Nervous System Functions
In order for those body parts to function, they must be stimulated and regulated by nerve impulses. Neurons have the ability to respond to a stimulus and convert it into a nerve impulse. They also have the ability to transmit that impulse to other neurons or the cells of muscles or glands. In neurons, information travels in the form of nerve impulses that are conducted along axons. Impulses do not travel in and between neurons like electric currents through telephone wires. For nerve impulses to be transmitted throughout the body, electrochemical reactions must occur in neurons. Dendrites are the points through which signals or impulses from adjacent neurons enter a particular neuron. If a dendrite of a neuron is stimulated, electrical and chemical changes take place throughout the cell.
- The master controlling and communicating system of the body
- 1. Sensory Input - gathering information
- To monitor changes occurring inside and outside the body
- Changes = stimuli
- 2. Integration
- To process and interpret sensory input and decide of action is needed
- 3. Motor Output
- A response to integrated stimuli
- The response activates muscles or glands
Structural Classification of the Nervous System
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Functional Classification of the Nervous System
Structures called sensory receptors at the ends of peripheral neurons provide the sensory function of the nervous system. These receptors gather information by detecting changes inside and outside the body. They monitor external environmental factors such as light and sound intensities as well as the temperature, oxygen concentration, and other conditions of the body's internal environment. Sensory receptors convert their information into nerve impulses, which are then transmitted over peripheral nerves to the CNS (The Gale Encyclopedia of Science, 2008). There, the signals are integrated and are brought together, creating sensations, adding to memory, or helping produce thoughts. Following integration, conscious or subconscious decisions are made and then acted upon by means of motor functions. The motor functions of the nervous system employ neurons that carry impulses from the CNS to responsive structures called effectors. These effectors are outside the nervous system and include muscles that contract in response to nerve impulse stimulation, and glands that secrete when stimulated. The motor portion of the PNS can be subdivided into the somatic and the autonomic nervous systems (2008). Generally the somatic nervous system oversees conscious (voluntary) activities, such as skeletal muscle contraction. The autonomic nervous system controls viscera, such as the heart and various glands, and thus controls subconscious (involuntary) actions.
Functional Classification of the Peripheral Nervous System
- Sensory (afferent) division
- Nerve fibers that carry information to the central nervous system
- Motor (efferent) division
- Nerve fibers that carry information away from the central nervous system
- Two divisions:
- 1. Somatic nervous system = voluntary
- Conscious control of skeletal muscles
- 2. Autonomic nervous system = involuntary
- Regulate smooth muscle, cardiac muscle, and glands
- Divisions - sympathetic and parasympathetic
http://academic.kellogg.edu/herbrandsonc/bio201_mckinley/Nervous%20System.htm
Supporting Cells of the Central Nervous Tissue
Known as neuroglia or glial cells
- Function - to support, insulate, and protect neurons
- Astrocytes - star shaped cells that connect neurons together and to their blood supply
- Microglia - function as phagocytes by engulfing foreign invaders
- Ependymal - (epithelial-like) provide a barrier between brain and spinal fluid
- Oligodendrocytes - connect thick neuronal fibers and produce an important insulating material called the myelin sheath
- Satellite Cells - protect neuron cell bodies
- Schwann Cells - form myelin sheath in the peripheral nervous system
The Neuron
The nervous system can detect changes in the body, make decisions on the basis of the information received, and stimulate muscles or glands to respond. Typically, these responses counteract the effects of the changes, and in this way, the nervous system helps maintain homeostasis. One major key component that helps the nervous system detect these subtle changes is with the help of the neuron.
Cell Body
- Conduct messages in the form of nerve impulses
- They number in the billions (much higher in anatomy teachers)
- How extremely longevity
- Most cannot divide (hippocampus is a rare exception; it is involved in memory)
- Have a high metabolic rate; require TONS of oxygen and glucose (aerobic respiration)
- 3 basic regions: dendrites, cell body, and axons
- Impulses travel from dendrites to cell body to axons
Cell Body
- Nucleus
- Large nucleolus
- Dendrites - conduct impulses towards the cell body
- Axons - conduct impulses away from the cell body
- Axons end in axonal terminals
- Axonal terminals contain vesicles with neurotransmitters
- Axonal terminals are separated from the next neuron by a gap
- Synaptic Cleft - gap between adjacent neurons
- Synapse - junction between nerves
- Myelin sheath - Whitish, fatty material covering the axons
- Schwann cells - produce myelin sheath in jelly roll-like fashion
- Nodes of Ranvier - gaps in myelin sheath along the axon
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Structural Classification of Neurons
- Multipolar Neurons - many extensions from the cell body; only one is an axon; the rest are dendrites. Most neurons whose cell bodies lie within the brain or spinal cord are of this type
- Bipolar Neurons - one axon and one dendrite; such neurons are found within specialized parts o f the eyes, nose, and ears
- Unipolar Neurons - have a short single process leaving the cell body; this process divides into two branches, which really function as a single axon: One branch (peripheral process ) is associated with dendrites near a peripheral body part. The other branch (central process ) enters the brain or spinal cord (Shier, 2007).
Essentials of Human Anatomy and Physiology, 2009
Nerve Impulses
Resting Neuron
*The exchange of ions initiates an action potential in the neuron*
Action Potential
- The plasma membrane at rest is polarized
- Fewer positive ions are inside the cell than outside the cell
- A stimulus depolarizes a neuron's membrane
- A depolarized membrane allows sodium (Na+) to flow inside the membrane
*The exchange of ions initiates an action potential in the neuron*
Action Potential
- If the action potential (nerve impulse) starts, propagated over the entire axon
- Impulses travel faster when fibers have a myelin sheath
- Potassium (K+) ions rush out of the neuron after sodium ions rush in, repolarizes the membrane
- Sodium-potassium pump, using ATP, restores the original configuration
Transmission of a Signal at Synapses
Impulses are able to cross the synapse to another nerve
- Neurotransmitter is released from a nerve's axon terminal
- The dendrite of the next neuron has receptors that are stimulated by the neurotransmitter
- An action potential is stated in the dendrite
- Action potential arrives
- Vesicle fuses with plasma membrane
- Neurotransmitter is released into synaptic cleft
- Neurotransmitter binds to receptor on the receiving neuron's membrane
- Ion channel opens
- Neurotransmitter is broken down and released and ion channel closes
Reflex Arc
Reflex - rapid, predictable, and involuntary response to a stimulus
- Occurs over pathways called reflex arcs
- Receptor (stimulus at distal end of neuron)
- Sensory neuron
- Integration center (spinal cord)
- Motor neuron
- Effector
Simple Reflex Arc
Essentials of Human Anatomy and Physiology, 2009
Types of Reflexes and Reglulation
Somatic Reflexes
Autonomic Reflexes
Patellar, or knee-jerk, reflex is an example of a two-neuron reflex arc
- Activation of skeletal muscles
- Example: when you move your hand away from a hot stove
Autonomic Reflexes
- Smooth muscle regulation
- Heart and blood pressure regulation
- Regulation of glands
- Digestive system regulation
Patellar, or knee-jerk, reflex is an example of a two-neuron reflex arc