Chapter 7: The Nervous System Flashcards
Sensory receptors
aka sense organs
- change energy into nerve impulses transmitted by sensory neurons to CNS
Proprioceptors
- receptors that provide CNS with information about body position
- help with movement control
- located in joints and muscles
Joint proprioceptor:
Free nerve endings
most abundant type of joint proprioceptors
- sensitive to touch and pressure
- at the beginning of movement they are strongly stimulated
- they adapt slightly at first, then transmit steady signal until movement is complete
Joint proprioceptor:
Golgi-type receptors
Functionally similar to free nerve ending but less abundant
- found in ligaments and around joints
Joint proprioceptor:
Pacinian corpuscles
- located in tissues around joints
- detect rate of joint rotation
- adapt rapidly following initiation of movement
Muscle proprioceptors
provide sensory feedback to nervous system regarding
- muscle length and rate of shortening – muscle spindles
- force development by muscle – golgi tendon organ
Muscle spindles
- regulate body movement and body posture
- respond to changes in muscle length
- large numbers in most human locomoter muscles
– highest density in muscles that require finest degree of control (i.e. hand muscles)
– in muscles responsible for gross movements, there are relatively few spindles
Intrafusal fibers in muscle spindles
- run parallel to normal muscle fibers (extrafusal fibers; contract and generate force)
- primary endings respond to dynamic change in muscle length
- secondary endings provide continuous information concerning static muscle length
Gamma motor neurons stimulate _______ fibers to contract with ______ fibers to prevent “slack” and maintain sensitivity
intrafusal; extrafusal
Muscle spindles and muscle contraction
- Muscle spindles detect stretch of the muscle
- sensory neurons conduct action potentials to the spinal cord
- Sensory neurons synapse with alpha motor neurons
- Stimulation of the alpha motor neurons causes the muscle to contract and resist being stretched
Golgi tendon organs (GTO)
monitor tension developed by muscle contraction
- “safety device”: prevents excessive force generation and muscle damage during muscle contraction
- provides a finer control over skeletal movements
Golgi tendon organs are located
within the tendon
Stimulation of golgi tendon organs
results in reflex relaxation of muscle
- excite inhibitory neurons that send IPSPs (inhibitory post synaptic potential) to muscle alpha motor neurons
- amount of force produced may depend on ability to voluntarily oppose GTO inhibition
Strength training may gradually reduce inhibition by GTOs which results in
greater muscle force, which results in better sport performance
Golgi tendon organ and muscle contraction
- Golgi tendon organs detect tension applied to a tendon
- Sensory neurons conduct action potentials to the spinal cord.
- Sensory neurons synapse with inhibitory interneurons that synapse with alpha motor neurons
- Inhibition of the alpha motor neurons causes muscle relaxation, relieving the tension applied to the tendon
–muscle contraction increases tension applied to tendons. In response, action potentials are conducted to the spinal cord.
Muscle chemoreceptors
specialized nerve endings that are sensitive to changes in the chemical environment surrounding a muscle
- H+ ions, CO2, and K+
- provide information to CNS about metabolic rate of muscular activity
- important in regulation of cardiovascular and pulmonary responses to exercise
Reflexes
Rapid, unconscious reaction to sensory stimuli
- not dependent on activation of higher brain centers
Reflex order of events
- sensory nerve sends impulse to spinal column
- interneurons are excited and stimulate motor neurons
- motor neurons control movement of muscles
stretch reflex
- rapid muscle stretching causes reflex contraction
- present in all muscles, but most dramatic in extensors of limbs
Knee-jerk reflex
- blow by rubber mallet on patellar tendon
- excites primary nerve endings located in muscle spindles
- these nerve endings synapse with alpha motor neuron at spinal cord level
- muscle fibers contract
Withdrawal reflex
during the withdrawal reflex, sensory neurons from pain receptors conduct action potentials to the spinal cord
sensory neurons synapse with excitatory interneurons that are part of the withdrawal reflex
the excitatory interneurons that are part of the withdrawal effect
Crossed extensor reflex
Collateral branches of the sensory neurons also synapse with excitatory neurons that cross to the opposite side of the spinal cord as part of the crossed extensor reflex
The excitatory interneurons that cross the spinal cord stimulate alpha motor neurons supplying extensor muscles in the opposite limb, causing them to contract and support body weight during the withdrawal reflex
Somatic motor neurons of PNS
Carry neural signals from spinal cord to skeletal muscles to contract
Motor neuron
also called an alpha motor neuron, is the somatic neuron that innervates skeletal muscle fibers
The cell body of motor neurons is located
in the spinal cord
the axon leaves the spinal cord and splits into collateral branches; each branch innervates a single muscle fiber
Motor unit
motor neuron and all the muscle fibers it innervates
Innervation ratio
number of muscle fibers/motor neuron
- ratio varies from muscle to muscle
innervation ratio is low in
muscles that require fine motor control
23/1 in extraocular muscles responsible for eye movement
higher innervation ratio in
other muscles
1,000/1 or greater in large muscles (e.g. leg muscles)
Activation of a single motor NEURON leads to
contraction of all the muscle fibers it innervates
Activation of a single motor UNIT results in
weak muscle contraction (i.e. limited force production)
to increase muscle force production
more motor units must be recruited
motor unit recruitment
progressive activation of more and more muscle fibers by the successive recruitment of additional motor units
Size principle
orderly and sequential motor unit recruitment. Smallest motor units recruited first.
Motor unit:
Type S
slow
small motor neurons innervate slow and high oxidative muscle (type 1) fibers
smallest motor units
Motor unit:
Type FR
fast, fatigue resistant
larger motor neurons innervate the intermediate muscle fibers (type IIa)
intermediate motor units
Motor unit:
Type FF
fast, fatiguable
largest motor neurons innervate the fast muscle fibers (type IIx)
largest motor units
Incremental tests of motor units
first stage
- low level muscle force production needed
- slow type S motor units recruited
as test progresses, to produce more muscle force, more and more type S motor units are recruited and eventually type FR motor units are recruited
as the test becomes more difficult, to increase muscle force production, type FF motor units are recruited
Brainstem is located
inside the base of the skull, just above the spinal cord
major structures of brainstem
- midbrain
- pons
- medulla oblongata
- reticular formation
Reticular formation
neurons scattered throughout the brain stem
- Receives and integrates information from all regions of the CNS
- works with higher brain centers in controlling muscular activity
Brain stem
Responsible for:
- many metabolic functions
- cardiorespiratory control (breathing, HR, BP)
- complex reflexes
- control of eye movement and muscle tone, equilibrium, maintenance of upright posture
Damage to brain stem results in
impaired movement control
Cerebrum
Cerebral cortex
- stores learned experiences
- receives sensory information
- organizes complex movement
Motor cortex
Portion of cerebral cortex that is most concerned with motor control and voluntary movement
final relay point
- recieves information from subcortical structures (e.g. cerebellum)
- information is summed
- final movement plan is formulated
- motor commands are sent to spinal cord
movement plan can be modified by
subcortical and spinal centers which supervise the fine details of movement
Cerebellum
coordinates and monitors complex movement
- incorporates feedback from proprioceptors
may initiate fast, ballistic movements
Cerebellum has connections to
- motor cortex
- brain stem
- spinal cord
Damage to cerebellum results in
- poor motor control
- muscular tremor (especially during rapid movements)
Details of movement are refined in
spinal cord via interaction of spinal neurons with higher brain centers
Spinal tuning
spinal mechanism by which voluntary movement is translated into appropriate muscle action
what causes the initial signal to move?
motor cortex does not give initial signal to move
First step of voluntary movement occurs
in subcortical and cortical motivation areas
- association areas of cortex form a “rough draft” of the movement
Cerebellum and basal ganglia
- convert “rough draft” into movement plan
Cerebellum: fast movements
Basal Ganglia: slow, deliberate movements
Movement plan send to motor cortex through thalamus
- motor cortex forwards message down spinal neurons for “spinal tuning” and finally to muscles
- feedback from muscle proprioceptors allows fine-tuning and improvement of motor program
Autonomic nervous system
Responsible for maintaining internal environment
- innervates effector organs not under voluntary control
(smooth muscle in blood vessels/airways/gut, cardiac muscle, and glands)
most organs receive dual innervation from both sympathetic and parasympathetic branches
Sympathetic division of autonomic nervous system
Releases norepinephrine
tends to activate an effector organ (e.g. increases heart rate)
Parasympathetic division of autonomic nervous system
Releases acetylcholine
tends to inhibit an effector organ (e.g. slows heart rate)
Activity of an organ is regulated by
the ratio of sympathetic/ parasympathetic impulses to the tissue
Autonomic nervous system activity during exercise
During exercise, activity of PNS decreases and SNS increases
Ganglia
group of cell bodies outside of the CNS
Sympathetic division processes
- cell bodies of sympathetic preganglionic neurons are located in thoracic and lumbar regions of the spinal cord
- preganglionic fibers leave spinal cord and enter sympathetic ganglia
- preganglionic fibers release acetylcholine
- postganglionic fibers leave sympathetic ganglia and innervate effector tissues
- postganglionic fibers release norepinephrine, which binds to alpha and beta adrenergic receptors on the membrane of target organs
Following stimulation, norepinephrine is removed from the synapse:
- taken up by the postganglionic fiber
- broken down into inactive byproducts by enzymes (monoamine oxidase)
Cell bodies of parasympathetic preganglionic neurons are located within the
brain stem
sacral portion of spinal cord
Parasympathetic nervous system processes
- Parasympathetic preganglionic fibers leave brain stem and spinal cord and enter parasympathetic ganglia
- both preganglionic and postganglionic fibers release acetylcholine
After parasympathetic nerve stimulation…
acetylcholine is released and rapidly degraded by the enzyme acetyl-cholinesterase
How does exercise enhance brain health?
- enhances learning and memory
- stimulates formation of new neurons
- improves brain vascular function and blood flow
- attenuates depression
- reduces peripheral factors for cognitive decline (inflammation, hypertension, and insulin resistance)
exercise improves brain function and decreases the risk of impairment with aging
Regular exercise can protect the brain against:
- disease (Alzheimer’s)
- certain types of brain injury (stroke)
Afferent fibers
sensory nerve fibers
conduct information from receptors to CNS
Efferent fibers
motor nerve fibers
conduct impulses from CNS to effector organs
Soma of neuron
center of operation
contains the nucleus
dendrites
receptive area
conduct electrical impulses toward cell body
axon
nerve fiber
carries electrical impulse away from cell body towards another neuron or effector organ
in large fibers like those that innervate skeletal muscle, the axons are covered by
Schwann cells
Schwann cells
form a discontinuous insulating layer (myelin sheath) along the length of the axon
- faster electrical impulses with myelin sheath
Gaps or spaces between myelin segments along the axon are called
Nodes of Ranvier
aid neural transduction
Axons with large myelin sheath
conduct impulses more rapidly than small nonmyelinated fibers
Damage of myelin results in
nervous system dysfunction
Irritability
ability of dendrites and neuron cell body to respond to a stimulus and convert it to a neural impulse (=electrical signal)
Conductivity
transmission of the impulse along the axon
Electrical signals are initiated by
a stimulus that causes a change in normal electrical charge of the neuron
At rest the inside of cells is
negatively charged relative to the charge on the exterior of the cell
- the negative charge is due to unequal distribution of charged ions (atoms) and it is called a resting membrane potential
- negatively charged fixed ions (anions) trapped inside the cell (proteins, phosphate groups, nucleotides) and cannot penetrate the membrane
- these anions attract positively charged ions (cations) from the extracellular fluid
magnitude of resting membrane potential is determined by
- the difference in ion concentrations across membrane
- permeability of plasma membrane to ions (Na+ and K+)
Ion channels
- channels that regulate the passage of ions across the membrane
- made of proteins that span the entire membrane from the inside to the outside surface
- ion passage is regulated by opening or closing of “gates” that serve as doors in the middle of the channel
Negative resting potential in a neuron is due to
primarily the diffusion of K+ out of the cell due to:
- the concentration gradient for K+ from inside to outside of the cell
- higher permeability of the membrane to K+ than Na+
– at rest all of the Na+ channels are closed whereas a few K+ channels are open
Sodium potassium pump
maintains resting membrane potential
- potassium tends to diffuse out of cell
- moves 2 K+ in and 3 Na+ out
Action potential
occurs when a stimulus of sufficient strength depolarizes the cell
- opens Na+ channels and Na+ diffuses into cell
- inside becomes more positive
Repolarization
Return to resting membrane potential
- K+ leaves the cell rapidly (K+ channels open)
- Na+ channels close
All-or-none law
once a nerve impulse is initiated, it will travel the length of the neuron without a decrease in voltage
Depolarization
Helps internal become more positive
- Na+ channels open
- K+ channels closed
For an impulse to cross from one neuron to another, it must cross the synaptic cleft at a
synapse
Excitatory transmitters
cause increase postsynaptic membrane permeability to sodium, leading to depolarization
if sufficient excitatory neurotransmitter, postsynaptic neuron is depolarized to threshold, and an action potential is generated
Excitatory post synaptic potentials (EPSP)
a series of graded depolarizations in the dendrites and cell body of a postsynaptic neuron
- can bring postsynaptic neuron to threshold and generate an action potential by temporal and spatial summation
temporal summation
summing several EPSPs from one presynaptic neuron that is active repeatedly over a time
spatial summation
summing EPSPs from several different presynaptic neurons that are active simultaneously
Inhibitory post synaptic potentials (IPSP)
cause hyperpolarization (post synaptic neuron becomes more negative)N of postsynaptic membrane
(increase negative neuron resting potential so it resists depolarization)
The ratio of EPSPs and IPSPs determines if a neuron reaches the threshold for an action potential to be generated
- if EPSPs= IPSPs then threshold is not reached, then no action potential
- if EPSPs > IPSPs then threshold is reached, then an action potential is generated
Acetylcholine
can be both excitatory and inhibitory depending on receptor
-in skeletal muscle— depolarization (Na+ enters cell)
-in heart— hyperpolarization (K+ exits cell)
Acetylcholinerase
breaks acetylcholine into acetate and choline
