THE HUMAN BRAIN

THE HUMAN BRAIN

Every animal we can think of -- mammals, birds, reptiles, fish, amphibians, etc. -- has a brain. But the human brain is unique. It gives us the power to think, plan, speak, imagine... It is truly an amazing organ.

The brain performs an incredible number of tasks:

·         It controls body temperature, blood pressure, heart rate and breathing.

·         It accepts a flood of information about the world around you from your various senses (seeing, hearing, smelling, tasting, touching, etc).

·         It handles physical motion when walking, talking, standing or sitting.

·         It lets you think, dream, reason and experience emotions.

All of these tasks are coordinated, controlled and regulated by an organ that is about the size of a small head of cauliflower: our brain.

Our brain, spinal cord and peripheral nerves make up a complex, integrated information-processing and control system. The scientific study of the brain and nervous system is called neuroscience or neurobiology. The field of neuroscience is so vast, and the brain and nervous system so complex.

The Structures of the Brain and What Each One Does

Neuron Structure

Our brain is made of approximately 100 billion nerve cells, called neurons. Neurons have the amazing ability to gather and transmit electrochemical signals -- they are something like the gates and wires in a computer. Neurons share the same characteristics and have the same parts as other cells, but the electrochemical aspect lets them transmit signals over long distances (up to several feet or a few meters) and pass messages to each other.

Neurons have three basic parts:

·         Cell body - This main part has all of the necessary components of the cell, such as the nucleus (contains DNA), endoplasmic reticulum and ribosome (for building proteins) and mitochondria (for making energy). If the cell body dies, the neuron dies.

·         Axon - This long, cable-like projection of the cell carries the electrochemical message (nerve impulse or action potential) along the length of the cell. Depending upon the type of neuron, axons can be covered with a thin layer of myelin, like an insulated electrical wire. Myelin is made of fat, and it helps to speed transmission of a nerve impulse down a long axon. Myelinated neurons are typically found in the peripheral nerves (sensory and motor neurons), while non-myelinated neurons are found in the brain and spinal cord.

·         Dendrites or nerve endings - These small, branch-like projections of the cell make connections to other cells and allow the neuron to talk with other cells or perceive the environment. Dendrites can be located on one or both ends of the cell.

Basic Neuron Types

Neurons come in many sizes. For example, a single sensory neuron from our fingertip has an axon that extends the length of your arm, while neurons within the brain may extend only a few millimetres. Neurons have different shapes depending on what they do. Motor neurons that control muscle contractions have a cell body on one end, a long axon in the middle and dendrites on the other end; sensory neurons have dendrites on both ends, connected by a long axon with a cell body in the middle.

Neurons also vary with respect to their functions:

·         Sensory neurons carry signals from the outer parts of your body (periphery) into the central nervous system.

·         Motor neurons (motoneurons) carry signals from the central nervous system to the outer parts (muscles, skin, and glands) of your body.

·         Receptors sense the environment (chemicals, light, sound, touch) and encode this information into electrochemical messages that are transmitted by sensory neurons.

·         Interneurons connect various neurons within the brain and spinal cord.

The simplest type of neural pathway is a monosynaptic (single connection) reflex pathway, like the knee-jerk reflex. When the doctor taps the right spot on our knee with a rubber hammer, receptors send a signal into the spinal cord through a sensory neuron. The sensory neuron passes the message to a motor neuron that controls your leg muscles. Nerve impulses travel down the motor neuron and stimulate the appropriate leg muscle to contract. The response is a muscular jerk that happens quickly and does not involve your brain. Humans have lots of hard-wired reflexes like this, but as tasks become more complex, the pathway "circuitry" gets more complicated and the brain gets involved.

Brain Parts

Brains evolved from ganglia of invertebrates. Regardless of the animal, brains have the following parts:

·         Brain stem - The brain stem consists of the medulla (an enlarged portion of the upper spinal cord), pons and midbrain (lower animals have only a medulla). The brain stem controls the reflexes and automatic functions (heart rate, blood pressure), limb movements and visceral functions (digestion, urination).

·         Cerebellum - The cerebellum integrates information from the vestibular system that indicates position and movement and uses this information to coordinate limb movements.

·         Hypothalamus and pituitary gland - These control visceral functions, body temperature and behavioral responses such as feeding, drinking, sexual response, aggression and pleasure.

·         Cerebrum (also called the cerebral cortex or just the cortex) - The cerebrum consists of the cortex, large fibber tracts (corpus callosum) and some deeper structures (basal ganglia, amygdale, and 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).

As we proceed from fish toward humans, you can see that the cortex gets bigger, takes up a larger portion of the total brain and becomes folded. The enlarged cortex takes on additional higher-order functions, such as information processing, speech, thought and memory. In addition, the part of the brain called the thalamus evolved to help relay information from the brain stem and spinal cord to the cerebral cortex.

Fish Brain ?

Lower animals (fish, amphibians, reptiles, birds) do not do much "thinking," but instead concern themselves with the everyday business of gathering food, eating, drinking, sleeping, reproducing and defending themselves. Therefore, their brains reflect the major centres that control these functions. We perform these functions as well, and so have a "reptilian" brain built into us.

Lower Brain

The basic lower brain consists of the spinal cord, brain stem and diencephalon (the cerebellum and cortex are also present, but will be discussed in later sections). Within each of these structures are centres of neuronal cell bodies, called nuclei; that are specialized for particular functions (breathing, heart-rate regulation, sleep):

·         Medulla - The medulla contains nuclei for regulating blood pressure and breathing, as well as nuclei for relaying information from the sense organs that comes in from the cranial nerves.

·         Pons - The Pons contains nuclei that relay movement and position information from the cerebellum to the cortex. It also contains nuclei that are involved in breathing, taste and sleep.

·         Midbrain - The midbrain contains nuclei that link the various sections of the brain involved in motor functions (cerebellum, basal ganglia, and cerebral cortex), eye movements and auditory control. One portion, called the substantial nigra, is involved in voluntary movements; when it does not function, you have the tremored movements of Parkinson's disease.

·         Thalamus - The thalamus relays incoming sensory pathways to appropriate areas of the cortex, determines which sensory information actually reaches consciousness and participates in motor-information exchange between the cerebellum, basal ganglia and cortex.

Hypothalamus - The hypothalamus contains nuclei that control hormonal secretions from the pituitary gland. These centres govern sexual reproduction, eating, drinking, growth, and maternal behavior such as lactation (milk-production in mammals). The hypothalamus is also involved in almost all aspects of behavior, including your biological "clock," which is linked to the daily light-dark cycle (circadian rhythms).

Balancing Act

The cerebellum is folded into many lobes and lies above and behind the Pons. It receives sensory input from the spinal cord, motor input from the cortex and basal ganglia and position information from the vestibular system. The cerebellum then integrates this information and influences outgoing motor pathways from the brain to coordinate movements. To demonstrate this, reach out and touch a point in front of you, such as the computer monitor -- your hand makes one smooth motion. If our cerebellum were damaged, same motion would be very jerky as our cortex initiated a series of small muscle contractions to home on the target point. The cerebellum may also be involved in language (fine muscle contractions of the lips and larynx), as well as other cognitive functions.