The central nervous system (CNS) is made up of the brain and spinal cord. It processes sensory input, controls movements, generates our thoughts and feelings, and manages decision-making. The more brain tissue an organism has, the more complex thinking it can do.
Some brain structures evolved earlier than others. The hindbrain contains the brain's oldest structures. It includes the medulla, the reticular formation, the cerebellum, and the pons. The midbrain contains structures important for orientation and moving around the environment. The brain's dopaminergic reward system also originates in this area of the brain. The brainstem is the central core of the brain. It includes the midbrain, medulla, and pons and connects the spinal cord to the brain. The medulla manages automatic survival functions, such as the heartbeat and breathing. The reticular formation is made up of connected circuits within the brain stem. Links from the reticular formation to the cortex control attention and the sleep-wake cycle by filtering incoming stimuli and controlling arousal. If the reticular formation is damaged or destroyed, coma results. If it is stimulated, people who are sleeping wake up, and those already awake become more alert. The cerebellum is involved in nonverbal learning, balance, and coordination. The pons relays information from the cerebellum to the rest of the brain.
Structures in the forebrain are more evolutionarily recent. The forebrain consists of the cerebral cortex and subcortical structures called the pituitary gland, the thalamus, and the limbic system. The pituitary gland is the body's master gland. It releases hormones that direct the function of many other glands in the body. The thalamus is the brain's sensory control center that directs messages between different brain regions. The limbic system is a brain region involved in motivation, emotion, learning, and memory. Some of the major structures it includes are the hypothalamus, the amygdala, and the hippocampus. The hypothalamus controls a number of vital functions. These include regulating body temperature, hunger and thirst, sexual behavior, circadian rhythm (daily physiological cycles), and hormone release from the pituitary gland. The hypothalamus also helps regulate emotional responses. The amygdala plays a central role in processing emotions and the formation of emotional memories. When emotionally arousing situations arise, the amygdala stimulates the hippocampus so that details of the event can be better remembered. The amygdala is especially involved in encoding fear-arousing events.The hippocampus is critical for creating new memories and integrating them into a network of general knowledge (semantic memory) and personal biographical events (episodic memory) that can be stored in other parts of the cerebral cortex. Damage to the hippocampus impairs the formation of new memories, although old memories can still be retrieved. Conditions such as Alzheimer's disease, Parkinson's disease, epilepsy, and high blood pressure can cause progressive shrinking of the hippocampus. The basal ganglia, which lie near the thalamus, direct intentional movement. Parkinson's disease and Huntington's disease attack these structures, resulting in uncontrollable trembling and jerking of the limbs.
The cerebral cortex is the part of the forebrain that surrounds older brain structures. The cerebral cortex is responsible for sensory processing and complex brain functions, such as reasoning, language, planning, and self-regulation. Only a small percentage of the cerebral cortex receives input from the body's sensory systems and directs actions. The rest of the cortex consists of multiple association areas. An association area interpret inputs from multiple sources, decides how to respond, and implements that response.
The cerebral cortex is divided into two separate hemispheres that communicate with each other by sending signals across a slender band of fibers called the corpus callosum. The left hemisphere controls the right side of the body, and the right hemisphere controls the left side of the body. The left hemisphere is specialized for sequential tasks, such as organizing words into grammatical sentences. The right hemisphere is specialized for holistic, spatial tasks, such as reading a map. The left hemisphere also stores verbal knowledge (such as vocabulary), while the right hemisphere processes the emotional aspects of language. Although each hemisphere has different strengths, these differences have been overstated in the popular press. People are not "right-brained" or "left-brained." Each person uses both hemispheres, and most tasks involve communication between the hemispheres.
If the corpus callosum is severed, as is sometimes done in cases of intractable epilepsy, the effects of hemispheric specialization become apparent. A blindfolded split-brain patient can verbally identify an object placed in the right hand but not an object placed in the left hand. Nonetheless, if the object is removed from the right hand and the blindfold is removed, the person is able to point to which object in an array was the one they felt. The right hemisphere knows what it experienced but cannot verbally identify it without assistance from the left hemisphere (where vocabulary is stored).
The different lobes of the brain are each involved in specific functions. The frontal lobe manages motor function, language, memory, and planning. A speech center called Broca's area is located in the left frontal lobe in most right-handed people. This area is located in the right hemisphere instead in about 27% of left-handed people. Broca's area is actively involved in processing the sequential aspects of language, as well as speech sounds and word meanings. It is named after French physician Paul Broca, who first identified this area in the late 19th century. Damage to Broca's area results in Broca's aphasia, in which speech production—but not comprehension—is severely impaired. This aphasia is characterized by labored speech, frequent pauses, and trouble using function words (such as of, the, and that).
The motor cortex is a strip of neural tissue located directly in front of the Rolandic fissure (in the cerebral cortex), which separates the frontal lobes from the parietal and temporal lobes. Different parts of the motor cortex correspond to and control different body parts. The larger the area of motor cortex devoted to a body part, the greater the control and dexterity. For example, more motor cortex is devoted to the fingers than the toes.
The prefrontal cortex is an area of the frontal lobe that is divided into three regions: the ventromedial, dorsolateral, and orbitofrontal regions. These areas are involved in planning, attention, problem-solving, error monitoring, decision-making, social cognition, and working memory. Damage to these areas results in loss of cognitive and emotional control. The first well-documented case demonstrating such effects was that of Phineas Gage, a 25-year-old railroad worker who suffered severe head trauma on September 13, 1848, when an explosion drove a 3-foot, 13-pound rod through his left cheek, eye, and prefrontal cortex. Before the accident, Gage had been a mild-mannered and conscientious worker. After the accident, he became irritable, irresponsible, indecisive, and prone to outbursts of profanity.
The parietal lobe manages sensation and perception. This lobe includes the somatosensory cortex, a strip of neural tissue located just behind the Rolandic fissure. Each part of the somatosensory cortex maps onto a particular part of the body. The larger the area devoted to a body part, the more sensitive that body part is. For example, larger areas of the somatosensory cortex are devoted to the lips and fingers than to the entire back.
The temporal lobe is involved in auditory processing, language, and memory storage. It contains a region called the auditory cortex that receives inputs from the ears. Another region, called Wernicke's area, handles language comprehension. This region is named after German physician Carl Wernicke, who first discovered it in 1874. Damage to this area results in Wernicke's aphasia, which is characterized by impaired language comprehension and fluent speech that is nonsensical.The occipital lobe is responsible for visual processing. There are two separate pathways from the eye to the cortex. One identifies objects, and the other locates them in space. A separate region in both occipital lobes, called the fusiform face area, is specialized for processing faces. Recognizing individuals is an important function that allows identification of one's offspring, kin, mate, friends, and foes.