory)	Hearing, balance	Sensory
IX. Glossopharyngeal	Swallowing, throat sensations, taste	Mixed
X. Vagus	Movement and sensations of abdominal organs	Mixed
XI. Accessory (Spinal)	Head and neck movement	Motor
XII. Hypoglossal	Tongue movement	Motor

Table 23.1 Functions of the Twelve Cranial Nerves

Link to Learning

Learn more about the cranial nerves and how to conduct a cranial nerve assessment in the human body.

Spinal Nerves

A spinal nerve is a nerve that is connected to the spinal cord. There are thirty-one spinal nerves, named for the level of the spinal cord at which each one emerges. There are eight pairs of cervical nerves designated C1 to C8, twelve pairs of thoracic nerves designated T1 to T12, five pairs of lumbar nerves designated L1 to L5, five pairs of sacral nerves designated S1 to S5, and one pair of coccygeal nerves. The nerves are numbered from the superior to inferior positions, and each emerges from the vertebral column through the intervertebral foramen at its level. Spinal nerves extend outward from the vertebral column to innervate the periphery. The nerves in the periphery are not straight continuations of the spinal nerves, but rather the reorganization of the axons in those nerves to follow different courses (Figure 23.11).

Figure 23.11 Each vertebrae is numbered and each numbered spinal nerve emerges from the vertebral column and branches off into alternate pathways to innervate different parts of the body. (credit: modification of work from Anatomy and Physiology 2e. attribution: Copyright Rice University, OpenStax, under CC BY 4.0 license)

Autonomic Nervous System

The autonomic nervous system (ANS) is a component of the peripheral nervous system (PNS) that is responsible for involuntary control of the body. This involuntary control is usually for the sake of homeostasis, a steady state of body systems that living organisms maintain by regulating their internal environment. Sensory input for autonomic functions can be from sensory structures tuned to external or internal environmental stimuli. The motor output extends to smooth and cardiac muscle as well as glandular tissue, which results in involuntary muscle or tissue action. The role of the ANS is to regulate the organ systems of the body to control homeostasis. Sweat glands, for example, are controlled by the ANS. When you are hot, sweating helps cool your body down. That is a homeostatic mechanism. But when you are nervous, you might start sweating also. That is not homeostatic; it is the physiological response to an emotional state. The ANS is divided into two different systems: the sympathetic nervous system and the parasympathetic nervous system.

Sympathetic Nervous System

The sympathetic nervous system becomes activated during times of tension or stress, resulting in “fight or flight” types of reactions. When the body senses an external danger or threat, the sympathetic nervous system causes involuntary reactions in the body that help combat the perceived threat. These reactions include the following:

• increased heart rate, which increases the amount of oxygen being delivered to the rest of the body
• airway muscle relaxation, allowing improved oxygen delivery to the lungs
• pupil dilation, resulting in improved vision
• slowed digestion, allowing for energy to be used for more important tasks such as running or fighting

Parasympathetic Nervous System

The parasympathetic nervous system is the part of the ANS that results in involuntary “rest and digest” types of reactions. These reactions include the following:

• pupil constriction to limit the amount of light entering the eyes, allowing for a state of rest
• saliva production in the mouth to aid with digestion
• tightening of airway muscles to reduce the work of breathing while at rest
• decreased heart rate to lessen the workload on the heart, allowing for rest and recovery
• increased rate of digestion to break down food to store as energy for later use

Link to Learning

Learn more about comparing the parasympathetic and sympathetic nervous systems of the human body.

Structures and Functions of the Musculoskeletal System

The musculoskeletal system is composed of bones, muscles, joints, and connective tissue. These structures are connected and allow the body to move and function while also protecting internal organs from injury.

Muscles

Muscle is one of the four primary tissue types of the body. The three different types of muscle tissue are cardiac muscle, smooth muscle, and skeletal muscle. The three muscle tissues have some properties in common; they all exhibit a quality called excitability, which occurs when the plasma membranes change their electrical states (from polarized to depolarized) and send an electrical wave called an action potential along the entire length of the membrane. The nervous system can influence the excitability of cardiac and smooth muscle to some degree. Skeletal muscle, however, completely depends on signaling from the nervous system to work properly. On the other hand, both cardiac muscle and smooth muscle can respond to other stimuli, such as hormones and local stimuli.

Cardiac Muscle

The cardiac muscle tissue is found only in the heart. Highly coordinated contractions of cardiac muscle pump blood into the vessels of the circulatory system. Contractions of the heart (heartbeats) are controlled by specialized cardiac muscle cells called pacemaker cells that directly control heart rate. Although cardiac muscle cannot be consciously controlled, the pacemaker cells respond to signals from the ANS to speed up or slow down the heart rate. The pacemaker cells can also respond to various hormones that modulate heart rate to control blood pressure.

Smooth Muscle

The smooth muscle is present in the walls of hollow organs such as the urinary bladder, uterus, stomach, and intestines. The walls of passageways (e.g., arteries and veins of the circulatory system) and the tracts of the respiratory, urinary, and reproductive systems also contain smooth muscle. Smooth muscle present in the eyes functions to change the size of the iris and alter the shape of the lens. Smooth muscle in the skin causes hair to stand erect in response to cold temperature or fear.

Skeletal System

The skeletal system forms the rigid internal framework of the body. The skeletal system is divided into axial and appendicular skeletons. The adult axial skeleton consists of eighty bones that form the head and trunk of the body. The appendicular skeleton consists of the limbs, which are attached to the axial skeleton. The appendicular skeleton has 126 bones. The skeletal system consists of bones, joints, muscle, and different types of connective tissue.

Bones

A hard, dense connective tissue that forms most of the adult skeleton is called a bone and it is the primary support structure of the body (Figure 23.12). There are 206 bones in the adult human body. The bones of the skeletal system perform the following critical functions for the human body:

• body support
• movement facilitation
• internal organ protection
• blood cell production
• mineral and fat storage and release

Figure 23.12 There are 206 bones within the adult skeletal system. (credit: modification of work from Anatomy and Physiology 2e. attribution: Copyright Rice University, OpenStax, under CC BY 4.0 license)

Joints

Except for the hyoid bone in the neck, each of the bones in the body is connected to at least one other bone. A joint is the location where bones come together. Many joints allow for movement between the bones. A ligament and a tendon are tissues that support a joint; the articulating surfaces of the adjacent bones can move smoothly against each other. However, the bones of other joints may be joined to each other by cartilage, a specific type of connective tissue that provides structural support and flexibility. These joints are designed for stability and provide for little or no movement. Importantly, joint stability and movement are related to each other. This means that stable joints allow for little or no mobility between the adjacent bones. Conversely, joints that provide the most movement between bones are the least stable. Understanding the relationship between joint structure and function will help to explain why particular types of joints are found in certain areas of the body.

The articulating surfaces of bones at joints considered stable with little or no mobility are strongly united to each other. For example, most of the joints of the skull are held together by fibrous connective tissue and do not allow for movement between the adjacent bones. This lack of mobility is important because the skull bones serve to protect the brain. Similarly, other joints united by fibrous connective tissue allow for very little movement, which provides stability and weight-bearing support for the body. For example, the tibia and fibula of the leg are tightly united to give stability to the body when standing.

At other joints, the bones are held together by cartilage, which permits limited movements between the bones. Thus, the joints of the vertebral column only allow for small movements between adjacent vertebrae, but when added together, these movements provide the flexibility that allows your body to twist, or bend to the front, back, or side.

In contrast, at joints that allow for wide ranges of motion, the articulating surfaces of the bones are not directly united to each other. Instead, these surfaces are enclosed within a space filled with lubricating fluid, which allows the bones to move smoothly against each other. These joints provide greater mobility, but because the bones are free to move in relation to each other, the joint is less stable. Most of the joints between the bones of the appendicular skeleton are this freely moveable type of joint. These joints allow the muscles of the body to pull on a bone and thereby produce movement of that body region. Your ability to kick a soccer ball, pick up a fork, and dance the tango depends on mobility at these types of joints.

Skeletal Muscle Tissue

The best-known feature of skeletal muscle is its ability to contract and cause movement. Skeletal muscles act not only to produce movement but also to stop movement, such as resisting gravity to maintain posture. Small, constant adjustments of the skeletal muscles are needed to hold a body upright or balanced in any position. Muscles also prevent excess movement of the bones and joints, maintaining skeletal stability and preventing skeletal structure damage or deformation. Joints can become misaligned or dislocated entirely by pulling on the associated bones; muscles work to keep joints stable. Skeletal muscles are located throughout the body at the openings of internal tracts to control the movement of various substances. These muscles allow functions, such as swallowing, urination, and defecation, to be under voluntary control. Skeletal muscles also protect internal organs (particularly abdominal and pelvic organs) by acting as an external barrier or shield to external trauma and by supporting the weight of the organs.

Connective Tissue

As indicated by its name, one of the major functions of connective tissue is to connect tissues and organs. The different types of connective tissue found in the musculoskeletal system include cartilage, ligaments, and tendons (Table 23.2).

Type	Description
Cartilage	Tough elastic fibrous tissue that covers bone surfaces at joints Provides structural support and flexibility and helps reduce friction during joint movement
Ligaments	Tough elastic bands of tissue surrounding joints that connect bones together Provides stability and limits joint movement
Tendons	Tough connective tissue located on each side of a joint that connects muscles to bones Controls joint movement

Table 23.2 Types of Connective Tissue Found in the Musculoskeletal System

Functions of the Neuromuscular System

The nervous system and musculoskeletal system work together to form the neuromuscular system. Signals within the nervous system act on the muscular system to provide body movement, posture control, breathing, and maintenance of homeostasis.

Essential Body Movement

The defining characteristic of the somatic nervous system is that it controls skeletal muscles. Somatic senses inform the nervous system about the external environment, but the response to that is through voluntary muscle movement. The term voluntary suggests that there is a conscious decision to make a movement. Another specialization of the skeletal muscle is the site where a motor neuron’s terminal meets the muscle fiber—called the neuromuscular junction (NMJ). This is where the muscle fiber first responds to signaling by the motor neuron. Every skeletal muscle fiber in every skeletal muscle is innervated by a motor neuron at the NMJ. Excitation signals from the neuron are the only way to functionally activate the fiber to contract.

Control of Posture

Maintaining good body posture is not as simple as it sounds. Even when muscles are not producing movement, they are contracted at least a small amount to produce muscle tone. The tension produced by muscle tone allows muscles to continuously stabilize joints and maintain posture. Muscle tone is accomplished by a complex interaction between the nervous system and skeletal muscles that results in the activation of a few motor units at a time, most likely in a cyclical manner. As a result, muscles never fatigue completely, because some motor units can recover while others are active.

Breathing

Unless you actively practice yoga and meditation, you may never have consciously thought about breathing. That is because breathing is an involuntary process controlled by the ANS. Signals from the external environment as well as internal signals from your body are sent to the brain, which acts as the command center to control breathing. Nerves from the spinal cord send these messages to breathing muscles such as the diaphragm and intercostal muscles to adjust breathing rates and patterns as needed. This all occurs without any conscious thought about breathing on your part—isn’t the human body amazing?

Maintaining Homeostasis

Maintaining homeostasis requires that the body continuously monitor its internal conditions. From body temperature to blood pressure to levels of certain nutrients, each physiological condition has a particular set point. A set point is the physiological value around which the normal range fluctuates. A normal range is the restricted set of values that is optimally healthy and stable. For example, the set point for normal human body temperature is approximately 98.6°F (37°C). Physiological parameters, such as body temperature and blood pressure, tend to fluctuate within a normal range that is a few degrees above and below that point. Control centers in the brain and other parts of the body monitor and react to deviations from homeostasis using negative feedback. A mechanism that reverses a deviation from the set point is called negative feedback. Therefore, negative feedback maintains body parameters within their normal range. The maintenance of homeostasis by negative feedback is continuously occurring throughout the body and is overseen by the neuromuscular system. For example, skin contains thermoreceptors that can detect an increase in body temperature. These receptors then use negative feedback mechanisms to send messages to the brain to initiate measures to cool down the body such as sweating or hyperventilating.