Chapter 14: Observations
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Source: Reproduced from Tortora and Derrickson ([199]) with permission of John Wiley & Sons.
Anatomy and physiology
The nervous system is the most complex of the body's systems and is responsible for the co‐ordination of all body functions and for adapting to changes in internal and external environments. The activities of the nervous system can be grouped into three basic functions (Tortora and Derrickson [199]):
- sensory (input): detection of internal or external stimuli; this information is carried to the spinal cord and the brain through the cranial and spinal nerves
- integrative (process): analysis of sensory information and initiating appropriate responses
- motor (output): eliciting an appropriate motor response by activating effectors (muscles and glands) through the cranial and spinal nerves.
The intricate network of neurones and neuroglia that comprise the nervous system are divided into two main subdivisions: the central nervous system and the peripheral nervous system.
The central nervous system
The central nervous system consists of the brain and spinal cord (Tortora and Derrickson [199]).
The brain
Located within the skull, the brain is the control centre for registering sensations, correlating them with one another and stored information, making decisions and taking actions; it is also the centre for intellect, emotions, behaviour and memories (Tortora and Derrickson [199]). The adult brain consists of four regions: the brainstem, the cerebellum, the diencephalon and the cerebrum (Figure 14.46).
Figure 14.46 The brain. Source: Reproduced from Peate and Wild ([157]) with permission of John Wiley & Sons.
The cerebrum is the largest part of the brain and provides us with the ability to read, write and speak; to make calculations; and to remember the past, plan for the future and imagine things (Wilkinson et al. [215]).
The cerebral hemispheres contain the greatest mass of brain tissue. Each hemisphere is subdivided into several lobes named after the bones that cover them: frontal, parietal, temporal and occipital (Figure 14.47). Each of these lobes has a particular function:
- The frontal lobe is located at the front of the brain and is associated with cognitive function (orientation, memory, insight, judgement, calculation and abstraction), expressive language (verbal and written) and voluntary motor function (through skeletal muscle).
- The parietal lobe is located in the middle section of the brain and is associated with sensory function, including integration of sensory information; awareness of body parts; interpretation of touch, pressure and pain; and recognition of object size, shape and texture.
- The occipital lobe is located at the back of the brain; it is associated with interpreting visual stimuli and receives impulses from the optic nerve.
- The temporal lobe is located in the bottom section of the brain and is primarily associated with hearing, speech, behaviour and memory (Adam et al. [2]).
Figure 14.47 The cerebrum. Source: Reproduced from Tortora and Derrickson ([199]) with permission of John Wiley & Sons.
The spinal cord
The spinal cord is a cylindrical mass of nerve tissue encased within the vertical canal of the vertebral column; it extends from the medulla oblongata of the brainstem to the superior border of the second lumbar vertebra (Tortora and Derrickson [199]). It contains important motor and sensory nerve pathways that exit and enter the cord through anterior and posterior nerve roots and spinal and peripheral nerves, and it also mediates reflex activity of the deep tendons from the spinal nerves (Bickley [18]). The spinal cord is divided into five segments: cervical, from C1 to C8; thoracic, from T1 to T12; lumbar, from L1 to L5; sacral, from S1 to S5; and coccygeal (Bickley [18]) (Figure 14.48).
Figure 14.48 External anatomy of the spinal cord and the spinal nerves (posterior view). Source: Reproduced from Tortora and Derrickson ([199]) with permission of John Wiley & Sons.
The peripheral nervous system
The cranial nerves
There are 12 pairs of cranial nerves that emerge from the brain. They are named according to their distribution and are numbered I to XII in order of their attachment to the brain (Tortora and Derrickson [199]). Cranial nerves II–XII arise from the diencephalon and the brainstem (Table 14.18). Cranial nerves I and II are actually fibre tracts emerging from the brain (Tortora and Derrickson [199]). Some cranial nerves are limited to general motor or sensory functions whereas others are specialized, enabling smell, vision or hearing (I, II and VIII) (Bickley [18]).
Table 14.18 Summary of cranial nerves
| Cranial nerve | Components | Principal functions |
|---|---|---|
| Olfactory (I) | Special sensory | Olfaction (smell) |
| Optic (II) | Special sensory | Vision (sight) |
| Oculomotor (III) | Motor | |
| Somatic | Movement of eyeballs and upper eyelid | |
| Motor (autonomic) | Adjusts lens for near vision (accommodation) | |
| Constriction of pupil | ||
| Trochlear (IV) | Motor | |
| Somatic | Movement of eyeballs | |
| Trigeminal (V) | Mixed | |
| Sensory | Touch, pain, and thermal sensations from scalp, face, and oral cavity (including teeth and anterior two‐thirds of tongue) | |
| Motor (branchial) | Chewing and controls middle ear muscle | |
| Abducens (VI) | Motor | |
| Somatic | Movement of eyeballs | |
| Facial (VII) | Mixed | |
| Sensory | Taste from anterior two‐thirds of tongue | |
| Touch, pain, and thermal sensations from skin in external ear canal | ||
| Motor (branchial) | Control of muscles of facial expression and middle ear muscle | |
| Motor (autonomic) | Secretion of tears and saliva | |
| Vestibulocochlear (VIII) | Special sensory | Hearing and equilibrium |
| Glossopharyngeal (IX) | Mixed | |
| Sensory | Taste from posterior one‐third of tongue | |
| Proprioception in some swallowing muscles | ||
| Monitors blood pressure and oxygen and carbon dioxide levels in blood | ||
| Touch, pain, and thermal sensations from skin of external ear and upper pharynx | ||
| Motor (branchial) | Assists in swallowing | |
| Motor (autonomic) | Secretion of saliva | |
| Vagus (X) | Mixed | |
| Sensory | Taste from epiglottis | |
| Proprioception from throat and voice box muscles | ||
| Monitors blood pressure and oxygen and carbon dioxide levels in blood | ||
| Touch, pain, and thermal sensations from skin of external ear | ||
| Sensations from thoracic and abdominal organs | ||
| Motor (branchial) | Swallowing, vocalization, and coughing | |
| Motor (autonomic) | Motility and secretion of gastrointestinal organs | |
| Constriction of respiratory passageways | ||
| Decreases heart rate | ||
| Accessory (XI) | Motor | |
| Branchial | Movement of head and pectoral girdle | |
| Hypoglossal (XII) | Motor | |
| Somatic | Speech, manipulation of food, and swallowing | |
The peripheral nerves
In addition to the cranial nerves, the peripheral nervous system includes spinal and peripheral nerves that carry impulses to and from the spinal cord. The 31 pairs of spinal nerves are named and numbered in accordance with the region and level of the spinal cord from which they emerge: eight cervical, twelve thoracic, five lumbar, five sacral and one coccygeal. The spinal nerves are typically connected to the spinal cord by a posterior root (containing sensory fibres) and an anterior root (containing motor fibres) (Tortora and Derrickson [199]).
