Unit 7: Nervous System

3 functions of the nervous system

3 functions of the nervous system

sensory input, integration, motor output

Sensory input (gathering info)

sensory receptors monitor changes called stimuli occurring inside and outside the body

Integration

nervous system processes and interprets sensory input and decides whether an action is needed; interneurons

Motor output

response or effect activates muscles or glands

Structural classification of the nervous system

central nervous system and peripheral nervous system

Functional classification of the nervous system

sensory (afferent) division and motor (efferent) division

Central nervous system (CNS)

includes brain and spinal cord and is involved in integration (command center), interpretation of incoming sensory info, and issues outgoing instructions

Peripheral nervous system (PNS)

includes nerves extending from the brain and spinal cord which serve as communication lines among sensory organs, brain, spinal cord, glands, and muscles

Sensory (afferent) division

nerve fibers that carry info to the CNS; somatic sensory (afferent) fibers carry info from skin, skeletal muscles, and joints

Motor (efferent) division

nerve fibers that carry impulses away from CNS organs to effector organs (muscles and glands); further divided into somatic and autonomic nervous systems

Somatic nervous system

consciously (voluntarily) controls skeletal muscles

Autonomic nervous system

automatically (involuntarily) controls smooth and cardiac muscles and glands; divided into sympathetic and parasympathetic nervous systems

Nervous tissue

composed of two types of cells; neurons and neuroglia (glial cells) which are supporting cells in CNS that support, insulate, and protect neurons; also resemble neurons, unable to conduct nerve impulses, and never lose ability to divide

Astrocytes

abundant star-shaped cells that brace and anchor neurons to blood capillaries, determine permeability, and exchanges between blood capillaries and neurons, protect neurons from harmful substances in blood and control chemical environment of brain

Microglia

spiderlike phagocytes that monitor health of nearby neurons and dispose of debris

Ependymal cells

line cavities of brain and spinal cord; cilia assist with circulation of cerebrospinal fluid

Oligodendrocytes

wrap around nerve fibers in CNS and produce myelin sheaths

Schwann cells

PNS glial cells that form myelin sheath around nerve fibers in PNS

Satellite cells

protect and cushion neuron cell bodies

Neuron

nerve cell specialized to transmit messages (nerve impulses)

Cell body

metabolic center of neuron; nucleus with large nucleolus, nissl bodies (rough ER); neurofibrils which are intermediate filaments that maintain cell shape

Dendrites

conduct impulses toward cell body; hundreds

Axons

conduct impulses away from cell body; neurons have one at axon hillock; end in axon terminals which contain vesicles with neurotransmitters; axon terminals separated from next neuron by gap

Synaptic cleft

gap between axon terminals and next neuron

Synapse

functional junction between nerves where nerve impulse is transmitted

Myelin sheaths

myelin is white fatty material covering axons that protects and insulates fibers and speeds nerve impulse transmission; Schwann cells (PNS) or Oligodendrocytes (CNS) wrap axons to form myelin sheath

Neurilemma

part of Schwann cell external to myelin sheath

Nodes of ranvier

gaps in myelin sheath along the axon

Nuclei

clusters of cell bodies in CNS

Ganglia

collections of cell bodies in PNS

Tracts

bundles of nerve fibers in CNS

Nerves

bundles of nerve fibers in PNS

White matter

collections of myelinated fibers (tracts)

Gray matter

mostly unmyelinated fibers and cell bodies

Sensory (afferent) neurons

carry impulses from sensory receptors to CNS; receptors include cutaneous sense organs which detect pain, temperature, touch, and pressure; proprioceptors in muscles and tendons detect stretch

Motor (efferent) neurons

carry impulses from CNS to viscera and/or muscles and glands

Interneurons (association neurons)

cell bodies located in CNS that connect sensory and motor neurons

Multipolar neurons

many extensions from cell body, all motor and interneurons, most common structural types

Bipolar neurons

one axon and one dendrite located in special sense organs such as nose and eye; rare in adults

Unipolar neurons

short single process leaving cell body that conduct impulses both toward and away from cell body; sensory neurons found in PNS ganglia

Irritability

ability to respond to a stimulus and convert it to a nerve impulse

Conductivity

ability to transmit the impulse to other neurons, muscles, or glands

Electrical conditions of a resting neuron’s membrane

plasma membrane at rest is inactive (polarized); fewer + ions inside neuron’s plasma membrane than outside (K+ inside cell and Na+ outside cell); polarized membrane more permeable to K+ than Na+; as long as inside more negative than outside, cell remains inactive

Action potential initiation and generation

stimulus changes permeability of neuron’s membrane to Na ions; Na channels now open and Na diffuses into neuron; inward rush of Na ions changes polarity at that site causing depolarization; graded potential (localized depolarization) exists where inside of membrane is more + than outside; if strong enough stimulus and sodium influx great enough, local depolarization activates neuron to conduct an action potential

Propagation of the action potential

if enough Na+ enters cell, action potential starts and is propagated over the entire axon; all-or-none response where nerve impulse either propagated or not; fibers with myelin sheaths conduct nerve impulses more quickly

Repolarization

membrane permeability changes again becoming impermeable to Na+ ions and permeable to K+ ions causing K+ ions to rapidly diffuse out of neuron, repolarizing the membrane; involves restoring inside of the membrane to - charge and outer to +; initial conditions of Na and K ions are restored using sodium-potassium pumps which uses ATP; 3 Na+ ions ejected from cell and 2 K+ ions returned to cell; until complete, neuron can’t conduct another impulse

Step 1 of transmission of the signal at synapses

when action potential reaches axon terminal, electrical charge opens calcium channels

Step 2 of transmission of the signal at synapses

calcium causes tiny vesicles containing neurotransmitter chemical to fuse with axonal membrane

Step 3 of transmission of the signal at synapses

entry of calcium into axon terminal causes porelike openings to form, releasing neurotransmitter into synaptic cleft

Step 4 of transmission of the signal at synapses

neurotransmitter molecules diffuse across synaptic cleft and bind to receptors on membrane of next neuron

Step 5 of transmission of the signal at synapses

if enough neurotransmitter released, a graded potential is generated; eventually an action potential will occur beyond synapse

Step 6 of transmission of the signal at synapses

electrical changes prompted by neurotransmitter binding are brief; neurotransmitter quickly removed from synapse either by reuptake or enzymatic activity; transmission of impulse is electrochemical (transmission down neuron is electrical and transmission to next neuron is chemical)

Reflexes

rapid, predictable, and involuntary responses to stimuli

Reflex arcs

neural pathways reflexes occur over

Somatic reflex

reflexes that stimulate the skeletal muscles and are involuntary although skeletal muscle normally under voluntary control; ex. pulling hard away from object

Autonomic reflex

regulation of smooth muscles, heart and blood pressure, glands and digestive system

5 elements of reflex arc

1. sensory receptor reacts to a stimulus

2. sensory neuron carries message to integration center

3. integration center (CNS) processes info and directs motor output

4. motor neuron carries message to effector

5. effector organ is the muscle or gland to be stimulated

Two-neuron reflex arcs

simplest type; ex. patellar reflex

Three-neuron reflex arcs

consists of 5 elements; receptor, sensory neuron, interneuron, motor neuron, and effector; ex. flexor (withdrawal) reflex

Regions of the brain

cerebral hemispheres, diencephalon, brain stem, and cerebellum

Cerebral hemispheres

paired superior parts of brain that include more than half of brain mass; surface is made of ridges (gyri) and grooves (sulci); fissures are deeper grooves; lobes named for cranial bones that lie over them; made up of cortex, white matter, and basal nuclei (deep pockets of gray matter)

Cortex (gray matter)

localizes and interprets sensory inputs, controls voluntary and skilled skeletal muscle activity, and acts in intellectual and emotional processing

Basal nuclei

subcortical motor centers that help control skeletal muscle movements; instead of gray matter, buried with white matter of cerebrum

Diencephalon (inter brain)

on top of brain stem; enclosed by cerebral hemispheres; includes thalamus, hypothalamus, and limbic system

Thalamus

relays sensory impulses to cerebral cortex, relays impulses between cerebral motor cortex and lower motor centers; involved in memory; encloses third ventricle

Hypothalamus

chief integration center of autonomic nervous system that regulates body temperature, food intake, water balance, and thirst; regulates hormonal output of anterior pituitary gland and acts as an endocrine organ (producing ADH and oxytocin); makes up floor of diencephalon

Limbic system

includes cerebral and diencephalon structures; mediates emotional response; involved in memory processing

Brain stem

provides pathway for ascending and descending tracts; produce programmed behaviors key to survival; includes midbrain, pons, and medulla oblongata

Midbrain

contains visual and auditory reflex centers; subcortical motor centers, and nuclei for cranial nerves III and IV; contains projection fibers (ex. fibers of pyramidal tracts); mamillary bodies to pons interiorly; cerebral aqueduct connects 3rd and 4th ventricles; 2 bulging fiber tracts, cerebral peduncles, convey ascending and descending impulses; 4 rounded protrusions

Pons

rounded structure protruding just below midbrain and mostly composed of fiber tracts and includes nuclei; relays info from cerebrum to cerebellum; cooperates with medullary centers to control respiratory rate and depth; contains nuclei of cranial nerves V-VII; contains projection fibers; involved in control of breathing

Medulla oblongata

most inferior part of brain stem that merges into spinal cord that relays ascending sensory pathway impulses from skin and proprioceptors; contains nuclei controlling heart rate, blood vessel diameter, respiratory rate, vomiting, etc.; relays sensory info to cerebellum, contains nuclei of cranial nerves VII-XII; contains projection fibers, site of crossover of pyramids; includes important fiber tracts and 4th ventricle lies posterior to pons and medulla

Epithalamus

forms roof of third ventricle; houses body (endocrine gland); includes choroid plexus (forms cerebrospinal fluid)

Reticular formation

diffuse mass of gray matter along brain stem; maintains cerebral cortical alertness, filters out repetitive stimuli, and helps regulate skeletal and visceral muscle activity

Cerebellum

processes info from cerebral motor cortex, proprioceptors, and visual and equilibrium pathways; provides instructions to cerebral motor cortex and subcortical motor centers resulting in smooth coordinated skeletal muscle movements; responsible for proper balance and posture

Cerebral cortex

primary somatic sensory area located in the parietal lobe posterior to central sulcus and receives impulses from the body’s sensory receptors (pain, temp, light touch); sensory homunculus is spatial map; left side of primary somatic sensory area receives impulses from right

Primary motor area

located anterior to central sulcus in frontal lobe and allows us to consciously move skeletal muscles; motor neurons form pyramidal (corticospinal) tract which descends to spinal cord; motor homunculus is spatial map

Broca’s area

motor speech area involved in ability to speak; usually left hemisphere at base of precentral gyrus

Other specialized areas of cerebral cortex

anterior association area (frontal lobe), posterior association area (posterior cortex), speech area (sounding out words)

Cerebral white matter

composed of fiber tracts deep to gray matter; corpus callosum connects hemispheres, tracts known as commissures; association fiber tracts connect areas within a hemisphere, projection fiber tracts connect cerebrum with lower cvs centers

Reticular activating system (RAS)

plays role in awake/sleep cycles and consciousness

Cerebrum

two hemispheres with convoluted surfaces; outer cortex of gray matter and inner region of white matter; controls balance; provides precise timing for skeletal muscle activities and coordination of body movements; fibers connect to cerebellum from inner ear, eye, proprioceptors of skeletal muscles and more

Protection of CNS

in addition to bony protection, also protected by meninges, cerebrospinal fluid (CSF), and blood-brain barrier

Meninges

dura mater, arachnoid layer, pia mater

Dura mater

outermost leathery layer; double-layered external covering made up of the periosteal layer attached to the inner surface of the skull and meningeal layer, the outer covering of the brain; folds inward in several areas (falx cerebri and tentorium cerebelli)

Arachnoid layer

middle layer; weblike extensions span the subarachnoid space to attach to pia mater; subarachnoid granulations protrude through dura mater and absorb cerebrospinal fluid into venous blood

Pia mater

internal layer that clings to surface of brain and spinal cord

Cerebrospinal fluid

similar to blood plasma in composition; formed continually by choroid plexuses which are capillaries in ventricles of brain; forms watery cushion to protect brain and spinal cord; circulated in arachnoid space, ventricles, and central canal of spinal cord

CSF circulation

CSF produced by choroid plexus of each ventricle and then flows through ventricles and into subarachnoid space via the median and later apertures; some CSF flows through central canal of spinal cord; CSF then flows through subarachnoid space and is absorbed into the dural venous sinuses via arachnoid villi

Blood-brain barrier

includes least permeable capillaries of body which allow water, glucose, and amino acids to pass through capillary walls; excludes many potentially harmful substances from entering brain (waste); useless as barrier againsts some substances

Concussion

slight brain injury; typically little permanent brain damage occurs

Contusion

marked nervous tissue destruction occurs; coma may occur

Death

may occur after head blows due to intracranial hemorrhage or cerebral edema

Cerebrovascular accident or stroke

results when blood circulation to a brain area is blocked and brain tissue dies; loss of some functions or death may result; Hemiplegia and Aphasia

Hemiplegia

one-sided paralysis

Aphasia

damage to speech center in left hemisphere

Transient ischemic attack (TIA)

temporary brain ischemia (restriction of blood flow); numbness, temporary paralysis, impaired speech

Spinal cord

extends from foramen magnum of skull to first or second lumbar vertebra; provides two-way conduction pathway to and from the brain; protected by vertebrae and meninges; 31 pairs of spinal nerves arise from spinal cord

Cauda equina

collection of spinal nerves at inferior end

Gray matter of spinal cord and spinal roots

internal gray matter is mostly cell bodies; posterior (dorsal) horns house interneurons which receive info from sensory neurons in dorsal root; cell bodies housed in dorsal root ganglion; ventral (anterior) horns house motor neurons of somatic (voluntary) nervous system which send motor info out ventral root; gray matter surround central canal filled with cerebrospinal fluid

White matter of spinal cord

composed of myelinated fiber tracts; 3 regions: dorsal, lateral, ventral columns; sensory (afferent) tracts conduct impulses toward brain; motor (efferent) tracts carry impulses from brain to skeletal muscles