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By the end of this section, you will be able to:
  • Describe the basic parts and functions of the central nervous system
  • Describe the basic parts and functions of the peripheral nervous system

The central nervous system

The central nervous system (CNS)    is made up of the brain and spinal cord and is covered with three layers of protective coverings called meninges    (“meninges” is derived from the Greek and means “membranes”) ( [link] ). The outermost layer is the dura mater, the middle layer is the web-like arachnoid mater, and the inner layer is the pia mater, which directly contacts and covers the brain and spinal cord. The space between the arachnoid and pia maters is filled with cerebrospinal fluid (CSF)    . The brain floats in CSF, which acts as a cushion and shock absorber.

Illustration shows the three meninges that protect the brain. The outermost layer, just beneath the skull, is the dura mater. The dura mater is the thickest meninx, and blood vessels run through it. Beneath the dura mater is the arachnoid mater, and beneath this is the pia mater.
The cerebral cortex is covered by three layers of meninges: the dura, arachnoid, and pia maters. (credit: modification of work by Gray's Anatomy)

The 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, cerebellum, brainstem, and retinas. The outermost part of the brain is a thick piece of nervous system tissue called the cerebral cortex    . The cerebral cortex, limbic system, and basal ganglia make up the two cerebral hemispheres. A thick fiber bundle called the corpus callosum    (corpus = “body”; callosum = “tough”) connects the two hemispheres. 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 verbally identify it.

Each hemisphere contains regions called lobes that are involved in different functions. Each hemisphere of the mammalian cerebral cortex can be broken down into four functionally and spatially defined lobes: frontal, parietal, temporal, and occipital ( [link] ).

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.
The human cerebral cortex includes the frontal, parietal, temporal, and occipital lobes.

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Source:  OpenStax, Human biology. OpenStax CNX. Dec 01, 2015 Download for free at http://legacy.cnx.org/content/col11903/1.3
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