NERVOUS SYSTEM

Introduction Most highly organized system of the body Fast, complex communication system that regulates thoughts, emotions, movements, impressions, reasoning, learning, memory, choices Basic Characteristics Master control system Master communication system Regulates, maintains homeostasis

Major Structures and Divisions Central nervous system (CNS) Peripheral nervous system (PNS) Automatic Nervous System Major structures are the brain, spinal cord, nerves and sensory organs Organization of the Nervous System – CNS, PNS, and ANS

Functions Monitors change (stimuli) - sensory input Integrates impulses - integration Effects responses - motor output The function of the nervous system is: To sense, interpret, and respond to internal and external environmental changes in order to maintain a steady state in the body’s homeostasis

Histology of Nervous Tissue Basic Characteristics Highly cellular 2 types of cells - neurons and supporting cells (neuroglia)

Neuroglia Characteristics Dense network of supporting cells for nerve tissue Over 900 billion CAN replace themselves Also known as glia cells (glia = glue) Supportive scaffolding; insulation; neuron health and growth (act as “glue” to support, bind, repair, and protect neurons) Another type of cell in the nervous system besides neurons is Neuoglia or glial cells Special nervous tissue cells that act as “glue” to support, bind, repair, and protect neurons

Neurons: Basic Characteristics Over 100 billion Highly specialized Conduct messages in form of nerve impulses Extreme longevity (>100 years) High metabolic rate 3 functional components in common: receptive/input regions, conducting component/trigger zone, secretory/output component Three types A = afferent (sensory) C = Connective (associative) E = Efferent (motor) -The brain is made of nerve cells (neurons) which communicate with each other at places called synapses. -There are about 100 billion neurons in the human brain and each has about 10,000 contacts with other neurons. Three types A = afferent (sensory) C = Connective (associative) E = Efferent (motor) Neurons Characteristics Excitability: the ability to react to a stimuli, physical or chemical Irritability: sensory adaptation, with prolonged stimulation, irritability is temporarily lost (i.e. smell) Conductivity: the ability to transmit an impulse Nonmyelinated fibers = 0.5 - 1 meter /sec (1 mph) Myelinated fibers = 80 - 130 meters/sec (300 mph)

Neurons: Characteristics Excitability: the ability to react to a stimuli, physical or chemical Irritability: sensory adaptation, with prolonged stimulation, irritability is temporarily lost (i.e. smell) Conductivity: the ability to transmit an impulse Nonmyelinated fibers = 0.5 - 1 meter /sec (1 mph) Myelinated fibers = 80 - 130 meters/sec (300 mph)

The Hand as a Neuron

A Closer Look: Parts of a Neuron nucleus terminal axon soma axon -The cell body is referred to as the soma. It contains the nucleus (DNA) of the cell. If the soma is damaged, a neuron will not recover. -The dendrites are multiple branches that come off the soma. Dendrites Short, tapering diffusely branched (tree-like) fibers Carry impulses toward the cell body from sensory receptors or other axons They receive nerve impulses from other neurons. -The single fiber coming off the soma that is thicker and longer than then dendrites is called the axon. Axons convey information to adjacent neurons. -Axons are Long, slender fiber that transmits impulses away from the cell body One per neuron Short, absent, or long (great toe - lumbar region: 3 to 4 feet = longest cells in body) Long are called nerve fibers Largest in diameter = most rapid conduction Distal tip of axon ends in synaptic knob or end plate are covered in a lipid/protein called the myelin sheath. This sheath insulates the neuron and helps electrical impulses travel smoothly and quickly from one neuron to another. myelin sheath dendrites

Myelin Sheath Whitish, fatty (protein lipoid) segmented covering of axons Myelinated fibers: conduct nerve impulses rapidly; electrical insulation Unmyelinated fibers: conduct impulses slowly White matter: myelinated sheaths around axons of the PNS gives the tissue a white color and forms myelinated nerves (axons = myelinated tracts) Gray matter: concentrations of cell bodies and unmyelinated fibers ( in PNS = ganglia; in CNS = nuclei)

Neurophysiology—Chemical Events At the Synapse

The Synapse the dynamic region between an axon terminal and a receiving neuron space between a terminal axon and receiving neuron is called the synaptic cleft synaptic cleft is where electrochemical transmission takes place, thus communication Impulses from one neuron are transmitted across the synapse to another neuron by a chemical called a neurotransmitter

Vesicles release Neurotransmitters 4 synapse -Chemicals are made in the soma (1 & 2) and stored in small vesicles at the tip of the axon (3). -An electrical impulse (4) travels from the soma to the axon. -Neurotransmitters are released (6) into the synapse (5). -The neurotransmitters stick to receptor proteins in the neighboring dendrite (7) and trigger an impulse that travels down the dendrite, across the soma, and down another axon.

Inactivation of Neurotransmitters The action of neurotransmitters can be stopped by four different mechanisms: Diffusion – neurotransmitters drifts away out of synaptic cleft 2. Enzyme deactivation – specific enzyme changes structure of neurotransmitter so it is not recognized by receptor Reuptake is a common way that action of neurotransmitters norepinephrine, dopamine, and serotonin are stopped.

Inactivation of Neurotransmitters continued Glia cells – astrocytes remove neurotransmitters from synaptic cleft 4. Reuptake – whole neurotransmitter molecule is taken back into axon terminal that released it

Some of the Better Known Neurotransmitters

Acetycholine Contributes to movement, learning, memory processes, and REM sleep Only transmitter between motor neurons and voluntary muscles EXCESS: muscle paralysis or convulsions, sometimes death DEFICIT: memory impairment (Alzheimer’s disease)

Dopamine Used by neurons that control voluntary movements Also used by neurons that are important for learning, attention, thought & emotion EXCESS: irrational thought, delusion, and/or hallucinations (Schizophrenia) DEFICIT: tremors, muscular rigidity, (Parkinson’s disease)

Serotonin Serotonin plays prominent role in regulation of mood, sleep, impulsivity, aggression, and appetite DEFICIT: related to depression, aggressive behavior

Norepinephrine Plays a role in eating, sleep, and mood DEFICIT: related to depression

Gamma-Aminobutyric Acid (GABA) Appears to have inhibitory effects at synapses Contributes to regulation of anxiety & levels of activity ABNORMALITY in GABA may cause epilepsy

Endorphins Opiate-like substances produced in the body Provide relief from pain and produce feelings of pleasure & well-being Drugs such as opium, morphine, and heroin bind with receptors for endorphins Endorphins may explain “runner’s high” experienced by long-distance runners

Divisions of the Nervous System

Central Nervous System – Brain and the Spinal Cord Integrates incoming pieces of sensory information, evaluates the information, and initiates the outgoing responses NO potential for regeneration The CNS - Integrates incoming pieces of sensory information, evaluates the information, and initiates the outgoing responses NO potential for regeneration

Brain Largest structure of the nervous system and one of the largest organs of the body 12 billion neurons and neuroglia The adult human brain weighs between 1300 g and 1400 g (approximately 3 lbs). A newborn human brain weighs between 350 and 400 g. For comparison: elephant brain = 6,000 g chimpanzee brain = 420 g rhesus monkey brain = 95 g beagle dog brain = 72 g cat brain = 30 g rat brain = 2 g Central Nervous System Brain: mass of 12 billion neurons and neuroglia weighing approximately 3 pounds, protected by cranial bones

Brain Covered by three layers of membranes called meninges Dura mater Arachnoid Pia mater Meninges: 3 membranous coverings with spaces between each Dura mater: “tough mother”; strong, white, fibrous tissue that lines the skull bones; has inward extensions into the fissures Epidural space: between the bone and the dura mater Subdural space: between the dura and arachnoid layers Arachnoid: resembles fine cobwebs with fluid (CSF) filling the spaces a. Subarachnoid space: between the arachnoid and pia layers Pia mater: “tender mother”; covers brain and spinal cord surface CSF: bathes the skull, brain, and spinal cord Serves as shock absorber for the brain and spinal cord 400-500 ml produced daily, yet only 140 ml is circulating at any time Circulates through the ventricles and into the central canal and subarachnoid spaces and is absorbed back into the blood Provides nutrients and waste removal for brain tissues It is clear, colorless, and composed of water, 40-60% glucose, NaCl, K+, protein, and a few white blood cells

Structure of the Brain The three main parts of the brain are the cerebrum, cerebellum and brain stem

Cerebrum Largest mass of brain (83% of brain mass); uppermost and least protective layer of the brain; responsible for higher mental functions and distribution of impulses Cerebral cortex: outer layer of gray matter; short and long term memory Cerebral medulla: white matter, conduction pathways Divided into right and left hemispheres (left side governs right side of body, right side governs left side of body) Two hemispheres connected by the corpus callosum Each hemisphere has four lobes Middle regions of the cerebrum are canals, called ventricles Cerebrum: largest mass of brain (83% of brain mass); responsible for higher mental functions and distribution of impulses Cerebral cortex: outer layer of gray matter; short and long term memory Cerebral medulla: white matter, conduction pathways Divided into right and left hemispheres (left side governs right side of body, right side governs left side of body) Cerebellum: below and posterior to cerebrum Right and left hemispheres connected by central vermis Outer gray, inner white forms arbor vitae Coordinates muscular movement, posture, balance, running, walking Damage produces ataxia (lack of coordination due to errors in speed, force, direction of movement Brainstem (damage = coma) Midbrain: upper part of brainstem Controls postural reflexes and walking Visual reflexes and auditory control, 3-4 cranial nerves Pons: a two-way conduction pathway, mixed gray and white fibers Controls inspiration Transverse fibers give it a bridge appearance Reflex mediation for 5-8 cranial nerves Medulla oblongata: the bulb (lowest part before the foramen magnum) made of white and gray fibers called reticular formation 75% of fibers cross here Controls vital functions: respiration and circulation Pyramids: bulges of white tracts on ventral surface Diencephalon: area between cerebrum and midbrain Thalamus: gray matter, relay station for sensory incoming and motor outgoing impulses; damage - increased sensitivity to pain, loss of consciousness Hypothalamus: forms floor of third ventricle Regulates autonomic control Cardiovascular control: dilates/constricts Temperature control Controls appetite: hunger and thirst Water balance GI control: peristalsis, intestinal secretions Emotional states: fear, anger, pleasure, pain, sexual reflexes Sleep control Regulates pituitary secretions CHO and fat metabolism Epithalamus: contains the pineal body/gland (melatonin)

Right vs. Left Hemisphere Left side processes: Speech Analysis Time Sequence                                   Right side processes: Creativity Patterns Spatial Awareness Context It Recognizes: Letters Numbers Words Faces Places Objects There are several myths associated with hemispheric dominance. Myths related to: Handedness - No correlation between hemispheric dominance and being right handed vs. left handed. Intelligence - No evidence to support right-sided hemispheric dominance is superior to left side hemispheric dominance and vice versa. There is no correlation to intelligence. Genetics - No evidence that dominance is hereditary. Gender - Females tend to be more left hemispheric dominant and males tend to be more right hemispheric dominant.

Lobes Frontal: voluntary motor control, learning, planning, L = motor, speech R = non-verbal abilities. Parietal: sensory, distance, size, shape, cognitive/intellectual processes Occipital: vision, visual memory Temporal: auditory, olfactory, speech, judgment, reasoning, will power There are important asymmetrical differences in the frontal lobes. The left frontal lobe is involved in controlling language related movement, whereas the right frontal lobe plays a role in non-verbal abilities. Some researchers emphasize that this rule is not absolute and that with many people, both lobes are involved in nearly all behavior.

Cerebellum Located in the lower back of the cranium; below and posterior to cerebrum Coordinates muscular movement, posture, balance, running, walking Damage produces ataxia (lack of coordination due to errors in speed, force, direction of movement Pons: a two-way conduction pathway, mixed gray and white fibers Controls inspiration Transverse fibers give it a bridge appearance Reflex mediation for 5-8 cranial nerves Medulla oblongata: the bulb (lowest part before the foramen magnum) made of white and gray fibers called reticular formation 75% of fibers cross here Controls vital functions: respiration and circulation Pyramids: bulges of white tracts on ventral surface Diencephalon: area between cerebrum and midbrain Thalamus: gray matter, relay station for sensory incoming and motor outgoing impulses; damage - increased sensitivity to pain, loss of consciousness Hypothalamus: forms floor of third ventricle Regulates autonomic control Cardiovascular control: dilates/constricts Temperature control Controls appetite: hunger and thirst Water balance GI control: peristalsis, intestinal secretions Emotional states: fear, anger, pleasure, pain, sexual reflexes Sleep control Regulates pituitary secretions CHO and fat metabolism Epithalamus: contains the pineal body/gland (melatonin)

Brainstem (damage = coma) Midbrain: upper part of brainstem * Controls postural reflexes and walking * Visual reflexes and auditory control, 3-4 cranial nerves Pons: (literally means “bridge”) a two-way conduction pathway that connect the cerebellum and the cerebrum with the rest of the brain, mixed gray and white fibers * Controls inspiration * Transverse fibers give it a bridge appearance * Reflex mediation for 5-8 cranial nerves Medulla oblongata: the bulb (lowest part before the foramen magnum) made of white and gray fibers called reticular formation * 75% of fibers cross here * Controls vital functions: respiration center, cardiac center, and vasomotor center (constricts or dilates the muscles in the blood vessel’s walls; thus influences BP

Diencephalon: area between cerebrum and midbrain Thalamus: gray matter, relay station for sensory incoming and motor outgoing impulses; damage - increased sensitivity to pain, loss of consciousness Hypothalamus: Regulates autonomic control Cardiovascular control: dilates/constricts Temperature control Controls appetite: hunger and thirst Water balance GI control: peristalsis, intestinal secretions Emotional states: fear, anger, pleasure, pain, sexual reflexes Sleep control Regulates pituitary secretions CHO and fat metabolism

Spinal Cord Deep grooves: anterior median fissure (deeper) and posterior median sulcus 2 bundles of nerve fibers called roots project from each side of cord Dorsal nerve root: sensory afferent fibers Dorsal root ganglion: sensory cell bodies Ventral nerve root: motor efferent fibers The nerve roots join together to form a single mixed nerve called a spinal nerve

Peripheral nervous system (PNS) Made of 12 pairs of cranial nerves and 31 pairs of spinal nerves Afferent (sensory) division Carries impulses toward the CNS Somatic (skin, skeletal muscles, joints) Visceral (organs within the ventral cavity) Connecting (associative) neurons Carry impulses from one neuron to another Efferent (motor) division Somatic: carries information from CNS to skeletal muscles (reflex and voluntary control)

Functions of the Peripheral Nervous System Cranial Nerves (12 pairs: “On Old Olympus’ Towering Top, A Finn and German Grew Some Hops”, “Some Say Marry Money But My Brother Says, Bad Business, Marry Money”) Olfactory: I, sensory, smell Optic: II, sensory, vision Oculomotor: III, motor, eye movement and pupil Trochlear: IV, motor, eye movement, peripheral vision Trigeminal: V, both, ophthalmic maxillary, mandibular (sensory); face and head (motor) Abducens: VI, motor, abducts eye Facial Nerve: VII, both, facial expression, taste, tongue movement Vestibulocochlear: VIII, sensory, hearing and balance Glossopharyngeal: IX, both, tongue, throat, swallowing Vagus: X, both, organ sense (thoracic and abdominal) inhibitor Accessory: XI, motor, spinal accessory, shoulder and head movement Hypoglossal: XII, motor, tongue and throat movement

Spinal Nerves 31 pairs of mixed nerves attached to spinal cord by ventral and dorsal roots 8 cervical 12 thoracic, 5 lumbar 5 sacral, 1 coccygeal Each nerve forms several large branches + rami, which subdivide to four complex networks called plexuses (cervical, brachial, lumbar, sacral) Dermatome: mapping of skin surface of nerve intervention Spinal Nerves: 31 pairs of mixed nerves attached to spinal cord by ventral and dorsal roots 8 cervical (pass through intervertebral foramina), 12 thoracic, 5 lumbar (exit cord at 1st lumbar vertebra, but do not exit spinal canal until reaching their intervertebral foramina so gives cord a “cauda equina” look), 5 sacral, 1 coccygeal Each nerve forms several large branches + rami, which subdivide to four complex networks called plexuses (cervical, brachial, lumbar, sacral) Dermatome: mapping of skin surface of nerve intervention

The Autonomic Nervous System Sympathetic system The sympathetic nerves are stimulated in situations that require action like the fight-or-flight reaction Parasympathetic system The parasympathetic nervous system functions in response to normal everyday situations Autonomic: includes peripheral nerves and ganglia on either side of the spinal cord; responsible for involuntary, regulates smooth muscles, cardiac muscle, glands Sympathetic: exit thoracic area of spinal cord and involved in preparing body for “fight or flight” Parasympathetic: exit cervical and lumbar areas of spinal cord and coordinates the body’s normal resting activities (“resting and digesting-repairing”)