The brain is the part of the body which lets us, and other animals, make sense of the world. It gets input from sense organs, and changes behaviour in response to this information. In humans, the brain also controls our use of language, and is capable of abstract thought. The brain is the control centre of the whole body. The brain is made up of a special type of cells. They are connected with each other and with the nerves in our body. In all animals the delicate brain is protected in some way. In ourselves, and all vertebrates, it is protected by the bones of the skull.
The brain does the thinking, learning, and feeling for the body. For humans, it is the source of consciousness. The brain also controls basic autonomic body actions, like breathing, digestion, heartbeat, that happen automatically. These activities, and much else, are governed by unconscious functions of the brain and nervous system. All the information about the world gathered by our senses is sent through nerves into the brain, allowing us to see, hear, smell, taste and feel things. The brain processes this information, and we experience it as pictures, sounds, and so on. The brain also uses nerves to tell the body what to do, for example by telling muscles to move or our heart to beat faster.
All vertebrates have brains and, over time, their brains have evolved to become more complex. Some simple animals, however, like sponges, do not have anything like a brain. Segmented invertebrates have ganglions in each segment, and a ring of nervous tissue around the alimentary canal at the front. This acts to bring sense data from the front into play with the movement of the body.
The human brain is provided with information about light, sound, the chemical composition of the atmosphere, temperature, head orientation, limb position, the chemical composition of the bloodstream, and more. In other animals additional senses are present, such as the infrared heat-sense of snakes, the magnetic field sense of some birds, or the electric field sense of some types of fish.
Each sensory system begins with specialized receptor cells,
Motor systems are areas of the brain that are involved in initiating body movements, that is, in activating muscles. Except for the muscles that control the eye, which are driven by nuclei in the midbrain, all the voluntary muscles in the body are directly innervated by motor neurons in the spinal cord and hindbrain.
The brain contains several motor areas that project directly to the spinal cord. At the lowest level are motor areas in the medulla and pons, which control stereotyped movements such as walking, breathing, or swallowing. At a higher level are areas in the midbrain, such as the red nucleus, which is responsible for coordinating movements of the arms and legs. At a higher level yet is the primary motor cortex, a strip of tissue located at the posterior edge of the frontal lobe. The primary motor cortex sends projections to the subcortical motor areas, but also sends a massive projection directly to the spinal cord, through the pyramidal tract. This direct corticospinal projection allows for precise voluntary control of the fine details of movements. Other motor-related brain areas exert secondary effects by projecting to the primary motor areas. Among the most important secondary areas are the premotor cortex, basal ganglia, and cerebellum.
|Ventral horn||Spinal cord||Contains motor neurons that directly activate muscles|
|Oculomotor nuclei||Midbrain||Contains motor neurons that directly activate the eye muscles|
|Cerebellum||Hindbrain||Calibrates precision and timing of movements|
|Basal ganglia||Forebrain||Action selection on the basis of motivation|
|Motor cortex||Frontal lobe||Direct cortical activation of spinal motor circuits|
|Premotor cortex||Frontal lobe||Groups elementary movements into coordinated patterns|
|Supplementary motor area||Frontal lobe||Sequences movements into temporal patterns|
|Prefrontal cortex||Frontal lobe||Planning and other executive functions|
In addition to all of the above, the brain and spinal cord contain extensive circuitry to control the autonomic nervous system, which works by secreting hormones and by modulating the "smooth" muscles of the gut.
- See also: Sleep
Many animals alternate between sleeping and waking in a daily cycle. Arousal and alertness are also modulated on a finer time scale by a network of brain areas.
A key component of the arousal system is the suprachiasmatic nucleus (SCN), a tiny part of the hypothalamus located directly above the point at which the optic nerves from the two eyes cross. The SCN contains the body's central biological clock. Neurons there show activity levels that rise and fall with a period of about 24 hours, circadian rhythms: these activity fluctuations are driven by rhythmic changes in expression of a set of "clock genes". The SCN continues to keep time even if it is excised from the brain and placed in a dish of warm nutrient solution, but it ordinarily receives input from the optic nerves, through the retinohypothalamic tract (RHT), that allows daily light-dark cycles to calibrate the clock.
The SCN projects to a set of areas in the hypothalamus, brainstem, and midbrain that are involved in implementing sleep-wake cycles. An important component of the system is the reticular formation, a group of neuron-clusters scattered diffusely through the core of the lower brain. Reticular neurons send signals to the thalamus, which in turn sends activity-level-controlling signals to every part of the cortex. Damage to the reticular formation can produce a permanent state of coma.
Sleep involves great changes in brain activity.
For any animal, survival requires maintaining a variety of parameters of bodily state within a limited range of variation: these include temperature, water content, salt concentration in the bloodstream, blood glucose levels, blood oxygen level, and others. The ability of an animal to regulate the internal environment of its body is known as homeostasis.
Maintaining homeostasis is a crucial function of the brain. The basic principle that underlies homeostasis is negative feedback: any time a parameter diverges from its set-point, sensors generate an error signal that evokes a response that causes the parameter to shift back toward its optimum value. (This principle is widely used in engineering, for example in the control of temperature using a thermostat.)
In vertebrates, the part of the brain that plays the greatest role is the hypothalamus, a small region at the base of the forebrain whose size does not reflect its complexity or the importance of its function. The hypothalamus is a collection of small nuclei, most of which are involved in basic biological functions. Some of these functions relate to arousal or to social interactions such as sexuality, aggression, or maternal behaviors; but many of them relate to homeostasis.
The individual animals need to express survival-promoting behaviors, such as seeking food, water, shelter, and a mate. The motivational system in the brain monitors the current state of satisfaction of these goals, and activates behaviors to meet any needs that arise. The motivational system works largely by a reward–punishment mechanism. When a particular behavior is followed by favorable consequences, the reward mechanism in the brain is activated, which induces structural changes inside the brain that cause the same behavior to be repeated later, whenever a similar situation arises. Conversely, when a behavior is followed by unfavorable consequences, the brain's punishment mechanism is activated, inducing structural changes that cause the behavior to be suppressed when similar situations arise in the future.
Most organisms studied to date utilize a reward–punishment mechanism: for instance, worms and insects can alter their behavior to seek food sources or to avoid dangers. In vertebrates, the reward-punishment system is implemented by a specific set of brain structures, at the heart of which lie the basal ganglia, a set of interconnected areas at the base of the forebrain. The basal ganglia are the central site at which decisions are made: the basal ganglia exert a sustained inhibitory control over most of the motor systems in the brain; when this inhibition is released, a motor system is permitted to execute the action it is programmed to carry out. Rewards and punishments function by altering the relationship between the inputs that the basal ganglia receive and the decision-signals that are emitted. The reward mechanism is better understood than the punishment mechanism, because its role in drug abuse has caused it to be studied very intensively. Research has shown that the neurotransmitter dopamine plays a central role: addictive drugs such as cocaine, amphetamine, and nicotine either cause dopamine levels to rise or cause the effects of dopamine inside the brain to be enhanced.
Learning and memory
Almost all animals are capable of modifying their behavior as a result of experience—even the most primitive types of worms. Because behavior is driven by brain activity, changes in behavior must somehow correspond to changes inside the brain. Already in the late 19th century theorists like Santiago Ramón y Cajal argued that the most plausible explanation is that learning and memory are expressed as changes in the synaptic connections between neurons. Until 1970, however, experimental evidence to support the synaptic plasticity hypothesis was lacking. In 1971 Tim Bliss and Terje Lømo published a paper on a phenomenon now called long-term potentiation: the paper showed clear evidence of activity-induced synaptic changes that lasted for at least several days. Since then technical advances have made these sorts of experiments much easier to carry out, and thousands of studies have been made that have clarified the mechanism of synaptic change, and uncovered other types of activity-driven synaptic change in a variety of brain areas, including the cerebral cortex, hippocampus, basal ganglia, and cerebellum. Brain-derived neurotrophic factor (BDNF) and physical activity appear to play a beneficial role in the process.
Neuroscientists currently distinguish several types of learning and memory that are implemented by the brain in distinct ways:
- Working memory is the ability of the brain to maintain a temporary representation of information about the task that an animal is currently engaged in. This sort of dynamic memory is thought to be mediated by the formation of cell assemblies—groups of activated neurons that maintain their activity by constantly stimulating one another.
- Episodic memory is the ability to remember the details of specific events. This sort of memory can last for a lifetime. Much evidence implicates the hippocampus in playing a crucial role: people with severe damage to the hippocampus sometimes show amnesia, that is, inability to form new long-lasting episodic memories.
- Semantic memory is the ability to learn facts and relationships. This sort of memory is probably stored largely in the cerebral cortex, mediated by changes in connections between cells that represent specific types of information.
- Instrumental learning is the ability for rewards and punishments to modify behavior. It is implemented by a network of brain areas centered on the basal ganglia.
- Motor learning is the ability to refine patterns of body movement by practicing, or more generally by repetition. A number of brain areas are involved, including the premotor cortex, basal ganglia, and especially the cerebellum, which functions as a large memory bank for microadjustments of the parameters of movement.
In mammals, the brain is made of three main parts: the cerebrum, the cerebellum and the brainstem. The surface of the cerebrum is the cerebral cortex, which all vertebrates have. Mammals also have an extra layer, the neocortex. This is the key to the behaviour which is typical of mammals, especially humans.
The cortex has sensory, motor, and association areas. The sensory areas are the areas that receive and process information from the senses. The motor areas control voluntary movements, especially fine movements performed by the hand. The right half of the motor area controls the left side of the body, and vice versa. Association areas produce a meaningful experience of the world, and supports abstract thinking and language. This enables us to interact effectively. Most connections are from one area of the cortex to another, rather than to subcortical areas; The figure may be as high as 99%.
The cerebellum coordinates muscles so they work together. It is also the centre of balance, a vital part of movement.
The brain stem is at the back of the brain (actually underneath it in humans). It joins the rest of the brain with the spinal cord. It has lots of different parts that control different jobs in the body: for instance, the brain stem controls breathing, heartbeat, sneezing, eye blinking, and swallowing. Body temperature and hunger are also controlled by parts of the brain stem.
The volume of the human brain (relative to the size of the whole body) is very large, compared to that of most other animals. The human brain also has a very large surface (called cortex) for its size, which is possible because it is very wrinkled. If the human cortex were flattened, it would be close to a square meter in area. Some other animals also have very wrinkled brains, such as dolphins and elephants. Here is a rule of thumb: the larger an animal is, the larger its brain will be.p15 Even allowing for that, the human brain, and in particular the neocortex, is very large. We know it increased in size four-fold over the last several million years of evolution.p79 There are ideas about why this happened, but no-one is quite sure. Most theories suggest complex social activity and the evolution of language would make a larger brain advantageous.p80 As an additional note, Einstein's brain weighed only 1,230 grams, which is less than the average adult male brain (about 1,400 grams). The detailed organisation of a brain obviously matters, but in ways which are not understood at present.
A human brain accounts for about 2% of the body's weight, but it uses about 20% of its energy. It has about 50–100 billion nerve cells (also called neurons), and roughly the same number of support cells, called glia. The job of neurons is to receive and send information to and from the rest of the body, while glia provide nutrients and guide blood flow to the neurons, allowing them to do their job. Each nerve cell has contact with as many as 10,000 other nerve cells through connections called synapses.
Neurons generate electrical signals that travel along their axons. When a pulse of electricity reaches a junction called a synapse, it causes a neurotransmitter chemical to be released, which binds to receptors on other cells and thereby alters their electrical activity.
Neurons often have extensive networks of dendrites, which receive synaptic connections. Shown is a pyramidal neuron from the hippocampus, stained for green fluorescent protein.
Illustration by René Descartes of how the brain implements a reflex response
Andreas Vesalius' Fabrica, published in 1543, showing the base of the human brain, including optic chiasma, cerebellum, olfactory bulbs, etc.
Drawing by Santiago Ramón y Cajal of two types of Golgi-stained neurons from the cerebellum of a pigeon
Fruit flies (Drosophila) have been extensively studied to gain insight into the role of genes in brain development.
The brain of a shark
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