Structure of Neurons
Typical structure of a neuron
Body – rounded area of the neuron which contains the nucleus, and can have dendrites and axons extending from the body.
Dendrites – numerous extensions that receive electrochemical messages.
Axons – extensions that send information from the body. Usually a neuron has only one axon.
Neuroglia – nerve helper cells.
Neuroglia provide physical support, insulation, and nutrients for neurons. Neurons have become very specialized and have lost many of the functions of typical cells, and rely on neuroglia for these functions and maintaining neuron health.
Sensory Nerves
Sensory receptors take information from the body and bring it through peripheral neurons to the spinal cord. Sensory receptors monitor changes in the external environmental factors such as light and sound. It also monitors the body’s internal environment such as temperature, oxygen levels, pH, and blood chemical levels.
Receptors take the sensory information and send it through nerve impulses to the central nervous system. The central nervous system integrates information to make decisions and appropriate responses. The brain and spinal cord respond to stimulus by a variety of mechanisms including motor functions to act appropriately to the changing stimulus.
Motor Control
Peripheral nerves carry impulses from the brain and spinal cord to the effector muscles. Motor functions can be broken into two categories
1. Those that are under conscious or voluntary control such as the somatic nervous system. The somatic nervous system controls skeletal muscle. You can consciously think about moving your finger and make it move.
2. Autonomic nervous system – controls involuntary responses such as cardiac, smooth, and glandular responses. You cannot consciously control stomach movements.
The body utilizes the somatic and autonomic nervous system to respond to changing stimulus in the external and internal environment to maintain homeostasis.
Questions
What would happen if the body was unable to respond to a changing environment? __________
What is the advantage to having automatic responses that do not require your conscious input? ___________
Which nervous system is made up of the brain and spinal cord? ________________________
Which nervous system is made up of nerves leaving the spinal cord? ____________________
How are impulses sent down a neuron? ____________________________________________
Where is the cell nucleus contained? ______________________________________________
What area of a neuron receives impulses from other neurons and feeds that information to the cell body? ________
What is the role of the axon? _____________________________________________________
Does an impulse travels down and ask on to the end receptor? __________________________
Why does the body need neuroglia? _______________________________________________
What are sensory receptors? _____________________________________________________
Would you imagine sensory receptors are very specific, such as for hot and cold? ___________
With those receptors be different than pressure receptors? ______________________________
What does the integrative function of the nervous system mean? _________________________
How does the nervous system help the body respond to changing environments? _______________
Does the nervous system help the body measure changing stimulus and respond accordingly? ___________
Neuroglia
Neurons need neuroglia to fill spaces, provide structural support, produce myelin, and protection with phagocytosis. In the central nervous system neuroglia outnumber neurons. Neuroglia have the ability to divide. Central nervous system neurons are unable to to divide or replicate. They lost that ability with their specialization and differentiation.
Specific neuroglia in the central nervous system include:
1. Microglial cells – support neurons and phagocytize bacteria and cellular debris. They can form scars in areas of neural damage.
2. Oligodendrocytes – produce insulating layers of myelin around axons within the central nervous system.
3. Astrocytes – found between neurons and blood vessels to provide structural support. Astrocytes help regulate concentration of nutrients and ions within neural tissue. Astrocytes also form scar tissue that fill spaces following injury to the central nervous system.
4. Ependymal cells – these cells form an epithelial like membrane that form the choroid plexus and inner linings of the ventricles in the brain and central canal of the spinal cord.
The neuroglial help assemble and form the blood brain barrier, which helps protect the central nervous system.
Schwann cells – neuroglia only located in the peripheral nervous system. Schwann cells produce myelin around myelinated axons in the peripheral nervous system.
Neuron Structure
The neuron cell body contains granular cytoplasm, cell membrane, and organelles. Neurofibrils are fine threads which extend into the axon. Membranous sacs called chromatophilic substances, or Nissl bodies, are similar to endoplasmic reticulum in other cells. Ribosomes attach to the chromatophilic substance for protein synthesis, similar to other cells. In the center of the cell body is a nucleus.
The dendrites are short and highly branched processes to increase receptive surfaces for communication with other neurons. The cell body transitions to a cone shaped thickening called the axon hillock. The axon hillock contains many mitochondria, microtubules, and neurofibrils and transitions to form the axon. The axon sends transmissions from the cell body to communicate with other cells.
Larger axons contain myelin sheaths produced by Schwann cells (neuroglia specific for the peripheral nervous system). Myelin wraps in layers tightly around axons, pushing most of the cytoplasm for myelin and nuclei to the outside layer called neurilemma. Small and narrow gaps between Schwann cells are called nodes of Ranvier. Myelin increases the speed of communication down an axon by allowing the signal to jump from node to node. Therefore myelinated axons transmit signals faster than unmyelinated. Myelination occurs both in the peripheral nervous system and in the central nervous system. In this CNS myelinated axons appear white because of the amount of myelin, and is therefore termed white matter. Unmyelinated axons and neuron cell bodies form the gray matter because of their appearance under the microscope.
Schwann cells in the neurilemma help peripheral nerves regenerate after damage. As previously mentioned CNS axons are myelinated by oligodendrocytes, which do not produce the neurilemma and cannot function for nerve regeneration in the brain and spinal cord.
Questions
A network of fine threads within a cell body is called? _________________________________
The area between the cell body and the axon is called the? ____________________________
Nissl bodies are scattered throughout the cytoplasm and are made up of? ________________
What is the function of the chromatophilic substance? ________________________________
What is the function of myelin? __________________________________________________
What cell makes myelin in the CNS? _____________________________________________
What cell makes myelin in the PNS? _____________________________________________
Myelin wraps around the neuron in tight sheets forming stacked layers of myelin, pushing the cytoplasm and Schwann cell nucleus to a region called the? __________________________
The area between each schwann cell is called the? ________________________________
Action potentials are said to skip down the myelin from? _____________________________
Narrow gaps between Schwann cells are called? ___________________________________
Are also axons myelinated? ____________________________________________________
Myelinated axons appear what color under the microscope? _________________________
Unmyelinated axons in cell bodies appear what color under a microscope? _____________
Types and Classification of Neurons
Neurons differ in size, shape, and structure. They can have different lengths and sizes of axons. In addition some neurons have many more dendrites than others. Neurons are classified into three major structural groups with specific functions to send nerve impulses in one direction.
1. Multipolar neurons – have many dendrite processes arising from the cell body. Multipolar neurons have a single axon extension. Multipolar neurons are the most common neurons in the brain and spinal cord.
2. Bipolar neurons – have only two processes extending from the cell body. One of the extensions is a dendrite, and the other is the axon leaves the cell body. Bipolar neurons are present in specialized parts of the eyes, nose, and ears.
3. Unipolar neurons – have a single process extending from the cell body. The single process then divides into two branches which functions as a single axon. Unipolar neurons are the major nerve component in ganglia which are located outside the brain and spinal cord. One branch of the unipolar neuron is associated with dendrites in the peripheral part of the body, while the other branch enters the brain or spinal cord.
Neurons can be classified based on their function
1. Sensory neurons – are also known as afferent neurons. These neurons send signals from receptors in the skin or sensory organs back to the spinal cord. Stimulated receptors sends a sensory impulse down the axon to the brain or spinal cord. Most sensory neurons are unipolar while some are bipolar.
2. Interneurons – Association or interneurons – lie completely within the brain or spinal cord. Interneurons are multipolar and link with other neurons for transmitting information. They are commonly utilized to transmit incoming sensory information to the appropriate parts of the central nervous system. Interneurons send sensory information for processing, interpretation, or to motor neurons. The cell bodies of some interneurons cluster in specialized groups or masses of nervous tissue called nuclei. The nuclei are similar to ganglia, except that it is located completely within the central nervous system.
3. motor neurons – effector neurons – transmit impulses from the brain or spinal cord to the effectors. Motor neurons are multipolar neurons. Motor impulses control skeletal muscle contraction and secretion of glands.
Questions
Sensory nerves transmit signals from the external environment to the brain and spinal cord. Often they pass information onto interneurons which passes that information off to? ___________________
Do interneurons allow for a significant amount of communication between neurons? _______
Do interneurons bring information from multiple sensory receptors and pass them off to the appropriate neurons within this CNS? _______
A factor neurons are also known as? _____________________________________________
Effector neurons are also known as? ___________________________________________
If I consciously cause flexion of my index finger, what type of neuron is being utilized from the spinal cord to the muscles in the finger? _____
Sensory neurons are commonly which type of neuron? ______________________________
Neurons in the eyes and nose are commonly this type of neuron? ______________________
Interneurons are most likely to be which type of neuron? _____________________________