The Central Nervous System

Brain: Cerebral Cortex and Brain Lobes

The cerebral cortex of the brain is divided into four lobes responsible for distinct functions: frontal, parietal, temporal, and occipital.

Learning Objectives

Describe the structure and function of the cerebral cortex

Key Takeaways

Key Points

  • The cerebral cortex is the outermost layer of the brain; it is easily recognizable by the grooves (sulci) and “hills” (gyri).
  • The brain contains two hemispheres, the left and the right, which are connected by a bundle of nerve fibers called the corpus callosum that transmits information between them.
  • The frontal lobe houses the olfactory bulb, which processes smells; the motor cortex, which controls movement; and it controls cognitive functions such as attention, speech, and decision-making.
  • The parietal lobe is involved in speech and reading, as well as interpreting touch sensations such as pressure, pain, heat, cold, along with sensing where each part of the body is in relation to the others and its environment.
  • The occipital lobe interprets visual cues, such as what we see and recognition of faces and objects.
  • The temporal lobe processes and interprets sounds and is also involved in forming new memories, a task for which the hippocampus, a structure inside the temporal lobe, is responsible.

Key Terms

  • corpus callosum: in mammals, a broad band of nerve fibers that connects the left and right hemispheres of the brain
  • proprioception: the sense of the position of parts of the body, relative to other neighbouring parts of the body
  • somatosensation: general senses which respond to stimuli like temperature, pain, pressure, and vibration
  • gyrus: a ridge or fold on the cerebral cortex
  • sulcus: any of the grooves that mark the convolutions of the surface of the brain

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.

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 composed of two hemispheres, right and left, which are separated by a large sulcus. A thick fiber bundle, the corpus callosum, connects the two hemispheres, allowing 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 relatively-undeveloped 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 syndrome, which gives insights into unique functions of the two hemispheres. For example, when an object is presented to patients’ left visual fields, they may be unable to verbally name the object (and may claim not to have seen an object at all). This is because the visual input from the left visual field crosses and enters the right hemisphere and is unable to 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.

The Four Brain Lobes

Each hemisphere of the mammalian cerebral cortex can be broken down into four functionally- and spatially-defined lobes: frontal, parietal, temporal, and occipital. 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; there is some organization to this map. 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 such as 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.

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Motor cortex control of muscle movement: Different parts of the motor cortex control different muscle groups. Muscle groups that are neighbors in the body are generally controlled by neighboring regions of the motor cortex as well. For example, the neurons that control shoulder movement are near the neurons that control elbow movement, which are themselves next to those that control wrist movement.

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Lobes of the cerebral cortex: The human cerebral cortex includes the frontal, parietal, temporal, and occipital lobes, each of which is involved in a different higher function.

The parietal lobe is located at the top of the brain. Neurons in the parietal lobe are involved in speech and reading. Two of the parietal lobe’s main functions are processing somatosensation (touch sensations such as 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 the ears. It is primarily involved in processing and interpreting sounds. It also contains the hippocampus (Greek for “seahorse”, which is what it resembles), a structure that processes memory formation. 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).

Brain: Midbrain and Brain Stem

Regions of the brain other than the cerebral cortex include those involved in sleep, memory, attention, motor coordination, and motivation.

Learning Objectives

Explain the structure and function of the non-cerebral cortex portions of the brain

Key Takeaways

Key Points

  • The basal ganglia control movement and posture; they also appear to be involved in self-motivation.
  • The thalamus communicates information from the cerebral cortex to the rest of the body and also helps regulate the states of consciousness versus sleep.
  • The hypothalamus regulates the pituitary gland, which controls the release of hormones throughout the body; it indirectly regulates functions such as water intake, body temperature, and sleep cycles.
  • The limbic system includes the amygdala, the hippocampus, and parts of the thalamus and hypothalamus; it regulates emotion, fear, and motivation.
  • The cerebellum controls motor reflexes and is, therefore, involved in balance and muscle coordination.
  • The brainstem connects and transmits signals from the brain to the spinal cord, controlling functions such as breathing, heart rate, and alertness.

Key Terms

  • cingulate gyrus: a section of the cerebral cortex, belonging to the fornicate gyrus, which arches over the corpus callosum
  • limbic system: part of the human brain involved in emotion, motivation, and emotional association with memory
  • endocrine system: a control system of ductless glands that secrete hormones which circulate via the bloodstream to affect cells within specific organs

Basal Ganglia

Interconnected brain areas called the basal ganglia (or basal nuclei) play important roles in movement control and posture. Damage to the basal ganglia, which occurs in Parkinson’s disease, leads to motor impairments such as 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”) 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, 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. 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.

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”). The two amygdale are important both for the sensation of fear and for recognizing fearful faces. The cingulate gyrus helps regulate emotions and pain.

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The limbic system: The limbic system regulates emotion and other behaviors. It includes parts of the cerebral cortex located near the center of the brain, including the cingulate gyrus and the hippocampus as well as the thalamus, hypothalamus, and amygdala.

Cerebellum

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 movement and learning new motor tasks.

Brainstem

The brainstem 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. It also controls several important functions of the body including alertness, arousal, breathing, blood pressure, digestion, heart rate, swallowing, walking, and sensory and motor information integration.

Spinal Cord

The spinal cord is a bundle of nerves that is connected to the brain and relays information from the brain to the body and vice versa.

Learning Objectives

Describe the structure and function of the spinal cord

Key Takeaways

Key Points

  • The spinal cord consists of a butterfly-shaped area of grey matter, containing neuronal and glial cell bodies, surrounded by white matter that contains the axons of the neurons.
  • Neurons at the back of the spinal cord ( dorsal ) generally transmit information from the body to the brain, while neurons at the front of the spinal cord ( ventral ) primarily transmit information from the brain to the body.
  • The spinal cord controls reflexes, which are incredibly fast reactions to stimuli; the speed at which they operate is due to the fact that they involve only a local connection between neurons and are not relayed through the brain.
  • Spinal cord injuries often result in paralysis; they do not heal, as spinal nerves lack the ability to regenerate.

Key Terms

  • grey matter: a collection of cell bodies and (usually) dendritic connections, in contrast to white matter
  • synapse: the junction between the terminal of a neuron and either another neuron or a muscle or gland cell, over which nerve impulses pass
  • axon: long slender projection of a nerve cell that conducts nerve impulses away from the cell body to other neurons, muscles, and organs
  • white matter: a region of the central nervous system containing myelinated nerve fibers and no dendrites
  • interneuron: a multipolar neuron that connects afferent and efferent neurons

Spinal Cord

Connecting to the brainstem and extending down the body through the spinal column is the spinal cord: 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 vertebral 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 (the part of neurons that send signals) compose the “white matter,” while neuron and glial cell bodies (neuronal “support” cells) compose the “grey matter.” Grey 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.

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Spinal cord cross-section: A cross-section of the spinal cord shows grey matter (containing cell bodies and interneurons) and white matter (containing axons).

The spinal cord also controls motor reflexes. These reflexes are quick, unconscious movements, such as 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.