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The Nervous System The nervous system enables the body to react to continuous changes in its internal and external environments. It also controls and integrates the various activities of the body, such as circulation and respiration The Nervous System Structurally : divided into CNS + PNS CNS: PNS: brain + spinal cord 12 pairs of cranial nerves 31 pairs of spinal nerves (8 cervical + 12 thoracic + 5 lumbar + 5 sacral + 1 coccygeal) Functionally : * somatic NS (control voluntary activities) * autonomic NS (control involuntary activities) Central nervous system (CNS) Brain Spinal cord Peripheral nervous system (PNS) Cranial nerves Ganglia outside CNS Spinal nerves Figure 48.19 1 coccygeal CENTRAL NERVOUS SYSTEM The human brain – Contains an estimated 100 billion nerve cells, or neurons Each neuron – May communicate with thousands of other neurons These neurons are supported by Neuroglia CENTRAL NERVOUS SYSTEM Basic nerve cell structure 3 main types of nerve cells sensory neurone relay neurone motor neurone Sensory neurons Carries impulses from receptors e.g pain receptors in skin to the CNS( brain or spinal cord) Relay neuron Carries impulses from sensory nerves to motor nerves. Motor neuron Carries impulses from CNS to effector e.g. muscle to bring about movement or gland to bring about secretion of hormone e.g ADH White matter Reflex arc Sensory neurone Receptors in skin cells Grey matter Relay neurone Motor neurone Effector (muscle) The three stages of information processing Are illustrated in the knee-jerk reflex 2 Sensors detect a sudden stretch in the quadriceps. Sensory neurons convey the information to the spinal cord. 3 Cell body of sensory neuron in dorsal root ganglion 4 The sensory neurons communicate with motor neurons that supply the quadriceps. The motor neurons convey signals to the quadriceps, causing it to contract and jerking the lower leg forward. Gray matter 5 Sensory neurons from the quadriceps also communicate with interneurons in the spinal cord. Quadriceps muscle The reflex is 1 initiated by tapping the tendon connected to the quadriceps 8.4 (extensor) muscle. White matter Hamstring muscle 6 Spinal cord (cross section) Sensory neuron Motor neuron Interneuron The interneurons inhibit motor neurons that supply the hamstring (flexor) muscle. This inhibition prevents the hamstring from contracting, which would resist the action of the quadriceps. Protecting the Brain Meninges 2 THE BRAIN The cerebrum has right and left cerebral hemispheres – That each consist of cerebral cortex overlying white matter and basal nuclei Left cerebral hemisphere Right cerebral hemisphere Corpus callosum Neocortex Figure 48.26 Basal nuclei In humans, the largest and most complex part of the brain –Is the cerebral cortex, where sensory information is analyzed, motor commands are issued, and language is generated A thick band of axons, the corpus callosum – Provides communication between the right and left cerebral cortices The cerebral cortex controls voluntary movement and cognitive functions Each side of the cerebral cortex has four lobes – Frontal, parietal, temporal, and occipital Frontal lobe Parietal lobe Speech Frontal association area Somatosensory association area Taste Reading Speech Hearing Smell Auditory association area Visual association area Vision Figure 48.27 Temporal lobe Occipital lobe THE SPINAL CORD THE SPINAL CORD THE SPINAL CORD PERIPHERAL NERVOUS SYSTEM consists of the cranial and spinal nerves and their associated ganglia Cranial Nerves: - 12 pairs that leave the brain and pass through foramina in the skull. - All are distributed in the head and neck except the Xth (vagus), which also supplies structures in the thorax and abdomen PERIPHERAL NERVOUS SYSTEM Spinal Nerves: - A total of 31 pairs leave the spinal cord and pass through intervertebral foramina in the vertebral column -The spinal nerves are named according to the region of the vertebral column with which they are associated: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal. PERIPHERAL NERVOUS SYSTEM Spinal Nerves: - Each spinal nerve is connected to the spinal cord by two roots: the anterior root and the posterior root. The anterior root carrying nerve impulses away from the central nervous system ( efferent fibers) go to skeletal muscle and cause them to contract are called motor fibers. Their cells of origin lie in the anterior gray horn of the spinal cord. PERIPHERAL NERVOUS SYSTEM Spinal Nerves: -The posterior root carry impulses to the central nervous system and are called afferent fibers concerned with conveying information about sensations of touch, pain, temperature, and vibrations, (sensory fibers). The cell bodies of these nerve fibers are situated in a swelling on the posterior root called the posterior root ganglion THORACIC SEGMENT PLEXUSES At the root of the limbs, the anterior rami join one another to form complicated nerve plexuses . The cervical and brachial plexuses are found at the root of the upper limbs, The lumbar and sacral plexuses are found at the root of the lower limbs. AUTONOMIC NERVOUS SYSTEM concerned with the innervation of involuntary structures such as the heart, smooth muscle, and glands distributed throughout the central and peripheral nervous system. divided into two sympathetic and parasympathetic and both parts have afferent and efferent nerve fibers. The hypothalamus of the brain controls the autonomic nervous system SYMPATHETIC ANS It prepare the body for an emergency. It accelerates the heart rate, causes constriction of the peripheral blood vessels and raises the blood pressure. redistribution of the blood so that it leaves the areas of the skin and intestine and becomes available to the brain, heart, and skeletal muscle. At the same time, it inhibits peristalsis of the intestinal tract and closes the sphincters. PARASYMPATHETIC ANS aim at conserving and restoring energy. They slow the heart rate, increase peristalsis of the intestine and glandular activity, and open the sphincters. Segmental innervation of skin The area of skin supplied by a single spinal nerve, and therefore a single segment of the spinal cord, is called a dermatome. On the trunk, adjacent dermatomes overlap considerably; Segmental Innervation of Muscle Biceps brachii tendon reflex: C5 and 6 (flexion of the elbow joint by tapping the biceps tendon) Triceps tendon reflex: C6, 7, and 8 (extension of the elbow joint by tapping the triceps tendon) Brachioradialis tendon reflex: C5, 6, and 7 (supination of the radioulnar joints by tapping the insertion of the brachioradialis tendon) Segmental Innervation of Muscle Abdominal superficial reflexes (contraction of underlying abdominal muscles by stroking the skin): Upper abdominal skin T6 to 7, middle abdominal skin T8 to 9, and lower abdominal skin T10 to 12 Patellar tendon reflex (knee jerk): L2, 3, and 4 (extension of the knee joint on tapping the patellar tendon) Achilles tendon reflex (ankle jerk): S1 and S2 (plantar flexion of the ankle joint on tapping the Achilles tendon)