UNIT 7 THE NERVOUS SYSTEM NEURAL TISSUE Contains

UNIT 7 – THE NERVOUS SYSTEM

NEURAL TISSUE Contains 2 kinds of cells: neurons: cells that send and receive signals neuroglia cells (glial cells): that support and protect neurons

ORGANS OF THE NERVOUS SYSTEM Brain and spinal cord Sensory receptors of sense organs (eyes, ears, etc. ) Nerves connect nervous system with other systems

ANATOMICAL DIVISIONS OF THE NERVOUS SYSTEM 1. 2. Central nervous system (CNS) Peripheral nervous system (PNS)

THE CENTRAL NERVOUS SYSTEM

THE CENTRAL NERVOUS SYSTEM (CNS) Consists of the spinal cord and brain Contain neural tissue, connective tissues, and blood vessels

FUNCTIONS OF THE CNS Are to process and coordinate: sensory data: motor commands: higher functions of brain: intelligence, memory, learning, emotion

THE BRAIN

THE HUMAN BRAIN Ranges from 750 cc to 2100 cc Contains almost 98% of the body’s neural tissue Average weight about 1. 4 kg (3 lb)

MENINGES There are 3 layers (meninges) that surround the brain and spinal cord Pia mater – covers the brain Arachnoid – middle layer Dura mater – outermost layer that is attached to the interior of the skull

6 REGIONS OF THE BRAIN Cerebrum Cerebellum Diencephalon Mesencephalon Pons Medulla oblongata

REGIONS OF BRAIN Cerebrum Largest part of brain Controls higher mental functions Memory storage Skeletal Muscle Control Divided into left and right cerebral hemispheres Surface layer of gray matter (neural cortex)

REGIONS OF BRAIN Cerebrum Also Neural Cortex called cerebral cortex Folded surface increases surface area Elevated ridges (gyri) Shallow depressions (sulci) Deep grooves (fissures)

REGIONS OF BRAIN

REGIONS OF BRAIN Cerebellum Second largest part of brain Coordinates repetitive body movements 2 hemispheres Covered with cerebellar cortex

REGIONS OF BRAIN Diencephalon Located under cerebrum and cerebellum Links cerebrum with brain stem Contains thalamus and hypothalamus

REGIONS OF BRAIN Diencephalon (Con’t) Thalamus: relays and processes sensory information Hypothalamus: hormone production emotion thirst/hunger body temp. controls circadian rhythms (day–night cycles)

REGIONS OF BRAIN The Brain Stem Processes spinal infomation between: cord and cerebrum or cerebellum Includes: mesencephalon pons medulla oblongata

REGIONS OF BRAIN The Brain Stem (Con’t) Mesencephalon Also called midbrain Processes sight, sound, and associated reflexes Maintains consciousness Pons Connects cerebellum to brain stem Is involved in somatic and visceral motor control

REGIONS OF BRAIN The Brain Stem (Con’t) Medulla Oblongata Connects brain to spinal cord Relays information Regulates autonomic functions: Cardiovascular, respiratory, and digestive systems

LIMBIC SYSTEM Includes tracts between the cerebrum and diencephalon. Functions are: Establishing emotional states Linking the conscious, intellectual functions of the cerebral cortex with the unconscious functions of the brain stem. Facilitating memory storage and retrieval. The cortices of the brain enable you to do tasks, the limbic system makes you want to do them.

THE LIMBIC SYSTEM Major parts to the limbic system: Amygdaloid body (amygdala) Interface between limbic system, cerebrum, and various systems. Role in: regulating heart rate control of the fight or flight response Linking emotions with specific memories Hippocampus Important in learning especially in the storage and retrieval of new long term memories. Studies have shown that the hypothalamus portion of the limbic system is responsible for emotions of rage, fear, pain, sexual arousal, and pleasure.

THE LIMBIC SYSTEM

Divisions of the cerebrum

THE CEREBRUM Is the largest part of the brain Controls all conscious thoughts and intellectual functions Processes somatic sensory and motor information

THE CEREBRAL CORTEX Figure 14– 12 b

STRUCTURES OF THE CEREBRUM (1 OF 3) Gyri of neural cortex: increase neurons) surface area (number of cortical

STRUCTURES OF THE CEREBRUM (2 OF 3) Longitudinal separates fissure: cerebral hemispheres Lobes: divisions of hemispheres

STRUCTURES OF THE CEREBRUM (3 OF 3) Central sulcus divides: anterior Lateral sulcus divides: frontal Corpus frontal lobe from posterior parietal lobe from temporal lobe Callosum Connects left and right hemispheres

STRUCTURES OF THE CEREBRUM

3 FUNCTIONAL PRINCIPLES OF THE CEREBRUM 1. 2. 3. Each cerebral hemisphere receives sensory information from, and sends motor commands to, the opposite side of body The 2 hemispheres have different functions although their structures are alike Correspondence between a specific function and a specific region of cerebral cortex is not precise

Sensory and Motor Areas of the Cerebrum

MOTOR AND SENSORY AREAS OF THE CORTEX Central sulcus separates motor and sensory areas Figure 14– 15 a

MOTOR AREAS Precentral directs gyrus of frontal lobe: voluntary movements

SENSORY AREAS Postcentral receives gyrus of parietal lobe: somatic sensory information (touch, pressure, pain, vibration, taste, and temperature)

SPECIAL SENSORY CORTEXES Visual cortex: information Auditory from sight receptors cortex: information Olfactory from sound receptors cortex: information Gustatory from odor receptors cortex: information from taste receptors

SPECIAL SENSORY CORTEXES

ASSOCIATION AREAS Sensory association areas: monitor and interpret arriving information at sensory areas of cortex Somatic motor association area (premotor cortex): coordinates motor responses (learned movements)

ASSOCIATION AREAS

SENSORY ASSOCIATION AREAS Somatic sensory association area: interprets input to primary sensory cortex (e. g. , recognizes and responds to touch) Visual association area: interprets Auditory activity in visual cortex association area: monitors auditory cortex

SPECIAL SENSORY CORTEXES

GENERAL INTERPRETIVE AREA Also called Wernicke’s area Present in only 1 hemisphere (left) Receives information from all sensory association areas Coordinates access to complex visual and auditory memories

OTHER INTEGRATIVE AREAS Speech center (Broca’s Area): is associated with general interpretive area coordinates all vocalization functions Prefrontal cortex of frontal lobe: integrates information from sensory association areas performs abstract intellectual activities (e. g. , predicting consequences of actions) Hippocampus sorts and integrates emotions and memories deep portion on the temporal lobe

HEMISPHERIC LATERALIZATION Functional differences between left and right hemispheres Figure 14– 16

THE LEFT HEMISPHERE In most people, left brain (dominant hemisphere) controls: reading, writing, and math decision-making speech and language

THE RIGHT HEMISPHERE Right cerebral hemisphere relates to: senses (touch, smell, sight, taste, feel) recognition (faces, voice inflections)

THE SPINAL CORD

SPINAL CORD Adult Spinal Cord About 18 inches (45 cm) long 1/2 inch (14 mm) wide Ends between vertebrae L 1 and L 2 Carries signals between brain and PNS Responsible for reflexes

THE PERIPHERAL NERVOUS SYSTEM

THE PERIPHERAL NERVOUS SYSTEM (PNS) Includes all neural tissue outside the CNS

FUNCTIONS OF THE PNS 1. 2. Deliver sensory information to the CNS Carry motor commands to peripheral tissues and systems

NERVES Also called peripheral nerves: bundles of axons with connective tissues and blood vessels carry sensory information and motor commands in PNS: cranial nerves—connect to brain spinal nerves—attach to spinal cord

RECEPTORS AND EFFECTORS Receptors: detect changes or respond to stimuli neurons and specialized cells complex sensory organs (e. g. , eyes, ears) Effectors: respond to efferent signals cells and organs

FUNCTIONAL DIVISIONS OF THE PNS Afferent division: carries sensory information from PNS sensory receptors to CNS Efferent carries division: motor commands from CNS to PNS muscles and glands

THE EFFERENT DIVISIONS OF THE PNS Somatic nervous system (SNS) Autonomic nervous system (ANS)

THE SOMATIC NERVOUS SYSTEM (SNS) Includes all somatic motor neurons that innervate skeletal muscles Controls skeletal muscle contractions: voluntary muscle contractions involuntary muscle contractions (reflexes)

THE AUTONOMIC NERVOUS SYSTEM (ANS) Visceral motor neurons innervate all other peripheral effectors: Controls subconscious actions: contractions of smooth muscle and cardiac muscle glandular secretions

DIVISIONS OF THE ANS Sympathetic has division: a stimulating effect Fight or flight Parasympathetic has division: a relaxing effect

NEURONS The basic functional units of the nervous system

FUNCTIONAL CLASSIFICATIONS OF NEURONS Sensory neurons: afferent Motor neurons of PNS neurons: efferent neurons of PNS

FUNCTIONS OF SENSORY NEURONS Monitor internal environment Monitor effects of external environment

MOTOR NEURONS Carry instructions from CNS to peripheral effectors Via efferent fibers (axons)

THE STRUCTURE OF NEURONS Figure 12– 1

THE MULTIPOLAR NEURON Common cell in the CNS: body (soma) short, branched dendrites long, single axon

DENDRITES Highly branched Dendritic spines: many fine processes receive information from other neurons 80– 90% of neuron surface area

THE AXON Are long Carries electrical signal (action potential) to target Axon structure is critical to function

THE SYNAPSE Area where a neuron communicates with another cell Synaptic Cleft The small gap that separates the presynaptic membrane and the postsynaptic membrane Synaptic Is Knob expanded area of axon Contains synaptic vesicles of neurotransmitters

THE SYNAPSE Figure 12– 2

NEUROTRANSMITTERS Are chemical messengers Are released at presynaptic membrane Affect receptors of postsynaptic membrane Are broken down by enzymes Are reassembled at synaptic knob

MYELINATION Increases speed of action potentials Myelin insulates myelinated axons Makes nerves appear white

WHITE MATTER AND GRAY MATTER White matter: regions Gray of CNS with many myelinated nerves matter: unmyelinated areas of CNS

SCHWANN CELLS Form myelin sheath around peripheral axons 1 Schwann cell sheaths 1 segment of axon: many Schwann cells sheath entire axon

SCHWANN CELLS Figure 12– 5 a

ELECTRICAL IMPULSES IN NERVES

ION MOVEMENTS AND ELECTRICAL SIGNALS All cell membranes produce electrical signals by ion movements Transmembrane potential is particularly important to neurons

MEMBRANE PROCESSES IN NEURAL ACTIVITIES Figure 12– 7 (Navigator)

MEMBRANE PROCESSES IN NEURAL ACTIVITIES Resting the potential: transmembrane potential of resting cell Graded potential: temporary, localized change in resting potential caused by stimulus Action is potential: an electrical impulse produced by graded potential propagates along surface of axon to synapse

RESTING POTENTIAL Figure 12– 8 (Navigator)

3 REQUIREMENTS FOR TRANSMEMBRANE POTENTIAL 1. 2. 3. Concentration gradient of ions (Na+, K+) Selectively permeable through channels Maintains charge difference across membrane (resting potential — 70 m. V)

PASSIVE FORCES ACROSS THE MEMBRANE Chemical gradients: concentration Electrical gradients of ions (Na+, K+) gradients: separated charges of positive and negative ions result in potential difference

EQUILIBRIUM POTENTIAL The transmembrane potential at which there is no net movement of a particular ion across the cell membrane

ACTIVE FORCES ACROSS THE MEMBRANE Sodium–potassium are ATPase (Na-K pump): powered by ATP + + carries 3 Na out and 2 K in balances passive forces of diffusion maintains resting potential (— 70 m. V)

CHANGES IN TRANSMEMBRANE POTENTIAL Transmembrane in potential rises or falls: response to temporary changes in membrane permeability resulting from opening or closing specific membrane channels

SODIUM AND POTASSIUM CHANNELS permeability to Na+ and K+ determines transmembrane potential Sodium and potassium channels are either passive or active Membrane

RESTING MEMBRANE POTENTIAL Resting membrane potential is -70 m. V Inside of cell is slightly negative compared to outside of cell Figure 12– 11 (Navigator)

GRADED POTENTIALS Also called local potentials Changes in transmembrane potential: that Any can’t spread far from site of stimulation stimulus that opens a gated channel: produces a graded potential

GRADED POTENTIALS Opening sodium channel produces graded potential Figure 12– 11 (Navigator)

GRADED POTENTIALS: STEP 1 Resting membrane exposed to chemical Sodium channel opens Sodium ions enter the cell Transmembrane potential rises Depolarization occurs

DEPOLARIZATION A shift in transmembrane potential toward 0 m. V

GRADED POTENTIALS: STEP 2 of Na+ through channel Produces local current Depolarizes nearby cell membrane (graded potential) Change in potential is proportional to stimulus Movement

REPOLARIZATION When the stimulus is removed, transmembrane potential returns to normal

EFFECTS OF GRADED POTENTIALS Also called local potentials At cell dendrites or cell bodies: trigger specific cell functions e. g. , exocytosis of glandular secretions At motor end plate: releases ACh into synaptic cleft

ACTION POTENTIALS

ACTION POTENTIALS Propagated changes in transmembrane potential Affect an entire excitable membrane Link graded potentials at cell body with motor end plate actions

INITIATING ACTION POTENTIAL Initial a stimulus: graded depolarization large enough (10 to 15 m. V) to change resting potential (— 70 m. V) to threshold level of voltage-regulated sodium channels (— 60 to — 55 m. V)

ALL-OR-NONE PRINCIPLE If a stimulus exceeds threshold amount: the action potential is the same no matter how large the stimulus Action potential is either triggered, or not

REMEMBER RESTING POTENTIAL

4 STEPS IN THE GENERATION OF ACTION POTENTIALS 1. 2. Depolarization to threshold Activation of Na+ channels: rapid depolarization Na+ ions rush into cytoplasm inner membrane changes from negative to positive

4 STEPS IN THE GENERATION OF ACTION POTENTIALS

4 STEPS IN THE GENERATION OF ACTION POTENTIALS 3. Inactivation of Na+ channels, activation of K+ channels: at +30 m. V Na+ channels close K+ channels open repolarization begins

4 STEPS IN THE GENERATION OF ACTION POTENTIALS 4. Return to normal permeability: K+ channels begin to close: K+ channels finish closing: when membrane reaches normal resting potential ( — 70 m. V) membrane is hyperpolarized to — 90 m. V Na+/K+ Pump returns ions to correct side of membrane, restoring resting membrane potential action potential is over

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SUMMARY: GENERATION OF ACTION POTENTIALS Table 12 -3

THE REFRACTORY PERIOD The time period: from beginning of action potential to return to resting state during which membrane will not respond normally to additional stimuli

2 DIVISIONS OF THE REFRACTORY PERIOD 1. Absolute refractory period: sodium channels open or inactivated no action potential possible

2 DIVISIONS OF THE REFRACTORY PERIOD 2. Relative refractory period: membrane potential almost normal very large stimulus can initiate action potential

POWERING THE SODIUM-POTASSIUM EXCHANGE PUMP maintain concentration gradients of Na+ and K+ over time: To energy (1 ATP for each 2 K+/3 Na+ exchange) requires Without ATP: neurons stop functioning

PROPAGATION OF ACTION POTENTIALS Propagation: moves action potentials generated in axon hillock along entire length of axon a series of repeated actions, not passive flow

2 METHODS OF PROPAGATING ACTION POTENTIALS 1. Continuous propagation: 2. unmyelinated axons Saltatory propagation: myelinated axons

CONTINUOUS PROPAGATION Of action potentials along an unmyelinated axon Affects 1 segment of axon at a time Figure 12– 14

CONTINUOUS PROPAGATION: STEP 1 Action potential in segment 1 Depolarizes membrane to +30 m. V Figure 12– 14 (Step 1)

CONTINUOUS PROPAGATION: STEP 2 Local current Depolarizes second segment to threshold Figure 12– 14 (Step 2)

CONTINUOUS PROPAGATION: STEP 3 Second segment develops action potential First segment enters refractory period Figure 12– 14 (Step 3)

CONTINUOUS PROPAGATION: STEP 4 Local current depolarizes next segment Cycle repeats Action potential travels in 1 direction (1 m/sec) Figure 12– 14 (Step 4)

SALTATORY PROPAGATION Of action potential along myelinated axon Figure 12– 15

SALTATORY PROPAGATION (2 OF 3) Figure 12– 15 (Steps 1, 2)

SALTATORY PROPAGATION (3 OF 3) Figure 12– 15 (Steps 3, 4)

SALTATORY PROPAGATION Faster and uses less energy than continuous propagation Speed: 15 -150 m/s Myelin insulates axon, prevents continuous propagation Local current “jumps” from node to node Depolarization occurs only at nodes http: //www. youtube. com/watch? v=m. Og. HC 5 G 8 Lu. I&feature=fvwrel

WHAT FACTORS AFFECT THE PROPAGATION SPEED OF ACTION POTENTIALS?

AXON DIAMETER AND PROPAGATION SPEED Ion movement is related to cytoplasm concentration Axon diameter affects action potential speed The larger diameter, the lower the resistance

IMPORTANT NEUROTRANSMITTERS Other than acetylcholine: norepinephrine (NE) dopamine serotonin gamma aminobutyric acid (GABA)

NOREPINEPHRINE (NE) Released by adrenergic synapses Excitatory and depolarizing effect Found in brain and portions of ANS

DOPAMINE A CNS neurotransmitter May be excitatory or inhibitory Involved in Parkinson’s disease, cocaine use

SEROTONIN A CNS neurotransmitter Affects attention and emotional states

GAMMA AMINOBUTYRIC ACID (GABA) Inhibitory effect Functions in CNS Not well understood