Chapter 7 - Nervous System
Basal nuclei
- "Islands" of gray matter buried deep within the white matter of the cerebrum - Regulate voluntary motor activities by modifying instructions sent to skeletal muscles by the primary motor cortex
Action potential initiation and generation (continued)
- A graded potential (localized depolarization) exists where the inside of the membrane is more positive and the outside is less positive - If the stimulus is strong enough and sodium influx great enough, local depolarization activates the neuron to conduct an action potential (nerve impulse)
Motor output
- A response, or effect, activates muscles or glands
Action potential initiation and generation
- A stimulus changes the permeability of the neuron's membrane to sodium ions - Sodium channels now open, and sodium (Na+) diffuses into the neuron - The inward rush of sodium ions changes the polarity at that site and is called depolarization
CNS glial cells: astrocytes
- Abundant, star-shaped cells - Brace and anchor neurons to blood capillaries - Determine permeability and exchanges between blood capillaries and neurons - Protect neurons from harmful substances in blood - Control the chemical environment of the brain
Other specialized areas
- Anterior association area (frontal lobe) - Posterior association area (posterior cortex) - Speech area (for sounding out words)
Brain stem
- Attaches to spinal cord
Autonomic nervous system (involuntary)
- Automatically controls smooth and cardiac muscles and glands - Further divided into the sympathetic and parasympathetic nervous systems
Sensory (afferent) neurons
- Carry impulses from the sensory receptors to the CNS -Receptors include: -- Cutaneous sense organs in skin -- Proprioceptors in muscles and tendons
Interneurons (association neurons)
- Cell bodies located in the CNS - Connect sensory and motor neurons
Major regions of all neurons
- Cell body (nucleus and metabolic center of the cell - Processes (fibers that extend from the cell body)
Cerebral white matter
- Composed of fiber tracts deep to the gray matter -- Corpus callosum connects hemispheres -- Tracts, such as the corpus callosum, are known as commissures -- Association fiber tracts connect areas within a hemisphere -- Projection fiber tracts connect the cerebrum with lower CNS centers
Three-neuron reflex arc
- Consists of five elements: receptor, sensory neuron, interneuron, motor neuron, and effector - Example: flexor (withdrawal) reflex
Processes (fibers)
- Dendrites - Axons
Diencephalon: thalamus
- Encloses the third ventricle - Relay station for sensory impulses passing upward to the cerebral cortex - Transfers impulses to the correct part of the cortex for localization and interpretation
Brain stem: midbrain
- Extends from the mammillary bodies to the pons inferiorly - Cerebral aqueduct (tiny canal) connects the third and fourth ventricles
Diencephalon: epithalamus
- Forms the roof of the third ventricle - Houses the pineal body (an endocrine gland) - Includes the choroid plexus—forms cerebrospinal fluid
Sensory Input
- Gathering information - Sensory receptors monitor changes, called stimuli occurring inside and outside the body
Transmission of the signal at synapses - step 5
- If enough neurotransmitter is released, a graded potential will be generated -- Eventually an action potential (nerve impulse) will occur in the neuron beyond the synapse
Propagation of the action potential
- If enough sodium enters the cell, the action potential (nerve impulse) starts and is propagated over the entire axon - All-or-none response means the nerve impulse either is propagated or is not - Fibers with myelin sheaths conduct nerve impulses more quickly
Cerebral hemispheres are paired (left and right) superior parts of the brain
- Include more than half of the brain mass - The surface is made of ridges (gyri) and grooves (sulci) - Fissures are deeper grooves - Lobes are named for the cranial bones that lie over them
Repolarization (continued)
- Initial conditions of sodium and potassium ions are restored using the sodium-potassium pump - This pump, using ATP, restores the original configuration - Three sodium ions are ejected from the cell while two potassium ions are returned to the cell - Until repolarization is complete, a neuron cannot conduct another nerve impulse
Central Nervous System - Functions
- Integration; command center - Interprets incoming sensory information - Issues outgoing instructions
- Cerebral cortex (continued) - Broca's area (motor speech area)
- Involved in our ability to speak - Usually in left hemisphere
Functional properties of neurons
- Irritability - Conductivity
CNS glial cells: ependymal cells
- Line cavities of the brain and spinal cord - Cilia assist with circulation of cerebrospinal fluid
- Cerebral cortex (continued) - Primary motor area
- Located anterior to the central sulcus in the frontal lobe - Allows us to consciously move skeletal muscles - Motor neurons form pyramidal (corticospinal) tract, which descends to spinal cord - Motor homunculus is a spatial map
Diencephalon: hypothalamus
- Makes up the floor of the diencephalon - Important autonomic nervous system center -- Regulates body temperature -- Regulates water balance -- Regulates metabolism - Houses the limbic center for emotions - Regulates the nearby pituitary gland - Houses mammillary bodies for olfaction (smell)
Repolarization
- Membrane permeability changes again—becoming impermeable to sodium ions and permeable to potassium ions - Potassium ions rapidly diffuse out of the neuron, repolarizing the membrane - Repolarization involves restoring the inside of the membrane to a negative charge and the outer surface to a positive charge
Protection of CNS
- Meninges - Cerebrospinal fluid (CSF) - Blood-brain barrier
Peripheral Nervous System (PNS)
- Nerves extending from the brain and spinal cord
Integration
- Nervous system processes and interprets sensory input and decides whether action is needed
Axons - conduct impulses AWAY from the cell body
- Neurons have only one axon arising from the cell body at the axon hillock - End in axon terminals, which contain vesicles with neurotransmitters - Axon terminals are separated from the next neuron by a gap
Cell body is the metabolic center of the neuron
- Nucleus with large nucleolus - Nissl bodies -- Rough endoplasmic reticulum - Neurofibrils -- Intermediate filaments that maintain cell shape
Cerebral cortex
- Primary somatic sensory area -- Located in parietal lobe posterior to central sulcus -- Receives impulses from the body's sensory receptors -- Pain, temperature, light touch (except for special senses) - Sensory homunculus is a spatial map - Left side of the primary somatic sensory area receives impulses from right side (and vice versa)
Somatic reflexes
- Reflexes that stimulate the skeletal muscles - Involuntary, although skeletal muscle is normally under voluntary control -- Example: pulling your hand away from a hot object
Autonomic reflexes
- Regulate the activity of smooth muscles, the heart, and glands -- Example: regulation of smooth muscles, heart and blood pressure, glands, digestive system
Supporting cells (called neuroglia, or glial cells, or glia)
- Resemble neurons - Unable to conduct nerve impulses - Never lose the ability to divide
Myelin Sheaths
- Schwann cells - Neurilemma - Nodes of Ranvier - Oligodendrocytes
PNS glial cells
- Schwann cells - Satellite cells
Motor (efferent) 2 subdivisions
- Somatic nervous system - Autonomic nervous system
CNS glial cells: microglia
- Spiderlike phagocytes - Monitor health of nearby neurons - Dispose of debris
Nervous system classifications are based on:
- Structures (structural classification) - Activities (functional classification)
Neuroglia functions
- Support - Insulate - Protect neurons
Nervous tissue is made up of two principal cell types
- Supporting cells - Neurons
Transmission of the signal at synapses - step 6
- The electrical changes prompted by neurotransmitter binding are brief - The neurotransmitter is quickly removed from the synapse either by reuptake or by enzymatic activity - Transmission of an impulse is electrochemical -- Transmission down neuron is electrical -- Transmission to next neuron is chemical
Brain stem: medulla oblongata
- The most inferior part of the brain stem that merges into the spinal cord - Includes important fiber tracts
Electrical conditions of resting neuron's membrane
- The plasma membrane at rest is inactive (polarized) - Fewer positive ions are inside the neuron's plasma membrane than outside -- K+ is the major positive ion inside the cell -- Na+ is the major positive ion outside the cell - As long as the inside of the membrane is more negative (fewer positive ions) than the outside, the cell remains inactive
Brain stems: pons
- The rounded structure protruding just below the midbrain - Mostly composed of fiber tracts - Includes nuclei involved in the control of breathing
Cerebellum
- Two hemispheres with convoluted surfaces - Outer cortex of gray matter and inner region of white matter
Cerebral areas involved in special senses
- Visual area (occipital lobe) - Auditory area (temporal lobe) - Olfactory area (temporal lobe)
Myelin
- White, fatty material covering axons - Protects and insulates fibers - Speeds nerve impulse transmission
CNS glial cells: oligodendrocytes
- Wrap around nerve fibers in the central nervous system - Produce myelin sheaths
Unipolar Neurons
- have a short singe process leaving the cell body - sensory neurons found in PNS ganglia - conduct impulses both toward and away from the cell body
Multipolar neurons
- many extensions from the cell body -- all motor and interneurons are multipolar -- most common structural type
Bipolar neurons
- one axon and one dendrite - located in special sense organs, such as nose/eye - rare in adults
Brain stem: reticuluar formation
- plays a role in awake/sleep cycles and consciousness
Oligodendrocytes
- produce myelin sheaths around axons of the CNS - Lack a neurilemma
Reflexes
- rapid, predictable, and involuntary responses to stimuli - occur over neural pathways called reflex arcs
Two-neuron reflex arcs
- simplest type - patellar (knee-jerk) reflex
Diencephalon
- sits on top of the brain stem - enclosed by the cerebral hemispheres
Two types of reflexes
- somatic reflexes - autonomic reflexes
Central Nervous System - Organs
-Brain -Spinal Cord
Three main regions of cerebral hemisphere
1. Cortex is superficial gray matter 2. White matter 3. Basal nuclei are deep pockets of gray matter
5 elements of a reflex arc
1. Sensory receptor—reacts to a stimulus 2. Sensory neuron—carries message to the integration center 3. Integration center(CNS)—processes information and directs motor output 4. Motorneuron—carries message to an effector 5. Effectororgan—is the muscle or gland to be stimulated
Diencephalon is made up of 3 structures
1. Thalamus 2. Hypothalamus 3. Epithalamus
Parts of the brain stem
1. midbrain 2. pons 3. Medulla oblongata
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
Structural Classification
Based on number of processes extending from the cell body
Central nervous system
Brain and spinal cord
Functional anatomy of the brain
Brain regions -- cerebral hemispheres -- diencephalon -- brain stem -- cerebellum
Tracts
Bundles of nerve fibers in the CNS
Nerves
Bundles of nerve fibers in the PNS
Transmission of the signal at synapses - step 2
Calcium, in turn, causes the tiny vesicles containing the neurotransmitter chemical to fuse with the axonal membrane
Motor (efferent) neurons
Carry impulses from the central nervous system to viscera and or muscles and glands
Cranial Nerves
Carry impulses to and from the brain
Spinal Nerves
Carry impulses to and from the spinal cord
Neuron = nerve cells
Cells specialized to transmit messages (nerve impulses)
Nuclei
Cluster of cell bodies in the central nervous system
Ganglia
Collections of cell bodies outside of the central nervous system in the PNS
Somatic nervous system (voluntary)
Consciously (voluntarily) controls skeletal muscles
Peripheral nervous system
Cranial and spinal nerves
Visceral sensory (afferent)
Fibers carry information from visceral organs
Somatic sensory (afferent)
Fibers that carry information from the skin, skeletal muscles and joints
Schwann cells
Form the myelin sheath around verve fibers in the PNS
Synapse
Functional junction between nerves where a nerve impulse is transmitted
Meninges
Have 3 layers: -Dura mater - Arachnoid layer - Pia mater
Saltatory Conduction
Myelin sheath separated by the node of ranvier
Motor (efferent) division
Nerve fibers that carry impulses away from the central nervous system organs to effector organs (muscles and glands)
Support cells in the CNS are grouped together as...
Neuroglia
Dendrites - conduct impulses TOWARD the cell body
Neurons may have hundreds of dendrites
Sensory (afferent) division
Never fibers that carry information to the central nervous system
Satellite Cells
Protects and cushion neuron cells bodies
Peripheral Nervous System - Function
Serve as communication lines among sensory organs, the brain and spinal cord and glands or muscles.
Transmission of the signal at synapses - step 3
The entry of calcium into the axon terminal causes porelike openings to form, releasing the neurotransmitter into the synaptic cleft
Transmission of the signal at synapses - step 4
The neurotransmitter molecules diffuse across the synaptic cleft and bind to receptors on the membrane of the next neuron
Transmission of the signal at synapses - Step 1
When the action potential reaches the axon terminal, the electrical charge opens calcium channels
White matter
collections of myelinated fibers (tracts)
Synaptic Cleft
gap between one axon terminal to the next neuron
Nodes of Ranvier
gaps in myelin sheath along the axon
Gray matter
mostly unmyelinated fibers and cell bodies
Neurilemma
part of the Schwann cell external to the myelin sheath
Schwann Cells
wrap axons in a jelly roll-like fashion (PNS) to form the myelin sheath
