The Body's Communication Systems: The Endocrine and  Nervous Systems


Two systems control all physiologic processes:

  • The endocrine system broadcasts its hormonal messages to essentially all cells by secretion into blood and extracellular fluid. Like a radio broadcast, it requires a receiver to get the message - in the case of endocrine messages, cells must bear a receptor for the hormone being broadcast in order to respond.
  • The nervous system exerts point-to-point control through nerves, similar to sending messages by conventional telephone. Nervous control is electrical in nature and fast.

    As will be repeatedly demonstrated, the nervous and endocrine systems often act together to regulate physiology. Indeed, some neurons function as endocrine cells.

The Endocrine System

What exactly are hormones and how are they different from "non-hormones"? 

In other words, a particular cell is a target cell for a hormone if it contains functional receptors for that hormone, and cells which do not have such a receptor cannot be influenced directly by that hormone. Reception of a radio broadcast provides a good analogy. Everyone within range of a transmitter for National Public Radio is exposed to that signal (even if they don't contribute!). However, in order to be a NPR target and thus influenced directly by their broadcasts, you have to have a receiver tuned to that frequency.

Hormone receptors are found either exposed on the surface of the cell or within the cell, depending on the type of hormone. In very basic terms, binding of hormone to receptor triggers a cascade of reactions within the cell that affects function. See Lecture 6 for more information on hormone action and signal transduction

A traditional part of the definition of hormones described them as being secreted into blood and affecting cells at distant sites. However, many of the hormones known to act in that manner have been shown to also affect neighboring cells or even have effects on the same cells that secreted the hormone. Nonetheless, it is useful to be able to describe how the signal is distributed for a particular hormonal pathway, and three actions are defined:

Two important terms are used to refer to molecules that bind to the hormone-binding sites of receptors:

Finally, a comment on the names given hormones and what some have called the tyranny of terminology. Hormones are inevitably named shortly after their discovery, when understanding is necessarily rudimentary. They are often named for the first physiologic effect observed or for their major site of synthesis. As knowledge and understanding of the hormone grow, the original name often appears inappropriate or too restrictive, but it has become entrenched in the literature and is rarely changed. In other situations, a single hormone will be referred to by more than one name. The problem is that the names given to hormones often end up being either confusing or misleading. The solution is to view names as identifiers rather than strict guidelines to source or function.

Credit Where Credit is Due:

This information was mostly stolen from the following webpage:  http://arbl.cvmbs.colostate.edu/hbooks/pathphys/endocrine/index.html

This is an excellent website with much more information on the Endocrine System


The Nervous System

Functions of the Nervous System

Sensory Input
Receptors are parts of the nervous system that sense changes in the internal or external environments. Sensory input can be in many forms, including pressure, taste, sound, light, blood pH, or hormone levels, that are converted to a signal and sent to the brain or spinal cord.

Integration and Output
In the sensory centers of the brain or in the spinal cord, the barrage of input is integrated and a response is generated. The response, a motor output, is a signal transmitted to organs than can convert the signal into some form of action, such as movement, changes in heart rate, release of hormones, etc. 

Divisions of the Nervous System

The nervous system monitors and controls almost every organ system through a series of positive and negative feedback loops.

The Central Nervous System

The Central Nervous System (CNS) is composed of the brain and spinal cord. The CNS is surrounded by bone-skull and vertebrae. Fluid and tissue also insulate the brain and spinal cord.

The Human Brain

The brain of all vertebrates develops from three swellings at the anterior end of the neural canal of the embryo. From front to back these develop into the

The Hindbrain

The main structures of the hindbrain are the

Medulla oblongata

The medulla looks like a swollen tip to the spinal cord. Nerve impulses arising here: The neurons controlling breathing have mu () receptors, the receptors to which opiates, like heroin bind. This accounts for the suppressive effect of opiates on breathing.  Destruction of the medulla causes instant death.

Pons

Cerebellum

The cerebellum consists of two deeply-convoluted hemispheres. Although it represents only 10% of the weight of the brain, it contains as many neurons as all the rest of the brain combined.

Its most clearly-understood function is to coordinate body movements. People with damage to their cerebellum are able to perceive the world as before and to contract their muscles, but their motions are jerky and uncoordinated.

The reticular formation is a region running through the middle of the hindbrain (and on into the midbrain). It receives sensory input (e.g., sound) from higher in the brain and passes these back up to the thalamus. The reticular formation is involved in sleep and arousal.

The Midbrain

The midbrain occupies only a small region in humans (it is relatively much larger in "lower" vertebrates). We shall look at only three features:
Link to discussion of how various psychoactive chemicals act on synapses within the central nervous system.

The Forebrain

The human forebrain is made up of

Diencephalon

Telencephalon

The cerebrum, the largest part of the human brain, is divided into left and right hemispheres connected to each other by the corpus callosum. The hemispheres are covered by a thin layer of gray matter known as the cerebral cortex, the most recently evolved region of the vertebrate brain. Fish have no cerebral cortex, amphibians and reptiles have only rudiments of this area.

The cortex in each hemisphere of the cerebrum is between 1 and 4 mm thick. Folds divide the cortex into four lobes: occipital, temporal, parietal, and frontal. No region of the brain functions alone, although major functions of various parts of the lobes have been determined.

The occipital lobe (back of the head) receives and processes visual information. The temporal lobe receives auditory signals, processing language and the meaning of words. The parietal lobe is associated with the sensory cortex and processes information about touch, taste, pressure, pain, and heat and cold. The frontal lobe conducts three functions:

  1. motor activity and integration of muscle activity
  2. speech
  3. thought processes

Functional areas of the brain. 

Most people who have been studied have their language and speech areas on the left hemisphere of their brain. Language comprehension is found in Wernicke's area. Speaking ability is in Broca's area. Damage to Broca's area causes speech impairment but not impairment of language comprehension. Lesions in Wernicke's area impairs ability to comprehend written and spoken words but not speech. The remaining parts of the cortex are associated with higher thought processes, planning, memory, personality and other human activities.

Parts of the cerebral cortex and the relative areas that are devoted to controlling various body regions. 

The Spinal Cord

The spinal cord runs along the dorsal side of the body and links the brain to the rest of the body. Vertebrates have their spinal cords encased in a series of (usually) bony vertebrae that comprise the vertebral column.

The gray matter of the spinal cord consists mostly of cell bodies and dendrites. The surrounding white matter is made up of bundles of interneuronal axons (tracts). Some tracts are ascending (carrying messages to the brain), others are descending (carrying messages from the brain). The spinal cord is also involved in reflexes that do not immediately involve the brain.


The Peripheral Nervous System 

The neuron is the functional unit of the peripheral nervous system. 

 

Three types of neurons:. 

Some axons are wrapped in a myelin sheath formed from the plasma membranes of specialized glial cells known as Schwann Cells. Schwann cells serve as supportive, nutritive, and service facilities for neurons. The gap between Schwann cells is known as the Node of Ranvier, and serves as points along the neuron for generating a signal. Signals jumping from node to node travel hundreds of times faster than signals traveling along the surface of the axon. This allows your brain to communicate with your toes in a few thousandths of a second.

Cross Section of a myelinated axon showing the many layers of the Schwann cell

The Transmission of Neural Impulses 

The Resting Potential

All cells (not just excitable cells) have a resting potential: an electrical charge across the plasma membrane, with the interior of the cell negative with respect to the exterior. The size of the resting potential varies, but in excitable cells runs about -70 millivolts (mv).

The resting potential arises from two activities:

Ionic Relations in the Cell

The sodium/potassium ATPase produces
  • a concentration of Na+ outside the cell that is some 10 times greater than that inside the cell
  • a concentration of K+ inside the cell some 20 times greater than that outside the cell.
The concentrations of chloride ions (Cl-) and calcium ions (Ca2+) are also maintained at greater levels outside the cell EXCEPT that some intracellular membrane-bound compartments may also have high concentrations of Ca2+ (green oval)

Depolarization

Certain external stimuli reduce the charge across the plasma membrane. In each case, the facilitated diffusion of sodium into the cell reduces the resting potential at that spot on the cell creating an excitatory postsynaptic potential or EPSP.

If the potential is reduced to the threshold voltage (about -50 mv in mammalian neurons), an action potential is generated in the cell.

Action Potentials

The nerve impulse. In the resting neuron, the interior of the axon membrane is negatively charged with respect to the exterior (A). As the action potential passes (B), the polarity is reversed. Then the outflow of K+ ions quickly restores normal polarity (C). At the instant pictured in the diagram, the moving spot, which has traced these changes on the oscilloscope as the impulse swept past the intracellular electrode, is at position C.

If depolarization at a spot on the cell reaches the threshold voltage, the reduced voltage now opens up hundreds of voltage-gated sodium channels in that portion of the plasma membrane. During the millisecond that the channels remain open, some 7000 Na+ rush into the cell. The sudden complete depolarization of the membrane opens up more of the voltage-gated sodium channels in adjacent portions of the membrane. In this way, a wave of depolarization sweeps along the cell. This is the action potential (In neurons, the action potential is also called the nerve impulse.)

The refractory period

A second stimulus applied to a neuron (or muscle fiber) less than 0.001 second after the first will not trigger another impulse. The membrane is depolarized (position B above), and the neuron is in its refractory period. Not until the -70 mv polarity is reestablished (position C above) will the neuron be ready to fire again.

Repolarization is first established by the facilitated diffusion of potassium ions out of the cell. Only when the neuron is finally rested are the sodium ions that came in at each impulse actively transported back out of the cell.

In some human neurons, the refractory period lasts only 0.001-0.002 seconds. This means that the neuron can transmit 500-1000 impulses per second.

The action potential is all-or-none

The strength of the action potential is an intrinsic property of the cell. So long as they can reach the threshold of the cell, strong stimuli produce no stronger action potentials than weak ones. However, the strength of the stimulus is encoded in the frequency of the action potentials that it generates.

Myelinated Neurons

The axons of many neurons are encased in a fatty sheath called the myelin sheath. It is the greatly expanded plasma membrane of an accessory cell called the Schwann cell. Where the sheath of one Schwann cell meets the next, the axon is unprotected. The voltage-gated sodium channels of myelinated neurons are confined to these spots (called nodes of Ranvier).

The inrush of sodium ions at one node creates just enough depolarization to reach the threshold of the next. In this way, the action potential jumps from one node to the next. This results in much faster propagation of the nerve impulse than is possible in nonmyelinated neurons.

Synapses

The junction between a nerve cell and another cell is called a synapse. Messages travel within the neuron as an electrical action potential. The space between two cells is known as the synaptic cleft. To cross the synaptic cleft requires the actions of neurotransmitters. Neurotransmitters are stored in small synaptic vesicles clustered at the tip of the axon.

A synapse.

Excitatory Synapse from the Central Nervous System (TEM x27,360). This image is copyright Dennis Kunkel at www.DennisKunkel.com, used without permission.

Arrival of the action potential causes some of the vesicles to move to the end of the axon and discharge their contents into the synaptic cleft. Released neurotransmitters diffuse across the cleft, and bind to receptors on the other cell's membrane, causing ion channels on that cell to open. Some neurotransmitters cause an action potential, others are inhibitory.

Neurotransmitters tend to be small molecules, some are even hormones. The time for neurotransmitter action is between 0.5 and 1 millisecond. Neurotransmitters are either destroyed by specific enzymes in the synaptic cleft, diffuse out of the cleft, or are reabsorbed by the cell. More than 30 organic molecules are thought to act as neurotransmitters. The neurotransmitters cross the cleft, binding to receptor molecules on the next cell, prompting transmission of the message along that cell's membrane. Acetylcholine is an example of a neurotransmitter, as is norepinephrine, although each acts in different responses. Once in the cleft, neurotransmitters are active for only a short time. Enzymes in the cleft inactivate the neurotransmitters. Inactivated neurotransmitters are taken back into the axon and recycled.