Chapter 28: the Nervous System

Chapter 28: Nervous Systems

28.1 Nervous systems receive sensory input, interpret it, and send out commands

The nervous system is the most intricately organized system capable of sending out signals from one location to another in a body. Nerve cells are called neurons and consist of a cell body containing the nucleus and organelles, and neuron fibers that send the signals.
The nervous system has two divisions:
Central nervous system made of the brain and spinal cord
Peripheral nervous system made up of nerves that carry signals into and out of the CNS. Nerves are bundles of neuron extensions wrapped in connective tissue. With the nerves, the PNS also has ganglia which are clusters of neuron cell bodies.
Functions of the nervous system:
Sensory input is conduction of signals from receptor to integration centers (sensory neurons)
Integration interprets signals and forms responses (interneurons)
Motor output is conduction of signals from integration centers to effector cells that perform bodily responses (motor neurons)
Reflexes are automatic responses to stimuli

28.2 Neurons are the functional units of nervous systems

The ability of neurons to send and receive signals is dependent on their structure. The organelles and nucleus are found in the cell body. Coming out of the cell body are many dendrites and one axon. The dendrites branch from the cell to receive signals from other neurons and relay that information to the cell body. The axon transmits signals to other cells, either other neurons or effector cells.
Neurons are only part of the nervous system. For every one neuron, up to 50 support cells, glia, are present. They are essential for normal functioning.
Schwann cells are found in the PNS. Oligodendrocytes are found in the CNS.
Axons are often covered with insulation called a myelin sheath. The sheath is made of several segments along the length of the axon. Each segment is a Schwann cell. Spaces between each cell are called the nodes of Ranvier and are the only places where the signal can be transmitted. The signal cannot pass along the myelin, so it jumps from node to node. This jumping allows it to travel much faster than if it had to move down the entire axon.
Axon ends in a cluster of branches, each with a synaptic terminal. The synapse is the site of communication between the synaptic terminal and another cell.

28.3 Nerve function depends on charge differences across neuron membranes

A resting neuron, one that is not sending signals, has potential energy that could be put to work sending signals. This is called the membrane potential, and it resides in an electrical charge difference in the plasma membrane. The cytoplasm in the membrane is negative, and the fluid outside the membrane is positive. Opposite charges attract and move towards each other and the membrane stores energy by holding charges apart. The voltage across the membrane in a resting neuron is resting potential, which is about -70millivolts. (negative because the charge inside is negative)
Resting potential exists because of differences in composition of ions inside and outside of the cell. In the membrane are channels and pumps that allow passage of inorganic ions potassium (K+) and sodium (Na+). More potassium is allowed to move than sodium, so it becomes more concentrated outside of the cell. As they diffuse out, a negative charge is left in the cell.
To maintain this charge, there is a sodium potassium pump. It actively transports to keep the sodium concentration low and the potassium concentration high. This gradient produces voltage across the membrane, resulting in the resting potential.

28.4 A nerve signal begins as a change in the membrane potential

A stimulus causes a nerve signal to be generated. Action potential is a nerve signal that carries information along an axon.
Stimulus is applied. If strong enough, voltage rises to a threshold (-50mv). The difference between the threshold and the resting potential is the minimum change in voltage that allows for an action potential.
The action potential is triggered when the threshold is reached. Membrane polarity is reversed quickly and the interior of the cell is more positive.
The membrane repolarizes as the voltage drops and returns to original resting potential.
The rapid change in charge is caused by movement of the ions across the membrane. Voltage gated channels open and close depending on changes in the membrane. The membrane change is caused by:
Resting membrane is positive on outside and negative on inside
Stimulus triggers sodium channels in membrane to allow sodium to enter the axon, making the inside surface slightly less negative. If the change is significant, then sodium channels open to allow sodium to move and trigger the threshold.
When threshold is reached, more sodium is allowed to move which causes voltage to peak
Peak voltage triggers the channels to close and inactivate. Meanwhile, the potassium channels open and potassium diffuses out quickly.
An undershoot of the resting potential closes the potassium channels and membrane returns to resting potential.

28.5 The action potential propagates itself along the neuron

For an action potential to function, it must travel the length of the axon from the cell body to the terminals. The signal starts out as an action potential generated near the cell body and is relayed to the terminals. Three steps occur to pass the signal:
When the sodium channels open, sodium rushes inward and the action potential is generated.
Potassium channels in the same region open and potassium diffuses out of the axon. When this happens, the sodium channels are closed.
After all channels close, the resting potential is restored.
Once the charge has been initiated, it spreads from the first action potential that was created. Electrical changes from the movement of sodium made the action potential and trigger the opening of the channels in the region to the right of the first action, causing a second action potential. The second triggers a third, and so on.
It can only be a one way flow. After the potassium channels open, the sodium channels close. Since those channels are closed in the first region, they cannot respond to the electrical change. Therefore only the ones next in line can respond and their sodium channels will open.
Action potentials are an all or none response. They are the same regardless of the trigger strength. The difference of the force of the contraction is made by the frequency of the action potential. More frequency means more response or a stronger response.

28.6 Neurons communicate at synapses

When the action potential reaches the end of the axon, it generally stops there but the information is transmitted. This transmission occurs at the synapse, which is the area between the synaptic terminal and another cell. Synapses have two varieties:
Electrical, where current passes from one cell to another. The receiving neuron is stimulated quickly by the same frequency of action potential of the sending neuron.
Chemical synapses have a narrow gap, the synaptic cleft separating the terminal from the sender to the receiver. The cleft stops the action potential in the sender from going directly to the receiver. Instead, the action potential is converted to a chemical signal and then sent to the receiver. The chemical signal consists of a neurotransmitter that can generate an action potential.
Neurotransmitter is contained in the synaptic vesicles in the sender’s terminals. An action potential arrives at the terminal. The action potential triggers chemical changes that fuse the vesicles to the sender cells. The fused vesicles release NT molecules by exocytosis in the cleft and the NT diffuses across.
What happens next depends on the synapse. In the most common type, the NT diffuse inward, opening gated channels to allow ions to diffuse. This diffusion triggers new action potentials.

28.7 Chemical synapses make complex information processing possible

One neuron may interact with many neurons, and in some cases, it could be hundreds that are also receiving signals from possibly thousands of other terminals. Inputs can be varied based on the NT that are received. The effect of the NT depends on the kind of channel that it opens.
If the NT triggers an action potential in another cell, then it is considered excitatory.
If the NT decreases the ability of an action potential to generate, then it is considered inhibitory.
The more NT that is used, then the stronger the response.

28.8 A variety of small molecules function as neurotransmitters

Many NT are small, nitrogen containing organic molecules.
Acetylcholine is one. It is important at the synapses between motor neuron and motor cells. It can be excitatory or inhibitory, depending on the receiver.
Biogenic amines come from amino acids. They include epinephrine, norepinephrine, serotonin, and dopamine. All of these function as hormones. They are important in the CNS and regulate sleep, mood, attention, and learning. Imbalances of these are associated with some diseases such as Parkinson’s, schizophrenia, or depression. Some psychotropic drugs, like LSD, work by binding to serotonin and dopamine receptors.
Four NT: aspartate, glutamate, glycine, and GABA (gamma aminobutyric acid) are all amino acid and important to the CNS. Aspartate and glutamate are excitatory. Glycine and GABA are inhibitory.
Substance P is a peptide (short chain of amino acid). It is an excitatory NT that mediates our perception of pain. Endorphins are peptides that decrease our perception of pain during emotional or physical stress.

28.9 Many drugs act at chemical synapses

Many psychoactive drugs, as well as caffeine, nicotine, and alcohol affect the action of NT in the brain’s synapses.

28.11The evolution of animal nervous systems reflects changes in body symmetry

There is a great variety in the animal kingdom in how nervous systems are organized. Some animals do not have a system at all, like sponges. Others, like cnidarians, have a simple nervous system that is specialized for generalized for sending and receiving nerve signals. This type is considered a nerve net, and has no brain or head, and no central or peripheral divisions.
In animals that are radially symmetrical, like the cnidarians or echinoderms, the nervous system is uncentralized. This does not mean they are functionally simple.
In bilaterally symmetrical animals, they have a distinct head and tail region. Generally, they move through an area head first, allowing the animal to receive new stimuli quickly. The head is usually equipped with a sense organs and a brain. There are two hallmarks associated with bilateral symmetry:
Cephalization – concentration of the nervous system at the head
Centralization – presence of a distinct CNS and PNS.
The simplest of the clearly defined CNS is made of two parallel nerve cords and ganglia (nerve clusters). Nerves that connect to these nerve cords make up the PNS.

28.12 Vertebrate nervous systems are highly centralized

All vertebrate nervous systems have some fundamental similarities. They have distinct central and peripheral divisions and they are centralized and cephalized.
The spinal cord runs lengthwise in the vertebral column. It conveys information to and from the brain and integrates simple responses to stimuli.
The brain includes homeostasis centers that keep the body functioning, sensory centers that integrate information from sensory receptors, and centers of emotion and intellect. It also sends commands out to the muscles.
A large amount of capillaries serve the brain, and they are much more selective than those that serve the body. They allow in essential nutrients and oxygen, but do not allow metabolic wastes and many chemicals to pass freely into the brain. This blood brain barrier maintains a stable environment for the brain.
Fluid-filled spaces, ventricles, are continuous with the central canal of the spinal cord. They are filled with cerebrospinal fluid, which is formed in the brain by filtration of the blood. CSF cushions the CNS and assists in the supply of nutrients and hormones as well as the removal of waste.
Protecting the brain and spinal cord is a layer of connective tissue called the meninges. CSF circulates in the meninges to add another layer of protection.
The CNS has two areas
White matter – has axons covered in myelin sheaths
Gray matter – made up of nerve cell bodies and dendrites
Cranial nerves originate in the brain and terminate mostly in the head and upper body (eyes, nose, ears), while spinal nerves originate in the spinal cord and extend to areas below the head.

28.13 The peripheral nervous system of vertebrates can be divided into functional components

The PNS has two functional components:
Somatic nervous system – carries signals to and from skeletal muscles, usually in response to external stimuli. Its actions are considered voluntary because they are under conscious control. Many skeletal muscle actions are reflexes from the brain and spinal cord.
Autonomic nervous system – regulates internal environment by controlling smooth and cardiac muscles, as well as the organs of the digestive, excretory, endocrine, and cardiovascular systems. This is involuntary.
The autonomic NS has two sets of neurons with opposing effects on body organs.
Parasympathetic division helps with activities that gain and conserve energy for the body (“rest and digest”). This can include stimulating the digestive organs, decreasing heart rate, and increasing glycogen production.
Sympathetic division has the opposite effect and prepares the body for energy-consuming activities (“fight or flight”). It inhibits digestive organs, dilates bronchi for more air to pass through, increases heart rate, secretes glucose, and allows the adrenal glands to secrete epinephrine and norepinephrine.
The body generally operates at an intermediate level and receives signals from both divisions.
The enteric division of the ANS is made of networks of neurons in the digestive tract, pancreas, and gall bladder. It controls the secretion of these organs and the movements of peristalsis by smooth muscle. Though it can function independently, it is usually regulated by the sympathetic and parasympathetic divisions.
Sympathetic and parasympathetic divisions arise from different areas of the CNS and use different NT. The parasympathetic usually arises from the brain and lower part of the spinal cord and use acetylcholine. The sympathetic emerge from the middle of the CNS and use the NT norepinephrine. While we consider separate divisions of the nervous system, it is important to understand that all divisions are working together to maintain homeostasis.

28.14 The vertebrate brain develops from three anterior bulges of the neural tube

During embryonic development in vertebrates, three bilaterally symmetrical bulges (the forebrain, midbrain, and hindbrain) appear at the anterior end of the neural tube. Gradually the forebrain and hindbrain become subdivided into regions that have specific responsibilities.
The cerebrum, and outgrowth of the forebrain, is the sophisticated center of homeostasis and integration. It is much larger in relation to other parts of the brain in birds and mammals that it is in other vertebrates. Behavior of mammals and birds is directly correlated to the size of the cerebrum.

28.15 The structure of a living supercomputer: the human brain

The brainstem is a functional unit composed of the medulla oblongata and the pons. The brainstem coordinates and filters the conduction of information from sensory and motor neurons to higher brain regions. It regulates sleep and arousal and coordinated body movements.
The cerebellum, also found in the hindbrain, is the planning center for body movements. It plays a role in learning, decision making, and remembering motor responses. The cerebellum receives sensory information about the position of joints and length of muscles, as well as from the auditory and visual centers. It uses the information to coordinate movement and balance, such as hand eye coordination.
The thalamus contains most of the neurons that relay information to the cerebral cortex. It sorts data into categories and sends the information to appropriate higher brain centers for processing.
The hypothalamus regulates body temperature, blood pressure, hunger, thirst, fight-or-flight reflexes, and helps us experience emotions.
A pleasure center is located here, and can also be called the addiction center because it is strongly affected by addictive substances.
Biological clock – is our internal timekeeper. It receives visual input (light/dark) from the eyes and maintains the circadian rhythm, which is our daily sleep/wake pattern.
The cerebrum is divided into right and left cerebral hemispheres, each of which is responsible for the opposite side of the body. Between them is a thick band of nerves, the corpus callosum, which allows communication between the hemisphere and process information together. Under the corpus callosum is a group of neurons called the basal nuclei, which are important in motor coordination. Damage to this area can lead to immobilization.

28.16 The cerebral cortex is a mosaic of specialized, interactive regions