A Review of Neuronal Pathways with Implications for Nursing

Abstract

INTRODUCTION:

Nurses are often responsible for establishing basic and nonverbal communication with patients whose level of consciousness fluctuates. Therefore, nurses are required to have knowledge of the clinical criteria and assessment scales for the various disorders of consciousness. This article provides working definitions associated with consciousness and discussions on the underlying neuronal pathways.

CONSCIOUSNESS:

Although the neurobiology of consciousness is complex and multifaceted, consciousness can be conceptualized as having two interrelated dimensions: arousal and awareness. While arousal has to do with one’s global state of responsiveness, awareness refers to one’s ability to perceive both the environment and self.

CONCLUSION:

The advancements of modern techniques provide a rich analysis of dynamic neural systems with a high spatiotemporal resolution. They will likely shape research, education, and knowledge for future care of patients who experience disorders of consciousness.

Introduction

Appropriate identification, assessment, evaluation, and communication of information requires empirical knowledge. Nurses are often responsible for establishing basic and nonverbal communication with patients experiencing varied and fluctuating levels of consciousness (LOC) in order to maintain optimal care and maximize recovery potential. Accurate communication of information regarding fluctuations in LOC to caregivers is also an important part of education and training for families who are seeking information about optimal management and, where appropriate, rehabilitation, and recovery.¹

It is important for nurses to understand terms related to consciousness to appropriately care for patients whose LOC is altered or fluctuating. This article provides working definitions with a discussion of the underlying neuronal pathways associated with consciousness; with a focus on the terms: consciousness, awareness, and arousal. Awareness and arousal are components that are subsumed under the term consciousness. Table 1 provides a list of additional key terms. Utilizing these definitions and discussions, the article provides an extension to nursing implications where appropriate. It is inherently critical for nurses to understand these concepts and implement them into practice when necessary.

Table 1.

Additional Key Terms

TERM	DEFINITION
Coma	a state of being in which a person is unconscious. The person is both unawake and unaware, with the general behavioral characteristics of closed eyes, immobility, and lack of reflexive movements
Vegetative state (VS)	similar pathophysiology as a coma (i.e., the patient remains unaware) except there generally seems to be some functioning of the upper brainstem, including the presence of sleep-wake cycles. Patients in this state of consciousness may open and close their eyes, breathe without assistance, and reflexively respond to stimuli, thus appearing awake. While a VS is typically seen as transitory between coma and some level of conscious recovery, it is possible that a person may remain in a VS for several weeks or months.
Persistent vegetative state (PVS)	VS state lasts more than 30 days
Permanent VS	VS state lasts more than three months.
Brain death	refers to the permanent cessation of critical neurological functions
Stupor	a state of excessively long or deep unresponsiveness where a patient can only be briefly aroused by vigorous, physical stimulation such as repeated shaking, talking loudly, or pinching.
Delirium	is a neurocognitive disorder that presents as an acute and fluctuating change in behavior secondary to impairments in consciousness and cognition
Dementia	an acquired chronic disorder of executive function in at least one cognitive domain (e.g. memory, language, judgment, or attention).

Consciousness

The concept of consciousness was popularized in the medical field by the co-discoverer of DNA, Francis Crick, whose research focused on visual awareness. He believed that since a majority was known about the visual parts of the brain, the neural basis of visual perception could serve as a foundation for exploring the neural bases of other aspects of consciousness. ² As such, Crick and his colleague Christof Koch published a series of papers in the 1990s exploring the neural correlates of consciousness (NCCs).^2–4 NCCs are the brain structures and neural activities responsible for generating states of conscious awareness.³ For example, the NCCs in visual awareness consist of finding the correlations between the firing properties of neurons in different parts of the brain as well as visual perception. It was hypothesized that there was a hierarchy of visual areas in the cerebral cortex and the response properties of neurons were thought to become more specific the “higher” the neuron was in the hierarchy, in that a single neuron at the top of the hierarchy would be the only one to respond to pictures of specific people or objects, coined the “grandmother cell.” ⁵ It is more likely, however, that there are thousands of cells across multiple pathways that are responsible for different endotypes of LOC and disorders of consciousness (DOC).⁶ When looking at temporal research of vision to explain the flash-lag effect, exemplified by how a flash and a moving object at the same location appear to be offset (e.g., an airplane with a flashing tail light), our brains are thought to be postdictive rather than predictive by constantly revising past recent events to make the conscious present consistent with the future.

Other researchers believe that while NCCs are important, they are not defining consciousness. Tononi and Koch ⁷ explain the integrated information theory, alluding to consciousness starting from experience itself through 5 phenomenological axioms: (1) intrinsic existence (the experience is in the present and exists independent of external observers); (2) composition (elementary or higher-order phenomenological distinctions structure consciousness); (3) information (experiences are particular and composed of a specific set of phenomenal distinctions separate from other possible experiences); (4) integration (unification of experiences to irreducible subsets), and (5) exclusion (definite in content and spatio-temporal grain). Together, these physical mechanisms produce the essential properties required to support consciousness.^7,8 Furthermore, the researchers extrapolate that consciousness can occur in very simple systems but predicts using feed-forward networks and aggregates (i.e., a group of individuals).⁷ The authors discuss how behavioral correlates of consciousness and reportability are the standard for operationalizing consciousness in research. However, they also describe the paradoxical nature of rapid eye movement (REM) sleep in which the body is typically immobile but an individual may be dreaming and vividly conscious of an imaginary environment. With the aid of advanced imaging, researchers have identified the ascending reticular activating system’s (ARAS) role in maintaining consciousness. The ARAS is composed of many neuronal circuits that ascend from the brainstem to the cerebral cortex. Its primary function is to control the overall degree of consciousness by arousing and activating the cerebral cortex through signals sent via ascending neuronal fibers.⁹ While additional studies are warranted, several patients have recovered from an injury to the ARAS, reflecting a recovery in consciousness.^10–12

The neurobiology of consciousness is complex and multifaceted, but can be conceptualized as having two distinct but interrelated dimensions: arousal and awareness. These fall along a continuum ranging from being fully awake to coma.¹³ As such, when examining the neuroanatomical regions that contribute to consciousness, it is helpful to differentiate wakefulness from simple awareness and more complex states. Generally, a person must be awake to be aware. There are noted exceptions such as with REM sleep and when specific injuries have led to DOC (e.g., coma, persistent vegetative state). Therefore, wakefulness and awareness are not directly associated. By examining the brains of individuals with DOC, the neuronal processes underlying both awareness and arousal can be studied.

Arousal

While awareness includes the content of consciousness, arousal is also considered one level of consciousness.^14,15 Despite arousal being a critical component of consciousness, the neuronal processes are not well understood. Arousal starts the process of becoming conscious and is the opposite of coma.¹⁵ Discrete areas in the pons and midbrain, specialized cells in the hypothalamus called the paraventricular nucleus, along with cholinergic and glutamatergic cells in the thalamus are thought to control the level of arousal.^16–18 This system, formerly called the thalamocortical arousal system, when activated, is responsible for the transition from a sleep or sedated state to an awakening state. Information via anatomical links occurs by way of the claustrum, which is directly connected to the neocortex. These links form a network over the entire brain, including an individual’s perception of their level of arousal.¹⁵

The process of awakening occurs within seconds followed by a slower process to obtain full awareness. This process begins by increasing blood flow to the brainstem and thalamus then to the frontal lobe.¹⁹ The use of positron-emission tomography (PET) scans suggest reactivation of the anterior cortex can take 15-20 minutes to transition from REM sleep to a lighter slow-wave (SWS) sleep then an awake state. As this occurs, electrical changes in EEG wave frequencies decrease from gamma toward alpha and higher.²⁰ As a result, the severity of alterations in gamma bands are reflected as varying levels of consciousness. These states can range from a coma, vegetative state, minimal consciousness to a locked-in syndrome. Maintenance of gamma activity is critical for consciousness, memory, learning, and attention. Dysregulation can result in abnormal motor and sensory responses as well as a variety of other neurological and psychiatric disorders.²⁰

Awareness

Awareness is one of two key components comprising human consciousness. Awareness is one’s ability to perceive both their environment and self while arousal is reflected as one’s global state of responsiveness. Awareness has been described as the phenomena that occurs when an organism is alive, awake, alert, and aware of the current moment, time, and place. It is not necessarily concerned with the past or future, but its presence is necessary for more complex levels of consciousness (e.g., having an elaborate sense of self).²¹ The content of one’s consciousness generally comprises an individual’s “sensations, thoughts, emotions, memory, imagination, and other major psychological processes (p. 108).” ²²

Although the underlying neural code responsible for awareness remains largely unclear, it is understood that awareness is a dynamic and complex neurological process involving a variety of active cerebral networks at any given time. Researchers have determined that awareness predominantly depends on the functional integrity of the cerebral cortex and its reciprocal subcortical connections, each of which resides in spatially defined areas of the brain.²¹ While not every neural region for awareness has been identified, neurons connecting the brainstem with the intralaminar nuclei (ILN) of the thalamus play a key role.²³ The thalamus has two sets of ILN, one on the right side and one on the left side. If lesions are found on both ILN or if the neurons connecting them with the brainstem are severed or blocked, thus preventing signal input, awareness is completely lost. ^23,24 Other neuronal networks have also been implicated in awareness including the default mode network (DMN) and the executive control network (ECN). The DMN is primarily composed of the medial prefrontal cortex and bilateral posterior parietal cortices and is responsible for intrinsic awareness processes such as mind-wandering and autobiographical thinking. The ECN consists of the parietal brain regions and is responsible for the awareness of external stimuli, such as awareness of the environment ²⁵ and provides an overview of neural regions implicated in consciousness using various neuroimaging techniques.^23,26

Given that awareness is a subjective first-person experience with indistinct neuronal substrates, the determination of whether someone is conscious requires the interpretation of several clinical signs, most of which are based on observations of the interaction between the patient, the examiner (i.e., nurse), and the environment. Clinical evaluation of awareness is limited to following commands and evaluating a patient’s motor responsiveness such as eye-tracking or responding to localized sources of pain.^27,28 These limitations to objectively measuring consciousness underlie much of the social, ethical, and economical challenges clinicians face when caring for patients with DOC.

Nursing implications

Nursing care of patients with altered or impaired neuronal pathways and functioning requires special attention, care, and compassion for the individual as a person.²⁹ The acute and chronic phases of care can be challenging.³⁰ Understanding the level of functioning and tailoring patient care are critical.^21,22 Nurses must be able to incorporate objective and observational findings, adapt their assessments, and provide individualized care.³¹ Awareness is not all-or-nothing; subtle changes and fluctuations in the patient’s awareness must be identified to ensure appropriate timely interventions.³² Prolonged contact with patients places nurses in a unique position requiring the contribution of relevant information and active participation. This requires nurses to have knowledge of the clinical criteria for the various disorders of consciousness, components of comprehensive neurological assessment and assessment scales such as the Nociception Coma Scale.³⁰ Differentiating reflexes from purposeful activity as well as response to interventions are central to caring for patients unable to communicate.^21,22

Conclusions

Understanding that consciousness is dynamic and that relative components of arousal and awareness may fluctuate within short periods of time is an important empirical knowledge area in nursing care. Nurses are in a position to understand these fluctuations, as they are intricately involved in the hourly, and sometimes minute-by-minute, management of patients with these impairments. Remaining current in education concerning the empirical knowledge of DOC and associated components is an important and continuing area of focus. As advances in care progress, the survival rate of patients in intensive care feasibly increases. The integration of nursing into the rapidly advancing area of coma and consciousness research is important. While the survival rate may increase, an associated downstream effect is that there are more patients who experience DOC.