Nervous Systems

Learning Objectives

  • Describe different types of nervous systems in different animal lineages
  • Identify and describe basic structures and functions of the central and peripheral nervous systems
  • Describe the roles of and differentiate between the divisions of the vertebrate nervous system (afferent, efferent, somatic, autonomic, sympathetic, parasympathetic)
  • Define learning and memory, and explain the roles of the hippocampus, the cerebral cortex, neuroplasticity, and synaptic plasticity in learning and memory

Neurons, Nerves, and the Nervous System

The information below was adapted from OpenStax Biology 35.0 and Khan Academy Overview of neuron structure and function. All Khan Academy content is available for free at www.khanacademy.org

When you’re reading this website, your nervous system is performing several functions simultaneously. The visual system is processing what is seen on the page; the motor system controls the click of the mouse; and (if you’re lucky) the prefrontal cortex maintains your attention. Even fundamental functions, like breathing and regulation of body temperature, are controlled by the nervous system. A nervous system is an organism’s control center: it processes sensory information from outside (and inside) the body and controls all behaviors: from eating to sleeping to finding a mate.

The nervous system is made up of neurons, specialized cells that can receive and transmit chemical or electrical signals, and glia, cells that provide support functions for the neurons by playing an information processing role that is complementary to neurons. Nerves are bundles of nervous tissue, often containing hundreds to thousands of axons wrapped in connective tissue. Nerves in the peripheral nervous system (PNS) carry information to and from neurons in the the central nervous system (CNS), where information is integrated and processed.

There are three different classes of neurons that make up the nervous system:

  • Afferent neurons (also called sensory neurons) get information about what’s going on inside and outside of the body and bring that information into the CNS so it can be processed. For instance, if you picked up a hot coal, sensory neurons with endings in your fingertips would convey the information to your CNS that it was really hot
  • Efferent neurons (also called motor neurons) get information from other neurons and convey commands to your muscles, organs and glands. For instance, if you picked up a hot coal, it motor neurons innervating the muscles in your fingers would cause your hand to let go.
  • Interneuons, which are found only in the CNS, connect one neuron to another. They receive information from other neurons (either sensory neurons or interneurons) and transmit information to other neurons (either motor neurons or interneurons). For instance, if you picked up a hot coal, the signal from the sensory neurons in your fingertips would travel to interneurons in your spinal cord. Some of these interneurons would signal to the motor neurons controlling your finger muscles (causing you to let go), while others would transmit the signal up the spinal cord to neurons in the brain, where it would be perceived as pain. Interneurons are the most numerous class of neurons and are involved in processing information, both in simple reflex circuits (like those triggered by hot objects) and in more complex circuits in the brain. It would be combinations of interneurons in your brain that would allow you to draw the conclusion that things that looked like hot coals weren’t good to pick up, and, hopefully, retain that information for future reference.

The material in the nervous system can also be classified based on whether it contains white matter (myelinated axons) and gray matter (uunmyelinated axons and cell bodies).

Diversity of Nervous Systems

The information below was adapted from OpenStax Biology 35.1

Nervous systems throughout the animal kingdom vary in structure and complexity, as illustrated by the variety of animals shown below:

Nervous systems vary in structure and complexity. In (a) cnidarians, nerve cells form a decentralized nerve net. In (b) echinoderms, nerve cells are bundled into fibers called nerves. In animals exhibiting bilateral symmetry such as (c) planarians, neurons cluster into an anterior brain that processes information. In addition to a brain, (d) arthropods have clusters of nerve cell bodies, called peripheral ganglia, located along the ventral nerve cord. Mollusks such as squid and (e) octopi, which must hunt to survive, have complex brains containing millions of neurons. In (f) vertebrates, the brain and spinal cord comprise the central nervous system, while neurons extending into the rest of the body comprise the peripheral nervous system. (credit: OpenStax Biology, e: modification of work by Michael Vecchione, Clyde F.E. Roper, and Michael J. Sweeney, NOAA; credit f: modification of work by NIH)

  • All animals have a true nervous system except sea sponges.
  • Cnidarians, such as jellyfish, lack a true brain but have a system of separate but connected neurons called a nerve net.
  • Echinoderms, such as sea stars, have neurons that are bundled into fibers called nerves.
  • Flatworms of the phylum Platyhelminthes have both a CNS made up of a small brain and two nerve cords, and PNS containing a system of nerves that extend throughout the body.
  • The insect nervous system is more complex but also fairly decentralized, with a brain, ventral nerve cord, and ganglia (clusters of connected neurons). These ganglia can control movements and behaviors without input from the brain.
  • Cephalopods, such as octopi, may have the most complicated of invertebrate nervous systems, with neurons that are organized in specialized lobes and eyes that are structurally similar to vertebrate species.
  • Compared to invertebrates, vertebrate nervous systems are more complex, centralized, and specialized. While there is great diversity among different vertebrate nervous systems, they all share a basic structure: a CNS that contains a brain and spinal cord and a PNS made up of peripheral sensory and motor nerves.

One interesting difference between the nervous systems of invertebrates and vertebrates is that the nerve cords of many invertebrates are located ventrally (along the belly) whereas the vertebrate spinal cords are located dorsally (along the back). There is debate among evolutionary biologists as to whether these different nervous system plans evolved separately or whether the original invertebrate body plan arrangement somehow “flipped” during the evolution of vertebrates.

The Central Nervous System in Vertebrates

The information below was adapted from OpenStax Biology 35.3 and Khan Academy Overview of neuron structure and function. All Khan Academy content is available for free at www.khanacademy.org

For the rest of this class session, we’ll focus on the vertebrate nervous system. In vertebrates, the nervous system can be broadly divided into two sections:

  • a central nervous system (CNS) consisting of:
    • the brain, a structure that processes information, composed of inter-connected neurons and glial cells
    • the spinal cord, a structure that transmits information, consisting of a thick bundle of nerve tissue that carries information about the body to the brain and from the brain to the body
  • a peripheral nervous system (PNS) that collects information and sends commands, containing nerves that extend to and from the spinal cord and are divided into:
    • afferent nerves that collect sensory information from the body and transmit it to the CNS; afferent nerves are also sometimes called sensory nerves
    • efferent nerves that carry commands from the CNS to the body; efferent nerves are also sometimes called motor nerves

The vertebrate nervous system consists of a CNS and PNS. Image credit, https://www.khanacademy.org/science/biology/ap-biology/human-biology/neuron-nervous-system/a/overview-of-neuron-structure-and-function, modified from “Nervous system diagram,” by Medium69 (CC BY-SA 4.0).

 

Anatomy of the Brain

The brain is the part of the central nervous system that is contained in the cranial cavity of the skull. It floats in a fluid called cerebrospinal fluid (CSF), which is filtered from arterial blood. CSF acts as a cushion and shock absorber, and makes the brain neutrally boyant. Ependymal cells (remember those?) help circulate the CSF to distribute and exchange chemical substances throughout the brain and into the spinal cord. The ependymal cells line the fluid-filled cavities in the center of the brain, called ventricals, which also contains CSF.

Modified from: John A Beal, PhDDep’t. of Cellular Biology & Anatomy, Louisiana State University Health Sciences Center Shreveport – http://www.healcentral.org/healapp/showMetadata?metadataId=40566 (Internet Archive of file description page), CC BY 2.5, https://commons.wikimedia.org/w/index.php?curid=879154

At the general anatomy level (shown above), the brain is organized with white matter (myelinated axons) toward the inside of the brain, and gray matter (unmyelinated axons and cell bodies) on the outside of the brain. Gray matter represents the information-processing centers of the brain, and white matter represents the networking between these processing centers. These processing centers include (but are not limited to!) the cerebrum and cerebral cortex, the diencephalon, the cerebellum, and the brain stem, further described below.

The vertebrate brain includes the cerebrum, cerebral cortex, diencephalon, cerebellum, and the brain stem. Image credit: OpenStax Biology.

  • The cerebrum and cerebral cortex make up the majority of the human brain.  The outermost part of the cerebrum is a thick piece of nervous system tissue called the cerebral cortex, which is folded into hills called gyri (singular: gyrus) and valleys called sulci (singular: sulcus). The cortex is made up of two hemispheres (right and left) and four lobes (frontal, parietal, temporal, occipital). The two hemispheres are joined by a thick fiber bundle called the corpus callosum (Latin: “tough body”) which connects the two hemispheres and allows information to be passed from one side to the other. Although there are some brain functions that are localized more to one hemisphere than the other, the functions of the two hemispheres are largely redundant.
  • The diencephalon controls homeostasis and acts as a relay station, transmitting sensory information from from sensory neurons to the cerebrum.  The structures located in the diencephalon that regulate these processes include the thalamus and the hypothalamus.  The thalamus (Greek for “inner chamber”) acts as a gateway to and from the cortex. It receives sensory and motor inputs from the body and also receives feedback from the cortex, and helps regulate consciousness, arousal, and sleep states. Below the thalamus is the hypothalamus, which controls the endocrine system by sending signals to the pituitary gland, a pea-sized endocrine gland that releases several different hormones that affect other glands as well as other cells. This relationship means that the hypothalamus regulates important behaviors that are controlled by these hormones. The hypothalamus is the body’s “thermostat” (think negative feedback loop); it makes sure key functions like food and water intake, energy expenditure, and body temperature are kept at appropriate levels. Neurons within the hypothalamus also regulate circadian rhythms, sometimes called sleep cycles.
  • The cerebellum (Latin for “little brain”) sits at the base of the brain on top of the brainstem. The cerebellum controls balance and aids in coordinating complex movements and learning new motor tasks.
  • The brainstem connects the rest of the brain with the spinal cord. Motor and sensory neurons extend through the brainstem allowing for the relay of signals between the brain and spinal cord. The brainstem controls several important functions of the body including alertness, arousal, breathing, blood pressure, digestion, heart rate, swallowing, walking, and sensory and motor information integration.

Each cortical hemisphere of the cerebrum contains regions called lobes that are involved in different functions, and include the frontal, parietal, temporal, and occipital lobes:

Sagittal, or side view of the human brain shows the different lobes of the cerebral cortex. The frontal lobe is at the front center of the brain. The parietal lobe is at the top back part of the brain. The occipital lobe is at the back of the brain, and the temporal lobe is at the bottom center of the brain. The motor cortex is the back of the frontal lobe, and the olfactory bulb is the bottom part. The somatosensory cortex is the front part of the parietal lobe. The brainstem is beneath the temporal lobe, and the cerebellum is beneath the occipital lobe. Image credit: OpenStax Biology

The human cerebral cortex can be further subdivided into four lobes, each of which specializes in different functions:

  • The frontal lobe is located at the front of the brain, over the eyes. This lobe contains the olfactory bulb, which processes smells. The frontal lobe also contains the motor cortex, which is important for planning and implementing movement. Neurons in the frontal lobe also control cognitive functions like maintaining attention, speech, and decision-making, and parts of this area are involved in personality, socialization, and assessing risk.
  • The parietal lobe is located at the top of the brain. Neurons in the parietal lobe are involved in speech and also reading. Two of the parietal lobe’s main functions are processing somatosensation (touch sensations like pressure, pain, heat, cold) and processing proprioception (the sense of how parts of the body are oriented in space).
  • The occipital lobe is located at the back of the brain. It is primarily involved in vision: seeing, recognizing, and identifying the visual world. Sensory neurons transmit information from the eyes at the front of the skull to the occipital lobe at the back of the brain via the occipital nerve.
  • The temporal lobe is located at the base of the brain by your ears and is primarily involved in processing and interpreting sounds. It also contains the hippocampus (Greek for “seahorse”); a structure that processes memory formation and is critical for learning.

Compared to other vertebrates, mammals have exceptionally large brains for their body size. An entire alligator’s brain, for example, would fill about one and a half teaspoons. This increase in brain to body size ratio is especially pronounced in apes, whales, and dolphins. While this increase in overall brain size doubtlessly played a role in the evolution of complex behaviors unique to mammals, it does not tell the whole story. Scientists have found a relationship between the relatively high surface area of the cortex and the intelligence and complex social behaviors exhibited by some mammals. This increased surface area is due, in part, to increased folding of the cortical sheet (more sulci and gyri). For example, a rat cortex is very smooth with very few sulci and gyri. Cat and sheep cortices have more sulci and gyri. Chimps, humans, and dolphins have even more.

Mammals have larger brain-to-body ratios than other vertebrates. Within mammals, increased cortical folding and surface area is correlated with complex behavior. Image credit: OpenStax Biology.

This video provides a summary of the functions of different parts of the vertebrate brain (watch the first four minutes):

Learning and Memory

The information below was adapted from OpenStax Biology 35.2

One of the key functions that the brain performs is the process of learning and memory. Learning is the ability to acquire new knowledge, and memory is the ability to recall it later. Learning and memory involve both specific brain structures as well as certain neuronal processes. The current hypothesis states that specific neurons in the cerebral cortex are responsible for physically storing memories, and that learning and memory are mediated by both chemical and structural changes in the synapses of these neurons.

Short-term memories are thought to be stored in the prefrontal cortex (part of the frontal lobe). The hippocampus in the temporal lobe is essential for consolidating these short-term memories into long-term memories, but the memories are not actually stored in the hippocampus. The precise location of memory storage is unknown, but it is though that different components of memories may be stored in different locations within the cerebral cortex, and that retrieval of long-term memories may involved the prefrontal cortex.

Storage and access are only half of the story for learning and memory; the other half is chemical and structural changes in synapses, or neural plasticity: formation of new and loss of existing neural connections. By the end of embryogenesis in humans, half of all embryonic neurons undergo programmed cell death, and half of the initial synapses are lost. This basic neural architecture is then continually remodeled during the individual’s lifetime. How does neural plasticity relate to learning and memory? Chemical and structural changes in synapses (synaptic plasticity, synaptic pruningsynaptogenesis) mediate access to and strength of these memories as follows:

  • Neurogenesis, or the growth of new neurons. At one time, scientists believed that people were born with all the neurons they would ever have, but research performed during the last few decades indicates that neurogenesis, the birth of new neurons, continues into adulthood. Neurogenesis was first discovered in songbirds that produce new neurons while learning songs. For mammals, new neurons also play an important role in learning: about 1000 new neurons develop in the hippocampus (a brain structure involved in learning and memory) each day. While most of the new neurons will die, researchers found that an increase in the number of surviving new neurons in the hippocampus correlated with how well rats learned a new task. Interestingly, both exercise and some antidepressant medications also promote neurogenesis in the hippocampus. Stress has the opposite effect.
  • Synaptogenesis, or the growth of new synapses between two existing neurons, and synaptic pruning, or the destruction of existing synapses between two neurons.
  • Synaptic plasticity, or the strengthening or weakening of existing synaptic connections. Two processes in particular, long-term potentiation (LTP) and long-term depression (LTD) are important forms of synaptic plasticity that occur in synapses in the hippocampus, a brain region that is involved in storing memories.
    • Long-term potentiation (LTP) is the long-term strengthening of a synaptic connection. LTP is based on the idea that “cells that fire together wire together.” There are various mechanisms underlying synaptic strengthening seen with LTP, including an increase in the amount of neurotransmitter released by the presynaptic neuron, and an increased response to the same amount of neurotransmitter by the postsynaptic neuron.  LTP can result in sensitization, where there is an increased response to the same external stimulus.
    • Long-term depression (LTD) is essentially the reverse of LTP: it is a long-term weakening of a synaptic connection. While it may seem counterintuitive, LTD may be just as important for learning and memory as LTP: the weakening unused synapses allows for unimportant connections to be lost and makes the synapses that have undergone LTP that much stronger by comparison. LTD can result in habituation, where there is a decreased response to the same external stimulus.

Long term potentiation can occur after repeated stimulation at a synaptic terminal (panel 1) via several mechanisms, including production of more neurotransmitter receptors on the postsynaptic neuron (panel 2) and production of more neurotransmitter molecules by the presynaptic neuron (panel 3). A stronger connection between the neurons (panel 4) will occur as a result of either of these changes. Image credit: modification of work by Tomwsulcer – Own work, CC0, https://commons.wikimedia.org/w/index.php?curid=15509518

This video provides a simplified overview of learning and memory in a commonly used model organism for studying these processes:

 

And finally, this (cropped) video provides a succinct overview of two of the common results of learning, sensitization or habituation:

via ytCropper

For your reference, the full length video can be viewed here: https://www.youtube.com/watch?v=ABUzqfD8vWg

The Spinal Cord

The information below was adapted from OpenStax Biology 35.3 and Khan Academy Overview of neuron structure and function. All Khan Academy content is available for free at www.khanacademy.org

In order to learn from and respond to information, the brain must receive that information from the environment. Detection of information occurs via the PNS, and is relayed to the brain via the spinal cord (which is part of the CNS). The spinal cord connects to the brainstem and extends down the body through the spinal column. The spinal cord is a thick bundle of nerve tissue that carries information about the body to the brain and from the brain to the body. The spinal cord is contained within the bones of the vertebrate column but is able to communicate signals to and from the body through its connections with spinal nerves (part of the peripheral nervous system).

A cross-section of the spinal cord looks like a white oval containing a gray butterfly-shape. Myelinated axons make up the white matter and neuron and glial cell bodies make up the gray matter. Gray matter is also composed of interneurons, which connect two neurons each located in different parts of the body. Relay of information to and from the spinal column is directional:

  • Axons and cell bodies in the dorsal (facing the back of the animal) spinal cord convey mostly sensory information from the body to the spinal cord then brain.
  • Axons and cell bodies in the ventral (facing the front of the animal) spinal cord primarily transmit signals controlling movement from the brain to out to the body.

 

Spinal nerves contain both sensory and motor axons. The somas of sensory neurons are located in dorsal root ganglia. The somas of motor neurons are found in the ventral portion of the gray matter of the spinal cord. Image credit: OpenStax Biology

As noted previously, a nerve is not the same thing as a neuron; nerves are actually bundles of nervous tissue and can contain hundreds to thousands of axons. Nerves contain both neuronal cells (axons and associated Schwann cells) and connective tissue (blood vessels and multiple layers of membranes bundling different groups of axons). The diagram below illustrates the general anatomy of a nerve:

Nerves contain blood vessels, neuronal tissues, and membranes composed of connective tissues bundling axons into different groups. By Ranjit530 – Drawn, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=29397234

 

The Peripheral Nervous System in Vertebrates

The information below was adapted from OpenStax Biology 35.4

The spinal column transmits information to the CNS for processing. How does the information get to the spinal column to begin with? And how do commands from the CNS get carried out? That is the job of nerves and the peripheral nervous system (PNS). If the CNS is like the power plant of the nervous system, creates the signals that control the functions of the body, then the PNS is like the wires that go to individual houses. Without those “wires,” the signals produced by the CNS could not control the body (and the CNS would not be able to receive sensory information from the body either).

The PNS is composed of the cranial and spinal nerves, and can be divided into the different divisions based on structure and function as follow:

  • The afferent (sensory) division: collects incoming sensory information; made up of cranial and spinal nerves that contain sensory neurons. Sensory neurons transmit sensory information from the skin, skeletal muscle, and sensory organs to the CNS. Without its afferent nervous system, an animal would be unable to receive or process any information about its environment (what it sees, feels, hears, and so on).
  • The efferent (motor) division: carries outgoing commands; made up of cranial and spinal nerves that contain motor neurons. Motor neurons transmit messages about desired movement from the CNS to the muscles to make them contract. Motor neurons may be under conscious or unconscious control.  The efferent division can be further divided into:
    • The somatic (conscious control) nervous system: sends motor commands from the CNS to voluntarily-controlled muscles; made up of cranial and spinal nerves that contain motor neurons under conscious control. Without the somatic nervous system, an animal would be unable to respond to its environment via controlled motor movements. Motor neurons of the somatic nervous system synapse with muscles under voluntary control, such as limb muscles.
    • The autonomic (unconscious control) nervous system, which controls bodily functions without conscious control; made of cranial and spinal nerves that contain motor neurons under unconscious control, such as the heart, smooth muscle, and exocrine and endocrine glands. Can be further divided into two systems with opposing effects:
      • The sympathetic nervous system, which controls the “fight or flight” reactions associated with the short-term stress response. Examples of functions controlled by the sympathetic nervous system include an accelerated heart rate and inhibited digestion. These functions help prepare an organism’s body for the physical strain required to escape a potentially dangerous situation or to fend off a predator. The sympathetic nervous system alters the behavior of may organs directly via synapsed neurons, including the adrenal glands which then release norepinephrine and epinephrine into the blood stream. These hormones then cause further changes, including dilation of the trachea and bronchi (making it easier for the animal to breathe), increasing heart rate, and moving blood from the skin to the heart, muscles, and brain (so the animal can think and run).
      • The parasympathetic nervous system: controls the “rest and digest” activities involved in conserving and restoring energy. The parasympathetic nervous system resets organ function after the sympathetic nervous system is activated (the common adrenaline dump you feel after a “fight-or-flight” event). Effects of the parasympathetic nervous system on target organs include slowing of heart rate, lowered blood pressure, and stimulation of digestion.

The opposing effects of the sympathetic vs parasympathetic nervous systems are illustrated below:

The sympathetic and parasympathetic nervous systems often have opposing effects on target organs, where the sympathetic system promotes the “fight or flight” response and the parasympathetic system promotes the “rest and digest” response. Image credit: OpenStax Biology

This video provides an overview of the CNS and PNS:

 

Reflex Arcs

Reflex arcs are an interesting phenomenon for considering how the PNS and CNS work together. Reflexes are quick, unconscious movements, like automatically removing a hand from a hot object. Reflexes are so fast because they involve local synaptic connections in the spinal cord, rather than relay of information to the brain. For example, the knee reflex that a doctor tests during a routine physical is controlled by a single synapse between a sensory neuron and a motor neuron. While a reflex may only require the involvement of one or two synapses, synapses with interneurons in the spinal column transmit information to the brain to convey what happened after the event is already over (the knee jerked, or the hand was hot). So this means that the brain is not involved at all in the movement associated with the reflex, but it is certainly involved in learning from the experience – most people only have to touch a hot stove once to learn that they should never do it again!

The simplest neuronal circuits are those that underlie muscle stretch responses, such as the knee-jerk reflex that occurs when someone hits the tendon below your knee (the patellar tendon) with a hammer. Tapping on that tendon stretches the quadriceps muscle of the thigh, stimulating the sensory neurons that innervate it to fire. Axons from these sensory neurons extend to the spinal cord, where they connect to the motor neurons that establish connections with (innervate) the quadriceps. The sensory neurons send an excitatory signal to the motor neurons, causing them to fire too. The motor neurons, in turn, stimulate the quadriceps to contract, straightening the knee. In the knee-jerk reflex, the sensory neurons from a particular muscle connect directly to the motor neurons that innervate that same muscle, causing it to contract after it has been stretched. Image credit: https://www.khanacademy.org/science/biology/ap-biology/human-biology/neuron-nervous-system/a/overview-of-neuron-structure-and-function, modified from “Patellar tendon reflex arc,” by Amiya Sarkar (CC BY-SA 4.0). The modified image is licensed under a CC BY-SA 4.0 license.

This video provides an overview of how reflex arcs work: