cranial nerves and
spinal cord, the
peripheral nervous system (PNS) and regulates virtually all human activity.
[1] Involuntary, or "lower," actions, such as
heart rate,
respiration, and
digestion, are unconsciously governed by the brain,
[1][2] specifically through the
autonomic nervous system. Complex, or "higher," mental activity, such as
thought,
reason, and
abstraction,
[2] is consciously controlled.
Anatomically, the brain can be divided into three parts: the
forebrain,
midbrain, and
hindbrain;
[3] the forebrain includes the several lobes of the
cerebral cortex that control higher functions, while the mid- and hindbrain are more involved with unconscious, autonomic functions. During
encephalization, human brain mass increased beyond that of other species relative to body mass. This process was especially pronounced in the
neocortex, a section of the brain involved with
language and
consciousness. The neocortex accounts for about 76% of the mass of the human brain;
[4] with a neocortex much larger than other animals, humans enjoy unique mental capacities despite having a
neuroarchitecture similar to that of more primitive
species. Basic systems that alert humans to stimuli, sense events in the environment, and maintain
homeostasis are similar to those of basic
vertebrates. Human
consciousness is founded upon the extended capacity of the modern neocortex, as well as the greatly developed structures of the
brain stem.
Neurophysiology
The human brain is the source of the conscious,
cognitive mind. The mind is the set of cognitive processes related to
perception, interpretation,
imagination,
memories, and crucially
language (cf.
Broca's area) of which a person may or may not be aware. Beyond cognitive functions, the brain regulates autonomic processes related to essential body functions such as respiration and heartbeat. The brain controls all movement from lifting a pencil to building a superstructure.
Extended neocortical capacity allows humans some control over
emotional behavior, but neural pathways between emotive centers of the brain stem and
cerebral motor control areas are shorter than those connecting complex cognitive areas in the neocortex with incoming sensory
information from the brain stem. Powerful emotional pathways can modulate spontaneous emotive expression regardless of attempts at cerebral self-control. Emotive stability in humans is associated with
planning,
experience, and an environment that is both stable and stimulating.
The 19th century discovery of the
primary motor cortex mapped to correspond with regions of the body led to popular belief that the brain was organized around a
homunculus. A distorted figure drawn to represent the body's motor map in the
prefrontal cortex was popularly recognized as the brain's homunculus, but function of the human brain is far more complex than this simple figure suggests. A similar, "sensory homunculus" can be drawn in the parietal lobe that parallels that in the frontal lobe.
The human brain appears to have no localized center of conscious control. The brain seems to derive consciousness from interaction among numerous systems within the brain.
Executive functions rely on cerebral activities, especially those of the
frontal lobes, but redundant and complementary processes within the brain result in a diffuse assignment of executive control that can be difficult to attribute to any single locale.
Visual perception generally is processed in the
occipital lobe, whereas the
primary auditory cortex resides in the
temporal lobe.
Midbrain functions include routing, selecting, mapping, and cataloging
information, including information perceived from the environment and information that is remembered and processed throughout the cerebral cortex.
Endocrine functions housed in the midbrain play a leading role in modulating
arousal of the cortex and of autonomic systems.
Nerves from the brain stem complex where autonomic functions are modulated join nerves routing messages to and from the cerebrum in a bundle that passes through the
spinal column to related parts of a body. Twelve pairs of
cranial nerves, including some that innervate parts of the head, follow pathways from the medulla oblongata outside the spinal cord.
A description of the
biological basis for consciousness so far eludes the best efforts of the current generation of
researchers. But reasonable assumptions based on observable behaviors and on related internal responses have provided the basis for general classification of elements of consciousness and of likely neural regions associated with those elements. Researchers know people lose consciousness and regain it, they have identified partial losses of consciousness associated with particular
neuropathologies and they know that certain conscious activities are impossible without particular neural structures.
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