Lesson Explainer: The Central Nervous System Biology

In this explainer, we will learn how to describe the structure and function of the different parts of the central nervous system.

The human nervous system has three functions: gathering sensory information from the internal and external environment, processing the gathered information, and responding with appropriate motor movements (e.g., facial expressions, gestures, digestion, and running). These functions are carried out by the two main parts of the human nervous system, the central nervous system (CNS) and the peripheral nervous system (PNS), which are shown in the diagram below.

The human CNS is often called the central processing unit of the body, because it controls our actions and our understanding of the environment and helps us connect with other humans. To perform these actions and process the sensory environment, the CNS consists of two major components: the brain and the spinal cord.

Key Term: Central Nervous System

The central nervous system is made up of the brain and the spinal cord.

Example 1: Describing the Central Nervous System

What is the central nervous system comprised of?

  1. The spine and the spinal nerves
  2. The peripheral nerves in external limbs
  3. The brain and the spinal cord
  4. The brain and the cranial nerves

Answer

You may recall that the human nervous system has three main functions: gathering sensory input, processing information, and responding via motor output. These functions are carried out by the two main parts of the human nervous system: the central nervous system (CNS) and the peripheral nervous system (PNS).

The illustration above shows the two main parts of the human nervous system. The central nervous system, highlighted in orange, consists of the brain and the spinal cord. The human CNS is often called the central processing unit of the body, because it controls our actions and our understanding of the environment and helps us connect with other humans. To perform these actions and process the sensory environment, the CNS consists of two major components: the brain and the spinal cord.

Therefore, the central nervous system is comprised of the brain and the spinal cord.

Since your brain and spinal cord perform some of the most critical functions in your body, they are heavily protected. The first layer of protection for the brain and spinal cord is the bones that encase these two structures. The brain is surrounded by the skull, whereas the spinal cord is shielded by small bones called vertebrae in your spine. Underneath the bone, there are three more layers of protection in the form of membranes called the meninges, shown in Figure 2.

Each membrane has a specific function and name. The outermost membrane, the dura mater, lines the skull bones. Dura mater is Latin for “tough mother” and aptly describes this thick, durable membrane, which also contains vein-like structures that carry blood from the brain to the heart. Underneath the dura mater is the arachnoid mater. This membrane is named for its resemblance to a spider’s web and is filled with brain fluid. The structure of this layer helps provide extra cushioning against physical trauma to the brain. The last membrane is called the pia mater, which is Latin for “soft mother.” The pia mater adheres to the surface of the brain and spinal cord like a plastic wrap.

Now that we know how the brain and spinal cord are protected from physical damage, let’s take a closer look at each component of the CNS in detail, starting with the brain.

The brain serves as the command center that coordinates all our activities. It controls the body by sending and receiving signals with the PNS via the spinal cord. The brain is composed of two main cell types: neurons (nerve cells) and support cells. Given that the brain is one of the most complex organs in the body, there are many highly specialized areas within the brain that work together. The brain has three divisions: the forebrain, the midbrain, and the hindbrain. These three divisions can be seen in Figure 3.

Key Term: Brain

The brain is composed of neurons and glial cells and acts as the command center that coordinates of all our activities.

The forebrain is the largest and most developed part of the human brain. This is because it consists primarily of the cerebrum and deep brain structures, such as the hippocampus (the seat of memory), optic nerves (which connect to the eyes), and olfactory bulb (necessary for smelling).

The cerebrum is responsible for reasoning, memory, language, sensory perception, and emotional response. As shown in Figure 3, the cerebrum is split into two halves, or hemispheres. Both hemispheres of the cerebrum can communicate through a thick tract of nerve fibers.

Key Term: Cerebrum

The cerebrum is the largest part of the brain that is responsible for memory, language, sensory perception, and emotional response.

The outermost layer of the cerebrum is called the cerebral cortex. The word cortex comes from the Latin word for “bark,” and it helps describe the thick piece of nervous system tissue that the cerebral cortex is made of. The folds in this thick piece of nervous system tissue are what gives the brain its notable wrinkled appearance. These folds are termed gyri (singular: gyrus), whereas the grooves are called sulci (singular: sulcus). These folds and grooves are not just for appearances.

Mammals, such as humans, have really large brains for their body size. In contrast, the brain of an alligator would only fill about one and a half teaspoons. The folding of the cortex helps increase the surface area of the brain tissue within the skull. This is important because an increased surface area means more brain cells and, hence, increased processing power for the brain. So, the more the folds or wrinkles in the cortex are, the more complex and potentially intelligent the brain is. The largest of these grooves in the brain are also used as landmarks to help researchers visualize the four distinct sections of the cerebrum.

Key Term: Cerebral Cortex

The cerebral cortex is the outer layer of the cerebrum that has folds and grooves, which give the brain its notable wrinkled appearance.

Each hemisphere of the cerebrum has four lobes: a frontal lobe, a parietal lobe, a temporal lobe, and an occipital lobe. With the two hemispheres of the cerebrum, this means that each of the lobes comes as a pair. These four lobes are shown in different colors in Figure 4.

Each of these four lobes has a distinct function.

The frontal lobe of the cerebrum is shown in red in Figure 4. This lobe controls voluntary movements, speech, reasoning, and memory. So, it is the frontal lobe that you use when you plan a schedule, pay attention to instructions, imagine the future, or use reasoned arguments. Additionally, the frontal lobe is highly specialized for short-term storage, like holding two different ideas in your mind at the same time.

Different areas of the frontal lobe carry out these functions. For example, the rearmost fold of the frontal lobe is a motor area because it helps control voluntary movement, whereas several folds in the left frontal lobe make up what is called Broca’s area, which controls the transformation of thoughts into the correct words.

Damage to the frontal lobe can have extensive effects on your personality and behavior. For example, Phineas Gage was a railroad foreman who survived a 5.9 kg (13-pound) iron rod penetrating through his brain, destroying his left frontal lobe. Although Gage lost sight in his left eye, he had no other deficits in his speech, motor ability, or memory. However, his personality drastically changed for several years. The previously even-tempered Gage became disrespectful, profane, unreliable, and impulsive.

Key Term: Frontal Lobe

The frontal lobe controls reasoning, voluntary movements, speech, and memory as well as social behaviors and personality.

Behind the frontal lobe is the parietal lobe (shown in green in Figure 4). The two main functions of the parietal lobe are processing touch sensations, such as pressure, pain, and temperature (also called somatosensation), and the sense of where our body parts are oriented in space (also called proprioception). These two functions, somatosensation and proprioception, are connected because the parietal lobe contains a somatosensory map of the body. This helps the brain process the “what” and “where” of sensory information.

Key Term: Parietal Lobe

The parietal lobe processes sensory information, such as pressure, pain, and temperature, and where our body parts are oriented in space.

The temporal lobe, shown in yellow in Figure 4, has several functions because it contains the centers for hearing and memory. At the top of each temporal lobe, there is an area responsible for receiving information from the ears, such as music and speech. Other parts, like the underside of each lobe, play a crucial role in forming and retrieving memories, including those that are tied to emotion or auditory information.

Damage to the temporal lobe can severely impact memory function. For example, Henry Molaison, better known as H.M. to neuroscientists everywhere, was a patient who at the age of 27 underwent a surgery to remove his temporal lobes to control his lifelong epileptic seizures. Although the surgery helped control his epilepsy, he was no longer able to form long-term memories. For example, he could remember a three-digit number for as long as 15 minutes if he used rehearsal, but as soon as he was distracted, he would forget the number and experiment. Without the ability to rehearse the information, H.M. could only retain information for 60 seconds.

Key Term: Temporal Lobe

The temporal lobe is involved in hearing and memory.

The occipital lobe, shown in blue in Figure 4, processes visual information.

Different areas of the occipital lobe process different types of visual information. The visual cortex processes information from the eyes and then interprets this information into the images that create our perception of the world. Other parts of the occipital lobe, such as the visual receiving and association areas, process visual images of language and interpret the imagery. These areas are extremely useful in reading and comprehension. So, whenever you can recognize the words of another language but have difficulty understanding them, you are only using the visual receiving area.

Key Term: Occipital Lobe

The occipital lobe processes visual information and houses the visual cortex.

Example 2: Identifying the Occipital Lobe of the Cerebrum

The diagram provided shows an outline of the major parts of the human brain. Which part of the cerebrum is responsible for interpreting and processing visual information?

  1. Occipital lobe
  2. Parietal lobe
  3. Frontal lobe
  4. Temporal lobe

Answer

The frontal lobe of the cerebrum, shown in red, processes information related to planning, motivation, attention, reward, voluntary movements, speech, memory, problem solving, and emotions. So, whenever you plan a schedule, imagine the future, or use reasoned arguments, it is your frontal lobe that is making all of those things possible.

The frontal lobe is highly specialized for short-term storage, allowing one idea to be kept in mind while other ideas are considered. Behind the frontal lobe is the parietal lobe (shown in green). The two main functions of the parietal lobe are processing somatosensation (touch sensations like pressure, pain, and temperature) and processing proprioception (the sense of where our body parts are oriented in space). The temporal lobe is shown in yellow. It has several functions because it contains the centers of smell, taste, sight, and touch. Other parts of this lobe seem to integrate memories and also help assign emotional meaning to long-term memories. The occipital lobe, shown in blue, processes visual information. When this part of the brain is injured, this can lead to blindness.

Therefore, the part of the cerebrum that is responsible for interpreting and processing visual information is the occipital lobe.

The insula can also be referred to as the fifth lobe of the brain and is considered to be the least known. This is because its location, hidden beneath the four other lobes (as shown in Figure 5) of the cerebrum, makes it difficult to study. However, newer methods of brain imaging have helped shed light on the role of the insula. For example, we now know that the insula is sort of a receiving zone that reads the state of the entire body and generates feelings that can bring about actions, like eating or decision-making. It also contains the gustatory cortex, which is responsible for our perception of taste.

Beneath the cerebrum is the diencephalon.

Another structure of the forebrain is the hypothalamus (shown in Figure 6), which regulates the rhythm of our sleep cycle and is considered the body’s thermostat. The hypothalamus also controls the endocrine system by sending signals to the pituitary gland, a pea-sized gland that releases different hormones. So, not only is the hypothalamus in charge of waking you up in the morning, but also it is responsible for keeping you hydrated and cool, as well as getting your adrenaline flowing during a test or performance.

Key Term: Hypothalamus

The hypothalamus regulates the autonomic nervous system and coordinates the nervous and endocrine systems to control different functions, such as sleep, hunger, thirst, and body temperature.

Example 3: Identifying the Functions of the Hypothalamus

The diagram provided shows a simplified outline of the brain, with the hypothalamus highlighted. What are the major functions of the hypothalamus?

  1. To process visual information
  2. To control voluntary movement and to help with memory and learning
  3. To control hunger and sleep cycles and to regulate body temperature
  4. To process auditory and visual information
  5. To control fine motor and involuntary movement

Answer

The structures of the midbrain help determine our emotional state, modify our perceptions and responses depending on that state, and allow us to initiate reflex arcs.

One of the structures of the midbrain is the hypothalamus, which regulates the rhythm of our sleep cycle and is considered the body’s thermostat. The hypothalamus also controls the endocrine system by sending signals to the pituitary gland, a pea-sized gland that releases different hormones. So, not only is the hypothalamus in charge of waking you up in the morning, but also it is responsible for keeping you hydrated and cool, as well as getting your adrenaline flowing during a test or performance.

Therefore, the major functions of the hypothalamus are to control hunger and the sleep cycle and to regulate the body temperature.

Beneath the forebrain is the brainstem, which connects the brain to the spinal cord. The brainstem consists of the midbrain and the hindbrain, shown in Figure 7. As many vital functions, such as breathing and heartbeat, as well as motor control for the head, neck, and chest are coordinated by the brainstem, it is considered a vitally important part of the CNS. For example, injury to the brainstem can lead to permanent changes in the quality of life, since the brainstem regulates almost all the daily activities of the body.

Key Term: Brainstem

The brainstem, which includes the midbrain and hindbrain, is a structure that conveys information about movement from the cerebrum in the forebrain to the cerebellum in the hindbrain.

The midbrain is relatively small compared to the forebrain, but it contains several important centers for vital reflex actions, visual and motor control, pain sensation, and aggression. The reflexes controlled by the midbrain are actions that occur without conscious thought, such as coughing or startling upon hearing a loud, sudden sound. The midbrain also contains a very important reward circuit that determines our motivation and possibly our addictions.

The hindbrain includes most of the pons, cerebellum, and medulla oblongata. The location of the hindbrain is shown in Figure 7.

The bulbous shape of the pons helps distinguish it from the other structures in the hindbrain. The word pons comes from the Latin word for “bridge,” as the pons connects the rest of the brainstem to the cerebral cortex, conveying information about movement from the cerebral cortex to the cerebellum. The pons, also sometimes called the pons varolii, serves as a coordination center for respiration, taste, and sleep.

Key Term: Pons (Pons Varolii)

The pons conveys information about movement from the cerebrum to the cerebellum and serves as a coordination center for respiration, taste, and sleep.

Behind the pons, at the base of the skull, is the cerebellum. This small, dense, coral-shaped structure contains more neurons than both hemispheres of the cerebrum and is crucially important for motor function. The word cerebellum comes from the Latin word for “little brain.” This is because it receives somatosensory information (e.g., pain, touch, pressure, and temperature) from the spinal cord, motor information from the cerebral cortex, and balance information from the inner ear. The cerebellum then processes all of this information to help coordinate our motor movements and maintain our posture.

Key Term: Cerebellum

The cerebellum is part of the hindbrain that functions to coordinate our motor movements and maintain our balanced posture.

Adjacent to the cerebellum is the medulla oblongata, the lowest part of the overall hindbrain. The medulla oblongata is where the brain transitions to the spinal cord. Despite being only about 3 centimetres in length, the medulla oblongata is very important to bodily functions because it contains the control centers for all of our autonomic and involuntary functions, such as heart rate, blood pressure, breathing, swallowing, and sneezing.

Key Term: Medulla Oblongata

The medulla oblongata is vital for the control of breathing, blood pressure, and heart rate.

Below the medulla oblongata is the spinal cord, which is the second organ of the central nervous system.

The spinal cord is a thick bundle of nerve fibers that starts in the medulla oblongata of the brain and rests inside the hollow center of the spine. It acts as a bridge between the brain and the PNS to relay messages between the brain and the body. Figure 8 shows a cross section of the spinal cord, which looks like a white oval containing a gray H shape.

Key Term: Spinal Cord

The spinal cord is the second component of the central nervous system and consists of a collection of nerve fibers that relay messages between the brain and the body.

In the cross section of the spinal cord, there is gray matter and white matter. The gray matter forms the core H shape.

The grey matter is composed mainly of relay neurons (also called intermediate neurons), which connect sensory neurons and motor neurons. The gray matter also contains the axons and dendrites of the sensory and motor neurons. In the brain, the gray matter is mainly found in layers, as in the cerebral cortex, or as clusters called nuclei. The gray matter found in the brain also appears more pink than gray because of the abundant blood supply to this tissue.

A layer of white matter surrounds the H-shaped gray matter. The white matter is made of the myelinated axons of neurons, which help nerve impulses travel quickly. The white color of myelin is what gives the white matter its characteristic color. In the brain, the white matter is buried under the gray matter and carries signals across different parts of the brain.

Key Term: Gray Matter

The gray matter is made of nonmyelinated neurons, dendrites, and cell bodies in the brain cortex, nuclei, and central part of the spinal cord.

Key Term: White Matter

The white matter is made of the myelinated axons of neurons, which help nerve impulses travel quickly.

The spinal cord is linked to the muscles and sensory receptors in the body and is bundled to 31 pairs of spinal nerves, which control motor, sensory, and other functions. At each level of the spinal cord, neurons are organized into a similar pathway described in Figure 9. Sensory receptors initiate sensory signals that are conducted by the dendrites of the sensory neurons toward the cell bodies, located in the dorsal ganglion. Then the sensory inputs travel in the axons into the spinal cord, forming the dorsal root. Once inside the spinal cord, the axons of the sensory neurons connect with relay neurons in the top arms of the H-shaped gray matter, which are called the dorsal horns.

In the ventral horns, or the lower arms of the H-shaped gray matter, the relay neurons connect to the motor neurons. As the motor neurons exit the spinal cord, they form the ventral root.

The circuit of neurons shown in Figure 9 describes the typical pathway of a reflex arc. The spinal cord contains thousands of them. These circuits of neurons control reflexes, which are rapid automatic motor responses triggered by sensory stimuli and do not require input from the brain. While reflexes occur involuntarily, voluntary commands sent from the brain to the gray matter of the spinal cord can stop a reflex if necessary.

Example 4: Distinguishing between the Dorsal and Ventral Roots of the Spinal Nerves

The diagram provided shows the anatomy of a cross section of the spinal cord. Each spinal nerve has a dorsal and a ventral root.

  1. Which root contains the cell bodies of sensory neurons?
    1. Dorsal
    2. Ventral
  2. Which root consists of motor neuron axons?
    1. Dorsal
    2. Ventral

Answer

The central nervous system is composed of two organs: the brain and the spinal cord. The spinal cord is a thick bundle of nerve fibers that starts in the medulla oblongata of the brain and rests inside the hollow center of the spinal canal. It acts as a bridge between the brain and the PNS. A cross section of the spinal cord looks like a white oval containing a gray butterfly shape, as illustrated in the figure below.

A layer of white matter surrounds the H-shaped gray matter. This white matter is made of the myelinated axons of neurons, which help nerve impulses travel quickly. The white color of myelin is what gives the white matter its characteristic color. In the brain, the white matter is buried under the gray matter and carries signals across different parts of the brain.

The spinal cord is linked to the muscles and sensory receptors in the body and is bundled to 31 pairs of spinal nerves, which control motor, sensory, and other functions. At each level of the spinal cord, neurons are organized into a similar pathway, shown below in the cross section of the spinal cord.

As the sensory neurons feed into the spinal cord, they form the dorsal root. Once inside the spinal cord, the axons of sensory neurons connect with relay neurons in the top arms of the H-shaped gray matter called the dorsal horns. In the ventral horns, or the lower arms of the H-shaped gray matter, the relay neurons connect to the motor neurons. As the motor neurons exit the spinal cord, they form the ventral root.

Part 1

Therefore, the root that contains the cell bodies of sensory neurons is the dorsal root.

Part 2

Therefore, the root that consists of motor neuron axons is the ventral root.

Let’s review some of the key points we have covered in this explainer.

Key Points

  • The central nervous system is made of the brain and spinal cord.
  • Both the brain and the spinal cord are surrounded by three membranes, or meninges, which are the dura mater, arachnoid mater, and pia mater.
  • The brain is divided into the forebrain, midbrain, and hindbrain.
  • The four lobes of the cerebrum are called the frontal, temporal, parietal, and occipital lobes.
  • A cross section of the white matter of the spinal cord looks like an oval that surrounds the H-shaped gray matter.
  • The white matter is white because of the coloring of myelin that is wrapped around the myelinated axons of neurons.

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