Every animal you can think of — mammals, birds, reptiles, fish, amphibians — has a brain. But the human brain is unique. Although it’s not the largest, it gives us the power to speak, imagine and problem solve. It is truly an amazing organ.
The brain performs an incredible number of tasks including the following:
- It controls body temperature, blood pressure, rate and breathing.
- It accepts a flood of information about the world around you from your various senses ( and touching).
- It handles your physical movement when walking, talking, standing or sitting.
- It lets you think 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.
Your brain, spinal cord and peripheral nerves make up a complex, integrated information-processing and control system known as your central nervous system. In tandem, they regulate all the conscious and unconscious facets of your life. The scientific study of the brain and nervous system is called neuroscience or neurobiology. Because the field of neuroscience is so vast — and the brain and nervous system are so complex — this article will start with the basics and give you an overview of this complicated organ.
We’ll examine the structures of the brain and how each section controls our daily functions, including motor control, visual processing, auditory processing, sensation, learning, memory and emotions.
Your brain is made of approximately 100 billion nerve called neurons. Neurons have the amazing ability to gather and transmit electrochemical signals — think of them like the gates and wires in a
Neurons share the same characteristics and have the same makeup as other , but the electrochemical aspect lets them transmit signals over long distances (up to several feet or a few meters) and send messages to each other.
- Cell body or soma. This main part has all of the necessary components of the cell, such as the nucleus (which contains), endoplasmic reticulum and ribosomes (for building proteins) and mitochondria (for making energy). If the cell body dies, the neuron dies.
- Axon . This long, cablelike 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 sheath, like an insulated electrical wire. Myelin is made of and protein, 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, branchlike 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 a cell.
Neurons come in many sizes. For example, a single sensory neuron from your fingertip has an axon that extends the length of your arm, while neurons within the brain may extend only a few millimeters.
They also have different shapes depending on their functions. Motor neurons that control contractions have a 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. Interneurons, or associative neurons, carry information between motor and sensory neurons.
These fundamental members of the nervous system 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, glands) of your body.
- 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 your 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 hardwired reflexes like this, but as tasks become more complex, the pathway circuitry gets more complicated and the brain gets involved.
The simplest possible creatures have incredibly basic nervous systems made up of nothing but reflex pathways. For example, flatworms and invertebrates don’t have centralized brains. They have loose associations of neurons arranged in straightforward reflex pathways. Flatworms have neural nets, or individual neurons linked together that form a net around the entire animal.
Most invertebrates (such as the lobster) have modest “brains” that consist of localized collections of neuronal cell bodies called ganglia. Each ganglion controls sensory and motor functions in its segment through reflex pathways, and the ganglia are linked together to form a simple nervous system. As nervous systems evolved, chains of ganglia evolved into more centralized simple brains.
Brains evolved from ganglia of invertebrates. Regardless of the animal, brains have the following parts:
- The brain stem, which 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 (), limb movements and visceral functions (digestion, urination).
- The cerebellum integrates information from the vestibular system that indicates position and movement and uses this data to coordinate limb movements.
- The hypothalamus and pituitary gland are responsible for visceral functions, body temperature and behavioral responses such as feeding, drinking, aggression and pleasure.
- The cerebrum (also called the cerebral cortex or just the cortex) consists of the cortex, large fiber tracts (corpus callosum) and some deeper structures (basal ganglia, amygdala and hippocampus). It integrates info 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).
Lower animals, such as fish, amphibians, reptiles and birds, don’t do much “thinking,” but instead concern themselves with the everyday business of gathering eating, drinking, , reproducing and defending themselves.
These are instinctual processes [source: ]. Therefore, their brains are organized along the major centers that control these function
We humans perform these functions as well, and so have a “reptilian” brain built into us. That means we have the same parts of the brain found in reptiles, namely the brain stem and the cerebellum.
Ready to learn about the lower brain? We’ll discuss that on the next page