26.1: Structure and Function
By the end of this section, you will be able to:
- Identify structures of the neurological system
- Recognize functions of the neurological system
- Recall effects of impaired function of the neurological system
The nervous system is the command center of the body. Many of the crucial jobs the nervous system undertakes happen automatically and regardless of our awareness, like a steady heartbeat and breathing pattern. At the same time, the nervous system is also the seat of our consciousness. It is what allows us to explore the world through our senses, process the input we get from the world around us and learn from it, and feel emotions in response to our experiences.
When the nervous system operates as expected, the body is able to function. However, dysregulation within the nervous system will lead to dysfunction and disturbance in the body, which may have a profound effect on a patient physically and mentally.
The nurse needs to understand the complex anatomy, structure, and function of the nervous system to fully appreciate the value of doing a neurological assessment. During the assessment, the nurse collects subjective and objective data through an interview and detailed physical exam of a patient’s central nervous and peripheral nervous systems.
Structures of the Neurological System
The neurological system consists of two parts: the central nervous system and the peripheral nervous system. The nervous system transmits signals between the brain and the rest of the body to control motor, sensory, cognitive, behavioral, and autonomic activities. In other words, the brain is the powerhouse that controls the ability to move, see, breathe, and think. In addition, the nervous system monitors bodily processes and responds to stimuli to maintain homeostasis and direct all physical, physiological, and biological activities.
Central Nervous System
The central nervous system (CNS) includes the brain and spinal cord . This nervous system is responsible for receiving, processing, and responding to sensory information. The brain is the organ responsible for sensation , movement, emotions, responses, thought processing, communication, and memory. The spinal cord sends motor commands from the brain to the peripheral body. The spinal cord also relays sensory information from sensory organs to the brain (Thau et al., 2022).
Brain
The major regions of the brain are the cerebrum and cerebral cortex , the diencephalon , the brain stem, and the cerebellum . The largest portion of the brain is the cerebrum (Figure 26.2), and it is composed of the right and left hemispheres, the thalamus, the hypothalamus, and the basal ganglia . The left and right hemispheres are in constant communication with each other but are responsible for different behaviors, this is known as brain lateralization (Figure 26.3). The left hemisphere is dominant for logic, language, and mathematical abilities. The right hemisphere is more creative and intuitive and is dominant in musical and artistical situations (Thau et al., 2022). The major communication pathway between the two hemispheres is the corpus callosum .
The cerebrum is covered by the cerebral cortex , a wrinkled outer layer of gray matter. The cerebral cortex is responsible for higher functions such as emotions, memory, and consciousness and is divided into the lobes described in (Table 26.1) (Thau et al., 2022).
| Lobe | Location | Function |
|---|---|---|
| Frontal | Front of the brain |
Concentration, information storage (memory), abstract thought, and motor function. The frontal lobe is also responsible for a person’s affect personality, judgment, and inhibitions.
Broca’s area is located in this lobe and is essential for language and motor control of speech. |
| Occipital | Posterior to the parietal lobe | Responsible for visual interpretation and memory |
| Parietal | Posterior to the frontal lobe | Analyzes sensory information and relays the interpretation to other cortical areas, and is essential to a person’s body awareness (size and shape discrimination, position in space, and right-left orientation) |
| Temporal | Inferior to the frontal and parietal lobes | Plays a role in memory of sound and understanding music and language. The auditory receptive areas are also contained in this lobe. Wernicke’s area is found here and plays a role in language comprehension (both written and spoken). |
The diencephalon , or the interbrain, contains the hypothalamus and thalamus (Figure 26.4). It acts as a primary relay and processing center for sensory information and autonomic control. The hypothalamus plays a role in the endocrine system by regulating the pituitary secretion of hormones that affect metabolism, stress response , reproduction, and urine production. It also helps regulate homeostasis such as thirst, sleep, hunger, and temperature regulation. Other parts of the hypothalamus play a role in memory and emotion for the limbic system. The thalamus processes, prioritizes, and relays motor and sensory information in collaboration with the cerebellum . The basal ganglia , masses of nuclei located in the deep cerebral hemispheres , are responsible for fine motor movements and coordination (Thau et al., 2022).
The brain stem includes the midbrain , pons, and medulla . The midbrain connects the pons and the cerebellum with the cerebral hemispheres. The midbrain also contains motor and sensory pathways and serves as the center for visual and auditory reflexes. The pons, located in front of the cerebellum between the midbrain and the medulla, is the bridge between the two halves of the cerebellum. The pons contains sensory and motor pathways. Portions of the pons help regulate respirations. The medulla contains reflex centers for respiration, heart rate, blood pressure, coughing, vomiting, sneezing, and swallowing. The reticular formation, which is responsible for arousal and the sleep-wake cycle, begins in the medulla and connects with various higher structures.
The cerebellum is located under the cerebrum and behind the brain stem. It is responsible for fine motor movements and coordination. It is also responsible for position (postural) sense or proprioception , one’s awareness of the position of one’s extremities without looking at them. Sensory feedback from the muscles and joints are sent back to the cerebellum to provide that information.
The brain is protected by the skull, meninges, and cerebrospinal fluid. The meninges consist of three layers: dura mater , arachnoid , and pia mater . The dura mater is the outermost layer covering the brain and spinal cord . The arachnoid is the middle layer and contains cerebrospinal fluid in the space below it, also known as the subarachnoid space . The innermost layer is the pia mater. It covers the brain closely and extends into the folds of the brain surface.
Spinal Cord
The spinal cord is an extension of the brain stem (the medulla) from the foramen magnum of the skull to the second lumbar vertebrae ; it transmits motor and sensory impulses. The length of the spinal cord is divided into four regions that correspond to the level at which spinal nerves pass through the vertebrae. In descending order, these are the cervical, thoracic, lumbar, and sacral regions. Each region has spinal nerves that innervate specific parts of the body. The spinal cord is protected by bone, meninges, and cerebrospinal fluid.
Neural Pathways
Ascending and descending neural pathways are in communication with the brain. The ascending pathways provide sensory information from the body to the spinal cord prior to reaching the brain. This information travels upward, using first-, second-, and third-order neurons. The first-order neurons send impulses to the spinal cord from the skin and proprioceptors. The second-order neurons then send those impulses to the thalamus and cerebellum . The third-order neurons then send those impulses to the somatosensory portion of the cerebrum. Those sensations include pressure, temperature, pain, and body senses (Thau et al., 2022).
The descending pathways are the communication from the brain to lower motor neurons. The lateral and anterior tracts conduct motor impulses and control voluntary muscle activity. Vestibulospinal tracts are involved in autonomic functions such as involuntary muscle control, sweating, circulation, and pupil dilation. The corticobulbar tract is responsible for impulses that control voluntary and facial movements. Lastly, the reticulospinal and rubrospinal tracts conduct involuntary muscle movement impulses (Thau et al., 2022).
Peripheral Nervous System
The peripheral nervous system (PNS) includes the cranial nerves, spinal nerves, and autonomic nervous system . The primary function of the PNS is to connect the CNS to the limbs and organs. Peripheral nerves are categorized as either sensory or motor nerves, or a combination of the two. Sensory nerves transmit impulses from the body to the brain for processing. Motor nerves conduct motor signals from the brain to the muscles to initiate movement.
Cranial Nerves
Cranial nerves are connected immediately to the brain. These nerves are responsible for motor and sensory functions of the head and neck. There are 12 cranial nerves (Figure 26.5) and these are numbered, using Roman numerals, in the order in which they occur from the brain. See Table 26.2 for the cranial nerves and their functions (Cleveland Clinic, 2021).
| Nerve | Type | Function |
|---|---|---|
| I (olfactory) | Sensory | Sense of smell |
| II (optic) | Sensory | Sense of vision |
| III (oculomotor) | Motor | Regulates eye and lid movements, blinking, pupil constriction |
| IV (trochlear) | Motor | Eye movement (up and down or back and forth) |
| V (trigeminal) | Both | Corneal reflex, facial sensations, mastication |
| VI (abducens) | Motor | Eye movement |
| VII (facial) | Both | Facial expression symmetry; upper and lower facial movement; tearing; salivation; taste; sensations in ear |
| VIII (acoustic) | Sensory | Sense of hearing, equilibrium |
| IX (glossopharyngeal) | Both | Taste, swallowing, pharyngeal muscles, sensation in pharynx and tongue |
| X (vagus) | Both | Homeostatic control of the thoracic and upper abdominal cavity muscles |
| XI (spinal accessory) | Motor | Shoulder and neck movement |
| XII (hypoglossal) | Motor | Tongue movement |
Spinal Nerves
Spinal nerves are connected to the brain through the spinal cord (Figure 26.6). There are 8 pairs of cervical nerves, designated C1 to C8; 12 thoracic nerves, designated T1 to T12; 5 pairs of lumbar nerves, designated L1 to L5; 5 pairs of sacral nerves, designated S1 to S5; and 1 pair of coccygeal nerves (Table 26.3). All spinal nerves are combined sensory and motor nerves. Spinal nerves extend outward from the vertebral column to innervate the periphery while also transmitting sensory information back to the CNS.
| Nerves | Location | Function |
|---|---|---|
| C1–C5 | Cervical |
C1: innervation to muscles at base of the skull
C2–C3: sensory and motor control for the back of the head C3–C5: phrenic nerve arises here and enables breathing by innervating the diaphragm |
| C5–C8 and T1 | Brachial | Motor and sensory innervation of the upper limbs and upper back |
| T1–T12 | Thoracic |
Cutaneous innervation of the viscera, skin, and musculoskeletal system
Motor innervation of the thorax, deep back, abdominal wall, and intestines |
| L1–L5 | Lumbar |
Innervates the transverse abdominal region to the anterior leg:
|
| L4 and L5 to S1–S4 | Sacral | Innervate the gluteal nerves to nerves of the feet |
Learn more about cranial nerve testing here.
Autonomic Nervous System
The autonomic nervous system is a division of the nervous system that regulates the involuntary body functions in order to maintain and restore homeostasis such as physiologic processes of respiration, digestion, blood pressure, and heart rate (Waxenbaum et al., 2024). This nervous system includes internal organs, such as the lungs, heart, blood vessels, digestive organs, and glands. The major divisions of the autonomic nervous system are the sympathetic and parasympathetic nervous system s.
Most organs of the body are innervated by the autonomic nervous system. The autonomic nervous system is regulated by centers in the hypothalamus, brain stem, and spinal cord . The hypothalamus links this system with the thalamus, pituitary gland, olfactory apparatus, and the cortex . Visceral and control mechanisms found in this region are essential for defense or attack modes in emotional states, metabolic processes, body temperature regulation, muscular and glandular activities of the gastrointestinal tract, the sleep cycle, and genital function control (Waxenbaum et al., 2024).
Sympathetic Nervous System
The sympathetic nervous system is the division best known for the “fight or flight” response . When there is a great emotional response of fear, anger, or anxiety, or even great physical stress, the body responds to sympathetic impulses. These impulses are designed to expend energy to combat trauma, threats, or stressful situations. Table 26.4 lists how parts of the body respond in a “fight or flight” response (Waxenbaum et al., 2024).
| Body Structure | Sympathetic Response |
|---|---|
| Blood vessels | Vasoconstriction |
| Digestive system | Blood vessel constriction, anal sphincter contraction, peristalsis inhibition |
| Eyes | Mydriasis (pupil dilation) |
| Heart | Increased heart rate, increased strength of contraction |
| Liver | Facilitates breakdown of glycogen to glucose for energy release |
| Lungs | Bronchodilation |
| Liver | Facilitates breakdown of glycogen to glucose for energy release |
| Salivary and lacrimal glands | Decreased secretions |
| Skin | Vasoconstriction, diaphoresis, arrector pili muscle contraction |
| Suprarenal gland | Epinephrine (adrenaline) release into the blood |
| Liver | Facilitates breakdown of glycogen to glucose for energy release |
| Urinary system | Decreased urine production, bladder sphincter contraction |
Parasympathetic Nervous System
The parasympathetic nervous system is the division best known for the “rest and digest” response. This system controls most visceral functions and dominates under nonstressful conditions. The parasympathetic nervous system only innervates the head, viscera, and external genitalia. In general, the structures affected by the parasympathetic nervous system respond in the opposition to how they respond with sympathetic impulses. For example, heart rate and force are decreased, bronchioles are constricted, pupils are constricted, and peristalsis is increased. The key functions of the parasympathetic nervous system promote respiration, digestion, and cardiac relaxation (Waxenbaum et al., 2024).
Watch this video to learn more about the autonomic nervous system and the sympathetic and parasympathetic divisions .
Functions of the Neurological System
The nervous system has a complex and multifaceted role in the body. Each facet of the system is involved in different tasks. However, taking a broader view of its role, one of the most important processes the nervous system plays a role in is communication and information processing. The system not only facilitates communication within the body, between systems, but outside of the body, with the external environment.
The nervous system takes in information about what is happening around the body in the world (sensation) and produces a reaction to that input (motor responses). The process of integration combines sensory perceptions and higher cognitive functions such as memories, learning, and emotion to form a response.
Sensation
We define sensation as receiving information about the environment. In humans, the major senses are taste, smell, touch, sight, and hearing. Additional sensory stimuli are also provided from inside the body, such as the stretch of an organ wall or the concentration of certain ions in the blood. Sensation is key for our survival. Think of the importance of being able to feel pain and temperature; for example, we learn quite young not to touch a hot stove because it will burn our hand.
The nurse will encounter patients with various sensory challenges or differences—from vision impairment to hearing loss or even anosmia (loss of sense of smell and taste). It is crucial that the nurse not only understands how to assess sensory function but understands how its impairment may affect a patient’s ability to function and quality of life.
Response
When the nervous system has taken in the information from the environment that’s gathered by the senses, it needs to process it and take action based on what has been perceived. The system will produce a response based on the stimuli perceived by sensory nerves.
For example, withdrawing a hand from a hot stove is an example of a response to a painfully hot stimulus. The sensations of heat and pain are gathered from the external source (stimulus) and processed by the nervous system , and an action is taken in response. In this case, a protective action is taken that helps a person avoid injury (and perhaps teaches them to avoid a danger).
Responses can be voluntary (e.g., contraction of a skeletal muscle) or involuntary (e.g., contraction of smooth muscle in the intestine). An involuntary movement, known as a reflex , occur without conscious thought. Voluntary responses are governed by the somatic nervous system, and involuntary responses are governed by the autonomic nervous system .
Breathing, digestion, blood pressure, and heart rate are autonomic responses that the nervous system carries out automatically. Motor responses, like those required to get up from a chair and walk into the kitchen, are intentional, conscious movements.
Integration
The process of integration occurs when stimuli received by sensory nerves are communicated to the nervous system, the information is processed, and a conscious response is generated. In this process, the different parts of the nervous system collaborate to make sense of the information and decide what to do with it.
Here’s an example: A batter in a baseball game does not automatically swing when they see the pitcher throw the ball. First, they have to consider the trajectory of the ball and its speed, and only then will they start to create the motor response of a swing. Integration will occur as the batter consciously decides to swing (or not). If the count is three balls and one strike, the batter may decide to let the pitch go by in the hope of getting a walk to first base. Or, maybe the batter is afraid to strike out and does not swing. Perhaps the batter is familiar with the pitcher’s nonverbal cues and is confident to take a swing at an anticipated fast ball. All these considerations are part of the batter’s integration response and the higher-level functioning that occurs in the cerebral cortex .
Effects of Impaired Function of the Neurological System
The neurological system can become damaged by infection, trauma, degeneration, tumors, blood flow disruption, structural defects, and autoimmune disorders. These can have both physical and psychosocial effects on a patient.
Impairments in the nervous system can affect any bodily system and, at times, may affect more than one simultaneously. For example, the global effects of a major stroke can affect a patient’s ability to walk, talk, and think. Specific conditions or injuries to bodily systems can affect specific senses, like glaucoma leading to blindness. A severe spinal cord injury may even impair autonomic functions (e.g., respiration). Nervous system impairments that involve the brain can also manifest in a patient’s emotional and psychological well-being, causing mood dysregulation, personality changes, and mental health conditions like depression and anxiety. The nurse needs to be aware of the many ways in which a dysfunctioning nervous system can affect a patient and be equipped to assess effectively for the different problems that can occur, as well as the far-reaching effects they can cause.
Physical Effects
Physical effects of an impaired neurological system include, but are not limited to, dizziness, loss of balance or gait issues, dysphagia (difficulty swallowing), muscle weakness, paralysis, loss of senses, headaches, tremors, seizures, slurred speech, and inabilities to carry out activities of daily living (ADLs). Examples of ADLs include eating, performing personal hygiene, toileting, getting dressed, walking, and transferring (e.g., from bed to chair).
Any damage to the brain, spinal cord, or nerves can greatly affect a person’s ability to perform various motor functions. Weakness, paralysis, spasticity, and coordination problems can have a significant effect on a patient’s ability to function independently and can lead to complications like falls. Sensory impairments not only affect a patient’s quality of life but can put them in danger (e.g., poor vision or hearing may prevent them from hearing or seeing an approaching car when they are walking on the street). Dysfunction of autonomic functions can have a range of effects, depending on the body system affected and the severity of the impairment. For example, dysregulation of blood pressure can lead to orthostatic hypotension, and issues with the body’s thermoregulatory system can lead to excessive sweating or feeling constantly cold.
When the nurse identifies physical effects of an impaired neurological system, the nurse should promote an environment that aids the patient with performing ADLs as independently and safely as possible. This may include ensuring a gait belt is available for ambulating the patient, ensuring a clutter-free environment to reduce the risk of falling, regular rounding on the patient and assessing for needs while in the room (e.g., call light in reach, toileting needs, items patient want are in reach, bed alarm is on), food changes if needed due to dysphagia (e.g., pureed, mashed, soft, liquid), signage reminding patient to call when they want to get up, and dressing tools (e.g., shoe horn, sock aid). A crucial component the nurse should include in the plan of care is to educate the patient about these interventions and their importance. Those with cognitive effects may need to be reminded or have signs posted in their room as well. The family, if present, should also be aware of these interventions to help promote patient safety as well.
Psychological Effects
Psychological effects of an impaired neurological system include, but are not limited to, depression, changes in cognition or mental ability, fear of losing control, and anxiety. Depression can be due to damage or impaired functioning of the nervous system or due to the physical or psychological effects of an impaired nervous system. Cognitive impairments affect a person’s ability to pay attention, make decisions, have sound judgment, speak and understand language, plan, reason, perceive things or ideas, and recall memories. This can contribute to depression and anxiety if the person is able to make the connection that they have lost a function they previously could perform. Fear of losing control and anxiety are common symptoms of cognitive impairments.