Brain

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For other uses of the term brain see brain (disambiguation). For information on the human brain specifically, please see its article.

Neuroscience Portal

In animals, the brain, or  encephalon (Greek for "in the head"), is  the control center of the central nervous system. In most animals, the brain is  located in the head, protected by the skull and  close to the primary sensory apparatus of vision, hearing, taste and  olfaction. In humans, it  is an  organ of thought. While all vertebrates have  a brain, invertebrates have  either a centralized brain or  collections of individual ganglia. Brains can be extremely complex. For example, the human brain contains more  than 100 billion neurons, each linked to as many as 10,000 others.

A sketch of the human brain by artist Priyan Weerappuli, imposed upon his sketch of the profile of Michaelangelo's David

[edit] Overview

Most brains exhibit a substantial distinction between gray matter and  white matter. Gray matter consists of the cell bodies of the neurons, while white matter consists of the fibers (axons) that connect neurons. The axons are  surrounded by a fatty insulating sheath called myelin, giving the white matter its distinctive color. The outer layer of the brain is  gray matter called cerebral cortex. Deep in the brain, compartments of white matter (fasciculi, fiber tracts), gray matter (nuclei) and  spaces filled with cerebrospinal fluid (ventricles) are  found.

The brain innervates the head through cranial nerves, and  it communicates with the spinal cord, which innervates the body through spinal nerves. Nervous fibers transmitting signal from the brain are  called efferent fibers. The fibers transmitting signals to the brain are  called afferent (or sensory) fibers. Nerves can be afferent, efferent or  mixed (i.e., containing both types of fibers).

The brain controls a wide variety of functions. it  is the site of reason and  intelligence, which include such components as cognition, perception, attention, memory and  emotion. The brain is  also responsible for control of posture and  movements. it  makes possible cognitive, motor and  other forms of learning. The brain can perform a variety of functions automatically, without the need for conscious awareness, such as coordination of sensory systems (eg. sensory gating and  multisensory integration), walking, and  homeostatic body functions such as heart rate, blood pressure, fluid balance, and  body temperature.

Many functions are  controlled by coordinated activity of the brain and  spinal cord. Moreover, some behaviors such as simple reflexes and  basic locomotion, can be executed under spinal cord control alone.

The brain undergoes transitions from wakefulness to sleep (and subtypes of these states). These state transitions are  crucially important for proper brain functioning. (For example, it  is believed that sleep is  important for knowledge consolidation, as the neurons appear to organize the day's stimuli during deep sleep by randomly firing off the most recently used neuron pathways; additionally, without sleep, normal subjects are  observed to develop symptoms resembling mental illness, even auditory hallucinations). Every brain state is  associated with characteristic brain waves.

Neurons are  electrically active brain cells that process information, whereas Glial cells perform supporting function. Brain cell metabolism consumes considerable amounts of energy. In addition to being electrically active, neurons constantly synthesise neurotransmitters. Neurons modify their properties (guided by gene expression) under the influence of their input signals. This plasticity underlies learning and  adaptation. it  is notable that some unused neuron pathways (constructions which have  become physically isolated from other cells) may continue to exist long after the memory is  absent from consciousness, possibly developing the subconscious.

The study of the brain is  known as neuroscience, a field of biology aimed at understanding the functions of the brain at every level, from the molecular up to the psychological. There is  also a branch of psychology that deals with the anatomy and  physiology of the brain, known as biological psychology. This field of study focuses on each individual part of the brain and  how it  assesses different parts of the body.

[edit] History

Main article: History of the brain

Early human views on the function of the brain regarded it  to be a form of “cranial stuffing” of sorts. In Egypt, from the late Middle Kingdom onwards, in preparation for mummification, the brain was regularly removed, for it  was the heart that was assumed to be the seat of intelligence. According to Herodotus, during the first step of mummification: ‘The most perfect practice is  to extract as much of the brain as possible with an  iron hook, and  what the hook cannot reach is  mixed with drugs.’ Over the next five-thousand years, this view came to be reversed; the brain is  now known to be the seat of intelligence, although colloquial variations of the former remain as in “memorizing something by heart”.

The first thoughts of the field of psychology actually came from ancient philosophers, including Aristotle. After philosophers have  become more  in tune with medical research over time, the idea of psychology had developed. From that point, different branches of psychology emerged with different individuals creating new theories. For example, Evolutionary Psychology was created with theories by Charles Darwin.

[ and  brain">edit] Mind and  brain

and  Brain"> and  Brain" width="32" height="32" longdesc="../../../p/o/r/Image%7EPortal.svg_9858.html" />

and  Brain">Mind and  Brain Portal

A distinction is  often made in the philosophy of mind between the mind and  the brain, and  there is  some controversy as to their exact relationship, leading to the mind-body problem. The brain is  defined as the physical and  biological matter contained within the skull, responsible for all electrochemical neuronal processes. The mind, however, is  seen in terms of mental attributes, such as beliefs or  desires. Some believe that the mind exists in some way independently of the brain, such as in a soul or  epiphenomenon. Others, such as strong AI theorists, say that the mind is  directly analogous to computer software and  the brain to hardware.

[edit] Comparative anatomy

A mouse brain.

Three groups of animals have  notably complex brains: the arthropods (insects and  crustaceans), the cephalopods (octopuses, squids, and  similar mollusks), and  the craniates (vertebrates).^[1] The brain of arthropods and  cephalopods arises from twin parallel nerve cords that extend through the body of the animal. Arthropods have  a central brain with three divisions and  large optical lobes behind each eye for visual processing.^[1]

The brain of craniates develops from the anterior section of a single dorsal nerve cord, which later becomes the spinal cord.^[2] In craniates, the brain is  protected by the bones of the skull. In vertebrates, increasing complexity in the cerebral cortex correlates with height on the phylogenetic and  evolutionary tree. Primitive vertebrates such as fish, reptiles, and  amphibians have  fewer than six layers of neurons in the outer layer of their brains. This cortical configuration is  called the allocortex (or heterotypic cortex).^[3]

More complex vertebrates such as mammals have  a six-layered neocortex (or homotypic cortex, neopallium), in addition to having some parts of the brain that are  allocortex.^[3] In mammals, increasing convolutions of the brain are  characteristic of animals with more  advanced brains. These convolutions provide a larger surface area for a greater number of neurons while keeping the volume of the brain compact enough to fit inside the skull. The folding allows more  grey matter to fit into a smaller volume, similar to a really long slinky being able to fit into a tiny box when completely pushed together. The folds are  called gyri, while the spaces between the folds are  called sulci.

Although the general histology of the brain is  similar from person to person, the structural anatomy can differ. Apart from the gross embryological divisions of the brain, the location of specific gyri and  sulci, primary sensory regions, and  other structures differs between species.

[edit] Invertebrates

In insects, the brain has four parts, the optical lobes, the protocerebrum, the deutocerebrum, and  the tritocerebrum. The optical lobes are  behind each eye and  process visual stimuli.^[1] The protocerebrum contains the mushroom bodies, which respond to smell, and  the central body complex. In some species such as bees, the mushroom body receives input from the visual pathway as well. The deutocerebrum includes the antennal lobes, which are  similar to the mammalian olfactory bulb, and  the mechanosensory neuropils which receive information from touch receptors on the head and  antennae. The antennal lobes of flies and  moths are  quite complex.

In cephalopods, the brain has two regions: the supraesophageal mass and  the subesophageal mass,^[1] separated by the esophagus. The supra- and  subesophageal masses are  connected to each other on either side of the esophagus by the basal lobes and  the dorsal magnocellular lobes.^[1] The large optic lobes are  sometimes not considered to be part of the brain, as they are  anatomically separate and  are joined to the brain by the optic stalks. However, the optic lobes perform much visual processing, and  so functionally are  part of the brain.

[edit] Vertebrates

 and  parietal lobes (orange). The cerebellum (blue) is  not part of the telencephalon. In vertebrates a gross division into three major parts is  used."> and  parietal lobes (orange). The cerebellum (blue) is  not part of the telencephalon. In vertebrates a gross division into three major parts is  used." width="350" height="207" longdesc="../../../b/r/a/Image%7EBrain-anatomy.jpg_fd85.html" />

The lobes of the cerebral cortex include the frontal (red), temporal (green), occipital (yellow), and  parietal lobes (orange). The cerebellum (blue) is  not part of the telencephalon. In vertebrates a gross division into three major parts is  used.

The telencephalon (cerebrum) is  the largest region of the mammalian brain. This is  the structure that is  most easily visible in brain specimens, and  is what most people associate with the "brain". In humans and  several other animals, the fissures (sulci) and  convolutions (gyri) give the brain a wrinkled appearance. In non-mammalian vertebrates with no cerebrum, the metencephalon is  the highest center in the brain. Because humans walk upright, there is  a flexure, or  bend, in the brain between the brain stem and  the cerebrum. Other vertebrates do not have  this flexure. Generally, comparing the locations of certain brain structures between humans and  other vertebrates often reveals a number of differences.

Behind (or in humans, below) the cerebrum is  the cerebellum. The cerebellum is  known to be involved in the control of movement,^[4] and  is connected by thick white matter fibers (cerebellar peduncles) to the pons.^[3] The cerebrum and  the cerebellum each have  two hemispheres. The telencephalic hemispheres are  connected by the corpus callosum, another large white matter tract. an  outgrowth of the telencephalon called the olfactory bulb is  a major structure in many animals, but in humans and  other primates it  is relatively small.

Vertebrate nervous systems are  distinguished by bilaterally symmetrical encephalization. Encephalization refers to the tendency for more  complex organisms to gain larger brains through evolutionary time. Larger vertebrates develop a complex, layered and  interconnected neuronal circuitry. In modern species most closely related to the first vertebrates, brains are  covered with gray matter that has a three-layer structure (allocortex). Their brains also contain deep brain nuclei and  fiber tracts forming the white matter. Most regions of the human cerebral cortex have  six layers of neurons (neocortex).^[3]

[edit] Vertebrate brain regions

(See related article at List of regions in the human brain)

 and  hindbrain structures."> and  hindbrain structures." width="300" height="247" longdesc="../../../e/m/b/Image%7EEmbryonicBrain.png_a77f.html" />

Diagram depicting the main subdivisions of the embryonic vertebrate brain. These regions will later differentiate into forebrain, midbrain and  hindbrain structures.

According to the hierarchy based on embryonic and  evolutionary development, chordate brains are  composed of the three regions that later develop into five total divisions:

• Rhombencephalon (hindbrain)
  • Myelencephalon
  • Metencephalon
• Mesencephalon (midbrain)
• Prosencephalon (forebrain)
  • Diencephalon
  • Telencephalon

The brain can also be classified according to function, including divisions such as:

• Limbic system
• Sensory systems
  • Visual system
  • Olfactory system
  • Gustatory system
  • Auditory system
  • Somatosensory system
• Motor system
• Associative areas

[edit] Humans

Main article: human brain

The structure of the human brain differs from that of other animals in several important ways. These differences allow for many abilities over and  above those of other animals, such as advanced cognitive skills. Human encephalization is  especially pronounced in the neocortex, the most complex part of the cerebral cortex. The proportion of the human brain that is  devoted to the neocortex—especially to the prefrontal cortex—is larger than in all other animals.

Humans have  unique neural capacities, but much of their brain structure is  similar to that of other mammals. Basic systems that alert the nervous system to stimulus, that sense events in the environment, and  monitor the condition of the body are  similar to those of even non-mammalian vertebrates. The neural circuitry underlying human consciousness includes both the advanced neocortex and  prototypical structures of the brainstem. The human brain also has a massive number of synaptic connections allowing for a great deal of parallel processing.

[edit] Neurobiology

The brain is  composed of two broad classes of cells, neurons and  glia, both of which contain several different cell types which perform different functions. Interconnected neurons form neural networks (or neural ensembles). These networks are  similar to man-made electrical circuits in that they contain circuit elements (neurons) connected by biological wires (nerve fibers). These do not form simple one-to-one electrical circuits like many man-made circuits, however. Typically neurons connect to at least a thousand other neurons.^[5] These highly specialized circuits make up systems which are  the basis of perception, different types of action, and  higher cognitive function.

[edit] Histology

Diagram of basic features of a neuron.

Neurons are  the cells that generate action potentials and  convey information to other cells; these constitute the essential class of brain cells.

In addition to neurons, the brain contains glial cells in a roughly 10:1 proportion to neurons. Glial cells ("glia" is  Greek for “glue”) form a support system for neurons. They create the insulating myelin, provide structure to the neuronal network, manage waste, and  clean up neurotransmitters. Most types of glia in the brain are  present in the entire nervous system. Exceptions include the oligodendrocytes which myelinate neural axons (a role performed by Schwann cells in the peripheral nervous system). The myelin in the oligodendrocytes insulates the axons of some neurons. White matter in the brain is  myelinated neurons, while grey matter contains mostly cell soma, dendrites, and  unmyelinated portions of axons and  glia. The space between neurons is  filled with dendrites as well as unmyelinated segments of axons; this area is  referred to as the neuropil.

In mammals, the brain is  surrounded by connective tissues called the meninges, a system of membranes that separate the skull from the brain. This three-layered covering is  composed of (from the outside in) the dura mater, arachnoid mater, and  pia mater. The arachnoid and  pia are  physically connected and  thus often considered as a single layer, the pia-arachnoid. Below the arachnoid is  the subarachnoid space which contains cerebrospinal fluid, a substance that protects the nervous system. Blood vessels enter the central nervous system through the perivascular space above the pia mater. The cells in the blood vessel walls are  joined tightly, forming the blood-brain barrier which protects the brain from toxins that might enter through the blood.

The brain is  bathed in cerebrospinal fluid (CSF), which circulates between layers of the meninges and  through cavities in the brain called ventricles. it  is important both chemically for metabolism and  mechanically for shock-prevention. For example, the human brain weighs about 1-1.5 kg. The mass and  density of the brain are  such that it  will begin to collapse under its own weight. The CSF allows the brain to float, easing the physical stress caused by the brain’s mass.

[edit] Function

Vertebrate brains receive signals through nerves arriving from the sensors of the organism. These signals are  then processed throughout the central nervous system; reactions are  formulated based upon reflex and  learned experiences. A similarly extensive nerve network delivers signals from a brain to control important muscles throughout the body. Anatomically, the majority of afferent and  efferent nerves (with the exception of the cranial nerves) are  connected to the spinal cord, which then transfers the signals to and  from the brain.

Sensory input is  processed by the brain to recognize danger, find food, identify potential mates, and  perform more  sophisticated functions. Visual, touch, and  auditory sensory pathways of vertebrates are  routed to specific nuclei of the thalamus and  then to regions of the cerebral cortex that are  specific to each sensory system. The visual system, the auditory system, and  the somatosensory system. Olfactory pathways are  routed to the olfactory bulb, then to various parts of the olfactory system. Taste is  routed through the brainstem and  then to other portions of the gustatory system.

To control movement the brain has several parallel systems of muscle control. The motor system controls voluntary muscle movement, aided by the motor cortex, cerebellum, and  the basal ganglia. The system eventually projects to the spinal cord and  then out to the muscle effectors. Nuclei in the brain stem control many involuntary muscle functions such as heart rate and  breathing. In addition, many automatic acts (simple reflexes, locomotion) can be controlled by the spinal cord alone.

Brains also produce a portion of the body's hormones that can influence organs and  glands elsewhere in a body—conversely, brains also react to hormones produced elsewhere in the body. In mammals, the hormones that regulate hormone production throughout the body are  produced in the brain by the structure called the pituitary gland.

It is  hypothesized that developed brains derive consciousness from the complex interactions between the numerous systems within the brain. Cognitive processing in mammals occurs in the cerebral cortex but relies on midbrain and  limbic functions as well. Among "younger" (in an  evolutionary sense) vertebrates, advanced processing involves progressively rostral (forward) regions of the brain.

Hormones, incoming sensory information, and  cognitive processing performed by the brain determine the brain state. Stimulus from any source can trigger a general arousal process that focuses cortical operations to processing of the new information. This focusing of cognition is  known as attention. Cognitive priorities are  constantly shifted by a variety of factors such as hunger, fatigue, belief, unfamiliar information, or  threat. The simplest dichotomy related to the processing of threats is  the fight-or-flight response mediated by the amygdala and  other limbic structures.

[edit] Brain pathology

A human brain showing frontotemporal lobar degeneration causing frontotemporal dementia.

Clinically, death is  defined as an  absence of brain activity as measured by EEG. Injuries to the brain tend to affect large areas of the organ, sometimes causing major deficits in intelligence, memory, and  movement. Head trauma caused, for example, by vehicle and  industrial accidents, is  a leading cause of death in youth and  middle age. In many cases, more  damage is  caused by resultant swelling (edema) than by the impact itself. Stroke, caused by the blockage or  rupturing of blood vessels in the brain, is  another major cause of death from brain damage.

Other problems in the brain can be more  accurately classified as diseases rather than injuries. Neurodegenerative diseases, such as Alzheimer's disease, Parkinson's disease, motor neurone disease, and  Huntington's disease are  caused by the gradual death of individual neurons, leading to decrements in movement control, memory, and  cognition. Currently only the symptoms of these diseases can be treated. Mental illnesses, such as clinical depression, schizophrenia, bipolar disorder, and  post-traumatic stress disorder are  brain diseases that impact personality and, typically, other aspects of mental and  somatic function. These disorders may be treated by psychiatric therapy, pharmaceutical intervention, or  through a combination of treatments; therapeutic effectiveness varies significantly among individuals.

Some infectious diseases affecting the brain are  caused by viruses and  bacteria. Infection of the meninges, the membrane that covers the brain, can lead to meningitis. Bovine spongiform encephalopathy (also known as mad cow disease), is  deadly in cattle and  humans and  is linked to prions. Kuru is  a similar prion-borne degenerative brain disease affecting humans. Both are  linked to the ingestion of neural tissue, and  may explain the tendency in some species to avoid cannibalism. Viral or  bacterial causes have  been substantiated in multiple sclerosis, Parkinson's disease, Lyme disease, encephalopathy, and  encephalomyelitis.

Some brain disorders are  congenital. Tay-Sachs disease, Fragile X syndrome, and  Down syndrome are  all linked to genetic and  chromosomal errors. Malfunctions in the embryonic development of the brain can be caused by genetic factors, by drug use, and  disease during a mother's pregnancy.

Certain brain disorders are  treated by brain surgeons (neurosurgeons) while others are  treated by neurologists and  psychiatrists.

[edit] Study of the brain

[edit] Fields of study

Neuroscience seeks to understand the nervous system, including the brain, from a biological and  computational perspective. Psychology seeks to understand behavior and  the brain. The terms neurology and  psychiatry usually refer to medical applications of neuroscience and  psychology respectively. Cognitive science seeks to unify neuroscience and  psychology with other fields that concern themselves with the brain, such as computer science (artificial intelligence and  similar fields) and  philosophy.

[edit] Methods of observation

Main article: neuroimaging

[edit] Electrophysiology

Each method for observing activity in the brain has its advantages and  drawbacks. Electrophysiology allows scientists to record the electrical activity of individual neurons or  groups of neurons.

[edit] EEG

By placing electrodes on the scalp one can record the summed electrical activity of the cortex in a technique known as electroencephalography (EEG). EEG measures the mass changes in electrical current from the cerebral cortex, but can only detect changes over large areas of the brain with very little sub-cortical activity.

[ and  PET">edit] fMRI and  PET

Functional magnetic resonance imaging (fMRI) measures changes in blood flow in the brain, but the activity of neurons is  not directly measured, nor can it  be distinguished whether this activity is  inhibitory or  excitatory. fMRI is  a noninvasive, indirect method for measuring neural activity that is  based on BOLD; Blood Oxygen Level Dependent changes. The changes in blood flow that occurs in capillary beds in specific regions of the brain are  thought to represent various neuronal activities. Similarly, a positron emission tomography (PET), is  able to monitor glucose metabolism in different areas within the brain which can be correlated to the level of activity in that region.

[edit] Behavioral

Behavioral tests can measure symptoms of disease and  mental performance, but can only provide indirect measurements of brain function and  may not be practical in all animals. In humans however, a neurological exam can be done to determine the location of any trauma, lesion, or  tumor within the brain, brain stem, or  spinal cord.

[edit] Anatomical

Autopsy analysis of the brain allows for the study of anatomy and  protein expression patterns, but is  only possible after the human or  animal is  dead. Magnetic resonance imaging (MRI) can be used to study the anatomy of a living creature and  is widely used in both research and  medicine.

[edit] Other methods

Attempts have  also been made to directly "read" the brain, which has been accomplished in a rudimentary manner through a brain-computer interface. Brain activity can be detected by implanted electrodes, raising the possibility of direct mind-computer interface. The reverse method has been successfully demonstrated: brain implants have  been used to generate artificial hearing and  (crude and  experimental) artificial vision for deaf and  blind people. Brain pacemakers are  now commonly used to regulate brain activity in conditions such as Parkinson's disease.

[edit] Other matters

Computer scientists have  produced simulated neural networks loosely based on the structure of neuron connections in the brain. Artificial intelligence seeks to replicate brain function—although not necessarily brain mechanisms—but as yet has been met with limited success.

Creating algorithms to mimic a biological brain is  very difficult because the brain is  not a static arrangement of circuits, but a network of vastly interconnected neurons that are  constantly changing their connectivity and  sensitivity. more  recent work in both neuroscience and  artificial intelligence models the brain using the mathematical tools of chaos theory and  dynamical systems. Current research has also focused on recreating the neural structure of the brain with the aim of producing human-like cognition and  artificial intelligence.

[edit] Brain as food

Pork brain, ready to be cooked

Like most other internal organs, the brain can serve as nourishment. For example, in the southern United States canned pork brain in gravy can be purchased for consumption as food. This form of brain is  often fried with scrambled eggs to produce the famous " and  brains">Eggs n' Brains".^[6] The brain of animals also features in French cuisine such as in the dish [tête de veau], or  head of calf. Although it  might consist only of the outer meat of the skull and  jaw, the full meal includes the brain, tongue, and  glands, with the latter form being the favorite food of French President Jacques Chirac.^[7] Similar delicacies from around the world include Mexican tacos de sesos made with cattle brain as well as squirrel brain in the US South.^[8] The Anyang tribe of Cameroon practiced a tradition in which a new tribal chief would consume the brain of a hunted gorilla while another senior member of the tribe would eat the heart.^[9]. Indonesian cuisine specialty in Padang cuisine also served beef brain in a gravy coconut milk named gulai otak (beef brain curry).

Consuming the brain and  other nerve tissue of animals is  not without risks. The first problem is  that the brain is  made up of 60% fat due to the myelin (which itself is  70% fat) insulating the axons of neurons and  glia.^[10] As an  example, a 140 g can of "pork brains in milk gravy", a single serving, contains 3500 milligrams of cholesterol, 1170% of our recommended daily intake.^[11]

Brain consumption can also result in contracting fatal transmissible spongiform encephalopathies such as Variant Creutzfeldt-Jakob disease and  other prion diseases in humans and  mad cow disease in cattle.^[12] Another prion disease called kuru has been traced to a funerary ritual among the Fore people of Papua New Guinea in which those close to the dead would eat the brain of the deceased to create a sense of immortality.^[13] Some archaeological evidence suggests that the mourning rituals of European Neanderthals also involved the consumption of the brain.^[14]

[edit] See also

Look up Brain in
Wiktionary, the free dictionary.

• Nervous system
• Central nervous system
• Neuroscience
• Neurology
• A/S ratio
• Brain damage
• Brain death
• Brain freeze
•  and  intelligence">Brain size
• Brain-computer interface
• Human brain
• Regions in the human brain
• Traumatic brain injury
•  and  neck anatomy">Head and  neck anatomy
• Neuroanthropology

[edit] Further reading

• Junqueira, L.C., and  J. Carneiro (2003). Basic Histology: Text and  Atlas, Tenth Edition. Lange Medical Books McGraw-Hill. ISBN 0-07-121565-4.
• Sala, Sergio Della, editor. (1999). Mind myths: Exploring popular assumptions about the mind and  brain. J. Wiley & Sons, New York. ISBN 0-471-98303-9.
• Vander, A., J. Sherman, D. Luciano (2001). Human Physiology: The Mechanisms of Body Function. McGraw Hill Higher Education. ISBN 0-07-118088-5.

[edit] References

[edit] External links

• Brain Tutorial
• Comparative Mammalian Brain Collection
• Brain Research News from ScienceDaily
• BrainInfo for Neuroanatomy
• Neuroscience for kids
• Everything you wanted to know about the brain — Provided by New Scientist.
• AllAboutCancers
• Fact sheets on brain injury - causes, effects and  coping strategies
• neuroscience wiki
• BrainMaps.org, interactive high-resolution digital brain atlas based on scanned images of serial sections of both primate and  non-primate brains
• Scientific American Magazine (September 2003 Issue) Ultimate Self-Improvement
• An orderly article about the brain
• Sample online brain test
• Spastic cerebral palsy symptoms