I. Overview of the nervous system. Fig. 8.1 shows an alternative schematic depiction of the nervous system compared to the one below.

Transcription

1 Chapter 8 The Nervous System. I. Overview of the nervous system. Fig. 8.1 shows an alternative schematic depiction of the nervous system compared to the one below. Nervous System CNS (Brain, Spinal Cord) All neuron cell bodies are found here, except those neuron cell bodies found in the ganglia. PNS (Nerves, Ganglia) Sensory (afferent) Motor (efferent) Somatic Sensory information from the skin, skeletal muscles, and tendons being sent to the CNS. Motor commands sent from the CNS to skeletal muscles. Voluntary. Reflexes of skeletal muscles. Involuntary. Single neuron from the receptor to the CNS; single neuron from the CNS to the effector. May have interneurons within the CNS. Autonomic Sensory and motor information sent to and from smooth muscle, glands and organs. Involuntary. Made up of pre- and postganglionic neurons in the PNS. Sympathetic Fight or flight; causes heightened awareness. Parasympathetic Resting and digesting, normal housekeeping tasks. II. Nervous System Terminology. a. Nerve a bundle of axons arising from many neurons. i. A nerve is classified as either sensory, motor, or mixed, depending on which way action potentials travel down its length. b. 31 pairs of spinal nerves arise from the spinal cord. c. 12 pairs of cranial nerves arise from the brain.

2 d. Ganglia cluster of neuron cell bodies outside the CNS. e. Tract (fiber tract) bundle of myelinated axons in the spinal cord or brain. f. White matter myelinated nerves and nerve tracts; appears white due to myelination. g. Gray matter neuron cell bodies, dendrites, interneurons, non-myelinated axons, and neuroglia; appears gray. h. Nuclei clusters of neuronal cell bodies deep within the brain. i. This term is different from cell nuclei, which references the DNA-containing structures within eukaryotic cells. III. Central nervous system. a. Meninges. Fig i. Three CT coverings that surround the brain and spinal cord: 1. Dura mater. 2. Arachnoid mater. 3. Pia mater. ii. Serve to cover and protect the brain, spinal cord, and their blood vessels; also enclose spaces filled with cerebrospinal fluid (CSF), which helps cushion and nourish the brain. b. Cerebrospinal fluid. i. There are four ventricles (chambers) within the brain: 1. Two lateral ventricles (L & R), one within each cerebral hemisphere. 2. A third ventricle, surrounded by the thalamus and hypothalamus. 3. A fourth ventricle, between the cerebellum, midbrain, and hindbrain. ii. Each ventricle contains specialized capillaries (choroid plexus) and neuroglial cells called ependymal cells that work together to secrete cerebrospinal fluid (CSF). iii. All ventricles are interconnected, and CSF circulates between them, down the central canal of the spinal cord, and between the meninges. iv. The CSF physically supports the brain and spinal cord within the dorsal body cavity and provides a protective cushion for them; also transports dissolved gases, nutrients, wastes, and other materials. c. Blood-brain barrier. i. Controls which blood-borne substances reach the neurons of the brain. ii. Neuroglial cells called astrocytes wrap around capillaries within the brain, to physically and biochemically isolate the CNS. 1. This is because hormones and other chemicals in the blood can alter neuron function. 2. Brain capillaries are relatively impermeable; therefore, substances must pass through the capillary cells rather than between them, as occurs in other tissues. 3. Transport proteins selectively transport glucose and some other water soluble substances across the barrier. 4. Lipid soluble substances easily pass through the barrier; therefore, caffeine, nicotine, alcohol, etc., quickly affect the brain. 5. The barrier helps to protect the brain from extreme shifts in extracellular solute concentrations. IV. Brain: command center. i. Contains about 100 billion neurons. ii. Receives, integrates, stores, and retrieves information. iii. Coordinates body activities by stimulating and inhibiting the activities of the different body parts. iv. Gray matter regions of the brain contain neuroglial cells, neuron cell bodies, dendrites, and unmyelinated axons. v. White matter regions of the brain contain myelinated axons. vi. Housed in the cranium. V. Regions of the brain. Fig. 8.3.

3 a. Forebrain. Most highly developed region of the brain. It includes: i. Cerebrum (2 cerebral hemispheres) large and highly convoluted to allow for maximization of surface area. Ridges = gyri (pl.); gyrus (sing.). Grooves = sulci (pl.); sulcus (sing.). 1. Contain sensory areas for skin senses, vision, hearing, and smell (olfaction), motor areas for voluntary control of movement, and association areas for interpreting sensations, language, thinking, decision making, self-awareness, creativity, abstract thought, and storage of memories. 2. Cerebral cortex thin layer of gray matter along the surface of the cerebral hemispheres. 3. Each hemisphere (R & L) can function separately, but the L. hemisphere responds primarily to signals from the R. side of the body, and vice versa. 4. Signals that coordinate the two hemispheres pass through the corpus callosum (a large tract). 5. Divided into lobes: Fig a. Frontal. b. Parietal. c. Temporal. d. Occipital. e. Insula (deep to temporal lobe). 6. Sensory areas. Fig a. Awareness of sensations is due to the cerebral cortex, with each sense processed in a different region. b. Sensory information from the skin, skeletal muscles, and joints is sent to the primary somatosensory cortex, found on the post-central gyrus of the parietal lobe. i. The green part of the figure shows the sensory homunculus. 7. Motor areas. a. The primary motor cortex, found on the pre-central gyrus of the frontal lobe, sends voluntary motor impulses to the skeletal muscles. i. The blue part of the figure shows the motor homunculus. b. The premotor cortex sits just in front of the primary motor cortex. i. It coordinates learned motor skills that are patterned or repetitive, and will stimulate the primary motor cortex in the pattern needed to perform a learned patterned task. (I.e.: playing a piano). 8. Association areas. a. Next to each sensory area is an association area, which communicates with the sensory, motor, and other areas of the brain to assign meaning to the information and integrate it with stored memories. b. When sensory input has been interpreted, it is sent to the prefrontal cortex, where predictions about possible responses are made, and an appropriate decision is made concerning any necessary response. i. The prefrontal cortex is the most complex association area, enabling us to reason, plan, comprehend abstract concepts; much of our personality is determined by the activities of this area. ii. Thalamus major coordinating center for sensory signals. Processes all sensory impulses (except olfaction) before relaying it to appropriate brain centers in the cerebrum. Involved in sensory experience, motor activity, stimulation of cerebral cortex, and memory. Fig Pineal gland control of some daily (circadian) rhythms; secretes melatonin. iii. Hypothalamus neural-endocrine coordination center of visceral homeostatic activities: water-solute balance/blood pressure, thirst & hunger, temperature control, carbohydrate

4 metabolism, heart rate, breathing rate; involved in emotions (part of limbic system); serves as a master biological clock. iv. Pituitary gland master endocrine gland (controlled by hypothalamus). Control of growth, metabolism, etc.; secretes TSH, GH, FSH, LH, ACTH, PRL, MSH, ADH, OXT. b. Midbrain (part of the brainstem). Responsible for visual and auditory reflex coordination. Contains nerve tracts ascending to the thalamus, descending tracts from the cerebrum and tracts to and from the cerebellum. c. Hindbrain (part of the brainstem). Connects the brain stem (midbrain + hindbrain) to the spinal cord. Contains: i. Pons bridge of nerve tracts from cerebrum to both sides of cerebellum. Also contains longitudinal tracts connecting forebrain to spinal cord. Contains respiratory centers. ii. Medulla oblongata relays sensory information to the thalamus; contains reflex centers involved in respiration, cardiovascular function, coughing reflex, digestion, sleep/wake centers, etc. iii. Cerebellum responsible for unconscious coordination of sensory-motor activity underlying limb movements, maintaining posture and balance, and spatial orientation. Integrates information from the eyes, inner ears, and muscle spindles with motor signals from the forebrain. Stores memory of learned motor patterns. d. Limbic system. Fig i. Plays a key role in our emotional states, and is associated with memory and olfaction. (Smells can trigger memories). Defined on basis of function rather than anatomy. 1. Memory information regarding your unique experiences as an individual. a. Short term memory (storage) lasts a few seconds to a few hours. Limited to bits of information; numbers, words, etc. b. Long term memory (storage) permanently (more or less) stored information; only some information from short term memory is transferred to long term memory. Some of the information stored in long term memory may eventually be lost. i. Facts explicit information: names, faces, words, dates, odors, etc. ii. Skills gained by practicing specific motor activities. e. Reticular formation netlike arrangement of small areas of gray and white matter. Extends from the upper spinal cord through the brainstem and radiates throughout the cerebrum. i. Has both sensory and motor functions. ii. Its primary sensory function is to alert the cerebral cortex to incoming sensory signals. iii. The reticular activating system (RAS), a part of the reticular formation, consists of fibers that project into the cerebral cortex. Incoming impulses from the ears, eyes, and skin are effective stimulators of the RAS. Its functions include: 1. Maintaining consciousness. 2. Awakening from sleep. 3. Helping to regulate muscle tone. VI. Spinal cord. Fig a. Extends from medulla oblongata to just below the last rib. b. Carries signals between the PNS and the brain. c. Control center for spinal reflexes that do not require input from the brain, and contributes to some autonomic reflexes. d. Housed in vertebral foramina. e. White matter grouped into ascending (sensory) tracts and descending (motor) tracts that are found around the periphery of the cord. f. Gray matter, composed of neuron cell bodies, neuroglia, unmyelinated axons, etc., is found in the interior of the cord (shaped like a butterfly).

5 g. 31 pairs of spinal nerves (part of the PNS) leave the spinal cord via the intervertebral foramina to service the body. VII. Withdrawal reflex (flexor reflex). Fig a. Consists of the following: i. Sensory receptors (free nerve endings) that respond to tissue damage (pain) from the foot. ii. Sensory neuron brings information to: iii. Spinal cord (control center) where the sensory neuron makes a synapse with an interneuron within the gray matter of the cord. The interneuron stimulates: iv. A motor neuron whose cell body is within the gray matter of the cord. The axon of the motor neuron leaves the cord to synapse with an effector (flexor muscles in the thigh). This causes the foot to be pulled away from the offending stimulus. v. The interneuron also synapses with and inhibits a motor neuron that normally excites the antagonistic muscles, causing them to relax. vi. This type of neural circuit, which simultaneously causes contraction of one muscle and the relaxation of its antagonists, is called reciprocal innervation. 1. This prevents conflict between opposing muscles and is vital in coordinating body movements. vii. The interneuron also synapses with other neurons within the cord that send sensory impulses to the brain, making you aware of the pain and movement of the limb. b. The figure in the book is only showing one reflex involved in the scenario above; at the same time, the crossed extensor reflex activates the appropriate muscles in the opposite leg so you don t fall down when you suddenly pull your foot off the ground VIII. The peripheral nervous system. Fig a. The PNS (nerves and ganglia) carry sensory and motor information between the CNS and the rest of the body. b. The spinal cord gives rise to 31 pairs of spinal nerves that service the body. i. Spinal nerves are mixed nerves (carry both sensory and motor commands). ii. Sensory signals enter the cord via the dorsal root. 1. Cell bodies of the sensory neurons reside in the dorsal root ganglia. iii. Motor signals exit the cord via the ventral root. 1. Cell bodies of the motor neurons reside in gray matter of the spinal cord. iv. The union of the dorsal and ventral roots forms the spinal nerve. c. The brain gives rise to 12 pairs of cranial nerves that service the head and some visceral organs. i. Some cranial nerves are strictly sensory, some are strictly motor, and some are mixed nerves. d. The PNS is divided into the somatic and autonomic divisions. i. Somatic division: 1. Carries sensory information from the skin, skeletal muscles, and tendons to the CNS. 2. Carries motor commands from the CNS to skeletal muscles. Voluntarily controlled. 3. Carries motor commands for reflexes of skeletal muscles. Involuntarily controlled. 4. Composed of a single sensory neuron from the receptor to the CNS; also, there is a single neuron from the CNS to the effector. The sensory and motor neurons may synapse with interneurons within the CNS. ii. Autonomic division: Fig Carries sensory and motor information sent to and from smooth muscle, glands, and organs. Involuntarily controlled. 2. Made up of pre- and post- ganglionic neurons. a. Cell bodies of pre-ganglionic neurons reside in the CNS. b. Cell bodies of post-ganglionic neurons reside in ganglia.

6 IX. 3. Involved in maintaining many body homeostasis mechanisms. 4. Subdivided into the sympathetic and parasympathetic divisions. a. Sympathetic division: i. Acts as a unified whole, causing mass activation of all structures it innervates. ii. Prepares the body for fight or flight. b. Parasympathetic division: i. Effects of this division occur more independently from one another. ii. Returns the body to a relaxed state after sympathetic stimulation. iii. Facilitates resting and digesting and is involved in normal housekeeping tasks within the body. c. Most organs have dual innervation via both the sympathetic and parasympathetic divisions; in these cases, the two divisions have opposing effects on that particular organ. Disorders of the nervous system. a. You will not be tested on this material, but it is interesting reading, because a number of familiar ailments are discussed. Study suggestions for this chapter: In the textbook at the end of the chapter, the sections entitled 1) Highlighting the Concepts, 2) Recognizing Key Terms, and 3) Reviewing the Concepts are all good for you to gauge your comprehension and focus your study efforts.