the nervous system. introduction neurons: masses of nerve cells nerves structural and functional...
TRANSCRIPT
THE NERVOUS SYSTEM
Introduction
Neurons: masses of nerve cells Nerves Structural and functional
units of nervous system Specialized to react to
physical and chemical changes
Neuroglical Cells: provide neurons with physiological requirements Functions like connective
tissue
Introduction
Nerve Impulse: information transmitted in form of electrochemical signal Travels along nerve fiber
Functions: sensory, integrative, motor Central Nervous System:
brain and spinal cord Peripheral Nervous System:
nerves
Sensory Functions
sensory receptors at the ends of peripheral neurons Detect changes outside the body
Light and sound intensities Temperature and oxygen concentration
Convert sensory information into nerve impulses Transmitted from peripheral nerves to CNS Integrative function: at CNS, signals are
integrated sensation, adding to memory, or produce thoughts
for perceptions
Sensory Functions
Motor Functions: after making conscious or subconscious decisions, act upon perceptions Effectors: signal from CNS
travels to PNS to respond Muscles and glands that
respond to nerve impulses Action counteracts the
change detected Maintain homeostasis
Neuron Structure
Cell Body: granular cytoplasm, cell membrane, and organelles Neurofibrils: network of fine threads which
extend into nerve fibers Chromatophilic substance (Nissl Bodies):
Scattered in cytoplasm Similar to rough ER protein synthesis
Nucleus: center of cell body Mature neurons don’t divide
Neuron Structure
Nerve Fibers: extend from cell body Dendrites: many
Shorter and highly branched Main receptive surface for communication
Axons: only one Conducts nerve impulse away from the body At end, fine extensions that contact
the receptive surfaces of other cells
Axon
Schwann Cell: enclosed axons Wind tightly around axon
Myelin: membrane layers of lipoproteins Myelin sheath: layers with more lipids Smaller axons lack a myelin sheath
Neurilemma: surrounds myelin sheath Nodes of Ranvier: narrow gaps in the myelin sheath
TYPES OF NERVE CELLS
Classification of Neurons by Structure
Bipolar Neurons: has only two nerve fibers Arise from each end 1 axon and 1 dendrite Eyes, nose, and ears
Classification of Neurons by Structure
Unipolar Neuron: single fiber that extends from cell body that divides into two branches Dendrite: connects to peripheral body part Axon: enters the brain or spinal cord Ganglia: aggregation of unipolar cells
Located outside brain and spinal cord
Classification of Neurons by Structure
Multipolar Neurons: many nerve fibers Axon: only one Dendrite: remaining fibers Neurons in brain and spinal cord
Classification of Neurons by Function
Sensory: afferent Unipolar or bipolar Carry impulses from PNS to CNS Specialized receptor ends at tips of dendrites Receptor cells: dendrites in skin or sensory
organs Changes inside or outside the body Single travels to brain or spinal cord
Classification of Neurons by Function
Interneurons: internuncial or association neuron multipolar Lie within brain or spinal
cord Link other neurons Transmit impulses from
one part of the brain or spinal cord to another Direct incoming sensory
impulses for appropriate processing
Classification of Neurons by Function
Motor Neurons: efferent neurons Multipolar Carry impulses out of the brain or spinal cord
to effectors Stimulate muscle to contract and glands to
release hormone
Classification of Neuroglical Cells
Fill spaces, support neurons, provide structural frameworks, produce myelin, and carryon phagocytosis
Outnumber neurons Schwann Cells Nicroglial Cells: scattered in
CNS Support neurons phagocytize bacterial cells and
cellular debris
Classification of Neuroglial Cells Oligodendrocytes: occur in rows along
nerve fibers Form myelin within the brain and spinal
cord Do not form sheaths
Classification of Neuroglical Cells Astrocytes: between neurons and blood
vessels Structural support, join parts, regulate
concentrations of nutrients and ions within tissue
Form scar tissue in CNS
Classification of Neuroglical Cells Ependymal Cells
form an epithelial like membrane that covers specialized brain parts
Forms the inner lining that enclose spaces within the brain and spinal cord
NERVE IMPULSES
Cell Membrane Potential
Surface of cell membrane is usually charge Polarized
Unequal distribution of positive and negative ions between sides of the membrane
Important in conducting nerve impulses
Distribution of Ion
Determined by pores or channels in the membrane
Some channels are always open Others can be closed Channels can be selective
Only allow certain ions to pass K pass easier than Na,
and less so than Ca
Resting Potential
Potential difference: difference in electrical charge between two regions Inside of cell: slightly negative Outside of cell: slightly positive
Resting Potential: potential difference in a nerve cell
Potential Changes
Nerve cell are excitable Respond to changes in their surroundings Temperature, light, or pressure Signals from other neurons
Changes in resting potential Depolarizing: inside of membrane becomes less
negative
Resting Potential
Resting potential is graded Amount of change in potential is
proportional to the intensity of stimulation Summation: accumulation of stimulation
Threshold potential Action Potential: 1/1000th of a second
Na diffuses freely inward Membrane becomes depolarized
K diffuses out of cell inside becomes negative repolarized
Nerve Impulse
When an action potential occurs in one region, it causes the bioelectric current to flow to the adjacent portions of the membrane
This stimulates the adjacent membrane Wave of action potentials down a nerve
fiber
Impulse Conduction
Unmyelinated fiber conducts an impulse over its entire surface
Myelinated surface prevents ion flow from the insulated membrane If the sheath were continuous, no impulse travel Nerve impulse jumps from node of Ranvier to next
node Contain Na and K channels
Impulse Conduction
Impulse is much faster than on an unmyelinated nerve
Speed of nerve impulse is proportional to the fiber diameter Skeletal Muscle fiber: 120
meters per second Sensory Fiber: 0.5 meters
per second Nerve stimulation is all-or-
none response Amount of stimulation
does not vary degree of response
The Synapse
Junction between two neurons Not a direction connection Gap: synaptic cleft
Synaptic Transmission: signal runs from dendrite of a neuron to the axon At axon, neurotransmitter
are released Bind with receptor on the
next neuron
Excitatory and Inhibitory Actions Excitatory: increases Na permeability
Trigger nerve impulses Inhibitory: decrease Na permeability
Lessens chance of nerve impulse
Neurotransmitters
~50 types have been identified Some neurons only release one type
where others release two or three Acetylcholine: stimulates muscles Monoamines: epinephrine,
norepinephrine, dopamine, serotonin Amino acids and peptides
Neurotransmitters
Synthesized in the cytoplasm of the synaptic knob
Stored in synaptic vesicles When action potential is reached, Ca ions
are transported inward Neurotransmitters are released Afterwards, some are reabsorbed while
others are broken down
Impulse Processing
Neuronal Pools: receive impulses, processes them, and conducts impulses away Facilitation: neuron receives a subthreshold
stimuli and becomes more excitable Convergence: two or more impulses from
different sources on a single neuron Additive effect on neurons
Divergence: impulses may pass into several output fibers Amplifies impulses
NERVE PATHWAYS, MENINGES, AND SPINAL CORD
Nerve Pathway
Routes nerve impulses follow through nervous system Reflex arc: involuntary actions
Begins with a receptor at the end of a sensory fiber
Leads to interneurons in CNS, or reflex center Communicate with motor neurons to effectors
Reflex Behavior; automatic subconscious responses to stimuli Heart rate, breathing, blood pressure, and
digestion Swallowing, sneezing, coughing, and vomiting
Reflex Behavior
Knee-jerk reflex: patellar tendon reflex sensory neuron communicating directly to a
motor neuron Helps maintain upright posture
Reflex Behavior
Withdrawal Reflex: touching a finger to something painful Motor neurons in the flexor muscles are
simulated Arm jerks back Signal is also sent to the
brain to signal pain Protective reflex
Meninges
Membranes located between the bone and soft tissues the nervous system
Protect the brain and spinal cord
Meninges: 3 layers
Dura mater: outermost layer Tough, white fibrous connective tissue Contains many blood vessels and nerves
Meninges: 3 layers
Arachnoid mater: thin, weblike membrane Lacks blood vessels Located b/w dura and pia maters
Meninges: 3 layers
Pia mater: very thin layer Contains many
nerves and blood vessels
Nourish the cells of the brain and spinal cord
Spinal Cord
Slender nerve column that passes downward from the brain into the vertebral canal
Spinal Cord
Structure of Spinal Cord
31segments Each gives rise to a pair of spinal nerves
Branch to various parts of the body Cervical Enlargement: thickening in the neck
Nerves to upper limbs Lumbar Enlargement: thickening in lower
back Nerves to lower limbs
Functions of the Spinal Cord
Conducting nerve impulses Ascending tract: nerve tract
or pathway that carries sensory information to the brain
Descending tracts: conduct motor impulses form the brain to the muscles and glands
Spinal reflexes: knee jerk and withdrawal Reflex arc passes through
spinal cord
BRAIN
Brain
Subdivides Cerebrum Cerebellum Brain Stem
Cerebrum
Two cerebral hemispheres Connected by corpus
callosum Cerebral Cortex: thin
layer of grey matter near the surface
White matter consists of myelinated nerve fibers that connect neurons within the nervous system and communicate with other body parts
Functions of Cerebrum
Higher brain function Sensory, motor, and association areas One hemisphere usually dominates for
certain intellectual functions
Diencephalon
Thalamus: central relay station for incoming sensory impulses
Hypothalamus: maintains homeostasis Limbic System: emotions and modifies
behaviors
Brain Stem
Midbrain, pons, and medulla oblongata
Midbrain: reflex centers Head and eye movement
Pons: transmits impulses b/w cerebrum and other parts of nervous system Rate and depth of breathing
Medulla Oblongata: all ascending and descending impulses Several vital and nonvital reflex centers
Brain Stem
Cerebellum: two hemispheres Reflex center for integrating sensory
information coordination of skeletal muscle movements Maintenance of equilibrium
PERIPHERAL NERVOUS SYSTEM AND AUTONOMIC NERVOUS SYSTEM
PNS
Cranial and spinal nerves Bunch form the brain
and spinal cord to all body parts
Somatic and autonomic systems
Cranial Nerves
12 pairs of cranial nerves connect the brain to parts of the head, neck, and trunk
Some sensory or primarily motor
Mostly mixed Some are somatic,
others autonomic
Spinal Nerves
31-pairs originate from the spinal cord Mixed nerves provide 2-way communication b/w
spinal cord and upper and lower limbs, neck, and trunk
Grouped according to the levels from which they arise
Emerges from a dorsal and ventral root Divides into branches beyond its foramen Most combine to form plexuses
Nerve fibers are sorted and recombined so that those associated with a particular region reach it together
Autonomic Nervous System
Functions without conscious effort Regulates visceral activities maintaining
homeostasis Autonomic functions are reflexes
controlled from nerve centers in the brain and spinal cord Sympathetic and parasympathetic Parasympathetic is most active
Autonomic Nerve Fibers
Motor fibers Sympathetic fibers leave the spinal cord
and synapse in paravertebral ganglia Parasympathetic fibers begin n the brain
stem and sacral region of the spinal cord and synapse in ganglia near viscera
Autonomic Neurotransmitters Differences in neurotransmitters
different effects of the autonomic divisions
Sympathetic and parasympathetic preganglionic fibers secrete acetylcholine
Parasympathetic postganglionic fibers secrete acetylcholine
Sympathetic postganglionic fibers secrete norepinephrine
Control of autonomic activity
Somewhat independent Control centers in medulla oblongata &
hypothalamus Utilize autonomic nerve pathways Limbic system and cerebral cortex control the
autonomic system during emotional distress