MCB 32 Introductory Human Physiology



MCB 32 Introductory Human Physiology

Notes for Tuesday, September 12

Chapter 3, pps. 63-71, Chapter 4, pps.75-84.

OUTLINE FOR LECTURE

l. Synaptic Transmission

ll. Chemical Communication

lll. Neurophysiology

lV. Brain structure

l. Synaptic Transmission (Figure 3.15, p. 65)

The transfer of information from a neuron to another cell by chemical means is called synaptic transmission.

The nerve impulse cannot pass from one neuron to the next because there is no direct connection between the two neurons.

Communication between a neuron and another cell is the function of specialized junction called the synapse.

The action potential produces the release of a chemical from the axon terminal of a presynaptic neuron. The neurotransmitter, in turn, acts on a postsynaptic neuron.

The released chemical is called a neurotransmitter because of its function as a transmitter of information and because it is released by a neuron.

Typically, the axon terminal of the presynaptic neuron synapses with the dendrite or cell body of the postsynaptic neuron.

Keep in mind that although we are describing how information is passed from one neuron to the another, neurons also form synapses with other cell types, including muscle cells, glands and other tissues.

The Synapse

The axon terminal of a presynaptic neuron ends in a rounded structure called the presynaptic bulb.

The synaptic bulb contains packets of chemical neurotransmitter packaged in small membrane-bound vesicles.

When an action potential arrives at the synaptic bulb, a series of events is initiated that ultimately result in movement of these vesicles to the cell membrane.

There the vesicles fuse with the cell membrane, releasing their contents into the space between the presynaptic and postsynaptic cells; this space is known as the synaptic cleft.

Because the synaptic cleft is rather narrow, some of the neurotransmitter can reach the postsynaptic cell membrane by diffusion. At the postsynaptic cell membrane the neurotransmitter binds to the receptor, which results in changes in membrane permeability to certain ion such as Na+.

This, in turn, produces a small graded depolarization of the postsynaptic cell membrane in the area of the synapse.

The production of the graded potential that we talked about in the last lecture is in fact, the graded potential produced by synaptic transmission.

The chemical substances that we alluded to earlier are the neurotransmitters released by presynaptic neurons.

The development of graded potentials at the synapses on dendrites and cell body of a neuron, if sufficiently summated result in an action potential. The action potential travels down the axon, producing the release of neurotransmitter, which, in turn produces a graded potential in the next neuron and so on.

Whether a postsynaptic neuron will generate an action potential depends on the sum of all the graded potentials it receives.

In fact, a single synapse is unlikely to cause the generation of an action potential in the postsynaptic neuron.

A postsynaptic cell may have hundreds synapses with dozens of presynaptic neurons.

Only when the entire sum of positive and negative infuences from many synapses produces a net graded depolarization at the axon hillock of about -55 mV will an action potential be initiated.

Termination of synaptic transmission

Earlier, we mentioned that graded potentials may be short-lived, decaying rather quickly over time.

This is because the neurotransmitter substances remain in the synaptic cleft for only a short period of time, and the increase in postsynaptic cell membrane permeability to Na+ is directly related to the concentration of neurotransmitter.

There are three possible fates of neurotransmitter that is released into the postsynaptic cleft:

1) it may diffuse out of the synaptic cleft into the general circulation, where it is

ultimately destroyed

2) it may be degraded by special enzymes produced by either the pre- or postsynaptic

cell or

3) it may be taken back up by the presynaptic cell and repackaged into membrane-

bound vesicles, to be used again.

The interference with neurotransmitter uptake accounts for most of the actions of of cocaine, some antidepressive drugs.

Together, these three mechanisms of neurotransmitter removal ensure the stimulus can be terminated as quickly as it can be initiated.

Speed of synaptic transmission

The rate of transmission of information across the synapse is considerably slower than the rate of conduction of a nerve impulse down an axon.

This synaptic delay, on the order of 1/2000th of a second is due to the time it takes for neurotransmitter to be released from the presynaptic cell, to diffuse across the synaptic cleft, and to activate the postsynaptic cell to become permeable to Na+, as well as the time for Na+ to enter the cell and produce a graded potential, and then a nerve impulse.

Given these steps, 1/2000th of a second seems relatively fast. Nevertheless, it is considerably slower than the speed of conduction of action potentials.

Neurotransmitter substances and receptors

The human body actually uses several dozen different neurotransmitter substances, each with its own particular mode of action and effect on the postsynaptic cell.

As mentioned earlier most neurotransmitters produce a depolarizing graded potential, increasing the chance that the threshold will be reached and that an action potential will be generated by the postsynaptic neuron or cell.

Others produce hyperpolarizing graded potentials in the postsynaptic cell, reducing the chance that threshold for an action potential will be reached.

Most neurons produce primarily one neurotransmitter at their axon terminals.

Some neurotransmitter substances exert an excitatory influence on one postsynaptic cell type and an inhibitory influence on another.

They can do this because different receptor subtypes can exist for a single neurotransmitter. Activation of different receptor subtypes can produce very different postsynaptic events.

Four of the most common neurotransmitter substances are acetylcholine (ACh), norepinephrine, (NE), epinephrine(E) and dopamine (DA).

Acetylcholine is the neurotransmitter found at the junction between a neuron and a skeletal muscle, where it stimulates the muscle to contract.

It also inhibits the rate at which the heart beats.

ACh is an example of one neurotransmitter producing both stimulatory and inhibitory actions, depending on type of receptor it actiates.

Norepinephrine, epinephrine, are found widely in the brain.

Norepinephrine is also the neurotransmitter for a branch of the nervous system that regulates many of the automatic functions, such as blood pressure and rate of breathing.

Neural Information Processing

So far we have described how an action potential, or nerve impulse is generated in a nerve cell and transmitted from a nerve cell to another cell. These nerve impulses form the basis of rapid communication throughout the body.

If all information is in the form of identical electrical impulses, how can one perceive all of the colors in a sunset, feel emotions, or guide a basketball into a hoop from a distance of 20 feet?

This is a very active area of research. Some of the biological phenomena that probably contribute to information processing and integration are discussed below.

The areas of the brain that receive the nerve impulses and how they are processed by the brain are very important in determining how information is perceived.

Convergence and divergence of neural information

Information is not transmitted strictly from one nerve cell to another in a straight line.

Many different presynaptic cells may form synapses with a single neuron; this joining of output of many presynaptic cells onto one postsynaptic cell is called convergence.

In humans terms, an example might be the sight of a steak, the sizzling sound it makes on a grill, and its smell may all provide information (via neurons and their synapses) to one neuron that contributes to the feeling of hunger.

Furthermore, one nerve cell can provide information to a variety of postsynaptic cells, a process called divergence.

In the example of a 20-foot jump shot, information from a single visual cell in the eye may provide information to different areas of the brain, involved in vision, control of muscle movement, memory, and even emotions.

Coding for stimulus intensity

The coding for stimulus intensity is in part determined by how many neurons in a given pathway are active and by the number of action potentials per unit time in a given neuron. The latter can vary from zero in an inactive neuron up to several hundred impulses per second.

Neurons vary considerably both in the threshold membrane potential required to initiate an action potential and in the maximun number of action potentials per unit time.

Because action potentials are conducted in an all-or-none fashion and do not vary in amplitude as they travel down a neuron, intensity is not encoded by action potential amplitude, but by frequency.

Inhibitory versus stimulatory input

As mentioned previously, synaptic input to a neuron can either increase of decrease the likelihood that the neuron will initiate an action potential. This depends on the type of neurotransmitter released and the type of receptor present on the postsynaptic membrane.

Direct electrical communication.

Electrical impulses generated by neurons are usually transmitted to adjacent neurons by a chemically mediated step because the distance between the two cells is too great for direct electrical transmission.

However, there are important exceptions. A few excitable cells are capable of direct electrical connections. The most important of these include certain muscle cells, particularly those of the heart and digestive tract.

Cells in these muscles are joined together by special connections known as gap junctions. Gap junctions are composed of membrane proteins that span the gap between adjacent cells, forming a direct channel between them.

Muscle cells joined by gap junctions can transmit electrical information in the form of ions directly from cell to cell, so that the action potential proceeds not just down the length of a single cell, but through all the muscle cells in the tissue.

This is the basis for the coordinated contraction of the heart, and for the process to move food through the digestive tract.

Some nerve cells also have gap junctions, called electrical synapses, but the chemical synapse is by far the more common.

Chemical Communication

Every form of communication except direct electrical connections between certain muscle cells relies, at least in part, on chemical messengers.

The classification of chemical messengers into subgroups relies primarily on the type of cell that releases the chemical and on how the chemical reaches its site of action.

Hormones

The classical definition of a hormone is a chemical that is secreted by a gland into the bloodstream traveling to other sites in the body to exert its effect.

The site at which the hormone acts is called the target. A single hormone may have only one target tissue or organ or it may have many diverse targets and thus many regulatory effects.

Hormones control processes as diverse as the rate of urine formation, the amount of sugar in the blood, and the rate of sexual maturation.

Hormones act on specific cells. This is because the target cell needs to have a specific receptor for that hormone in order for it to respond.

To make things more complicated, different cells may have different receptor types for the same hormone. It is the special properties of these receptors that determine the effect elicited by the hormone.

Paracrine and Autocrine Factors.

Numerous chemical messengers have been discovered that do not fit the classical definition of hormone.

Some cells communicate with nearby cells by releasing chemicals into the extracellular space. These chemicals function as messengers even though they do not circulate in blood. Such local chemical messengers are called paracrine factors.

Examples of paracrine factors are histamine and prostaglandins.

Histamine is a chemical released from specialized cells called mast cells that are present in most tissues. It is responsible for local swelling of a tissue when it is damaged, and for some of the effects of allergic reactions.

Prostaglandins are a family of fatty acids found in almost every tissue and organ in the body. Prostaglandins have a variety of roles in local tissue function, including constriction or dilation of blood vessels, blood clotting and promotion of inflammation. The various growth factors that have been identified in a number of tissues and organs are also paracrine factors.

In general, paracrines factors are synthesized and released in the vicinity of their target cells.

Autocrines

Chemical messengers that act directly on the cells that produce them. Inhibitors of cell reproduction are paracrines and autocrines. One of the defects of cancer cells may be that they lack these inhibitors or do not respond to them, so that cell division continues unchecked.

Neurohormones

Some neurons called neuronendocrine cells do not synapse with another neuron; instead, their axon terminals are located near blood vessels. Neuroendocrine cells release their chemical messengers, call neurohormones, directly into the bloodstream.

An example of a neurohormone is oxytocin. Oxytocin is carried in the blood to the mammary gland, where it is responsible for milk ejection.

Neurophysiology

The brain receives input from the external world and your internal world and integrates those signals so as to adjust the output in order to maintain homeostatic balance.

It integrates many inputs such as vision, sound, touch and position and responds appropriately.

It Initiates movement, thought, speech and all that makes us uniquely human.

The brain is also the organ that controls that most illusive property, consciousness.

Our understanding of brain function came slowly from careful anatomical observations and from many animal experiments in which parts of the brain were removed or stimulated electrically.

Early anatomists recognized that the brain was not a homogeneous organ but rather is composed of discrete parts, many of which could be easily identified.

Furthermore, the brain is attached to the spinal cord, from which numerous nerves exit and and travel to the muscles and organs of the body.

Indeed, no part of the body is free of innervation. Thus, the brain seemed to be an organ centrally located and well designed to coordinate body functions.

By the middle of the nineteenth century, anatomists performed ablation (surgical removal) experiments in an attempt to understand the brain’s function.

Among the first were studies in which they removed small parts of the brain from hens. Depending on which part of the brain was surgically removed, they observed a different deficit.

Removing parts of the lower brain, the medulla, interrupted normal respiration.

If parts of the outermost brain, or cerebrum, were ablated the hen “lost instincts”, meaning it would no longer eat or perform other normal activities.

These types of crude studies clearly indicated that the brain controls many functions, and that different areas of the brain serve different functions.

Electrical studies were an outgrowth of the realization that specific brain areas have specific functions.

As you learned in earlier lecture, a weak electrical current applied to a nerve causes the nerve to generate an action potential that is then propagated down the nerve. This led researchers to test the results of stimulating the brain surface with a weak electrical current.

They demonstrated that electrical stimulation of a discrete area of the brain of an anesthetized animal might cause a limb to move in a particular direction. Stimulation of another spot on the surface of the brain might cause a digit to contract. Stimulation of a specific brain area always led to the same effect.

Anatomical studies, coupled with electrophysiological ones, detailed much of what we know about brain function.

Structure of the Nervous System

The nervous system is composed of all the neural elements of the body.

A clear division can be made between the elements contained in the bony structures of the body, the skull and the spinal column, and the neural elements outside the bony case.

The former is called the central nervous system (CNS).

The latter, which includes all the neural elements not contained in the CNS, is called the periperal nervous system (PNS).

The peripheral nervous system is composed of two branches. The first is the somatic nervous system, which carries motor information from the CNS to the skeletal (voluntary) muscles via the efferent nerves; and afferent nerves which carry sensory information from the muscles, joints, skin, and bones back to the CNS

The other branch is called the autonomic nervous system (ANS), which carries information to and from the glands, smooth muscles, and heart via the sympathetic and parasympathetic subdivisions, and is involved in the regulation of homeostatic control.

Human Brain Anatomy

Viewed from the top, (superior view), it is evident that the brain is divided into two similar parts, the left and right hemispheres. Running down the middle of the brain is the longitudinal fissure, making the separation visible.

Below this fissure is a large tract of nerve fibers connecting the two hemispheres, called the corpus callosum.

The outer portion of the brain, called the cerebral cortex or gray matter, as it lacks the white myelin coating, is composed of a series of folded convolutions.

The elevated parts of the cortex are called gyri, and the depressions between the gyri are called sulci. Gray matter contains mainly cell bodies and dendrites of the neurons.

Viewed from the lateral or side view, the cerebral cortex is divided into four separate areas, or lobes; the occipital lobe, the parietal lobe, the frontal lobe and the temporal lobe.

The central sulcus separates the frontal lobe from the pariental lobe and the lateral sulcus separates the temporal lobe from the parietal lobe.

A less evident separation exists between the parietal and occipital lobes.

Each of these lobes has a separate and unique function.

Lying just below the cortex is a thicker mass of brain tissue composed of myelinated fibers having a whiter cast, appropriately called the white matter. Both gray matter and white matter are also seen in the spinal cord.

Surrounding the brain is a composite of four tough membranes called the meninges, which contain the jelly-like substance of the brain.

The meninges along with the bony skull, help to protect the brain from injury.

Cellular Elements of the Nervous System

The CNS is composed of nerve cells and associated cells.

The neurons can be considered the major working elements of the brain and spinal cord, despite the fact that many other non-neural cell types are present. Each has a unique function, and the proper functioning of each is a requirement for normal brain activity.

First, let us describe the non-neural elements of the brain.

Glia

Approximately half of the volume of the brain cavity is filled with a set of cells called glia. The precise function of these elements is not completely known, but we do have some knowledge of a few of the glial cell types. Glia act as a structural as well as functional units.

Astrocytes

star-shaped

regulate the ionic environment in which the neuronal elements reside.

Because precise ionic concentrations of K+ and Na+ in the fluids that bathe neurons are necessary for normal activity, the astrocyte is an important cell.

may regulate blood flow to certain areas of the brain as neural activity is altered

Oligodendrocytes

produce the myelin sheath that wraps around the myelinated fibers.

in the periphery, the Schwann cells supply the axons with myelin

This is an extremely important function, as impairment in myelination results in severe neuromuscular deficits.

In the disease multiple sclerosis, the insulating myelin becomes impaired, resulting in a severe deficit in muscular control along with other neurological symptoms, such as alterations in personality, memory loss, emotional instability, and pain. All these disorders may be traced to the impairment of electrical conduction along the central neurons.

Vascular Cells

Most capillaries throughout the body are highly permeable, allowing most solutes to pass freely across them into the interstitial space.

The capillaries of the brain are an exception. Oxygen, carbon dioxide and glucose may freely cross the capillary membrane; however, many other bloodborne solutes that can easily cross other capillaries are restricted in their movement across brain capillaries.

This limitation of the brain capillaries to the movement of certain solutes is called the blood-brain barrier.

Adjacent endothelial cells of the brain capillaries are joined very tightly, forming what are called tight junctions, so that solutes are unable to pass between adjacent cells.

In addition, astrocytes attach to the capillaries with foot-like processes.

This anatomical arrangement causes the capillary system of the brain to be less permeable to bloodborne solutes than the other capillary networks of the body.

The blood-brain barrier serves to protect the brain from sudden changes in the environment surrounding the neural elements.

This protection presents a serious problem, though, when physicians try to treat infections of the brain.

Many drugs are unable to cross the blood-brain barrier and cannot reach the site of infection, requiring the use of special drugs that are able to cross the blood-brain barrier.

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