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The Nervous System: Organization

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  • Nerve Nets

    Cnidarians are radially symmetrical and have the simplest nervous system. Nerve network conducts signals from sensory cells to muscle cells and there is no centralization of the nervous system.

    Cephalization and Bilateral Symmetry

    The anterior end of bilaterally symmetrical animals contains most of the sense organs because this end of the animal moves through the environment first. As evolution proceeded, the anterior end of the central nervous system became larger to accommodate these sense organs. The larger, anterior end of the central nervous system is called the brain. The development of the brain is called cephalization; highly cephalized animals have a large brain.


    Bilateral symmetry has led to paired structures (nerves, muscles, sense organs, brain). Some flatworms have a nerve net like Cnidarians but others show more organization including a brain and nerve cords. Planarians

    • The nervous system of planarians resembles a ladder. It has two nerve cords with ganglia ("a brain") at the anterior end.
    • Sensory receptors are located in the auricles.
    • The eyespots contain photoreceptors.
    • Transverse nerves that connect the two cords keep movements of the two sides coordinated.


    Mollusks show a great diversity of nervous systems. Some mollusks such as bivalves have no cephalization. Slow-moving animals have some cephalization, enabling sensory reception as the animal moves through the environment. The active predatory lifestyle of cephalopods require complex sense organs; they are highly cephalized.

    Annelids, Arthropods

    Annelids and arthropods have repeating segments and an anterior brain. Each segment contains a ganglion; the nerve cord extends through all of the segments. The ganglion in each segment controls the muscles of that segment. The brain exerts overall control to coordinate the animal.


    Sea stars have a central nerve ring and a nerve that extends from the ring into each arm. Each arm also contains a nerve net.


    Vertebrates have complex sense organs and exhibit complex behaviors. These require a complex nervous system. The vertebrate nervous system is extremely cephalized.

    Divisions of the Vertebrate Nervous System

    The central nervous system (CNS) is the brain and spinal cord. The peripheral nervous system (PNS) is composed of the nerves and ganglia. Ganglia are clusters of nerve cell bodies outside the CNS.

    Peripheral Nervous System


    Nerves are bundles of neurons; either long dendrites and/or long axons.

    There are no cell bodies in nerves. The cell bodies are in the ganglia (PNS) or nuclei (in gray matter of the CNS).

    Most nerves contain both kinds of neurons (sensory and motor). The sensory neurons conduct information to the CNS, the motor neurons conduct away from the CNS.

    All of the neurons in some nerves conduct in the same direction.   These nerves contain either sensory or motor neurons.

    Cranial Nerves and Spinal Nerves

    Humans have 12 pairs of cranial nerves and 31 pairs of spinal nerves.

    Cranial nerves are sensory, motor, or mixed, and all but the vagus are involved with the head and neck region; the vagus nerve manages the internal organs.

    Spinal nerves are all mixed nerves. Their regular arrangement reflects the segmentation of the human body.

    Spinal nerves are connected to the spinal cord by two branches called roots.

    • The dorsal root contains sensory neurons. The dorsal root ganglion contains the cell bodies of sensory neurons. Sensory neurons therefore have long dendrites.
    • The ventral root contains motor neurons. Motor neurons have short dendrites and long axons.

    Somatic Nervous System

    The somatic nervous system provides conscious, voluntary control.

    It includes all of the nerves that serve the skeletal muscles and the exterior sense organs. 

    It also includes reflexes.

    Reflex arcs

    Reflexes are simple, stereotyped and repeatable motor actions (example: movements) brought about by a specific sensory stimulus. The reflex is involuntary but may involve the use of voluntary (skeletal) muscle and nerves.

    Reflexes are quick and produce behaviors that are typically beneficial. For example, when you fall, reflex arcs immediately act to extend your arm so that your arm prevents your head and body from hitting the ground.

    Some reflexes involve the brain, others do not.

    A whole series of responses may occur since some sensory neurons stimulate several interneurons which, in turn send impulses to other parts of the CNS. If you were to fall forward, interneurons would use information from the ears to determine the direction of the fall and extend the arms in a forward direction. If you were to fall toward the left side, interneurons would select neurons that activate muscles to extend your arm to the left side.

    Example: The stretch reflex

    The stretch reflex is involved in helping the body maintain its position without having to consciously think about it.

    Stretch-sensitive receptors in the muscles contain stretch-gated channels. When the muscle is stretched, the channels open, causing the neuron to depolarize. Action potentials are conducted to the spinal cord. The axon terminals synapse with motor neurons leading right back to the muscles. This causes the muscle to contract to its original position.

    Autonomic Nervous System

    This part of the nervous system sends signals to the heart, smooth muscle, glands, and all internal organs.

    It is generally without conscious control.

    The autonomic nervous system uses two or more motor neurons:

    • The cell body of one of the motor neurons is in the CNS. The cell body of the other one is in a ganglion.
    Sympathetic Division

    The sympathetic nervous system stimulates the body. For example, it helps prepare the body to deal with emergency situations. This is often called the "fight or flight" response.

    Stimulation from sympathetic nerves dilates the pupils, accelerates the heartbeat, increases the breathing rate, and inhibits the digestive tract.

    The neurotransmitter is norepinephrine.

    Sympathetic nerves arise from the middle (thoracic-lumbar) portion of the spinal cord.

    Parasympathetic Division

    When there is little stress, the parasympathetic system tends to slow down the overall activity of the body.

    It causes the pupils to contract, it promotes digestion, and it slows the rate of heartbeat.

    The neurotransmitter is acetylcholine.

    The actual rate of stimulus to each organ is determined by the sum of opposing signals from the sympathetic and parasympathetic systems.

    Parasympathetic nerves arise from the brain and sacral (near the legs) portion of the cord.

    Enteric Division

    The enteric division contains neurons that control the digestive tract, pancreas, and gallbladder.

    Activity of the enteric division is usually regulated by the sympathetic and parasympathetic divisions.

    Evolution of Vertebrate Central Nervous Systems

    The central nervous system evolved in vertebrates by adding on to what was there. The oldest parts of the human nervous system deal with reflexes. Newer additions and modifications are associated with memory, learning, and thinking.

    Central Nervous System

    The central nervous system is the brain and spinal cord.

    It is wrapped in 3 layers of membranes called meninges. Meningitis is an infection of these coverings.

    The brain contains fluid-filled ventricles that are continuous with the central canal of the cord. Fluid within the ventricles and central canal originates from the blood. It slowly circulates, carrying nutrients and wastes from cells. The fluid eventually returns to the circulatory system and is replaced by fresh fluid.

    Divisions of the Brain

    Generally, many body functions involve cells in several areas of the brain. However, certain areas of the brain tend to be more important in some functions while other areas dominate the control of other functions.

    Some major parts of the brain are listed below.

    • Hindbrain: medulla oblongata, cerebellum, pons
    • Midbrain
    • Forebrain: thalamus, hypothalamus, cerebrum


    Medulla oblongata

    The medulla controls vital functions such as breathing, heart rate, and blood pressure.

    It also contains reflexes such as vomiting, coughing, sneezing, hiccupping, swallowing, and digestion.

    Information that passes between the spinal cord and the rest of the brain must pass through the medulla.  In the medulla, sensory and motor axons on the right side cross to the left side and axons on the left side cross to the right side.  As a result, stimuli passing through from the left side of the body are sent to the right side of the brain and signals passing through from the right side of the brain stimulate the left side of the body.


    The cerebellum coordinates and refines complex muscle movements.   Movement information that is initiated in higher brain centers (the cerebral cortex) is compared to the actual position of the limbs.  The cerebellum then adjusts and refines the movement.

    It is large in birds because flight requires considerable coordination.


    The pons is involved in some of the same activities as the medulla.  For example, it assists the medulla in controlling breathing.

    The pons functions as a connection between higher brain regions, the cerebellum, and the spinal cord.


    The midbrain receives some sensory information and sends it to the appropriate part of the forebrain.

    The midbrain originally functioned for reflexes associated with visual input. It is the most prominent part of the brain in fishes and amphibians and has major control of the body.

    The midbrain of reptiles, birds, and mammals controls visual reflexes such as the pupil response to light intensity but the forebrain of these vertebrates processes the visual information (see diagram below). 

    The midbrain also controls some auditory reflexes and helps control posture.


    The medulla oblongata, pons, and midbrain look like the spinal cord and appear to connect the rest of the brain to the spinal cord. They are collectively referred to as the brainstem.



    Like the midbrain of mammals, the thalamus serves as a relay area to the cerebrum from other parts of the spinal cord and brain. For example, it receives sensory input (except smell) and sends to appropriate areas of the cerebral cortex.

    The Thalamus contains part of the reticular formation (see below).

    Reticular Formation

    The reticular formation is a net of nerve cells extending from the thalamus through the brain stem (midbrain, pons and medulla oblongata) to the spinal cord. 

    It acts as a filter to incoming stimuli and discriminates important from unimportant. Hundreds of millions of sensory receptors flood the brain; the brain does not have the capacity to deal with even a small fraction of this information, so much of it must be ignored.


    • You may be unaware of conversation in a crowded room but the system alerts you when you hear your name.
    • You can sleep in the presence of some kinds of sounds but others will wake you.

    The reticular activating system (RAS) is the part of the reticular formation that controls wakefulness.

    Sleep centers are located in the reticular formation. Neurons in one sleep center secrete serotonin, a chemical that inhibits the RAS and thus causes drowsiness and sleep.

    Another sleep center secretes factors that counteract serotonin and bring about wakefulness.

    Damage to these centers can lead to unconsciousness or coma.


    The hypothalamus regulates the endocrine system by controlling the secretions of the pituitary gland or by producing some of the hormones that are secreted by the pituitary. These hormones affect the body or affect other glands in the body. Their overall affect is to maintain homeostasis.

    The hypothalamus also contains neurons associated with the limbic system (below).

    Limbic System

    The limbic system contains neural pathways that connect portions of the cortex, thalamus, hypothalamus, and basal nuclei (several areas deep within the cerebrum).

    It causes pleasant or unpleasant feelings about experiences (rage, pain, pleasure, sorrow). This helps guide the individual into appropriate behavior that is more likely to be beneficial.


    The cerebrum became greatly enlarged as evolution progressed from the earliest vertebrates to mammals.   In reptiles, birds, and mammals, it receives sensory information and coordinates motor responses.  

    Motor responses to the skeletal muscles originate in the cerebrum but are refined and coordinated by the cerebellum.

    In humans, the cerebrum is the largest part of the brain. Characteristics such as thinking, intelligence, and emotion are controlled here.

    Olfactory Bulbs-  The anterior parts of the cerebral hemispheres are called the olfactory bulbs. It receives input from the olfactory nerves (smell). The olfactory bulbs of primitive vertebrates comprise a large proportion of the cerebrum.

    Cerebral Cortex-  Over evolutionary time, gray matter developed over the cerebrum. This is the cerebral cortex and it is an information-processing center. It increased in size more rapidly than the skull so that it has become folded (convoluted) in order to fit in the skull.

    The human cerebral cortex is thin (1.5-4 mm thick) and is highly folded to increase its surface area.

    Intelligence, emotion, creativity, learning, and memory are localized in the cerebral cortex.

    Lobes of the cerebral cortex

    The cerebral cortex is divided into four lobes, each receives information from particular senses and processes the information into higher levels of consciousness.   

    Lobe Function

    motor functions; permits conscious control of skeletal muscles; contains the primary motor cortex

    conscious thought

    Parietal sensory areas from the skin; contains the primary sensory cortex
    Occipital The primary visual cortex is located within the occipital lobe.
    Temporal hearing and smell

    Primary Sensory and Primary Motor Cortex- The primary sensory cortex is a narrow band of cortex tissue that extends from one side of the cortex near the ear over the top of the brain to the other side. Information from sensory receptors in the skin arrive at this area. The motor cortex is a band of cortex tissue directly anterior (in front) of the primary sensory cortex. Signals that control the skeletal muscles originate in this area.

    Corpus Callosum

    The corpus callosum contains neurons that cross from one side of the brain to the other, allowing each half to communicate with each other. 

    Summary of Brain Structure

    Brain Structure


    Medulla oblongata

    Vital functions such as breathing, heart rate, and blood pressure

    Reflexes such as vomiting, coughing, sneezing, hiccupping, swallowing, and digestion

    Neurons cross

    Pons Breathing, connects spinal cord, cerebellum and higher brain centers
    Cerebellum Motor coordination

    Receives visual, auditory, and tactile information

    In mammals, this information is sent to the thalamus and higher brain centers.  In lower vertebrates, the information is further processed in the midbrain.


    Relays sensory information to the cerebral cortex.

    Contains part of the reticular formation (controls arousal).


    Maintains homeostasis, regulates the endocrine system

    Contains part of the Limbic system (controls emotion)

    Cerebrum Processes sensory information and produces signals that move the skeletal muscles.
    Cerebral Cortex

    This is the outer layer of the cerebrum.

    Thinking, intelligence, and cognitive functions are located here.

    Processing of sensory information and motor responses


    The limbic system is involved in memory formation.

    The hippocampus, a structure that is deep in the cerebrum and a part of limbic system, is necessary to form new memories. People with a damaged hippocampus cannot remember things since the time the damage occurred but can remember from before that time.

    Short-term memory is probably stored as electrical differences because they can be removed by the application of an electrical shock.

    Long-term memory is probably stored as new or different synapses. Research on snails shows that learning is associated with an increased number of synapses. Forgetting is associated with a decreased number.

    Disuse can cause a synapse to wither and sever the connection between two neurons. Intensively stimulated synapses form stronger connections, grow, or sprout buds to form more connections.

    Memory appears to be stored in sensory areas of the cerebrum.

    The Spinal Cord

    The vertebrae surround and protect the spinal cord.

    Cerebrospinal fluid within the central canal functions to cushion the spinal cord.

    Many sensory - motor reflex connections are in the spinal cord. Interneurons often lie between sensory and motor neurons.

    White matter

    White matter contains tracts that connect the brain and the spinal cord.

    The white color is due to the myelin sheaths.

    Gray matter

    Gray matter looks gray because it is unmyelinated.

    It contains the short interneurons that connect many sensory and motor neurons. Sensory neurons enter the gray matter and the axons of motor neurons leave the gray matter.

    The cell bodies of these motor neurons are located in the gray matter.