The central nervous system (CNS) comprises the spinal cord and brain (Figure 6.1). The spinal cord is a thick communications tract that relays sensory and motor signals between the peripheral nervous system (PNS) and the brain. The cord also contains intrinsic circuits that support certain muscle reflexes. The brain is a highly sophisticated data processor containing neural circuits that analyze sensory data and then execute appropriate responses via the spinal cord and PNS efferents.
Large portions of the brain are devoted to associative functions that integrate information from the various senses and allow us to assign meaning to sounds, associate smells with specific memories, and recognize objects and faces, for example. Associative regions also provide for abstract thinking, language skills, social interactions, and learning and memory. The human body has bilateral symmetry, and the structures of the spinal cord and brain are, for the most part, mirrored about a midline.
Sensory and motor information generally crosses the midline at some point in its journey between the brain and the periphery. In practice, this means that the left side of the brain controls the right side of the body and vice versa. For the purposes of discussion, the brain can be divided into four principal areas: the brainstem, cerebellum, diencephalon, and the cerebral hemispheres (triencephalon). A full discussion of CNS function is beyond the scope of this book, which focuses on the sensory and motor aspects of CNS function. For more information on higher brain functions, see LIR Neuroscience.
Figure 6.1 Central nervous system.
C1–C8, T1–T12, L1–L5, and S1–S5 are spinal nerves.
II. Spinal Cord
The spinal cord is housed within the vertebral canal. It extends from the foramen magnum at the base of the skull caudally to the second lumbar vertebra.
The vertebral column consists of a series of stacked vertebrae divided anatomically into five regions: cervical, thoracic, lumbar, sacral, and coccygeal. The cervical, thoracic, and lumbar vertebrae are separated by intervertebral disks that allow the bones to articulate, but the sacral and coccygeal vertebrae are fused to form the sacrum and coccyx, respectively. The spinal cord can be divided into 31 named segments. Thirty-one pairs of spinal nerves (one on each side of the body) emerge from corresponding segments (see Figure 6.1). Although the spinal cord terminates before it reaches the sacrum, spinal nerves continue caudally within the vertebral canal until they reach an appropriate exit level.
Rostral and caudal are anatomical terms meaning “beak” (or mouth) and “tail,” respectively. They are commonly used to indicate direction of information flow in the CNS.
Spinal nerves are a component of the PNS. The nerves contain sensory afferent and motor efferent fibers (spinal nerves are sometimes called mixed spinal nerves for this reason) that generally serve tissues on the same level as the nerves. Thus, nerves emerging from the cervical region (C2) control head and neck movements, whereas sacral nerves (S2 and S3) project to the bladder and large intestine.
Somatic and autonomic sensory fibers travel to the spinal cord via peripheral nerves (Figure 6.2). They relay sensations of pain, temperature, and touch from the skin; proprioceptive signals from muscle and joint receptors; and sensory signals from numerous visceral receptors. Multiple peripheral nerves come together to form the posterior root of a spinal nerve and enter the vertebral canal via an intervertebral foramen.
The cell bodies of these nerves cluster within a prominent spinal ganglion located within the foramen. The posterior root then divides into a number of rootlets and joins the spinal cord. Sensory nerves travel rostrally to synapse within nuclei en route to the brain. Branches of sensory afferents may also synapse directly with motor neurons or on interneurons that synapse with motor neurons, which makes local spinal cord–mediated reflexes possible (see 11?III).
Figure 6.2 Sensory and motor pathways.
Motor efferents from the brain travel caudally and synapse with peripheral motor nerves within the spinal cord. These nerves include both somatic and autonomic motor efferents. They leave the spinal cord via anterior rootlets, which join to form an anterior root and then travel out to the periphery alongside sensory fibers in spinal nerves.
The spinal cord’s interior is roughly organized into a butterfly-shaped central area of gray matter surrounded by white matter (Figure 6.3). The white matter contains bundles of nerve fibers with common origins and destinations that relay information between the PNS and the brain. Sensory nerve fibers from the periphery travel rostrally to the brain in discrete ascending tracts. Descending tracts carry bundles of motor efferents from the CNS en route to the periphery. The tracts are named according to their origin and destination. For example, the spinothalamic tract carries pain fibers from the spine upward to the thalamus.
The corticospinal tract carries motor fibers from the cortex downward to the spine. The tracts (also known as fasciculi) are grouped in posterior, lateral, and anterior columns (also known as funiculi). The “wings” of the gray butterflies are divided into posterior and anterior horns and act as synaptic relay stations for information flow between neurons. They contain neuronal cell bodies, which may be clustered in functionally related groups, or nuclei. The gray matter on either side of the cord is connected by commissures containing bundles of fibers that allow for information flow across the midline.
Figure 6.3 Spinal cord organization.
CNS tissue typically appears white or gray in color. White matter is largely composed of myelinated nerve axons (it gets its white color from myelin). Gray matter is composed of cell bodies, dendrites, and unmyelinated axons.
All sensory and motor information flowing to and from the brain passes through the brainstem (Figure 6.4). The brainstem contains several important nuclei that act as relay stations for information flow between brain and periphery. Many of the 12 cranial nerves (CNs) originate from nuclei within the brainstem also (Figure 6.5). The CNs provide sensory and motor innervation to the head and neck and include nerves that mediate vision, hearing, smell, and taste, among many other functions. Intrinsic circuits within the brainstem create control centers that allow for reflex responses to sensory data. The location and functions of these centers are discussed in more detail in Chapter 7. The brainstem can be subdivided anatomically into three areas:
- Medulla: The medulla contains autonomic nuclei involved in the control of respiration and blood pressure and in coordination of swallowing, vomiting, coughing, and sneezing reflexes.
- Pons: The pons helps control respiration.
- Midbrain: The midbrain contains areas involved in controlling eye movements.
Figure 6.4 Brainstem organization.
CSF = cerebrospinal fluid.
Figure 6.5 Cranial nerve (CN) functions.
C1 = first cervical vertebra.
Modified from Krebs, C., Weinberg, J., and Akesson, E. Lippincott’s Illustrated Review of Neuroscience. Lippincott Williams & Wilkins, 2012
CN I and CN II do not originate in the brainstem. CN I, the olfactory nerve, is a sensory nerve that relays information from the olfactory epithelium in the roof of the nasal cavities directly to the olfactory bulb. CN II, the optic nerve, enters the brain at the level of the diencephalon.
The cerebellum fine-tunes motor control and facilitates smooth execution of learned motor sequences (see 11?IV?C). Cerebellar function requires massive integrative and computational capabilities, which is why this small area contains more neurons than the rest of the brain combined, even though it accounts for only ~10% of total brain mass! The cerebellum is attached to the brainstem by three peduncles that contain thick afferent and efferent nerve fiber bundles.
The cerebellum receives sensory data from muscles, tendons, joints, skin, and the visual and vestibular systems and inputs from all regions of the CNS involved in motor control. It also sends signals back to most of these areas and modifies their output (Figure 6.6). Integration of sensory data with motor commands is achieved using feedback and feedforward circuits that include the Purkinje cell, a neuronal type renowned for its immense dendritic tree. The dendrites are sites of information flow from hundreds of thousands of presynaptic neurons. The cerebellar circuits allow movements to be finessed with reference to incoming sensory data, even as they are being executed.
Figure 6.6 Functional relationships among central nervous system components.
Modified from Siegel, A. and Sapru, H.N. Essential Neuroscience. Second Edition. Lippincott Williams & Wilkins, 2011
Cerebellar injury does not cause paralysis, but it does have profound motor effects (ataxia, or an inability to coordinate muscle activity). Patients with cerebellar damage walk with a staggering gait that mimics alcohol intoxication. They may also have slurred speech and difficulties with swallowing and eye movement.
The diencephalon and telencephalon together make up the forebrain. The diencephalon contains two major structures: the thalamus and hypothalamus (Figure 6.7).
Figure 6.7 Thalamus and hypothalamus location.
Modified from Krebs, C., Weinberg, J., and Akesson, E. Lippincott’s Illustrated Review of Neuroscience. Lippincott Williams ; Wilkins, 2012
Sensory information from the periphery passes through the thalamus for processing before reaching a conscious level. Output from the olfactory system is the single exception, insofar as it bypasses the thalamus and feeds raw olfactory data to the cortex directly. The thalamus also controls sleep and wakefulness and is required for consciousness. Damage to the thalamus can result in deep coma. The thalamus is also involved in motor control and has areas that project to the cortical motor regions.
The hypothalamus is a major autonomic nervous system control center that is discussed in detail in Chapter 7. Its functions include control of body temperature, food intake, thirst and water balance, and blood pressure, and it also controls aggression and rage. The hypothalamus exerts control through direct neural connections to autonomic centers in the brainstem, but it also controls the endocrine system. Endocrine control occurs directly through hormonal synthesis and release (oxytocin and antidiuretic hormone) and indirectly by secreting hormones that affect release of pituitary hormones.
The telencephalon, or cerebrum, is the seat of human intellect. It is organized into two cerebral hemispheres comprising the basal ganglia and the cerebral cortex.
A. Basal ganglia
The basal ganglia are a group of functionally related nuclei (Figure 6.8) that work closely with the cerebral cortex and thalamus to effect motor control. Their function is discussed at length in Chapter 11. Major structures within the basal ganglia include the caudate nucleus and putamen (together forming the striatum) and the globus pallidus.
Figure 6.8 0Basal ganglia.
Modified from Siegel, A. and Sapru, H.N. Essential Neuroscience. Second Edition. Lippincott Williams ; Wilkins, 2011
B. Cerebral cortex
The cerebral cortex is involved in conscious thought, awareness, language, and learning and memory.
The cortex comprises a sheet of neural tissue organized in six layers that is folded to accommodate the 15 to 20 billion (1.5–2.0 × 1010) neurons contained within. The folds (gyri) are separated by sulci (grooves). Deep fissures separate the cortex into four lobes: frontal, parietal, occipital, and temporal (Figure 6.9). The lobes contain discrete areas that can be distinguished on a cytoarchitectural basis and that correlate with regions of specialized function.
Figure 6.9 Cortical lobes.
The cortex can be functionally divided into three general areas that stretch across both hemispheres: sensory, motor, and associative.
Sensory regions process information from the sensory organs (see Chapters 8, 9 and 10). Primary sensory regions receive and process information directly from the thalamus. Spatial information is preserved as data flows from the senses to the sensory areas and then accurately maps onto the cortex (topographic mapping). Thus, the pattern of light falling on the retina is faithfully replicated in the pattern of excitation within the primary visual cortex.
Motor areas are involved with planning and executing motor commands. Primary motor areas execute movements. Axons from these areas project to the spinal cord, where they synapse with and excite motor neurons. Supplementary motor areas are involved with planning and fine control of such movements (see Chapter 11).
The majority of cortical neurons are involved in associative functions. Each cortical sensory region feeds information to a corresponding association area. Here, patterns of color, light, and shade are recognized as a human face, for example, or a series of notes can be recognized as coming from a songbird. Other associative areas integrate sensory information from other parts of the brain to allow for higher mental functions. These include abstract thinking, acquisition of language, musical and mathematical skills, and the ability to engage in social interactions.