Figure 20.6
The movement of an action potential down an axon. A chemical message from another nerve causes the sodium ion channels at one point in the axon to open. Sodium ions rush across the membrane and cause the interior of the axon to become positively charged (depolarized) because the cell now contains more positive charges. Potassium ion channels then open and potassium ions flow out of the cell, which end the action potential. The action potential then moves down the axon membrane toward the synapse.
Figure 20.7
The changes in membrane potential during an action potential. Note the failed initiations that did not start an action potential. These depolarizations did not reach the threshold level (of about -55mV), so they did not start an action potential.
Types of Neurons
Neurons are highly specialized for the processing and transmission of cellular signals and can be classified by their structure or function. Structural classification is based on the number of dendrites and axons that a cell has. Functional classification groups neurons according to the direction in which the nerve impulse is moving in relation to the CNS.
We will discuss the three functional groups of nerves:
Sensory neurons carry signals from tissues and organs to the central nervous system and are sometimes also called afferent neurons. Sensory neurons typically have a long dendrite and short axon. Sensory neurons are found in reflex arcs and are involved in several forms of involuntary behavior, including pain avoidance.
Motor neurons carry signals from the central nervous system to muscles and glands and are sometimes called efferent neurons. Motor neurons have a long axon and short dendrites.
Interneurons connect sensory and motor neurons in neural pathways that go through the central nervous system. Interneurons are also called association or relay neurons. Interneurons are found only in the central nervous system where they connect neuron to neuron.
Communication Between Neurons
Neurons communicate with each other at specialized junctions called synapses. Synapses are also found at junctions between neurons and other cells, such as muscle cells like the one shown in Figure below. To see a synapse between two neurons, refer to Figure above.
There are two types of synapses:
chemical synapses use chemical signaling molecules as messengers
electrical synapses use ions as messengers
We will primarily discuss chemical synapses in this chapter. The axon terminal of one neuron usually does not touch the other cell at a chemical synapse. Between the axon terminal and the receiving cell is a gap called a synaptic cleft. The transmitting cell is called the presynaptic neuron, and the receiving cell is called the postsynaptic cell or if it is another neuron, a postsynaptic neuron.
Figure 20.8
A synapse between a neuron and a muscle cell. The connection between a neuron and a muscle cell, called a . The finger-like projections of the axon are the axon terminals. An action potential moves down to the axon terminals where it causes a chemical message called a neurotransmitter to be released into the synaptic cleft. The neurotransmitter then causes an action potential to start on the membrane of the muscle cell.
Your brain has a huge number of synapses. Each of your 1012 (one trillion) neurons—including glial cells—has on average 7,000 synaptic connections to other neurons. It has been estimated that the brain of a three-year-old child has about 1016 synapses (10 quadrillion). This number declines with age, and levels off by adulthood. An adult has between 1015 and 5 x 1015 synapses (1 to 5 quadrillion).
Neurotransmitter Release
When an action potential reaches the axon terminal, it causes the neurotransmitter vesicles to fuse with the terminal membrane, and the neurotransmitter is released into the synaptic cleft. A neurotransmitter is a chemical message that is used to relay electrical signals between a neuron and another cell. Neurotransmitter molecules are made inside the presynaptic neuron and stored in vesicles at the axon terminal. Some neurons make only one type of neurotransmitter, but most neurons make two or more types of neurotransmitters.
When an action potential reaches the axon terminal, it causes the neurotransmitter vesicles to fuse with the terminal membrane. Neurotransmitter is released into the synaptic cleft. The neurotransmitters then diffuse across the synaptic cleft and bind to receptor proteins on the membrane of the postsynaptic cell, as shown in Figure below.
Figure 20.9
The synaptic cleft. Neurotransmitter that is released into the synaptic cleft diffuses across the synaptic membrane and binds to its receptor protein on the post synaptic cell.
Neurotransmitter Action
Many types of neurotransmitters exist, a few of which are listed in Table below. Neurotransmitters can have an excitatory or inhibitory effect on the postsynaptic cell. An excitatory neurotransmitter initiates an action potential and an inhibitory neurotransmitter prevents one from starting. Glutamate is the most common excitatory transmitter in the body while GABA and glycine are inhibitory neurotransmitters. The release of acetylcholine, an excitatory neurotransmitter causes an inflow of positively charged sodium ions (Na+) into the postsynaptic neuron. This inflow of positive charge causes a depolarization of the membrane at that point. The depolarization then spreads to the rest of the postsynaptic neuron. Acetylcholine is the neurotransmitter that initiates muscle movement.
The effect of a neurotransmitter also can depend on the receptor it binds to. That is, a single neurotransmitter may be excitatory to the receiving neuron, or it may inhibit such an impulse by causing a change in the membrane potential of the cell. Synapses too can be excitatory or inhibitory and will either increase or decrease activity in the target neuron, based on the opening or closing of ion channels.
Common Neurotransmitters and Their Receptors Name Receptor Name and Type Ions Involved
Glutamate (glutamic acid) Glutamate receptors (ligand-gated ion channels and G protein-coupled receptors) Ca2+, K+, Na+
Acetylcholine Acetylcholine receptors (ligand-gated ion channel) Na+
Norepinephrine (noradrenaline) Adrenoceptors (G protein-coupled receptors) Ca2+
Epinephrine (adrenaline) Adrenoceptors (G protein-coupled receptors) Ca2+
Serotonin (5-hydroxytryptamine) 5-HT receptors 5-HT3 is a ligand-gated ion channel
5-HT1, 5-HT2, 5-HT4, 5-HT5A, 5-HT7 are G protein-coupled receptors
K+, Na+
Gamma-aminobutyric acid (GABA) GABAA and GABAC (ligand-gated ion channels) GABAB (G protein-coupled receptors) Cl- K+
Histamine Histamine receptors (H1, H2, H3, H4) (G protein-coupled receptors)
Neurotransmitter receptors can be gated ion channels that open or close through neurotransmitter binding or they can be protein-linked receptors. Protein-linked receptors are not ion channels; instead they cause a signal transduction that involves enzymes and other molecules (called second messengers) in the postsynaptic cell. Refer to the Cell Structure and Function chapter for more information about signal transduction mechanisms.
Neurotransmitter Reuptake
Many neurotransmitters are removed from the synaptic cleft by neurotransmitter transporters in a process called reuptake. Reuptake is the removal of a neurotransmitter from the synapse by the pre-synaptic neuron. Reuptake happens after the neurotransmitter has transmitted a nerve impulse. Without reuptake, the neurotransmitter molecules might continue to stimulate or inhibit an action potential in the post-synaptic neuron. The process of release and reuptake of neurotransmitters is shown in Figure below.
Figure 20.10
A synapse before and during reuptake. Neurotransmitter transporter proteins (also called reuptake pumps) release the neurotransmitter and also reuptake it from the synaptic cleft. Reuptake is a way of controlling the effect the neurotransmitter has on the post-synaptic cell.
Re-uptake is carried out by transporter proteins which bind to the released transmitter and actively transport it across the plasma membrane into the pre-synaptic neuron. The reuptak
e of neurotransmitter is the target of some types of medicine. For example, serotonin is a neurotransmitter that is produced by neurons in the brain. Serotonin is believed to play an important role in the regulation of mood, emotions, and appetite. After release into the synaptic cleft, serotonin molecules either attach to the serotonin receptors (called 5-HT receptors) of the post-synaptic neuron, or they attach to receptors on the surface of the presynaptic neuron that produced the serotonin molecules, for reuptake. Reuptake is a form of recycling because the neuron takes back the released neurotransmitter for later use. Medicines called selective serotonin reuptake inhibitors (SSRIs) block the reuptake of the neurotransmitter serotonin. This blocking action increases the amount of serotonin in the synaptic cleft, which prolongs the effect of the serotonin on the postsynaptic neuron. Some scientists hypothesize that decreased levels of serotonin in the brain are linked to clinical depression and other mental illnesses. So SSRI medications such as sertraline and fluoxetine are often prescribed for depression and anxiety disorders.
Another way that a neurotransmitter is removed from a synapse is digestion by an enzyme. At cholinergic synapses (where acetylcholine is the neurotransmitter), the enzyme acetylcholinesterase breaks down the acetylcholine.
Neurotransmitters and Disease
Diseases that affect nerve communication can have serious consequences. A person with Parkinson's disease has a deficiency of the neurotransmitter dopamine. Progressive death of brain cells that produce dopamine increases this deficit, which causes tremors, and a stiff, unstable posture. L-dopa is a chemical related to dopamine that when given as a medicine, eases some of the symptoms of Parkinson’s disease. The L-dopa acts as a substitute neurotransmitter, but it cannot reverse the disease.
The soil bacterium Clostridium tetani produces a neurotoxin that causes the disease tetanus. The bacteria usually get into the body through an injury caused by an object that is contaminated with C. tetani spores, such as a puncture wound caused by stepping on a nail. The C. tetani neurotoxin blocks the release of the neurotransmitter GABA, which causes skeletal muscles to relax after contraction. When the release of GABA is blocked, the muscle tissue does not relax and remains contracted. Tetanus can be fatal when it affects the muscles used in breathing. Thankfully, tetanus is treatable and can be prevented by vaccination.
Another bacterium called Clostridium botulinum produces a toxin that is occasionally found in preserved foods that have been improperly sterilized. The toxin causes a disease called botulism. Botulin toxin blocks the release of the excitatory neurotransmitter acetylcholine. Blockage of acetylcholine causes the progressive relaxation of muscles because they are unable to contract. Paralysis of the muscles used for breathing can be fatal unless the patient is treated with a respirator.
Synapses and Recent Research
Recent studies have found that electrical synapses are more common in the central nervous system than were previously thought. An electrical synapse is a link between two neighboring neurons that is formed at a narrow gap between the pre- and postsynaptic cells called a gap junction. At gap junctions, cells are about 3.5 nm from each other, a much shorter distance than the 20 to 40 nm distance that separates cells at chemical synapses.
Figure 20.11
Electrical synapses. Electrical synapses are more common in the nervous system than was once thought. Cell signaling at electrical synapses is much faster than signaling at chemical synapses. The image at the bottom left of the figure shows the location of gap junctions between cells.
Each gap junction has many channels which cross the plasma membranes of both cells, as is shown in Figure above. Gap junction channels are wide enough to allow ions and even medium sized molecules like signaling molecules to flow from one cell to the next. For example, when positive ions move through the channel into the next cell, the extra positive charges depolarize the postsynaptic cell.
Signaling at electrical synapses is faster than the chemical signaling that occurs across chemical synapses. Ions directly depolarize the cell without the need for receptors to recognize chemical messengers, which occurs at chemical synapses. Such fast communication between neurons may indicate that in some parts of the brain large groups of neurons can work as a single unit to process information. Electrical synapses are numerous in the retina and cerebral cortex.
In addition to neurons, glial cells are an important part of the nervous system. The word glia means "glue" in Greek. Glial cells can be thought of as partners to neurons by aiding in the maintenance of homeostasis, signal transduction, formation of myelin and providing support and nutrition. The importance of neurons as the conductive cells of the nervous system, known as the neuron doctrine, has been questioned by recent research. The role of glial cells in processing neural information has begun to be appreciated more. There are far more glial cells than neurons, it has been estimated that glial cells outnumber neurons by as many as 50:1.
Central Nervous System
The central nervous system (CNS), which includes the brain and the spinal cord, shown in Figure below, represents the largest part of the nervous system. The brain is the central control of the nervous system. The spinal cord carries nerve impulses from the brain to the body and from the body to the brain. Together with the peripheral nervous system (PNS), which includes all nervous tissue outside of the central nervous system, it controls virtually every activity in the body. The brain is protected by the skull and the spinal cord is protected by the vertebrae. An overview of the CNS can be viewed at http://vimeo.com/2024719.
Figure 20.12
The components of the central nervous system (CNS).
The Brain
The brain is the most complex organ in the body. The brain contains about 100 billion neurons each of which can be connected to tens of thousands of other neurons within the brain. The brain is the source of what makes us human; the conscious mind. The mind is the set of cognitive processes related to perception, interpretation, imagination, memories, and language. Beyond cognitive functions, the brain regulates processes related to homeostasis such as respiration and heartbeat. An average adult human brain weighs between 1 and 1.5 kg (3 lb). An adult brain uses about 20-25% of the total energy used by the body, while the developing brain of an infant consumes around 60% of total energy used by the body.
The brain can be classified by the processes its different parts control. The cerebrum generally controls conscious functions such as problem-solving and speech, while the midbrain and the brain stem are more involved with unconscious (autonomic) functions such as breathing, heartbeat, and temperature regulation. The cerebellum is involved in coordination and control of body movement. For a video of "Brain Basics," see http://www.teachers.tv/video/13838.
Cerebrum
The cerebrum is what most people would think of as the "brain." The cerebrum lies on top of the brainstem. It is made up of two cerebral hemispheres, which are shown in Figure below. The two cerebral hemispheres are connected to each other at the corpus callosum, the light-colored X-shaped structure in the center of the image. The corpus callosum is a wide, flat bundle of axons found deep inside the brain. Mammals (including humans), have the largest and most well-developed cerebrum among all species.
Figure 20.13
A magnetic resonance image (MRI) of the human brain in which the two hemispheres of the cerebrum can be seen.
Each hemisphere of the cerebrum can be divided into four parts, or lobes. These are: the frontal lobe, the parietal lobe, the temporal lobe, and occipital lobe. Researchers have identified a number of functional areas within each lobe, some of which are listed in Table below. Both hemispheres look identical, but there are functional differences between them. For example, there are differences between the centers of function for spatial awareness between right and left-handed people. Each cerebral hemisphere receives sensory information and controls muscle movements of the opposite side of the body. The right hemisphere controls the left side of the body, and the left hemisphere contr
ols the right side of the body.
Functions Controlled by the Cerebral Lobes Lobe Functions
Frontal Speech, intellectual function (reasoning, abstract thought), touch
Parietal Speech, taste, reading
Temporal Hearing, smell
Occipital Vision
Figure 20.14
The lobes of the cerebral cortex-frontal, temporal, occipital, and parietal. The cerebellum (purple) and brain stem (gray) are not part of the hindbrain. In vertebrates, a gross division into three major parts is used.
The cerebral cortex is the highly-folded outer layer of the cerebrum that is between 2 mm and 4 mm thick. The lobes that make up the cerebral cortex, shown in Figure above, are named after the skull bones that cover those areas of the brain. The many folds in the cortex allow for the large surface area of the brain to fit inside the skull. The cerebral cortex controls higher functions, such as consciousness, reasoning, emotions, and language. It also controls sensory functions such as touch, taste, smell, and responses to external stimuli. In the cerebrum, and found below the cerebral cortex, is the white matter. White matter is made up of myelinated axons that act as “cables” that link up certain parts of the right and left hemispheres.
CK-12 Biology I - Honors Page 93