Anatomy & Physiology – The Nervous System

 

The Central Nervous System

The main function of the central nervous system (CNS) is to receive information from the body, interpret it, and then send out instructions. The CNS is made up of the spinal cord, brain, ears, eyes, and sensory organs related to taste, smell, and touch.

Spinal Cord

The spinal cord is a thick bundle of nerves that connects your brain to the rest of your body. It is located within the bones of your spine, which protect it from damage. It is made up of 31 segments: 7 cervical vertebrae (8 cervical segments), 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal. A pair of spinal nerves exits from each segment of the spinal cord.

The spinal cord has two main functions:

1. Nerve impulse transmission. Neurons located in the white matter of the spinal cord transmit sensory signals from peripheral regions to the brain and motor signals from the brain to peripheral regions.
2. Reflexes. Neurons in the gray matter of the spinal cord interpret incoming sensory information and respond with motor impulses that control muscles (such has moving your hand away from a hot stove).

Brain 

The brain controls the body’s systems and organs and keeps them working and interrelating properly. The brain enables us to think, remember, and imagine. In large part, the brain is what makes us human. The brain communicates with the rest of the body through the spinal cord and the nerves, which inform the brain about what is happening in the body.

The brain of all vertebrates develops into the forebrain (prosencephalon), midbrain (mesencephalon), and hindbrain (rhombencephalon) areas. Within these areas, the brain has several distinct parts that work together: cerebrum, cerebellum, brain stem, limbic system, and pituitary gland.

Cerebrum

The cerebrum makes up 85 percent of the brain and is associated with higher thought. It controls the voluntary muscles and is what enables us to think and reason. It is also where short and long term memories are stored. So when we do math, when we write a sentence, when we play basketball, when we remember what we watched on TV last night, or what we got for Christmas when we were five, all those activities are controlled by the cerebrum.

The cerebrum has two halves called the left and right hemispheres. The right hemisphere is often associated with abstract thought and creativity, while the left hemisphere is more analytical. Because the nerves cross coming out of the brain, the right side of the cerebrum controls the left side of the body, and the left half controls the right side.

The cerebral cortex is divided into four lobes, or sections.

• Frontal Lobe. Includes reasoning, planning, parts of speech, movement, emotions, and problem solving.
• Parietal Lobe. Includes movement, orientation, recognition, perception of stimuli.
• Occipital Lobe. Includes visual processing.
• Temporal Lobe. Includes perception and recognition of auditory stimuli, memory, and speech.

 

The distinctive wrinkled appearance of the cerebral cortex, the outer most layer of the cerebrum, makes the brain more efficient by increasing the surface area, which increases the number of neurons. The gray colored surface of the cerebral cortex is a layer of neurons. Beneath that layer are white nerve fibers that carry signals between the nerve cells and other parts of the brain and the body.

 

The cerebral cortex, sometimes referred to as the neocortex, is the most recent part of the mammalian brain to evolve. It consists of six cell layers and is found only in more advanced mammals including humans, primates, and dolphins.

Cerebellum

The cerebellum, or little brain, is located at the back of the brain, below the cerebrum. Just an eighth the size of the cortex, the cerebellum controls balance, movement, and coordination.

Structurally, the cerebellum has a wrinkled surface and two hemispheres. Considering evolution, it is a much older brain structure that predates humans. Scientists believe this because reptiles have a cerebellum although they do not have a cerebrum.

Limbic System

The limbic system, sometimes called the emotional brain, is found buried within the cerebrum. Like the cerebellum, evolutionarily the structure is rather old. It is made up of the several structures:

Amygdala. This is involved in emotional responses, hormonal secretions, and memory.

Cingulate Gyrus. This is a fold in the brain associated with emotions and the regulation of aggressive behavior.

Fornix. This is a fibrous band of nerve fibers that connect the hippocampus to the hypothalamus.

Hippocampus. A tiny structure involved with learning, it stores and files memories, turning short term memories into permanent memories.

Hypothalamus. This is a pearl size structure that directs many bodily functions: temperature, hunger, thirst, sleep, sexual behavior, and emotional responses, through the production of eight hormones.

OlfactoryCortex. This is involved in the identification of odors.

Thalamus. This relays sensory signals to and from the spinal cord and the cerebrum.

Brain Stem 

The brain stem is positioned beneath the cerebrum and in front of the cerebellum. It connects the brain to the spinal cord. The brain stem, which is associated with more basic functions, is made up of the midbrain, pons, and medulla oblongata.

Midbrain. This serves as the nerve pathway of the cerebral hemispheres; it contains auditory and visual reflex centers.

Medulla oblongata. This functions primarily as a relay station between the spinal cord and the brain; it also controls reflex activities such as coughing, gagging, swallowing, and vomiting.

Pituitary Gland 

The size of a pea, the pituitary gland is the body’s master gland that controls hormone production by the thyroid gland, adrenal glands, ovaries, and testes. These hormones affect metabolism, blood pressure, sexuality, reproduction, and other vital body functions. The pituitary gland also produces growth hormone, which controls height, and prolactin for milk production.

Peripheral Nervous System

The peripheral nervous system includes the nerves outside of the brain and spinal cord. The nerves radiate out from the spinal cord and reach every part of the body. Nerves carry messages back and forth from the glands and muscles to the brain. The biggest nerve is an inch thick and the smallest nerve is thinner than a human hair. Each nerve is made up of specialized nerve cells called neurons.

Neurons

The nervous system is made up of billions of neurons. It was once thought that neurons were not replaced when they die. But research in 1998 showed that new neurons are constantly being produced in two areas of the brain. Although most quickly die, many are integrated into the surrounding brain tissue.

Neurons are typically flat, star shaped cells that are wired together, like a body-wide communications system, so that they can carry messages to and from the brain through an electrochemical process.

The main portion of the neuron is the cell body, or soma. Nerve cells come in different shapes and sizes, but in general they all have finger like projections called dendrites that give neurons their star shape. One extension, called the axon, is longer. It is often lined by myelin, a fatty substance that improves the efficiency of nerve transmissions.

When a neuron is stimulated it generates a tiny electrical charge. These pulses travel along neurons, going from the axon of one cell to the dendrite of the next.

But axons and dendrites never touch. So in order for the signals to pass from one neuron to the next, they have to jump the gap between the dendrite and axon, called the synapse. The electrical pulse triggers the release of chemicals, called neurotransmitters, which carry the pulse to the next cell.

How Axons and Dendrites Differ

Neurotransmitters

Acetylcholine is called an excitatory neurotransmitter because it typically excites cells. It controls muscle contractions and causes glands to secrete hormones. Alzheimer’s disease is associated with a shortage of acetylcholine.

Gamma-aminobutyric acid (GABA) is called an inhibitory neurotransmitter because it generally makes cells less excitable. It helps control muscle activity and plays an integral role in the visual system. Drugs that increase GABA levels in the brain are used to treat epileptic seizures and tremors in patients with Huntington’s disease.

Serotonin is an inhibitory neurotransmitter that constricts blood vessels and induces sleep. It is also involved in temperature regulation.

Dopamine is another inhibitory neurotransmitter that affects our moods and the control of complex movements. The loss of dopamine activity in certain portions of the brain causes some of the muscular symptoms seen in Parkinson’s disease. Many medications used to treat behavioral disorders work by modifying dopamine’s effects on the brain.

Although they work in the same basic way, not all nerve cells are alike. The peripheral nervous system is divided into two systems. The autonomic nervous system primarily controls involuntary functions or actions, while the sensory-somatic system controls voluntary actions. In the autonomic system, two groups of motor neurons are used to stimulate the effectors instead of one.

The Sensory-Somatic Nervous System

The sensory-somatic system consists of 12 pairs of cranial nerves and 31 pairs of spinal nerves, which contain both sensory and motor neurons. All of our conscious awareness of the external environment, and all of our motor activity to respond to it, comes through the sensory-somatic portion of the peripheral nervous system.

The Autonomic Nervous System

The autonomic nervous system (ANS) consists of sensory neurons and motor neurons that run between the central nervous system, especially the hypothalamus and medulla oblongata, and various internal organs including the heart, lungs, viscera, and glands. Autonomic means that these functions are not controlled by the conscious mind.

The sympathetic nervous system is constantly monitoring conditions inside our bodies. It is located next to the parasympathetic chain, which connects to skin, blood vessels, and organs in the body cavity. The sympathetic chain is located on both sides of the spine and consists of ganglias, which are bundles of nerves. Stimulation of the sympathetic system prepares the body for emergencies; in other words, it triggers our fight or flight instinct. When that happens, the lungs widen to give us more oxygen, blood is diverted from the skin and organs to the brain, muscle tension is increased, and our heart beats faster.

When there is no danger present, the autonomic nervous system then works to maintain normal internal functions and works with the somatic nervous system.

The autonomic nervous system has two parts: the sympathetic nervous system and the parasympathetic nervous system.

These systems work in balance with each other and affect almost every structure in the body. The sympathetic nervous system has an exerting function, while the parasympathetic has mainly a relaxing function.

The main nerves of the parasympathetic system are the vagus nerves, which originate in the medulla oblongata.

Sensory System

Sensory neurons are nerve cells within the nervous system responsible for converting external stimuli from the organism’s environment into internal electrical impulses. There are five main senses: touch, smell, taste, hearing, and sight. The body’s sensory organs constantly send signals about what is happening outside and inside it to the brain.

Receptors are specialized cells that detect changes in external and internal environmental stimuli and convert those changes into electrical impulses. They may be grouped together to form a sense organ, such as the eye or ear, or they may be scattered, like those in skin and viscera. Each organ has receptors sensitive to a specific stimulus.

• Skin. Skin is sensitive to touch, pressure, pain, and temperature.
• Tongue. The tongue is sensitive to chemicals in food.
• Nose. The nose is sensitive to chemicals in the air.
• Eyes. The eyes are sensitive to light.
• Ears. The ears are sensitive to sound.

The sensory system has three different types of neurons, each having a slightly different function:

• Sensory neurons (or afferent neurons) carry signals from receptors to the spinal cord and brain.
• Relay neurons (or interneurons) carry messages from one part of the CNS to another.
• Motor neurons (or efferent neurons) carry signals from the CNS to effectors.

Conclusion