The Nervous System (Pgs. 196-217)

The Nervous System Organs of the nervous system are divided into Central Nervous System (CNS) Peripheral Nervous System (PNS)

Vertebrate Nervous System Central Nervous System Brain Spinal Cord Peripheral Nervous system – cranial & spinal nerves & ganglia Somatic (voluntary) – connect to skin and Skeletal Muscles Autonomic (involuntary, homeostatic control) – cardiac muscle, smooth muscle, and glands Sympathetic – dominates in times of stress; “fight or flight” syndrome (increases heart rate, blood pressure, breathing) Parasympathetic – acts as counterbalance, conserves energy (decreases heart rate, blood pressure, breathing rate)

Anatomical and Functional Organizaton

Central Nervous System Brain – largest & most complex part of NS. Contains nerve centers associated with sensations. Issues motor commands & carries on higher mental functions: Fig. 8-9, 8-10

Brain

Brain stem Extends from base of the cerebrum to the spinal cord Consists of midbrain, pons, & medulla oblongata Medulla oblongata – transmits all ascending & descending impulses, & contains several vital & nonvital relex centers (cardiac, respiratory, vasomotor) Pons- transmits impulses between the cerebrum & other parts of the NS, and contains centers that help regulate the rate & depth of breathing Midbrain – contains reflex centers associated with eye & head movements (visual & auditory impulses)

Diencephalon Thalamus – central relay station for incoming sensory impulses; emotions and alerting or arousal mechanisms Hypothalamus – maintain homeostasis, regulation of body temp., water balance, sleep-cycle control, appetite & sexual arousal (**very important gland of the endocrine system)

Cerebellum Consists of two hemispheres that are connected by the vermis Composed of white matter surrounded by a thing cortex of gray matter Functions primarily as a reflex center in the co-ordination of skeletal muscle movements & the maintenance of equilibrium

Cerebrum Two hemisphere connected by the corpus callosum Surface marked by convolutions (ridges) and gyri (grooves); 4 lobes each hemisphere – frontal, parietal, temporal, occipital Composed of thin layer of gray matter near surface; white matter found deeper Higher brain functions such as thought, reasoning, interpretations of sensory impulses, control of voluntary muscles, and storage of memory

Spinal Cord Nerve column that extends from the brain into the vertebral canal. It terminates at the level between L1 and L2; fig. 8-11, 8-12

Spinal Cord - Structure Composed of 31 segments, each of which gives rise to a pair of spinal nerves Characterized by two deep longitudinal grooves that divide it into right and left halves Has a central core of gray matter that is surrounded by white matter White matter is composed of bundles of myelinated nerve fibers

Spinal Cord - Function Provides a two way communications system between the brain & body parts outside the NS Primary reflex center Ascending tracts carry sensory impulses to the brain; descending tracts carry motor impulses to muscles and glands Many of the fibers in the ascending and descending tracts cross over in the spinal cord or brain

Coverings & Fluid spaces of the Brain & Spinal Cord Meninges – protective membrane covering brain & spinal cord; three layers; Fig. 8-13 Dura mater – outer layer Arachnoid – middle layer Pia mater – inner layer Cerebrospinal fluid occupies the space between the arachnoid & pia mater

Coverings & Fluid spaces of the Brain & Spinal Cord Ventricles – interconnected cavities within the cerebral hemisphere & brain stem that are filled with cerebrospinal fluid: Fig. 8-14

Peripheral Nervous System Consists of cranial and spinal nerves that branch out from the brain and spinal cord to all body parts. Subdivided into somatic & autonomic portions.

Basic Structure of a Nerve

Cranial Nerves Fig. 8-16, Table 8-2 12 pairs that connect the brain to parts in the head, neck, & trunk Most cranial nerves are mixed (sensory & motor); some are pure sensory & other primarily motor Some cranial nerve fibers are somatic & others are autonomic

Spinal Nerves Fig. 8-17, 8-18 31 pairs originate from the spinal cord These mixed nerves provide a two way communication system between the spinal cord & parts in the arm, legs, neck, and trunk Each nerve emerges by a dorsal and ventral root Dorsal (posterior) root contains sensory fibers and is characterized by the presence of a dorsal root ganglion Ventral root contains motor fibers

Spinal Nerves Fig. 8-17, 8-18 Each spinal nerve divides into several branches & then combines with other spinal nerves to form plexuses in which nerve fibers are sorted and recombined so that those fibers associated with a particular part reach it together.

Somatic Nervous System Skeletal muscle & skin Conduction of nerve impulse is all the way from the spinal cord or brain to the effector No synapses Fig. 8-19 (left side of picture)

Autonomic Nervous System Portion of nervous system that function without conscious effort Concerned primarily with the regulation of visceral activities that aid in maintaining homeostasis Fig. 8-19 (right side of picture) Made up of motor neurons that conduct impulses to cardiac and smooth muscle tissue & glandular epithelial tissue

Autonomic Nervous System Conduction pathway is a 2 neuron relay – in the ganglia the impulses are integrated before passing out to effectors ANS is divided into 2 divisions Sympathetic Parasympathetic

Sympathetic Prepares the body for stressful and emergency conditions “fight or flight syndrome” Increase in heart rate, breathing, decrease in activities of digestive tract Sympathetic preganglionic dendrites and cell bodies are located in the gray matter of the thoracic and upper lumbar segments of the spinal cord Sympathetic preganglionic axons synapse with many postganglionic neurons in sympathetic ganglion & these postganglionic neurons frequently terminate in widely separated organs; therefore, sympathetic responses are usually widespread

Parasympathetic Most active under ordinary conditions Decreases heart rate and breathing Opposite (antagonist) to sympathetic Parasympathetic preganglionic dendrites and cell bodies are located in gray matter of brain stem and sacral segments of the spinal cord; axons extend some distance before terminating in parasympathetic ganglion Parasympathetic postganglionic neurons have short axons that extend into nearby structures; therefore parasympathetic stimulation usually involved responses by only one organ

Neurotransmittters – chemical compounds released by axons The different effects of the autonomic divisions are due to the different neurotransmitters released by the postganglionic fibers There are various types of receptors present on the cell membranes and a cell’s response to neurotransmitters depends upon the number and type of receptors present in their membranes. Fig. 8-20