Nervous system - A network of cells that carries information to and from all parts of the body.

Neuroscience - A branch of the life sciences that deals with the structure and functioning of the brain and the neurons, nerves, and nervous tissue that form the nervous system.

Biological Psychology or Behavioral neuroscience - A branch of neuroscience that focuses on the biological bases of psychological processes, behavior, and learning, and it is the primary area associated with the biological perspective in psychology.

STRUCTURE OF THE NEURON: THE NERVOUS SYSTEM’S BUILDING BLOCK

Santiago Ramón y Cajal (1887) - A doctor studying slides of brain tissue first theorized that the nervous system was made up of individual cells.

Although the entire body is composed of cells, each type of cell has a special purpose and function and, therefore, a special structure.

Neuron - Is a specialized cell in the nervous system that receives and send messages within that system. They are one of the messengers of the body, and that means they have a very special structure.

Dendrite - parts of the neuron that receive messages from other cells. The name dendrite means “tree-like,” or “branch,” and this structure does indeed look like the branches of a tree. They are attached to the cell body, or the soma.

Soma - The part of the cell that contains the nucleus and keeps the entire cell alive and functioning. The word soma means “body”.

Axon - From the Greek word “axis” is a fiber attached to the soma, and its job is to carry messages out to other cells.

Axon Terminals - The end of the axon that branches out into the several shorter fibers that have swellings or little knobs on the ends (may also be called presynaptic terminals, terminal buttons, or synaptic knobs), are responsible for communicating with other nerve cells.

Neurons make up a large part of the brain, but they are not the only cells that affect out thinking, learning, memory, perception, and all of the other facets of life that make us who we are.

Glia or Glial cells - These are other primary cells that serve a variety of functions. Some glia serves as a sort of structure on which the neurons develop and work and that hold the neurons in place. During early brain development, radial glial cells (extending from inner to outer areas like the spokes of a wheel) help guide migrating neurons to form the outer layers of the brain.
Other glia are involved in getting nutrients to the neurons, cleaning up the remains of neurons that have died, communicating with neurons and other glial cells, and providing insulation for neurons.

Glial cells affect both the functioning and structure of neurons and specific types also have properties similar to stem cells, which allow them to develop into new neurons, both during prenatal development and in adult mammals.

Glial cells are also being investigated for their possible role in a variety of psychiatric disorders, including major depressive disorder and schizophrenia. It appears in some areas of the brain, major depressive disorder is characterized by lower numbers of specific glial cells whereas in schizophrenia, parts of the brain have a greater number (Blank & Prinz, 2013).

Two special types of glial cells, called oligodendrocytes and Schwann cells, generate a layer of fatty substances called myelin.

Oligodendrocytes produce myelin for the neurons in the brain and spinal cord (the central nervous system);
Schwann cells produce myelin for the neurons of the body (the peripheral nervous system).

Myelin - Wraps around the shaft of the axons, forming an insulating and protective sheath. Bundles of myelin-coated axons travel together as “cables” in the central nervous system called tracts, and in the peripheral nervous system bundles of axons are called nerves.

The myelin sheath is a very important part of the neuron. It not only insulates and protects the neuron, it also speeds up the neural message traveling down the axon.

The places where the myelin seems to bump are actually small spaces on the axon called nodes, which are not covered in myelin. Myelinated and unmyelinated sections of axons have slightly different electrical properties.

When the electrical impulse that is the neural message travels down an axon coated with myelin, the electrical impulse is regenerated at each node and appears to “jump” or skip rapidly from node to node down the axon (Koester & Siegelbaum, 2013; Schwartz et al., 2013). That makes the message go much faster down the coated axon than it would down an uncoated axon of a neuron in the brain.

Multiple sclerosis (MS) - the myelin sheath is destroyed (possibly by the individual’s own immune system), which leads to diminished or complete loss of neural functioning in those damaged cells. Early symptoms of MS may include fatigue, changes in vision, balance problems, and numbness, tingling, or muscle weakness in the arms or legs

GENERATING THE MESSAGE WITHIN THE NEURON: THE NEURAL IMPULSE

A neuron that’s at rest—not currently firing a neural impulse or message—is actually electrically charged. Inside and outside of the cell is a semiliquid (jelly-like) solution in which there are charged particles, or ions.

Although both positive and negative ions are located inside and outside of the cell, the relative charge of ions inside the cell is mostly negative, and the relative charge of ions outside the cell is mostly positive due to both diffusions, the process of ions moving from areas of high concentration to areas of low concentration, and electrostatic pressure, the relative electrical charges when the ions are at rest.

The cell membrane itself is semipermeable. This means some substances that are outside the cell can enter through tiny protein openings, or channels, in the membrane, while other substances in the cell can go outside.

Many of these channels are gated—they open or close based on the electrical potential of the membrane.

When the cell is resting (the electrical potential is in a state called the resting potential, because the cell is at rest).

This electrical charge reversal is known as the action potential because the electrical potential is now in action rather than at rest.

To sum all that up, when the cell is stimulated, the first ion channel opens and the electrical charge at that ion channel is reversed. Then the next channel opens and that charge is reversed, but in the meantime the first ion channel has been closed and the charge is returning to what it was when it was at rest. The action potential is the sequence of ion channels opening all down the length of the cell’s axon.The Neuron at Rest - During the resting potential, the neuron is negatively charged inside and positively charged outside.

The Neural Impulse - The action potential occurs when positive sodium ions enter into the cell, causing a reversal of the electrical charge from negative to positive.

The Neural Impulse Continues - As the action potential moves down the axon toward the axon terminals, the cell areas behind the action potential return to their resting state of a negative charge as the positive sodium ions are pumped to the outside of the cell, and the positive potassium ions rapidly leave.

SENDING THE MESSAGE TO OTHER CELLS: THE SYNAPSE

Inside the synaptic vesicles are chemicals suspended in fluid, which are molecules of substances called neurotransmitters. The name is simple enough—they are inside a neuron, and they are going to transmit a message.

Next to the axon terminal is the dendrite of another neuron (see Figure 2.3). Between them is a fluid-filled space called the synapse or the synaptic gap.

Figure 2.3 The Synapse The nerve impulse reaches the axon terminal, triggering the release of neurotransmitters from the synaptic vesicles. The molecules of neurotransmitter cross the synaptic gap to fit into the receptor sites that fit the shape of the molecule, opening the ion channel and allowing sodium ions to rush in.

But the neurons must have a way to be turned off as well as on. Otherwise, when a person burns a finger, the pain signals from those neurons would not stop until the burn was completely healed. Muscles are told to contract or relax, and glands are told to secrete or stop secreting their chemicals.

The neurotransmitters found at various synapses around the nervous system can either turn cells on (called an excitatory effect) or turn cells off (called an inhibitory effect), depending on exactly what synapse is being affected. Although some people refer to neurotransmitters that turn cells on as excitatory neurotransmitters and the ones that turn cells off as inhibitory neurotransmitters, it’s really more correct to refer to excitatory synapses and inhibitory synapses.

NEUROTRANSMITTERS: MESSENGERS OF THE NETWORK

The first neurotransmitter to be identified was named acetylcholine (ACh). It is found at the synapses between neurons and muscle cells. Acetylcholine stimulates the skeletal muscles to contract but actually slows contractions in the heart muscle.

If acetylcholine receptor sites on the muscle cells are blocked in some way, then the acetylcholine can’t get to the site and the muscle will be incapable of contracting—paralyzed, in other words. This is exactly what happens when curare, a drug used by South American Indians on their blow darts, gets into the nervous system.

Antagonist (a chemical substance that blocks or reduces the effects of a neurotransmitter) for ACh.

Agonist (a chemical substance that mimics or enhances the effects of a neurotransmitter) for ACh.

ACh also plays a key role in memory, arousal, and attention. For example, ACh is found in the hippocampus, an area of the brain that is responsible for forming new memories, and low levels of ACh have been associated with Alzheimer’s disease, the most common type of dementia.

Dopamine (DA) is a neurotransmitter found in the brain, and like some of the other neurotransmitters, it can have different effects depending on the exact location of its activity.

For example, if too little DA is released in a certain area of the brain, the result is Parkinson’s disease—the disease currently being battled by former boxing champ Muhammad Ali and actor Michael J. Fox (Ahlskog, 2003). If too much DA is released in other areas, the result is a cluster of symptoms that may be part of schizophrenia (Akil et al., 2003).

Serotonin (5-HT) is a neurotransmitter originating in the lower part of the brain that can have either an excitatory or inhibitory effect, depending on the particular synapses being affected. It is associated with sleep, mood, anxiety, and appetite. For example, low levels of 5-HT activity have been linked to depression.

Although ACh was the first neurotransmitter found to have an excitatory effect at the synapse, the nervous system’s major excitatory neurotransmitter is glutamate.

Glutamate plays an important role in learning and memory and may also be involved in the development of the nervous system and in synaptic plasticity (the ability of the brain to change connections among its neurons). However, an excess of glutamate results in overactivation and neuronal damage, and may be associated with the cell death that occurs after stroke, head injury, or in degenerative diseases like Alzheimer’s disease and Huntington disease (Julien et al., 2011; Siegelbaum et al., 2013)

Gaba-aminobutyric acid or GABA is the most common neurotransmitter producing inhibition in the brain. GABA can help to calm anxiety, for example, by binding to the same receptor sites that are affected by tranquilizing drugs and alcohol. In fact, the effect of alcohol is to enhance the effect of GABA, which causes the general inhibition of the nervous system associated with getting drunk.

A group of substances known as neuropeptides can serve as neurotransmitters, hormones, or influence the action of other neurotransmitters (Schwartz & Javitch, 2013). You may have heard of the set of neuropeptides called endorphins— pain-controlling chemicals in the body.

When a person is hurt, a neurotransmitter that signals pain is released. When the brain gets this message, it triggers the release of endorphins. The endorphins bind to receptors that open the ion channels on the axon. This causes the cell to be unable to fire its pain signal and the pain sensations eventually lessen. For example, you might bump your elbow and experience a lot of pain at first, but the pain will quickly subside to a much lower level.

The name endorphin comes from the term endogenous morphine. (Endogenous means “native to the area”—in this case, native to the body.) Endorphins are one reason that heroin and the other drugs derived from opium are so addictive—when people take morphine or heroin, their bodies neglect to produce endorphins. When the drug wears off, they are left with no protection against pain at all, and everything hurts. This pain is one reason why most people want more heroin, creating an addictive cycle of abuse (withdrawal symptoms?).

Acetylcholine (ACh) - Excitatory or inhibitory; involved in arousal, attention, memory, and controls muscle contractions.

Norepinephrine (NE) - Mainly excitatory; involved in arousal and mood.

Dopamine (DA) - Excitatory or inhibitory; involved in control of movement and sensations of pleasure.

Serotonin (5-HT) - Excitatory or inhibitory; involved in sleep, mood, anxiety, and appetite.

Gaba-aminobutyric acid (GABA) - Major inhibitory neurotransmitter; involved in sleep and inhibits movement.

Glutamate Major - excitatory neurotransmitter; involved in learning, memory formation, nervous system development, and synaptic plasticity.

Endorphins - Inhibitory neural regulators; involved in pain relief.

CLEANING UP THE SYNAPSE: REUPTAKE AND ENZYMES

The neurotransmitters have to get out of the receptor sites before the next stimulation can occur. Some just drift away through the process of diffusion, but most will end up back in the synaptic vesicles in a process called reuptake.

There is one neurotransmitter that is not taken back into the vesicles, however. Instead, an enzyme* specifically designed to break apart ACh clears the synaptic gap very quickly (a process called enzymatic degradation.) There are enzymes that break down other neurotransmitters as well.

An Overview of the Nervous System

• Central Idea: The nervous system is a complex network of cells and tissues that coordinates and controls the body's functions.

Main Branches:

1. Central Nervous System (CNS)

   • Brain

     Interprets and stores information and sends orders to muscles, glands, and organs.

     • Cerebrum

     • Cerebellum

     • Brainstem

   • Spinal Cord

     Pathway connecting the brain and the peripheral nervous system.

2. Peripheral Nervous System (PNS)

   Transmits information to and from the central nervous system.

   • Somatic Nervous System

     Automatically regulates glands, internal organs and blood vessels, pupil dilation, digestion, and blood pressure.

     • Sensory Neurons

       Carries messages from senses to CNS.

     • Motor Neurons

       Carries messages from CNS to muscles and glands.

   • Autonomic Nervous System

     Carries sensory information and controls movement of the skeletal muscles.

     • Sympathetic Nervous System

       Prepares the body to react and expend energy in times of stress.

     • Parasympathetic Nervous System

       Maintains body functions under ordinary conditions; saves energy.

Sub-Branches:

Central Nervous System (CNS)

• Brain

  • Cerebrum

    • Frontal Lobe

    • Parietal Lobe

    • Temporal Lobe

    • Occipital Lobe

  • Cerebellum

    • Coordination of Movement

    • Balance

  • Brainstem

    • Medulla Oblongata

    • Pons

    • Midbrain

• Spinal Cord

  • Ascending Tracts

  • Descending Tracts

Peripheral Nervous System (PNS)

• Somatic Nervous System

  • Sensory Neurons

    • Touch

    • Temperature

    • Pain

    • Proprioception

  • Motor Neurons

    • Skeletal Muscles

    • Voluntary Movements

• Autonomic Nervous System

  • Sympathetic Nervous System

    • Fight or Flight Response

    • Increased Heart Rate

    • Dilated Pupils

  • Parasympathetic Nervous System

    • Rest and Digest Response

    • Decreased Heart Rate

    • Constricted Pupils

The central nervous system (CNS) is composed of the brain and the spinal cord. Both the brain and the spinal cord are composed of neurons and glial cells that control the life sustaining functions of the body as well as all thought, emotion, and behavior.

THE BRAIN: The brain is the core of the nervous system, the part that makes sense of the information received from the senses, makes decisions, and sends commands out to the muscles and the rest of the body, if needed. The brain is responsible for cognition and thoughts, including learning, memory, and language.

THE SPINAL CORD: The spinal cord is a long bundle of neurons that serves two vital functions for the nervous system.

If it were a real spinal cord, the outer section would appear to be white and the inner section would seem gray.

That’s because the outer section is composed mainly of myelinated axons and nerves, which appear white, whereas the inner section is mainly composed of cell bodies of neurons, which appear gray.

The purpose of the outer section is to carry messages from the body up to the brain and from the brain down to the body. It is simply a message “pipeline.”

The inside section, which is made up of cell bodies separated by glial cells, is actually a primitive sort of “brain.” This part of the spinal cord is responsible for certain reflexes—very fast, lifesaving reflexes.

There are three basic types of neurons:
afferent (sensory) neurons that carry messages from the senses to the spinal cord,
efferent (motor) neurons that carry messages from the spinal cord to the muscles and glands, and
interneurons that connect the afferent neurons to the motor neurons

A good way to avoid mixing up the terms afferent and efferent is to remember “afferent neurons access the spinal cord, efferent neurons exit.”

DAMAGE TO THE CENTRAL NERVOUS SYSTEM: Damage to the central nervous system was once thought to be permanent. Neurons in the brain and spinal cord were not seen as capable of repairing themselves.

The brain actually exhibits a great deal of neuroplasticity, the ability to constantly change both the structure and function of many cells in the brain in response to experience and even trauma.

Scientists have been able to implant nerve fibers from outside the spinal cord onto a damaged area and then “coax” the damaged spinal nerves to grow through these “tunnels” of implanted fibers (Cheng et al., 1996).

Researchers are also examining the effects of implanting Schwann cells from the peripheral nervous system to the central nervous system to aid in treating spinal cord injuries (Deng et al., 2013).

The brain can change itself quite a bit by adapting neurons to serve new functions when old neurons die or are damaged. Dendrites grow and new synapses are formed in at least some areas of the brain, as people learn new things throughout life.

THE PERIPHERAL NERVOUS SYSTEM: NERVES ON THE EDGE

The peripheral nervous system or PNS (see Figure 2.7 and also refer back to Figure 2.5) is made up of all the nerves and neurons that are not contained in the brain and spinal cord.

It is this system that allows the brain and spinal cord to communicate with the sensory systems of the eyes, ears, skin, and mouth and allows the brain and spinal cord to control the muscles and glands of the body.

Somatic nervous system - consists of nerves that control the voluntary muscles of the body.

Autonomic nervous system (ANS) - consists of nerves that control the involuntary muscles, organs, and glands.

THE SOMATIC NERVOUS SYSTEM

The somatic nervous system is made up of the sensory pathway, which comprises all the nerves carrying messages from the senses to the central nervous system (those nerves containing afferent neurons), and

the motor pathway, which is all of the nerves carrying messages from the central nervous system to the voluntary, or skeletal,* muscles of the body—muscles that allow people to move their bodies.

When people are walking, raising their hands in class, lifting a flower to smell, or directing their gaze toward the person they are talking to or to look at a pretty picture, they are using the somatic nervous system.

Involuntary** muscles, such as the heart, stomach, and intestines, together with glands such as the adrenal glands and the pancreas, are all controlled by clumps of neurons located on or near the spinal column.

THE AUTONOMIC NERVOUS SYSTEM

The word autonomic suggests that the functions of this system are more or less automatic, which is basically correct.

Whereas the somatic division of the peripheral nervous system controls the senses and voluntary muscles, the autonomic division controls everything else in the body—organs, glands, and involuntary muscles. The autonomic nervous system is divided into two systems, the sympathetic division and the parasympathetic division.

THE SYMPATHETIC DIVISION

The sympathetic division of the autonomic nervous system is primarily located on the middle of the spinal column—running from near the top of the ribcage to the waist area.

It may help to think of the name in these terms: The sympathetic division is in sympathy with one’s emotions. In fact, the sympathetic division is usually called the “fight-or-flight system” because it allows people and animals to deal with all kinds of stressful events.

Emotions during these events might be anger (hence, the term fight) or fear (that’s the “flight” part, obviously) or even extreme joy or excitement. Yes, even joy can be stressful.

The sympathetic division’s job is to get the body ready to deal with the stress. Many of us have experienced a fight-or-flight moment at least once in our lives.

THE PARASYMPATHETIC DIVISION

Called the “eat-drink-and-rest” system. The neurons of this division are located at the top and bottom of the spinal column, on either side of the sympathetic division neurons (para means “beyond” or “next to” and in this sense refers to the neurons located on either side of the sympathetic division neurons).

The parasympathetic division’s job is to return the body to normal functioning after a stressful situation ends. It slows the heart and breathing, constricts the pupils, and reactivates digestion and excretion. Signals to the adrenal glands stop because the parasympathetic division isn’t connected to the adrenal glands. In a sense, the parasympathetic division allows the body to restore all the energy it burned—which is why people are often very hungry after the stress is all over.

It is the parasympathetic division that is responsible for most of the ordinary, day-to-day bodily functioning, such as regular heartbeat and normal breathing and digestion. People spend the greater part of their 24-hour day eating, sleeping, digesting, and excreting. So it is the parasympathetic division that is typically active. At any given moment, then, one or the other of these divisions, sympathetic or parasympathetic, will determine whether people are aroused or relaxed.

Distant Connections: The Endocrine Glands

Other glands, called endocrine glands, have no ducts and secrete their chemicals directly into the bloodstream (see Figure 2.9 on the next page). The chemicals secreted by this type of gland are called hormones.

THE PITUITARY: MASTER OF THE HORMONAL UNIVERSE

The pituitary gland is located in the brain itself, just below the hypothalamus. The hypothalamus controls the glandular system by influencing the pituitary. That is because the pituitary gland is the master gland, the one that controls or influences all of the other endocrine glands. One part of the pituitary controls things associated with pregnancy and levels of water in the body.

The hormone that controls aspects of pregnancy is called oxytocin, and it is involved in a variety of ways with both reproduction and parental behavior. It stimulates contractions of the uterus in childbirth. The word itself comes from the Greek word oxys, meaning “rapid,” and tokos, meaning “childbirth,” and injections of oxytocin are frequently used to induce or speed up labor and delivery. It is also responsible for the milk letdown reflex, which involves contraction of the mammary gland cells to release milk for the nursing infant.

The hormone that controls levels of water in our body is called vasopressin, and it essentially acts an antidiuretic, helping the body to conserve water

Another part of the pituitary secretes several hormones that influence the activity of the other glands.

One of these hormones is a growth hormone that controls and regulates the increase in size as children grow from infancy to adulthood.

THE PINEAL GLAND:

The pineal gland is also located in the brain, near the back, directly above the brain stem. It plays an important role in several biological rhythms. The pineal gland secretes a hormone called melatonin, which helps track day length (and seasons). In some animals, this influences seasonal behaviors such as breeding and molting. In humans, melatonin levels are more influential in regulating the sleep–wake cycle.

THE THYROID GLAND:

The thyroid gland is located inside the neck and secretes hormones that regulate growth and metabolism. One of these, a hormone called thyroxin, regulates metabolism (how fast the body burns its available energy). As related to growth, the thyroid plays a crucial role in body and brain development.

PANCREAS

The pancreas controls the level of blood sugar in the body by secreting insulin and glucagon. If the pancreas secretes too little insulin, it results in diabetes. If it secretes too much insulin, it results in hypoglycemia, or low blood sugar, which causes a person to feel hungry all the time and often become overweight as a result.

THE GONADS

The gonads are the sex glands, including the ovaries in the female and the testes in the male. They secrete hormones that regulate sexual behavior and reproduction. They do not control all sexual behavior, though. In a very real sense, the brain itself is the master of the sexual system—human sexual behavior is not controlled totally by instincts and the actions of the glands as in some parts of the animal world, but it is also affected by psychological factors such as attractiveness.

THE ADRENAL GLANDS

Everyone has two adrenal glands, one on top of each kidney. The origin of the name is simple enough; renal comes from a Latin word meaning “kidney” and ad is Latin for “to,” so adrenal means “to or on the kidney.” Each adrenal gland is actually divided into two sections, the adrenal medulla and the adrenal cortex.

It is the adrenal medulla that releases epinephrine and norepinephrine, when people are under stress, and aids in sympathetic arousal.

The adrenal cortex produces over 30 different hormones called corticoids (also called steroids) that regulate salt intake, help initiate* and control stress reactions, and also provide a source of sex hormones in addition to those provided by the gonads.

One of the most important of these adrenal hormones is cortisol, released when the body experiences stress, both physical stress (such as illness, surgery, or extreme heat or cold) and psychological stress (such as an emotional upset).

Cortisol is important in the release of glucose into the bloodstream during stress, providing energy for the brain itself, and the release of fatty acids from the fat cells that provide the muscles with energy.

Parts of the Brain and Their Functions

1. Cerebrum

   • Largest part of the brain.

   • Divided into two hemispheres: left and right.

   • Responsible for higher cognitive functions, such as thinking, memory, perception, and voluntary movement.

2. Cerebral Cortex

   • Outer layer of the cerebrum.

   • Divided into four lobes: frontal, parietal, temporal, and occipital.

   • Frontal lobe: involved in decision-making, problem-solving, and motor control.

   • Parietal lobe: processes sensory information, including touch and spatial awareness.

   • Temporal lobe: responsible for auditory processing, language, and memory.

   • Occipital lobe: primarily involved in visual processing.

3. Thalamus

   • Located at the center of the brain.

   • Acts as a relay station, receiving sensory information from various parts of the body and sending it to the appropriate areas of the cerebral cortex.

4. Hypothalamus

   • Situated below the thalamus.

   • Regulates essential functions like body temperature, hunger, thirst, and sleep.

   • Controls the release of hormones from the pituitary gland.

5. Brainstem

   • Connects the brain to the spinal cord.

   • Composed of three parts: midbrain, pons, and medulla oblongata.

   • Midbrain: involved in sensory and motor functions, as well as sleep and arousal.

   • Pons: aids in relaying messages between the cerebrum and cerebellum, and controls certain autonomic functions.

   • Medulla oblongata: regulates vital functions like breathing, heart rate, and blood pressure.

6. Cerebellum

   • Located at the back of the brain, below the cerebrum.

   • Coordinates voluntary movements, balance, posture, and muscle tone.

   • Plays a role in motor learning and cognitive functions related to attention and language.

7. Limbic System

   • Comprised of several structures, including the hippocampus, amygdala, and hypothalamus.

   • Involved in emotions, memory, and motivation.

   • Hippocampus: crucial for forming and retrieving memories.

   • Amygdala: responsible for processing emotions, particularly fear and aggression.