Learning Objectives
In this section, you will explore the following questions:
- What are the major areas of the brain?
- What are the primary functions of the spinal cord, cerebral lobes, cerebellum, and brainstem?
Connection for AP® Courses
The central nervous system (CNS) consists of the brain and spinal cord, both of which are protected by the skull and vertebral column, respectively. The CNS receives sensory information, integrates this information, and initiates a motor response, with the brain serving as the control center for processing sensory information and directing responses. Different parts of the vertebrate brain (including ours) have different functions, and brain development in animals reveals a unique evolutionary progression. You do not have to memorize all the different parts of the brain and their functions for AP. However, as a student of biology, you should have a general understanding of the three major parts of the brain and their functions.
In mammals, the parts of the brain include the cerebrum or cortex (which can be broken down into four primary lobes: frontal, temporal, occipital, and parietal), basal ganglia, thalamus, hypothalamus, limbic system, cerebellum, and brainstem. Information traveling up the spinal cord to the brain is directed to one of the specialized areas of the cerebrum; for example, association areas for hearing are localized in the temporal lobe. The cerebellum helps coordinate skeletal muscle activity, and the medulla oblongata and pons in the brainstem are centers for vital functions, such as breathing and heart rate. Although localization of functions occurs, most complex functions, like language, involve neurons in multiple brain regions. In terms of energy, since the brain consumes about 20 percent of the body’s resources (ATP), is it any wonder that you’re exhausted after taking an AP® test? Information from the brain travels down the spinal cord, making connections with peripheral nerves; thus, the spinal cord transmits sensory and motor input and controls motor reflexes, like the automatic responses when the pupils of your eye constrict in bright sunlight or when your jerk your hand away from something hot.
Information presented and the examples highlighted in the section support concepts outlined in Big Idea 3 of the AP® Biology Curriculum Framework. The AP® Learning Objectives listed in the Curriculum Framework provide a transparent foundation for the AP® Biology course, an inquiry-based laboratory experience, instructional activities, and AP® exam questions. A learning objective merges required content with one or more of the seven science practices.
| Big Idea 3 | Living systems store, retrieve, transmit and respond to information essential to life processes. |
| Enduring Understanding 3.E | Transmission of information results in changes within and between biological systems. |
| Essential Knowledge | 3.E.2 Animals have nervous systems that detect external and internal signals, transmit and integrate information, and produce responses. |
| Science Practice | 1.1 The student can create representations and models of natural or man-made phenomena and systems in the domain. |
| Learning Objective | 3.49 The student is able to create a visual representation to describe how the vertebrate brain integrates information to produce a response. |
| Essential Knowledge | 3.E.2 Animals have nervous systems that detect external and internal signals, transmit and integrate information, and produce responses. |
| Science Practice | 1.2 The student can describe representations and models of natural or man-made phenomena and systems in the domain. |
| Learning Objective | 3.46 The student is able to describe how the vertebrate brain integrates information to produce a response. |
| Essential Knowledge | 3.E.2 Animals have nervous systems that detect external and internal signals, transmit and integrate information, and produce responses. |
| Science Practice | 1.1 The student can create representations and models of natural or man-made phenomena and systems in the domain. |
| Learning Objective | 3.50 The student is able to create a visual representation to describe how the vertebrate brain integrates information to produce a response. |
As mentioned above, the central nervous system (CNS) is made up of the brain, a part of which is shown in Figure 26.19 and spinal cord and is covered with three layers of protective coverings called meninges (from the Greek word for membrane). The outermost layer is the dura mater (Latin for “hard mother”). As the Latin suggests, the primary function for this thick layer is to protect the brain and spinal cord. The dura mater also contains vein-like structures that carry blood from the brain back to the heart. The middle layer is the web-like arachnoid mater. The last layer is the pia mater (Latin for “soft mother”), which directly contacts and covers the brain and spinal cord like plastic wrap. The space between the arachnoid and pia maters is filled with cerebrospinal fluid (CSF). CSF is produced by a tissue called choroid plexus in fluid-filled compartments in the CNS called ventricles. The brain floats in CSF, which acts as a cushion and shock absorber and makes the brain neutrally buoyant. CSF also functions to circulate chemical substances throughout the brain and into the spinal cord.
The entire brain contains only about 8.5 tablespoons of CSF, but CSF is constantly produced in the ventricles. This creates a problem when a ventricle is blocked—the CSF builds up and creates swelling and the brain is pushed against the skull. This swelling condition is called hydrocephalus (“water head”) and can cause seizures, cognitive problems, and even death if a shunt is not inserted to remove the fluid and pressure.
Brain
The brain is the part of the central nervous system that is contained in the cranial cavity of the skull. It includes the cerebral cortex, limbic system, basal ganglia, thalamus, hypothalamus, and cerebellum. There are three different ways that a brain can be sectioned in order to view internal structures: a sagittal section cuts the brain left to right, as shown in Figure 26.21b, a coronal section cuts the brain front to back, as shown in Figure 26.20a, and a horizontal section cuts the brain top to bottom.
Cerebral Cortex
The outermost part of the brain 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—which are separated by a large sulcus. A thick fiber bundle called the corpus callosum (Latin: “tough body”) 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. In fact, sometimes (very rarely) an entire hemisphere is removed to treat severe epilepsy. While patients do suffer some deficits following the surgery, they can have surprisingly few problems, especially when the surgery is performed on children who have very immature nervous systems.
In other surgeries to treat severe epilepsy, the corpus callosum is cut instead of removing an entire hemisphere. This causes a condition called split-brain, which gives insights into unique functions of the two hemispheres. For example, when an object is presented to patients’ left visual field, they may be unable to verbally name the object (and may claim to not have seen an object at all). This is because the visual input from the left visual field crosses and enters the right hemisphere and cannot then signal to the speech center, which generally is found in the left side of the brain. Remarkably, if a split-brain patient is asked to pick up a specific object out of a group of objects with the left hand, the patient will be able to do so but will still be unable to vocally identify it.
Link to Learning
See this website to learn more about split-brain patients and to play a game where you can model the split-brain experiments yourself.
MISSING EXERCISE: tag:apbio-ch26-ex046Each cortical hemisphere contains regions called lobes that are involved in different functions. Scientists use various techniques to determine what brain areas are involved in different functions: they examine patients who have had injuries or diseases that affect specific areas and see how those areas are related to functional deficits. They also conduct animal studies where they stimulate brain areas and see if there are any behavioral changes. They use a technique called transcranial magnetic stimulation (TMS) to temporarily deactivate specific parts of the cortex using strong magnets placed outside the head; and they use functional magnetic resonance imaging (fMRI) to look at changes in oxygenated blood flow in particular brain regions that correlate with specific behavioral tasks. These techniques, and others, have given great insight into the functions of different brain regions but have also showed that any given brain area can be involved in more than one behavior or process, and any given behavior or process generally involves neurons in multiple brain areas. That being said, each hemisphere of the mammalian cerebral cortex can be broken down into four functionally and spatially defined lobes: frontal, parietal, temporal, and occipital. Figure 26.21 illustrates these four lobes of the human cerebral cortex.
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. Areas within the motor cortex map to different muscle groups, and there is some organization to this map, as shown in Figure 26.22. For example, the neurons that control movement of the fingers are next to the neurons that control movement of the hand. Neurons in the frontal lobe also control cognitive functions like maintaining attention, speech, and decision-making. Studies of humans who have damaged their frontal lobes show that 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 parietal lobe contains a somatosensory map of the body similar to the motor cortex.
The occipital lobe is located at the back of the brain. It is primarily involved in vision—seeing, recognizing, and identifying the visual world.
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. The hippocampus is illustrated in Figure 26.24. The role of the hippocampus in memory was partially determined by studying one famous epileptic patient, HM, who had both sides of his hippocampus removed in an attempt to cure his epilepsy. His seizures went away, but he could no longer form new memories (although he could remember some facts from before his surgery and could learn new motor tasks).
Evolution Connection
Cerebral Cortex
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.
Basal Ganglia
Interconnected brain areas called the basal ganglia (or basal nuclei), shown in Figure 26.20b, play important roles in movement control and posture. Damage to the basal ganglia, as in Parkinson’s disease, leads to motor impairments like a shuffling gait when walking. The basal ganglia also regulate motivation. For example, when a wasp sting led to bilateral basal ganglia damage in a 25-year-old businessman, he began to spend all his days in bed and showed no interest in anything or anybody. But when he was externally stimulated—as when someone asked to play a card game with him—he was able to function normally. Interestingly, he and other similar patients do not report feeling bored or frustrated by their state.
Thalamus
The thalamus (Greek for “inner chamber”), illustrated in Figure 26.24, 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. This feedback mechanism can modulate conscious awareness of sensory and motor inputs depending on the attention and arousal state of the animal. The thalamus helps regulate consciousness, arousal, and sleep states. A rare genetic disorder called fatal familial insomnia causes the degeneration of thalamic neurons and glia. This disorder prevents affected patients from being able to sleep, among other symptoms, and is eventually fatal.
Hypothalamus
Below the thalamus is the hypothalamus, shown in Figure 26.24. The hypothalamus 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—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.
Limbic System
The limbic system is a connected set of structures that regulates emotion, as well as behaviors related to fear and motivation. It plays a role in memory formation and includes parts of the thalamus and hypothalamus as well as the hippocampus. One important structure within the limbic system is a temporal lobe structure called the amygdala (Greek for “almond”), illustrated in Figure 26.24. The two amygdala are important both for the sensation of fear and for recognizing fearful faces. The cingulate gyrus helps regulate emotions and pain.
Cerebellum
The cerebellum (Latin for “little brain”), shown in Figure 26.21, sits at the base of the brain on top of the brainstem. The cerebellum controls balance and aids in coordinating movement and learning new motor tasks.
Brainstem
The brainstem, illustrated in Figure 26.21, connects the rest of the brain with the spinal cord. It consists of the midbrain, medulla oblongata, and the pons. Motor and sensory neurons extend through the brainstem allowing for the relay of signals between the brain and spinal cord. Ascending neural pathways cross in this section of the brain allowing the left hemisphere of the cerebrum to control the right side of the body and vice versa. The brainstem coordinates motor control signals sent from the brain to the body. 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.
Science Practice Connection for AP® Courses
Activity
Create a representation to illustrate what parts of the brain allow you to perform a favorite daily activity, like kicking a soccer ball, learning a new dance move, or reading the information in this section of the text and jotting down a few notes.
Teacher Support
The activity is an application of AP® Learning Objective 3.50 and Science Practice 1.1 because students are creating a diagram to describe how the brain integrates and coordinates information to produce a response (for example, modeling the chronology and location of signaling that must take place between different parts of the brain, spinal cord, and muscle groups in order to kick a soccer ball).
Students should identify the cerebellum, hindbrain (pons), thalamus and cerebrum as responsible for kicking a soccer ball. The motor cortex and premotor areas of the cerebrum create the impulses for movement, as well as process the sensory information needed to see the soccer ball. The thalamus, cerebellum and hindbrain coordinate the movement with the body’s current posture to maintain balance and coordination.
Spinal Cord
Connecting to the brainstem and extending down the body through the spinal column is the spinal cord, shown in Figure 26.21. 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, as illustrated in Figure 26.25. 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. Axons and cell bodies in the dorsal (facing the back of the animal) spinal cord convey mostly sensory information from the body to the 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 the body.
The spinal cord also controls motor reflexes. These reflexes are quick, unconscious movements—like automatically removing a hand from a hot object. Reflexes are so fast because they involve local synaptic connections. 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 (the knee jerked, or the hand was hot).
In the United States, there around 10,000 spinal cord injuries each year. Because the spinal cord is the information superhighway connecting the brain with the body, damage to the spinal cord can lead to paralysis. The extent of the paralysis depends on the location of the injury along the spinal cord and whether the spinal cord was completely severed. For example, if the spinal cord is damaged at the level of the neck, it can cause paralysis from the neck down, whereas damage to the spinal column further down may limit paralysis to the legs. Spinal cord injuries are notoriously difficult to treat because spinal nerves do not regenerate, although ongoing research suggests that stem cell transplants may be able to act as a bridge to reconnect severed nerves. Researchers are also looking at ways to prevent the inflammation that worsens nerve damage after injury. One such treatment is to pump the body with cold saline to induce hypothermia. This cooling can prevent swelling and other processes that are thought to worsen spinal cord injuries.