4. The Nervous System

One primary difference between the somatic and autonomic nervous systems is that the peripheral component of the autonomic nervous system contains two neurons.

A motor neuron in the somatic nervous system goes directly from the spinal cord to the muscle without synapsing. In the autonomic nervous system, two neurons work in series to transmit messages from the spinal cord. The first neuron is known as the preganglionic neuron, whereas the second is the postganglionic neuron. The soma of the preganglionic neuron is in the CNS, and its axon travels to a ganglion in the PNS. Here it synapses on the cell body of the postganglionic neuron, which then affects the target tissue.

KEY CONCEPT 2

Action potentials rely on both electrical and chemical gradients. The neuron starts at the resting potential, around -70 mV. At the resting potential, potassium is high inside the cell and low outside the cell, while sodium is high outside the cell and low inside the cell. Once the cell reaches threshold, sodium channels open and sodium floods the cell, making it more positive inside (depolarization). Then, sodium channels are inactivated and the potassium channels open. This allows potassium to flow out of the cell, bringing the potential to the negative range (repolarization), and actually overshooting the resting potential (hyperpolarization). The Na /K ATPase then works to restore the resting potential.

The positive potential inside the cell not only triggers the voltage-gated sodium channels to inactivate, but also triggers the voltage-gated potassium channels to open. Once sodium has depolarized the cell, there is an electrochemical gradient favoring the efflux of potassium from the neuron.

As positively charged potassium cations are driven out of the cell, there will be a restoration of the negative membrane potential called repolarization. The efflux of K causes an overshoot of the resting membrane potential, hyperpolarizing the neuron. This hyperpolarization serves an important function: it makes the neuron refractory to further action potentials.

Impulse Propagation

As sodium rushes into one segment of the axon, it will cause depolarization in the surrounding regions of the axon. This depolarization will bring subsequent segments of the axon to threshold, opening the sodium channels in those segments. Each of these segments then continues through the rest of the action potential in a wavelike fashion until the action potential reaches the nerve terminal. After the action potential has fired in one segment of axon, that segment becomes momentarily refractory, as described previously. The functional consequence of this is that information can only flow in one direction.

Myelin is produced by oligodendrocytes in the central nervous system and Schwann cells in the peripheral nervous system.

At certain intervals along the axon, there are small breaks in the myelin sheath with exposed areas of axon membrane called nodes of Ranvier. As will be explored in the discussion of action potentials to follow, nodes of Ranvier are critical for rapid signal conduction. Finally, at the end of the axon is the nerve terminal or synaptic bouton (knob). This structure is enlarged and flattened to maximize neurotransmission to the next neuron and ensure proper release of neurotransmitters, the chemicals that transmit information between neurons.

KEY CONCEPT 5

Consider the purpose of reflexes. Although it may be amusing to make your friends' legs jump when you tap them, there is a more functional reason why this response occurs. The stretch on the patellar tendon makes the body think that the muscle may be getting overstretched. In response, the muscle contracts in order to prevent injury.

axon hillock

Excitatory input causes depolarization (raising the membrane potential, V , from its resting potential) and thus makes the neuron more likely to fire an action potential. Inhibitory input causes hyperpolarization (lowering the membrane potential from its resting potential) and thus makes the neuron less likely to fire an action potential. If the axon hillock receives enough excitatory input to be depolarized to the threshold value (usually in the range of -55 to -40 mV), an action potential will be triggered.

Neurotransmission must be regulated—there are almost no circumstances under which constant signaling to the postsynaptic cell would be desirable. Therefore, the neurotransmitter must be removed from the synaptic cleft. There are three main mechanisms to accomplish this goal.

First, neurotransmitters can be broken down by enzymatic reactions. The breakdown of acetylcholine (ACh) by acetylcholinesterase (AChE), is a classic example. Second, neurotransmitters can be brought back into the presynaptic neuron using reuptake carriers. The reuptake of serotonin (5-HT), is a classic example of this mechanism. Dopamine (DA) and norepinephrine (NE) also use reuptake carriers. Third, neurotransmitters may simply diffuse out of the synaptic cleft. Nitric oxide (NO), a gaseous signaling molecule, fits into this category.

Action Potential Generation

If the cell is brought to threshold, voltage-gated sodium channels open in the membrane. These ion channels open in response to the change in potential of the membrane (depolarization) and permit the passage of sodium ions. There is a strong electrochemical gradient that promotes the migration of sodium into the cell. From an electric standpoint, the interior of the cell is more negative than the exterior of the cell, which favors the movement of positively charged sodium cations into the cell. From a chemical standpoint, there is a higher concentration of sodium outside the cell than inside, which also favors the movement of sodium into the cell. As sodium passes through these ion channels, the membrane potential becomes more positive; that is, the cell rapidly depolarizes. Sodium channels not only open in response to changes in membrane potential, but are also inactivated by them. When V approaches +35 mV, the sodium channels are inactivated and will have to be brought back near the resting potential to be deinactivated. Thus, these sodium channels can exist in three states: closed (before the cell reaches threshold, and after inactivation has been reversed), open (from threshold to approximately +35 mV), and inactive (from approximately +35 mV to the resting potential).

parasympathetic nervous system

It is associated with resting and sleeping states and acts to reduce heart rate and constrict the bronchi. The parasympathetic nervous system is also responsible for managing digestion by increasing peristalsis and exocrine secretions. Acetylcholine is the neurotransmitter responsible for parasympathetic responses in the body and is released by both preganglionic and postganglionic neurons. The vagus nerve (cranial nerve X), is responsible for much of the parasympathetic innervation of the thoracic and abdominal cavity.

KEY CONCEPT 3

It is critical to understand the difference between electrical and chemical transmission. Within a neuron, electricity is used to pass signals down the length of the axon. Between neurons, chemicals (neurotransmitters) are used to pass signals to the subsequent neuron (or gland or muscle).

The Na /K ATPase is important for restoring this gradient after action potentials have been fired.

It transports three Na out of the cell for every two K into the cell at the expense of one ATP. ATP is necessary because both Na and K are moved against their gradients by this process; thus, this qualifies as primary active transport. Each time the pump works, it results in the inside of the cell becoming relatively more negative, as only two positive charges are moved in for every three that are moved out.

Because the plasma membrane contains a thick nonpolar barrier (fatty acid tails), it is not energetically favorable for ions to cross this barrier. Inside the neuron, [K ] is high and [Na ] is low. Outside of the neuron, [Na ] is high, whereas [K ] is low. The negative resting potential is generated by both negatively charged proteins within the cell and the relatively greater permeability of the membrane to K compared with Na . If the cell membrane is more permeable to K and the ion's concentration is higher inside, K will diffuse down its gradient out of the cell. What does this mean in terms of charge movement?

K is positively charged, so its movement out of the cell results in a cell interior that is negative. Put another way, if we assume that the membrane starts at zero, and we take away a positive charge, we end up with a negative charge inside the cell: 0 - (+1) = -1. Na cannot readily enter at rest, so the negative potential is maintained.

KEY CONCEPT 1

Na wants to go into the cell because the cell is more negative inside (electrical gradient) and has a lower concentration of Na inside (chemical gradient).

Once released into the synapse, the neurotransmitter molecules diffuse across the cleft and bind to receptors on the postsynaptic membrane. This allows the message to be passed from one neuron to the next.

Neurons may be either excitatory or inhibitory; this distinction truly comes at the level of the neurotransmitter receptors, which tend to be either ligand-gated ion channels or G protein-coupled receptors. If the receptor is a ligand-gated ion channel, the postsynaptic cell will either be depolarized or hyperpolarized. If it is a G protein-coupled receptor, it will cause either changes in the levels of cyclic AMP (cAMP) or an influx of calcium

Neurotransmitters

Prior to release, neurotransmitter molecules are stored in membrane-bound vesicles in the nerve terminal. When the action potential reaches the nerve terminal, voltage-gated calcium channels open, allowing calcium to flow into the cell. This sudden increase in intracellular calcium triggers fusion of the membrane-bound vesicles with the cell membrane at the synapse, causing exocytosis of the neurotransmitter.

KEY CONCEPT 4

The first neuron in the autonomic nervous system is called the preganglionic neuron. The second neuron is the postganglionic neuron.

The spinal cord extends downward from the brainstem and can be divided into four divisions: cervical, thoracic, lumbar, and sacral. Almost all of the structures below the neck receive sensory and motor innervation from the spinal cord.

The spinal cord is protected by the vertebral column, which transmits nerves at the space between adjacent vertebrae. Like the brain, the spinal cord also consists of white and grey matter. The white matter lies on the outside of the cord, and the grey matter is deep within it. The axons of motor and sensory neurons are in the spinal cord. The sensory neurons bring information in from the periphery and enter on the dorsal (back) side of the spinal cord. The cell bodies of these sensory neurons are found in the dorsal root ganglia. Motor neurons exit the spinal cord ventrally, or on the side closest to the front of the body.

The brain consists of white matter and grey matter.

The white matter consists of axons encased in myelin sheaths. The grey matter consists of unmyelinated cell bodies and dendrites. In the brain, the white matter lies deeper than the grey matter. At the base of the brain is the brainstem, which is largely responsible for basic life functions such as breathing.

Multiple neurons may be bundled together to form a nerve in the peripheral nervous system.

These nerves may be sensory, motor, or mixed, which refers to the type(s) of information they carry; mixed nerves carry both sensory and motor information. The cell bodies of neurons of the same type are clustered together into ganglia

axon

a long appendage that terminates in close proximity to a target structure (a muscle, a gland, or another neuron). Most mammalian nerve fibers are insulated by myelin to prevent signal loss or crossing of signals.

The axon hillock plays an important role in _________ _________, or the transmission of electrical impulses down the axon. Signals arriving from the dendrites can be either excitatory or inhibitory; the axon hillock sums these signals, and if the result is excitatory enough (reaching threshold, as discussed later in this chapter), it will initiate an action potential.

action potentials

resting membrane potential

an electrical potential difference (voltage) between the inside of the neuron and the extracellular space. Usually, this is about -70 mV, with the inside of the neuron being negative relative to the outside. Neurons use selective permeability to ions and the Na /K ATPase to maintain this negative internal environment

The information received from the dendrites is transmitted through the cell body before it reaches the _____ _________, which integrates the incoming signals.

axon hillock

The somatic nervous system

consists of sensory and motor neurons distributed throughout the skin, joints, and muscles. Sensory neurons transmit information through afferent fibers. Motor impulses, in contrast, travel along efferent fibers.

The cell has many appendages emanating directly from the soma called ______, which receive incoming messages from other cells.

dendrites

speed of action potentials

depends on the length and cross-sectional area of the axon. Increased length of the axon results in higher resistance and slower conduction. Greater crosssectional areas allow for faster propagation due to decreased resistance. The effect of cross-sectional area is more significant than the effect of length. In order to maximize the speed of transmission, mammals have myelin. Myelin is an extraordinarily good insulator, preventing the dissipation of the electric signal. The insulation is so effective that the membrane is only permeable to ion movement at the nodes of Ranvier. Thus, the signal "hops" from node to node—what is called saltatory conduction. All action potentials within the same type of neuron have the same potential difference during depolarization. Increased intensity of a stimulus does not result in an increased potential difference of the action potential, but rather an increased frequency of firing.

If a neuron signals to a gland or muscle, rather than another neuron, the postsynaptic cell is termed an

effector.

Interneurons

found between other neurons and are the most numerous of the three types. Interneurons are located predominantly in the brain and spinal cord and are often linked to reflexive behavior.

The autonomic nervous system (ANS)

generally regulates heartbeat, respiration, digestion, and glandular secretions. In other words, the ANS manages the involuntary muscles associated with many internal organs and glands. The ANS also helps regulate body temperature by activating sweating or piloerection, depending on whether we are too hot or too cold. The main thing to understand about these functions is that they are automatic, or independent of conscious control. The ANS has two subdivisions: the sympathetic nervous system and the parasympathetic nervous system. These two branches often act in opposition to one another, meaning that they are antagonistic.

sympathetic nervous system

is activated by stress. The sympathetic nervous system is closely associated with rage and fear reactions, also known as "fight-or-flight" reactions. When activated, the sympathetic nervous system: Increases heart rate Redistributes blood to muscles of locomotion Increases blood glucose concentration Relaxes the bronchi

The central nervous system (CNS)

is composed of the brain and spinal cord.

Ependymal cells

line the ventricles of the brain and produce cerebrospinal fluid, which physically supports the brain and serves as a shock absorber.

The peripheral nervous system (PNS)

made up of nerve tissue and fibers outside the brain and spinal cord, such as the 12 pairs of cranial and 31 pairs of spinal nerves. The PNS thus connects the CNS to the rest of the body and can itself be subdivided into the somatic and autonomic nervous systems.

temporal summation

multiple signals are integrated during a relatively short period of time. A number of small excitatory signals firing at nearly the same moment could bring a postsynaptic cell to threshold, enabling an action potential.

Just like insulation prevents wires next to each other from accidentally discharging each other, the ______ _______ maintains the electric signal within one neuron. In addition, myelin increases the speed of conduction in the axon

myelin sheath

Most synapses are chemical in nature; they use small molecules referred to as ____________ to send messages from one cell to the next.

neurotransmitters

absolute refractory period

no amount of stimulation can cause another action potential to occur.

Astrocytes

nourish neurons and form the blood-brain barrier, which controls the transmission of solutes from the bloodstream into nervous tissue

In the central nervous system, axons may be bundled together to form tracts. Unlike nerves, tracts only carry one type of information. The cell bodies of neurons in the same tract are grouped into

nuclei

Microglia

phagocytic cells that ingest and break down waste products and pathogens in the central nervous system.

the neuron after the synaptic cleft is called the

postsynaptic neuron.

the neuron preceding the synaptic cleft is called the

presynaptic neuron

Oligodendrocytes (CNS) and Schwann cells (PNS)

produce myelin around axons.

The nucleus is located in the cell body, also called the ______ It is also the location of the endoplasmic reticulum and ribosomes.

soma

Neurons

specialized cells capable of transmitting electrical impulses and then translating those electrical impulses to chemical signals.

the nerve terminal, synaptic cleft, and postsynaptic membrane are known as a ______. Neurotransmitters released from the axon terminal traverse the synaptic cleft and bind to receptors on the postsynaptic neuron

synapse

Neurons are not physically connected to each other. Between the neurons, there is a small space into which the terminal portion of the axon releases neurotransmitters, which bind to the dendrites of the postsynaptic neuron.

synaptic cleft

spatial summation

the additive effects are based on the number and location of the incoming signals. A large number of inhibitory signals firing directly on the soma will cause more profound hyperpolarization of the axon hillock than the depolarization caused by a few excitatory signals firing on the dendrites of a neuron.

monosynaptic reflex arc

there is a single synapse between the sensory neuron that receives the stimulus and the motor neuron that responds to it. A classic example is the knee-jerk reflex. When the patellar tendon is stretched, information travels up the sensory (afferent, presynaptic) neuron to the spinal cord, where it interfaces with the motor (efferent, postsynaptic) neuron that contracts the quadriceps muscle. The net result is extension of the leg, which lessens the tension on the patellar tendon. Note that the reflex is simply a feedback loop and a response to potential injury. If the patellar tendon or quadriceps muscles are stretched too far, they may tear, damaging the knee joint. Thus, the reflex serves to protect the muscle.

polysynaptic reflex arc

there is at least one interneuron between the sensory and motor neurons. A real-life example is the reaction to stepping on a nail, which involves the withdrawal reflex. The leg with which one steps on the nail will be stimulated to flex, using the hip muscles and hamstring muscles, pulling the foot away from the nail. This is a monosynaptic reflex, similar to the knee-jerk reflex described previously. However, if the person is to maintain balance, the other foot must be planted firmly on the ground. For this to occur, the motor neuron that controls the quadriceps muscles in the opposite leg must be stimulated, extending that leg. Interneurons in the spinal cord provide the connections from the incoming sensory information to the motor neurons in the supporting leg.

relative refractory period

there must be greater than normal stimulation to cause an action potential because the membrane is starting from a potential that is more negative than its resting value.

Motor neurons (also known as efferent neurons)

transmit motor information from the brain and spinal cord to muscles and glands.

Sensory neurons (also known as afferent neurons)

transmit sensory information from receptors to the spinal cord and brain.