General Functions of the Nervous System
Types of Neurons and Neuroglial Cells
Classification of Neurons and Classification of Neuroglial Cells
Distribution of Ions, Resting Potential, Potential Changes, and Action Potential
Impulse Conduction and the All-or-None Response
Synaptic Transmission, Excitatory and Inhibitory Actions, and Neurotransmitters
Neuronal Pools, Facilitation, Convergence, and Divergence
Reflex Arcs and Reflex Behaviors
Structure of the Spinal Cord and Functions of the Spinal Cord
Structure of the Cerebrum and Functions of the Cerebrum
Ventricles and Cerebrospinal Fluid
Cranial Nerves and Spinal Nerves
The nervous system can be divided into two parts - 1) the central nervous system (CNS) consisting of the brain and spinal cord and 2) the peripheral nervous system consisting of peripheral nerves.
Click here to view the organs of the nervous system.
The nervous system has 3 functions:
1) A Sensory Function - Sensory receptors at the end of peripheral nerves pick up information about the body's internal and external environment. These receptors also detect changes that occur. For example, when you feel pain, a sensory receptor is picking up that information. All sensory information is picked up in the peripheral nervous system and sent to the central nervous system.
2) An Integrative Function - The integrative function takes place in the brain or spinal cord. These organs receive sensory information and make decisions regarding the information. The decision making is the integrative function. For example, if you feel pain your brain might decide you need to move away from the painful stimulus.
3) A Motor Function - Once the CNS makes a decision, it then carries out a motor function. The motor function is the stimulation of a muscle (skeletal, smooth or cardiac muscle) or a gland. For example if your brain decides you are touching something hot with your finger, the motor function would be to stimulate the skeletal muscles in the arm to pull your finger away from the painful stimulus. When a motor function is carried out, neurons in the CNS carry an impulse along a peripheral nerve to either a muscle or a gland; these are called effectors.
Neurons are the functional cells of the nervous system. They transmit electrochemical messages called nerve impulses to other neurons and effectors (muscles or glands).
In general neurons have a cell body and processes called nerve fibers that extend from the cell body. The cell body is the portion of the neuron that contains the nucleus and typical organelles such as mitochondria, lysosomes, and a Golgi apparatus. The cell body also contains thread-like structures called neurofibrils which extend into the nerve fibers and chromatophilic substance which similar to RER of other cells and functions in protein synthesis.
Extending from the cell body are nerve fibers. The two types of nerve fibers are axons and dendrites. A neuron may have one or more dendrites but typically only has one axon. Dendrites are usually short and branch profusely near the cell body; they specialize to receive information for the neuron. Axons are typically long and branch away from the cell body; they specialize to send information (nerve impulses) away from the cell body.
Click here to view the structure of a neuron.
Neuroglial cells, called Schwann cells, in the peripheral nervous system wrap themselves axons. Therefore the axons are coated by the cell membranes of Schwann cells. The cell membranes contain large amounts of myelin which is a fatty substance that acts to insulate the axon. Narrow gaps in the myelin sheath between Schwann cells are called Nodes of Ranvier. Axons tightly wrapped in myelin are called myelinated axons; nonmyelinated axons lack these tight myelin layers.
Click here to view a Schwann cell around an axon.
Neurons can be classified as follows based on their structures:
1) Bipolar neurons - These neurons have two nerve fibers - one axon and one dendrite. Bipolar neurons are found in parts of the eyes, ears, and nose.
2) Multipolar neurons - These neurons have many nerve fibers - one axon and many dendrites. Multipolar neurons are found in the central nervous system.
3) Unipolar neurons - These neurons have a singe nerve fiber that extends from the cell body and eventually divides into an axon and a dendrite. Unipolar neurons are found in ganglia. Ganglia are groups of neuron cell bodies outside the central nervous system.
Neurons can be classified as follows based on their functions:
1) Sensory (afferent) neurons - These neurons carry sensory information from the periphery to the CNS. Sensory neurons pick up sensory information from their receptors which are usually at the tips of their dendrites. Sensory neurons are unipolar and bipolar in shape.
2) Interneurons - Interneurons are multipolar in shape, found in the CNS, and they function to link neurons together. Interneurons transmit impulses from one part of the spinal cord or brain to another. They pick up information from sensory neurons and direct the information to appropriate parts of the brain for interpretation and decision making. They can also direct information to motor neurons.
3) Motor (efferent) neurons - Motor neurons carry information from the CNS to effectors (muscles or glands) in peripheral nervous system. Most motor neurons are multipolar.
Click here to view the different types of neurons.
The smaller cells of the central nervous system are called neuroglial cells. They do not send nerve impulses like neurons. The various types of neuroglial cells are as follows:
1) Microglial cells - phagocytic cells found in the CNS.
2) Oligodendrocytes - form myelin in the brain and spinal cord.
3) Astrocytes - connect neurons to blood vessels. These cells provide structural support and help regulate nutrients and ion concentrations in nervous tissue. They also form scar tissue that fills spaces following injury to nervous tissue.
4) Ependymal cells - form an epithelial membrane that lines the ventricles of the brain and the central canal of the spinal cord.
Click here to view the different types of neuroglial cells.
Most cell membranes are said to have a cell membrane potential. This means the membrane is polarized. Just like a battery is polar (one end is negative and the other end is positive) cell membranes are polar because the inside is negatively charged and the outside is positively charged. In most of our body cells, the outside of cell membranes is positively charged because more positive ions are on the outside. The inside of cell membranes is negatively charged because more negative ions are on the inside. This membrane potential is very important for the function of neurons.
Potassium ions pass through the cell membrane of neurons quite easily while sodium ions pass through less easily. Potassium and sodium ions are both positively charged. We will now see how these ions play a role in nerve impulses.
When a cell membrane is at rest or unstimulated, the outside of the membrane is slightly positively charged and the inside is slightly negatively charged. This is because there are more total sodium and potassium ions (both positively charged) outside the membrane. When sodium enters the cell through diffusion, the cell actively pumps sodium out. When the cell pumps sodium out, it also pumps potassium in. However, potassium will quickly diffuse out of the cell again. The net result or bottom line is that more positively charged ions collect on the outside of the cell membrane thus creating a polarization of the membrane. This polarized state is termed the resting potential of the cell membrane. As long as the membrane remains unstimulated, it will remain in this polarized state.
The cell membrane of a neuron will respond to stimuli such as heat, pressure, and chemicals by changing the amount of polarization across its membrane. If a cell membrane is at rest (inside negative, outside positive) and it responds to a stimulus by making the outside of its membrane less positive, we say it has depolarized. In other words, it became less polar. To make the outside of the membrane less positive, some of the sodium ions (which are positive) flow through channels to the inside of the cell membrane.
If the membrane of an axon becomes depolarized enough, an action potential is created.
When a stimulus causes depolarization of an axon membrane, it can lead to an action potential. The minimum amount of stimulus needed to cause the action potential is called the threshold stimulus. In an action potential, a portion of the axon membrane depolarizes because sodium is entering the cell. In other words, it is becoming less polar because more positive charges are entering the cell. Very quickly in an action potential, the cell membrane will become polar again. We call this process repolarization. What happens to repolarize the membrane is that potassium exits the cell. Remember potassium is also positively charged so by exiting the cell, the outside becomes positive again. The axon membrane will then return to its resting potential. Eventually the sodium is pumped out of the cell and potassium is pumped in and then potassium diffuses out. The resting potential is reestablished since more positively charged ions are on the outside of the membrane again.
A nerve impulse is created by a wave of action potentials flowing down an axon membrane. When an action potential is created in one area of the axon membrane, it causes an action potential to be created on the region of the axon membrane right next to it. Eventually the nerve impulse (wave of action potentials) will reach the end of the axon.
Click here to view a nerve impulse and a summary table of events of impulse conduction.
An unmyelinated axon does not conduct a nerve impulse as quickly as a myelinated axon. Also, the speed of the nerve impulse is related to the diameter of the axon. The larger the diameter, the faster the nerve impulse travels to its end.
Like muscle fiber contraction, nerve impulse is an all-or-nothing response. In other words, once a stimulus starts a nerve impulse, the impulse will travel to the end of the axon. Increasing the strength of the stimulus will not change the response.
A synapse is simply defined as the junction between two neurons.
Recall that axons branch profusely. When a nerve impulse travels down an axon, the impulse eventually reached the ends of all these branches. These ends of axons are called synaptic knobs. These synaptic knobs will contact dendrites, cell bodies and axons of other neurons. Whatever the synaptic knob is contacting is called a postsynaptic structure. Within synaptic knobs are vesicles that contain chemicals called neurotransmitters. When the nerve impulse reaches the synaptic knobs, the neurotransmitters are released onto postsynaptic structures.
Click here to view different types of synapses.
When a neurotransmitter is released onto another neuron, it can cause the neuron cell membrane to depolarize. Remember that if a cell membrane depolarizes enough, an action potential can be created. Therefore, neurotransmitters that cause depolarizations are called excitatory.
A neurotransmitter may prevent depolarization. This type of neurotransmitter is called inhibitory because it inhibits the generation of an action potential.
A neuron will be contacted by many different synaptic knobs of thousands of different neurons. If the synaptic knobs with excitatory neurotransmitters release their chemicals, then the postsynaptic neuron will be excited, depolarized, generate an action potential, and send a nerve impulse. Conversely, if the synaptic knobs with inhibitory neurotransmitters release their chemicals, then the postsynaptic neuron will be inhibited from sending a nerve impulse. Most of the time a combination of excitatory and inhibitory neurotransmitters are released onto a neuron at the same time. Whichever neurotransmitter type is released in the greatest amount will determine whether or not the postsynaptic neuron will generate a nerve impulse or not.
There are about 50 different neurotransmitters. Most neurons release only one type of neurotransmitter but some will release more than one type. Remember that neurotransmitters are in vesicles inside synaptic knobs. When a nerve impulse travels down an axon and reaches a synaptic knob, calcium flows into the synaptic knob. The calcium causes the vesicles to empty the neurotransmitter onto the postsynaptic structure through exocytosis.
Click here to view a synapse and a summary table of synaptic events.
The following is a list of common neurotransmitters: GABA, acetylcholine, glutamate, norepinephrine, epinephrine, dopamine, and serotonin.
Click here for a list of neurotransmitters and their actions.
Neurons in the central nervous system are organized into groups called neuronal pools. Each pool receives inputs from other areas of the central nervous system. The pool also sends information to other areas of the central nervous system.
A particular neuron of a neuronal pool may receive both excitatory and inhibitory input. If the net effect of the input is excitatory enough to start an action potential, the neuron will send a nerve impulse. However, it the net effect is not excitatory enough to start an action potential, the neuron will not send a nerve impulse. But the neuron is more excitable to incoming stimulation than before. We now say the neuron is in a state of facilitation.
A neuron will receive input from many different other neurons. This process of receiving information from different sources is called convergence. For example, one neuron may receive visual information, hearing information, and touch information all at the same time.
Remember that axons of neurons branch profusely. Therefore, one neuron can send its message to many different neurons. The process of sending information to many different cells is called divergence. For example, one neuron may send its message to thousands of different muscle fibers causing contraction of the muscle.
Click here to view the difference between divergence and convergence.
Recall that a nerve fiber is a cytoplasmic extension of a neuron cell body. Nerves are groups of nerve fibers held together by tough connective tissue in the peripheral nervous system. There are three general types of nerves:
1) Sensory nerves - nerves that conduct impulses to the CNS. They carry only sensory information.
2) Motor nerves - nerves that conduct impulses away from the CNS to muscles or glands. They carry only motor information.
3) Mixed nerves - nerves that conduct impulse to the CNS and away from the CNS to muscles or glands. They carry sensory and motor information. Most nerves in the body are mixed.
Click here to view a mixed nerve.
The paths that nerve impulses travel as they flow through the nervous system are called nerve pathways. The reflex arc is a very simple nerve pathway.
Information flows through a typical reflex arc are as follows:
1) It begins on receptors. Receptors are usually found on the tips of dendrites of sensory neurons.
2) The information then flows through the sensory neuron containing the receptors.
3) The information then flows to a group of interneruons.
4) The information then flows to motor neurons.
5) The information then reaches effectors (glands or muscles).
Click here to view the parts of a reflex arc.
A reflex is a predictable, automatic response to a stimulus. For example, if you touch something very hot, the predictable response is that you will pull your finger away from the hot surface; this is a type of reflex called a withdrawal reflex.
In this withdrawal reflex, the receptors are in the skin at the tips of the fingers. These receptors send their information to sensory neurons and the sensory neurons relay the information to interneurons in the brain and spinal cord. The interneurons then relay the information to parts of the brain that make you aware of the pain. The interneurons also relay the information to motor neurons that activate the muscles in the arm. The muscles in the arm coordinate the movement of pulling away your finger from the painful stimulus.
Meninges are membranes that protect the brain and spinal cord. There are three layers of meninges:
1) Dura mater - This is the outermost layer of the meninges. It is also the toughest; hence the name dura which means durable.
2) Arachnoid mater - This is the middle layer of the meninges. It is wispy in appearance like a spider's web; hence the name arachnoid meaning spider.
3) Pia mater - This is the innermost layer of the meninges. It sits directly on top of the brain and spinal cord. It is a very delicate layer (pia means soft).
Between the arachnoid mater and pia mater is a space called the subarachnoid space. It contains cerebrospinal fluid (CSF) which cushion the brain and spinal cord.
Click here to view meninges of the brain.
The spinal cord is a slender nerve column that is continuous with the brain. The spinal cord descends into the vertebral canal and ends around the level of the first or second lumbar verterbra.
The spinal cord is divided into 31 spinal segments. There are 8 cervical segments, 12 thoracic segments, 5 lumbar segments, 5 sacral segments and 1 coccygeal segment.
There is a thickening of the spinal cord in the neck region. This thickening is called the cervical enlargement and contains the motor neurons that innervate the muscles of the arms. Another thickening of the spinal cord occurs in the lumbar region. This thickening is called the lumbar enlargement and it contains the motor neurons that innervate the muscles of the legs.
Two grooves run down the midline of the spinal cord. The anterior groove is called the anterior median fissure while the posterior groove is termed the posterior median sulcus. A sulcus is a groove that is more shallow than a fissure.
When you look at a cross section of the spinal cord, you observe two differently colored areas. The inner tissue is termed gray mater as its color is darker than the outer tissue which is termed white mater. The gray mater contains neuron cell bodies and their dendrites while the white matter contains myelinated axons.
The divisions of the gray matter are called horns. So, the front divisions are called anterior horns and the back divisions are called posterior horns. In some places in the spinal cord, there are also lateral horns. Most motor neurons that innervate skeletal muscles are located in the ventral horns.
The divisions of the white matter are called funiculi. The gray matter divides the white matter up into anterior, lateral, and posterior funiculi. The funiculi contain myelinated axons. Groups of myelinated axons in the brain or spinal cord are called nerve tracts.
The two sides of gray matter are connected by an area of gray matter called the gray commissure. A canal runs through the gray commissure down the entire length of the spinal cord. This canal is called the central canal and contains cerebrospinal fluid.
Click here to view the structure of the spinal cord.
Click here to view a cross section of the spinal cord.
One function of the spinal cord is to carry sensory information up to the brain. The tracts that carry sensory information up to the brain are called ascending tracts.
Another function of the spinal cord is to carry motor information down from the brain to muscles and glands. These tracts that carry information down from the brain are called descending tracts.
Recall that tracts in the spinal cord and brain are bundles of myelinated axons. Tracts are often name for where they begin and end. For example, the spinothalamic tract begins in the spinal cord and ends in the thalamus of the brain. This is an ascending tract; therefore it is carrying sensory information. Other tracts are called corticospinal tracts. They begins in the cortex of the cerebrum and end in the spinal cord. These tracts are descending; therefore they carry motor information.
One last function of the spinal cord is to participate in reflexes like the withdrawal reflex previously described.
The brain can be divided up into four major areas: 1) the cerebrum 2) the diencephalon 3) the cerebellum and 4) the brain stem.
Click here to view the major divisions of the brain.
The cerebrum is the largest part of the brain. It is divided into two halves called cerebral hemispheres. A thick bundle of nerve fibers corpus callosum connects the two hemispheres. The grooves on the surface of the cerebrum are called sulci. The "bumps" of brain matter between the sulci are called gyri. A deep groove called the longitudinal fissure runs between the two longitudinal hemispheres.
The cerebrum is divided up into 5 lobes:
1) Frontal lobe - forms anterior portion of each cerebral hemisphere; lies beneath frontal bone
2) Parietal lobe - posterior to the frontal lobes; lies beneath parietal bones
3) Temporal lobe - inferior to the frontal and parietal lobes; lies beneath temporal bones
4) Occipital lobe - most posterior portion of each cerebral hemisphere; lies beneath the occipital bone
5) Insula - insulated or covered by the frontal, temporal and parietal lobes.
The cerebral cortex is the outermost layer of the cerebrum. It is made of gray matter and therefore contains neuron cell bodies and dendrites. This layer contains nearly 75% of all neurons in the entire nervous system.
Beneath the cerebral cortex is white matter. Recall that white matter is composed of myelinated axons.
Click here to view the lobes of the cerebrum.
The cerebrum interprets sensory information and initiates body movements. It also contains memory, learning and emotion areas.
The cerebral cortex is divided into the following functional areas:
1) Primary motor areas - located in the frontal lobes; they initiate skeletal muscle movements.
2) Motor speech area (Broca's area) - located in the frontal lobes; initiate skeletal muscle movements needed for speech.
3) Sensory areas - located in many lobes of the cerebrum; these areas interpret sensory information.
- visual sensory area - located in the occipital lobes; interprets visual information (what you see)
- auditory sensory area - located in the temporal lobes; interprets auditory information (what you hear)
- cutaneous sensation area - located in the parietal lobes; interprets feelings on the skin such as touch, itch, pain, etc.
4) Association areas - located in all lobes of the cerebrum; they interpret experiences, oversee memory, reasoning, judgement and emotions.
Click here to see the functional areas of the cerebral cortex.
Click here for a summary table of functional areas of the cerebral cortex.
Most people have a dominant cerebral hemisphere. The dominant hemisphere is used primarily for language related activities of speech, writing, reading and for complex intellectual functions such as analysis. The other hemisphere is called the nondominant hemisphere and is used to carry out nonverbal functions such as interpreting musical patterns, controlling emotions and intuitive thinking. For about 90% of the population, the left hemisphere is dominant.
Ventricles are interconnected cavities within the brain. They are filled with cerebrospinal fluid (CSF) that circulates continuously. The CSF is generated by a mass of capillaries called the choroid plexus. Recall that cerebrospinal fluid is found in the subarachnoid space of the meninges and the central canal of the spinal cord. Therefore, CSF is within the brain and spinal cord and around the brain and spinal cord. This fluid protects and cushions these structures. CSF also maintains stable ionic concentrations in the CNS.
Click here to view the ventricles of the brain.
The diencephalon is located between the cerebral hemispheres and above the brain stem. The diencephalon is divided into the following structures:
1) Thalamus - serves as a relay station for sensory information heading to cerebral cortex for interpretation. If sensory information does not pass through the thalamus before it reaches the cerebral cortex, it can be misinterpreted. For example, let's say you are feeling pain in your left forearm. This information goes up the spinal cord and through the thalamus and then to the cerebral cortex for interpretation. If the information did not go through the thalamus, the cerebral cortex may interpret that you are feeling coldness instead of pain in your left forearm.
2) Hypothalamus - maintains homeostasis by regulating many visceral activities such as heart rate, blood pressure, breathing rate, etc.
3) Optic tracts - tracts receiving visual information from optic nerves.
4) Infundibulum - structure that holds the pituitary gland to the brain
5) Posterior pituitary gland - hangs from the hypothalamus; secreted hormones
6) Mammillary body - rounded structures behind the infundibulum.
7) Pineal gland - small gland above the midbrain; secretes hormones.
The brain stem is a structure that connects the cerebrum to the spinal cord.
Click here to view the brain stem.
The three parts of the brain stem are as follows:
The midbrain is the portion of the brain stem just beneath the diencephalon. It controls visual reflexes and auditory reflexes. An example of a visual reflex is when you see something in your peripheral vision and you automatically turn your head to view it more clearly. An example of an auditory reflex is when you automatically turn your head to hear something more clearly.
The pons is a rounded bulge on the underside of the brain stem situated between the midbrain and the medulla oblongata. The pons contains nerve tracts to connect the cerebrum to the cerebellum. It also contains tracts that connect the medulla oblongata to the cerebrum. The pons also regulated breathing.
The medulla oblongata is the most inferior portion of the brain stem directly connected to the spinal cord. It controls many vital visceral activities such as heart rate, blood pressure, and breathing. It also controls reflexes associated with coughing, sneezing, and vomiting.
The reticular formation is a group of neuron cell bodies and fibers that are spead throughout the brain stem, diencephalon, cerebrum, and cerebellum. The reticular system acts to "wake" the cerebral cortex up so it can interpret sensory information. When you are asleep your reticular formation is "off" so you do not interpret sensations (touch, hearing, smells, etc) while you are sleeping.
The cerebellum is under the occipital lobes of the cerebrum and posterior to the pons and medulla oblongata. It coordinates complex skeletal muscle contractions needed for body movements. For example, when you walk, many muscles have to contract and relax at appropriate times. You cerebellum coordinates these activities. The cerebellum also coordinates fine movements such as threading a needle, playing a piano, etc.
Click here to view the structure of the cerebellum.
The peripheral nervous system consists of nerves that branch off the central nervous system. These nerves are called peripheral nerves and are of 2 types - cranial nerves and spinal nerves.
The peripheral nervous system can be subdivided into a somatic nervous system and an autonomic nervous system. The somatic nervous system consists of nerves that connect the CNS to the skin and skeletal muscle. The autonomic nervous system consists of nerves that connect the CNS to viscera such as the heart, stomach, intestines, glands, blood vessels, bladder, etc.
Cranial nerves are peripheral nerves that originate from the brain. Roman numeral and names designate the different cranial nerves. The 12 pairs of cranial nerves are as follows:
I. Olfactory nerves - carry smell information to the brain for interpretation
II. Optic nerves - carry visual information to the brain for interpretation
III. Oculomotor nerves - innervate muscles that move the eyeball, eyelid and iris
IV. Trochlear nerves - innervate muscles that move the eyeball
V. Trigeminal nerves - carry sensory information from the surface of the eye, skin of the scalp, skin of face, lining of gums, and palate to the brain for interpretation. Also innervates muscles needed for chewing.
VI. Abducens nerves - innervate muscles that move the eyeball.
VII. Facial nerves - innervate muscles of facial expression, salivary glands and tear glands; also carries sensory information from the tongue
VIII. Vestibulocochlear nerves - carry hearing and equilibrium information from the inner ear to the brain for interpretation
IX. Glossopharyngeal nerves - carry sensory information from the throat and tongue to brain for interpretation; also innervates muscles of the throat.
X. Vagus nerves - carry sensory information from thoracic and abdominal organs to the brain for interpretation; also innervates muscles in the throat, stomach, intestines and the heart.
XI. Accessory nerves - innervates muscles of the throat, neck, back, and voice box.
XII. Hypoglossal nerves - innervates muscles of the tongue.
Click here to view the cranial nerves.
Spinal nerves are peripheral nerves that originate from the spinal cord. There are 31 pairs of spinal nerves. There are 8 pairs of cervical nerves (numbered C1 through C8), 12 pairs of thoracic nerves (numbered T1 through T12), 5 pairs of lumbar nerves (numbered L1through L5), 5 pairs of sacral nerves (numbered S1 through S5), and one pair of coccygeal nerves (Co).
Each spinal nerve is formed by two roots - a ventral root and a dorsal root. These two roots emerge from the spinal cord and fuse together to make a spinal nerve. The ventral root carries axons of motor neurons only. The ventral root carries axons of sensory neurons only. The dorsal root also contains a dorsal root ganglion which contains the cell bodies of sensory neurons.
Except in the thoracic region, the main portions of spinal nerves fuse together to form nerve plexuses.
Click here to view the spinal nerves.
Click here to see the structure of a spinal nerve.
The cervical plexus is formed by the first four cervical spinal nerves. Nerves coming off this plexus supply the skin and the muscles of the neck. A portion of the phrenic nerve also comes off this plexus. The phrenic nerve innervates the diaphragm (muscle needed for breathing).
The brachial plexus is formed by the last four cervical nerves and the first thoracic nerve. Nerves coming off this plexus supply the arms.
The lumbosacral plexus is formed by the last thoracic nerves, the lumbar nerves, the sacral nerves and the coccygeal nerves. Nerves coming off this plexus supply the lower abdominal wall, external genitalia, buttocks, thighs, legs and feet.
The autonomic nervous system controls organs not under conscious control. For example, it controls thoracic organs, abdominopelvic organs, blood vessels and glands throughout the body.
In general, receptors located in visceral organs, blood vessels and glands relay information along sensory neurons to the brain and spinal cord. In response to this information, motor nerve fibers can be activated. In the autonomic nervous system, motor nerve from the brain and spinal cord innervate other motor neurons in ganglia which are collections of neuron cell bodies outside the CNS. The motor neurons of ganglia then innervate smooth muscle of various organs and blood vessels, glands or cardiac muscles.
The two divisions of the autonomic nervous system are the sympathetic division and the parasympathetic division. The sympathetic division prepares organs for "fight or flight" situations; in other words scary or emergency situations. So, the sympathetic division would prepare the heart for a scary situation by increasing the rate at which it beats. The parasympathetic division prepares the body for resting and digesting. So, the parasympathetic division would prepare the heart for resting by keeping its heart rate relatively slos. Most of the time your visceral organs are under parasympathetic control.
Nerve fibers of the autonomic nervous system are motor fibers. There are two types of motor neurons in the autonomic nervous system. One neuron is termed a preganglionic neuron and its cell body is always located in the brain or spinal cord. The other neuron is called a postganglionic neuron and its cell body is always in a ganglion outside the brain and spinal cord. The axon of the preganglionic neuron is called a preganglionic fiber; it will synapse with the postganglionic neuron. The axon of the postganglionic neuron is called a postganglionic fiber; it will synapse with glands, smooth muscle in visceral organs or blood vessels or fibers in the heart.
The preganglionic neurons of the sympathetic division are located in the thoracic and lumbar regions of the spinal cord.
The sympathetic division has short preganglionic fibers; these fibers release acetylcholine onto the postganglionic neuron. The postganglionic fibers of this division are long and they release norepinephrine onto organs and glands. Norepinephrine increases heart rate, increases blood pressue, increases breathing rate, slows down the activity of digestive glands, slows down the muscles of the stomach and intestines, and dilates pupils.
The preganglionic neurons of the parasympathetic division are located in the brain and the sacral regions of the spinal cord.
The parasympathetic division has long preganglionic fibers; these fibers release acetylcholine onto the postganglionic neuron. The postganglionic fibers of this division are short; they release acetylcholine onto organs and glands. Acetylcholine slows heart rate, slows breathing rate, contricts pupils, decreases blood pressure, activates digestive glands, activates muscles of stomach and intestines.
Click here to see the organizations of the parasympathetic and sympathetic nervous systems.
Click here to see a summary of the actions of the sympathetic and parasympathetic nervous systems.
Acetylcholine and norepinephrine are the two neurotransmitters of the autonomic nervous system. Fibers that release acetylcholine are called cholinergic fibers. Fibers that release norepinephrine are called adrenergic fibers. The cholinergic fibers of the autonomic nervous system are the preganglionic and postganglionic fibers of the parasympathetic division and the preganglionic fibers of the sympathetic division. The only cholinergic fibers of the autonomic nervous system are the postganglionic fibers of the sympathetic division.
The autonomic nervous system is constantly being influences by the brain stem, the hypothalamus, and the cerebral cortex.