Autonomic (Involuntary) Nervous System - Reacts with Internal Environment.
(internal organs and glands. Largely involuntary muscles.)
Sympathetic Branch
- Causes release of neurotransmitter noradrenaline, which among other things stimulates the heartbeat and raises blood pressure. The release of adrenaline from the adrenal medulla into the blood ensures that all the cells of the body will be exposed to sympathetic stimulation even if no neurons reach them directly.
Parasympathetic Branch
- Mainly the vagus nerves, which returns the body functions to normal after they have been altered by sympathetic stimulation. The vagus nerves also help keep inflammation under control.
The brain is made up of many cells, but the neurons are the primary players in brain functions.
There are 100 Billion neurons in the brain. A typical neuron has about 1,000 to 10,000 synapses (that is, it communicates with many
other neurons, muscle cells, glands, etc.).
It has been estimated that there are 1 quadrillion synapses in the human brain. That's 1015.
This wiring system surpasses by many orders of magnitude the complexity of even the most advanced supercomputers.
See Also: Functional brain map from Science Daily
The brain is divided into the left and right hemispheres and with the exception of the pineal gland in the center, every brain module below is duplicated in each hemisphere.
The Corpus Callosum is a bundle of nerve fibers (millions of axons from the cerebral cortex) that allows the two hemispheres to communicate.
Left Hemisphere -
Communicates by using words, has highly developed verbal abilities, is logical and systematic, concerned with matters as they are.
Right Hemisphere - Communicates using images (pictures), has highly developed spatial abilities, is intuitive and imaginative, concerned with emotions and feelings.
Sensory and motor neurons on the left side of the body are connected to the right hemisphere of the brain and vice versa.
So, a stroke in the left hemisphere may cause speech problems and paralysis of the right side of the body.
Whereas a stroke in the right hemisphere may cause paralysis to the left side and problems with spatial and perceptual abilities causing misjudgement of distances and instability. See Effects of Stroke.
The main components of the brain are most apparent in the embryonic development of the brain, where
three swellings occur for the forebrain, midbrain and hindbrain. All vertibrates have these 3 parts.
The Forebrain
Cerebrum (also called the Cerebral Cortex, Cerebral Hemispheres or just the Cortex) (Telencephalon)
- The cerebrum consists of the cortex, large fiber tracts (corpus
callosum) and some deeper structures (basal ganglia, amygdala,
hippocampus). It integrates information from all of the sense organs,
initiates motor functions, controls emotions and holds memory and
thought processes (emotional expression and thinking are more prevalent
in higher mammals).
The neopallium is the top layer of the cerebral hemispheres, about 2 mm thick, and is involved in higher functions such as sensory perception, generation of motor commands, spatial reasoning, and in humans, language and conscious thought. Other names for the neopallium include neocortex , isocortex and homotypical cortex.
Frontal lobe - Involved in motor skills (including speech) and higher cognitive functions.
Parietal lobe - Receives and processes all somatosensory input from the body (touch, pain)
Temporal lobe - The processes auditory information from the ears, relates it to areas of the parietal lobe and the motor cortex of the frontal lobe.
Occipital lobe - Receives and processes visual information directly from the eyes and relates this information to Wernicke's area in the parietal lobe and frontal lobe.
Limbic system - More evolutionarily primitive brain structures deep inside the brain. They are involved in many of our emotions and motivations, particularly those that are related to survival, such as fear, anger, and emotions related to sexual behavior. The limbic system is also involved in feelings of pleasure that are related to our survival, such as those experienced from eating and sex.
(Sometimes the Limbic System is classified as distinct from the Cerebrum)
Basal Ganglia - The basal ganglia are a group of structures, including the globus pallidus, caudate nucleus, subthalamic nucleus, putamen and substantia nigra, that are important in coordinating movement.
Amygdala - Reaction to fear. Rreceives signals of the potential danger and begins to set off a series of reactions that will help you protect yourself.
Hippocampus - Control of the emotional response to a given situation. Also important for memory and learning..
Cingulate Gyrus - Coordinates Sensory Input With Emotions. Emotional Responses to Pain.
Regulates Aggressive Behavior.
Fornix - Connects the Hippocampus to the Hypothalamus
Diencephalon
Thalamus - Involved in sensory perception and regulation of motor functions (i.e., movement). It connects areas of the cerebral cortex that are involved in sensory perception and movement with other parts of the brain and spinal cord that also have a role in sensation and movement
Hypothalamus and pituitary gland - These
control visceral functions, body temperature and behavioral responses
such as feeding, drinking, sexual response, aggression and pleasure.
Posterior lobe of the pituitary - Receives antidiuretic hormone (ADH) and oxytocin
from the hypothalamus and releases them into the blood.
Brainstem
- The brainstem connects the brain and the spinal cord. It consists of the medulla (an enlarged portion of the upper spinal cord), pons and midbrain
(lower animals have only a medulla). The brainstem controls the
reflexes and automatic functions (heart rate, blood pressure), limb
movements and visceral functions (digestion, urination).
The Midbrain (mesencephalon)
The midbrain (mesencephalon) occupies only a small region in humans (it is relatively much larger in "lower" vertebrates).
Tectum - Controls Auditory and Visual Responses.
Tegmentum - Attention Control Mechanism and awareness.
Controls Motor Functions. Regulates Some Autonomic Functions.
Ventral Tegmental Area (VTA): Packed with dopamine-releasing neurons, which
relay messages about pleasure through their nerve fibers to nerve cells in a limbic system structure called the nucleus accumbens. Still other fibers reach to a related part of the frontal region of the cerebral cortex. So, the pleasure circuit, which is known as the mesolimbic dopamine system, spans the survival-oriented brainstem, the emotional limbic system, and the frontal cerebral cortex.
The Hindbrain (rhombencephalon)
Medulla Oblongata - Controls autonomic functions (such as breathing, heartbeat,
sneeze, cough, swallow, vomiting).
Processing of inter-aural time differences for sound localization.
Pons - Contains many cranial nerves (Nerves such as the facial nerves, which emerge from the brainstem rather than the spinal cord). It contains the Pontine Sleep Center, which is important for the level of consciousness, and the Respiratory Centers, which along with the Medullary Respiratory Centers help control Respiratory Movements.
Cerebellum - The cerebellum is located behind the brain stem.
The cerebellum integrates information
from the vestibular system that indicates position and movement and
uses this information to coordinate balance, posture and limb movements.
The brain functions by sending electrical signals from one place to another. Very small charges pass between nerve cells, accompanied by changes in electrical potential, in voltage.
This activity can be measured and displayed as a wave form called brain wave or brain rhythm.
Brain waves are measured with an electroencephalogram (EEG) by placing electrodes on the scalp and measuring voltage differences.
Frequency
band (cycles/second)
Name
of Wave Band
Description
1
- 3
Delta
Generally
strongest when a person is in a deep dreamless sleep.
4
- 7
Theta
May
be associated with dreamy, creative, intuitive states.
8
- 12
Alpha
Associated
with a calm and relaxed state when the person is not
thinking.
12
- 16
SMR
Sensorimotor rhythm (SMR) is associated with physical stillness and body presence.
15
- 30
Beta
Associated
with being alert, with normal thinking, with processing
information.
40+
Gamma
(Sometimes included in Beta). Associated
with intensely focused thought. Brain state of hyperalertness, perception, and integration of sensory input. Tibetan Buddhist monks had high Gamma activity during meditation.
The brain and spinal cord are made up of many cells, including neurons and glial cells.
90% of the brain is glial cells; they provide support functions for the neurons.
Source: NIH
There are 100 Billion neurons in the brain.
The neuron body contains organelles such as mitochondria, endoplasmic reticulum,
and a nucleus (which contains DNA).
The dendrites radiate out from the cell body. They are specialized for
receiving information from other neurons. Dendrites act like antennae for nerve cells.
The axon generally carries nerve impulses away from the cell body.
Usually only one axon sprouts off of the cell body, but it may have many branches.
Axons convey information from one part of the brain to antoher.
They may be up to a meter long.
(A Giraffe's primary afferent axon (from toe to neck) is 15 feet long.)
The Synapse is the point of connection between two neurons (axon
and dendrite) or between a neuron (axon) and a muscle or gland.
Boutons are where synapes occur.
Different Types Of Neurons
There are different types of neurons. They all carry electro-chemical nerve signals, but differ in structure (the number of processes, or axons, emanating from the cell body) and are found in different parts of the body.
Sensory neurons or Bipolar neurons carry messages from the body's sense receptors (eyes, ears, etc.) to the Central Nervous system (CNS). These neurons have two processes. Sensory neuron account for 0.9% of all neurons. (Examples are retinal cells, olfactory epithelium cells.)
Motoneurons or Multipolar neurons carry signals from the CNS to the muscles and glands. These neurons have many processes originating from the cell body. Motoneurons account for 9% of all neurons. (Examples are spinal motor neurons, pyramidal neurons, Purkinje cells.)
Interneurons or Pseudopolare (Spelling) cells form all the neural wiring within the CNS. These have two axons (instead of an axon and a dendrite). One axon communicates with the spinal cord; one with either the skin or muscle. These neurons have two processes. (Examples are dorsal root ganglia cells.)
Source: NIH
Note: Some articles state that appendages of a sensory neuron that comes from the nerve ending is a dendrite.
Women have 10% more neurons.
You loose 10% over a lifetime.
A typical neuron is 10 microns (µm) (.01 mm) in diameter. (See size)
They vary in size from 4 microns (.004 mm) to 100 microns (.1 mm) in diameter.
Their length varies from a fraction of an inch to several feet.
The cell body of a motor neuron is approximately 100 microns (0.1 millimeter) in diameter.
Science
Source: NIH
Source: 
(A Giraffe's primary afferent axon (from toe to neck) is 15 feet long.)
The Synapse is the point of connection between two neurons (axon
and dendrite) or between a neuron (axon) and a muscle or gland.
Boutons are where synapes occur.