Brain Anatomy and Function
Understanding the Human Brain and Nervous System
The brain is the control center for the entire nervous system. It can be divided into three main sections: the cerebrum, the cerebellum, and the brain stem.
The Cerebrum
The cerebrum is divided into two halves, called hemispheres, which are connected by bundles of nerves. The cerebrum controls all conscious activities, intelligence, memory, language, skeletal muscle movements, and senses.
The outer surface of the cerebrum, called the cerebral cortex, is made up of gray matter. The cerebral cortex contains numerous folds and grooves that increase its total surface area. This increase in surface area played an important role in the evolution of human intelligence, allowing for more complex thought processes.
The Cerebellum
The cerebellum is located at the back of the brain. It controls balance, posture, and coordination. If the cerebellum is injured, movements can become jerky.
The Brain Stem
The brain stem is made up of the medulla oblongata, the pons, and the midbrain. The medulla oblongata is the part of the brain that controls involuntary activities such as breathing and heart rate. The pons and midbrain act as pathways connecting various parts of the brain to each other.
The Peripheral Nervous System (PNS)
The PNS carries impulses between the body and the central nervous system (CNS). For example, when a stimulus is picked up by receptors in your skin, it initiates an impulse in the sensory neurons. The impulse is carried to the CNS, where it transfers to the motor neurons, which carry the impulse to a muscle.
The PNS can be separated into two divisions: the somatic nervous system and the autonomic nervous system.
The Somatic Nervous System
The somatic nervous system is made up of 12 pairs of cranial nerves from the brain, 31 pairs of spinal nerves from the spinal cord, and all of their branches. These nerves are bundles of neuron axons bound together by connective tissue. The cell bodies of the neurons are found in clusters along the spinal column. Most nerves contain both sensory and motor axons.
The nerves of the somatic system relay information mainly between your skin, the CNS, and skeletal muscles. This pathway is voluntary, meaning that you can decide whether or not to move body parts under the control of the system.
Sometimes a stimulus results in an automatic, unconscious response within the somatic system. When you touch something hot, you automatically jerk your hand away. Such an action is a reflex, an automatic response to a stimulus. A reflex impulse travels to the spinal column or brain stem where it causes an impulse to be sent directly back to a muscle. It does not go to the brain for interpretation. The brain becomes aware of the reflex only after it occurs.
The Autonomic Nervous System
Internal reactions to being scared are controlled by the autonomic nervous system. The autonomic nervous system carries impulses from the CNS to internal organs. These impulses produce responses that are involuntary, meaning they are not under conscious control.
There are two divisions of the autonomic nervous system: the sympathetic nervous system and the parasympathetic nervous system.
The Sympathetic Nervous System
The sympathetic nervous system controls many internal functions during times of stress. When something scares you, the sympathetic nervous system causes the release of hormones, such as epinephrine and norepinephrine, which results in a fight-or-flight response.
The Parasympathetic Nervous System
The parasympathetic nervous system controls many of the body’s internal functions when it is at rest. It is in control when you are reading quietly. Both the sympathetic and parasympathetic systems send signals to the same internal organs. The resulting activity of the organ depends on the intensities of the opposing signals.