sensory function
detects changes in internal and external environment via receptors
integrative function
processes sensory information
motor function
carries information from integrator to effector for a response
somatic receptors
sensory receptors in skin, skeletal muscle, joints
visceral receptors
sensory receptors in internal organs
sensory (afferent) division
somatic and visceral sensory nerve fibers; conducts impulses from receptors to the CNS
peripheral nervous system (PNS)
cranial nerves and spinal nerves; communication between CNS and body
central nervous system (CNS)
brain and spinal cord; integrative and control centers
motor (efferent) division
motor nerve fibers; conducts impulses from the CNS to effectors (muscles and glands)
somatic nervous system
somatic motor (voluntary); conducts impulses from the CNS to skeletal muscles
autonomic nervous system (ANS)
visceral motor (involuntary); conducts impulses from the CNS to cardiac muscles, smooth muscles, and glands
sympathetic division
mobilizes body systems during activity; "fight or flight
parasympathetic division
conserves energy; promotes housekeeping functions during rest; "resting and digesting
neurons
nerve cells; excitable; amitotic; high metabolic rate
cell body
biosynthetic center of the neuron; contains nucleus and organelles
nissl bodies
rough endoplasmic reticulum (site of protein synthesis); located in the cell body
neurofibrils
part of cytoskeleton (maintains cell shape and integrity); located in the cell body
dendrites
receptive regions; branched projections off cell body; always unmyelinated
axon (nerve fiber)
impulse (action potential) generating and conducting region; long, thin projection off of cell body
axon hillock
region at junction between cell body and axon; point where action potential is initiated
telodendria
terminal branches of axon
axon terminals
distal ends of telodendria; secretes neurotransmitters
neuroglia (glial cells)
not excitable; provide support, nourishment, protection of neurons; mitotic
ependymal cells
line brain and spinal cord cavities; produce CSF that fills cavities; cilia circulate CSF (CNS)
microglial cells (microglia)
phagocytic- remove microorganisms and debris; protect neurons (CNS)
astrocytes
must abundant glial cells in CNS; anchor neurons to other structures (ex. blood capillaries); control the chemical environment of neurons (maintains the ion balance that is essential for neurons to produce action potentials)
oligodendrocytes
wrap their processes around nerve fibers (axons) to form myelin sheaths
myelin sheaths
form insulative coverings; has a white appearance
white matter
collection of myelinated axons
gray matter
collection of cell bodies, dendrites, and unmyelinated axons
satellite cells
surround neuron cell body; function unknown (PNS)
Schwann cells
wrap their processes around axons to produce myelin sheaths
nodes of Ranvier
unmyelinated gaps along axon (between Schwann cells)
classification of neurons
structural and functional
structural types of neurons
mulitpolar, bipolar, and unipolar neurons
mulitpolar neurons
many processes extend from the cell body; all are dendrites except for a single axon (most common- found in CNS and motor neurons of PNS)
bipolar neurons
two processes extend from the cell body: one is a fused dendrite, the other is an axon (receptors in eye, ear, nose)
pseudounipolar neurons
one process extends from the cell body and forms central and peripheral processes, which together comprise an axon (sensory neurons in PNS)
peripheral process on pseudounipolar neuron
carries signal toward cell body, but also can generate and conduct an impulse and is myelinated- classifying it as an axon
functional types of neurons
sensory (afferent) neurons, motor (efferent) neurons, interneurons (association/connection neurons)
sensory (afferent) neurons
carries sensory information (from receptors to CNS; from low to high brain centers)
motor (efferent) neurons
carries motor (processed) information (from CNS to effectors; from high to low brain centers)
interneurons (association/connection neurons)
any neuron between 1st sensory neuron and last motor neuron; only in CNS
sensory receptors
allow us to detect stimuli (ex. touch/pressure, hot/cold, pain, relative position and movement of body parts)
stimulus: touch, pressure
receptors: mechanoreceptors (merkel discs, pacinion corpuscles)
stimulus: hot, cold
receptors: thermoreceptors (free nerve endings)
stimulus: pain
receptors: nocioreceptors (free nerve endings)
stimulus: change in body position
receptors: proprioreceptors (muscle spindle, goldi tendon organ; monitor change in body position by how much organs (muscles) are stretched)
neurolemma
cell membrane
Resting Membrane Potential (RMP)
potential difference in the resting neuron; same RMP as in muscle cells= -70 mV
RMP: outside of cell more positive than inside
Na/K ATPase pumps 3 Na out for every 2 K in; more K leaks out than Na leaks in; Anions inside cell
Membrane Channels
ion channels, chemically-gated ion channels (ligand gated), and voltage-gated ion channels
ion channels
passive: always open (although size and charge restrictions apply)
chemically-gated ion channels (ligand gated)
integral proteins that change shape to function as gates to restrict/allow movement across membrane in response to binding of NEUROTRANSMITTER
voltage-gated ion channels
integral proteins that change shape in response to ELECTRICAL CHANGES in membrane (important in action potentials)
voltage-gated ion channels in NEURONS
voltage-gated Na and K channels are present ONLY on surface of axon and axon hillock
voltage-gated ion channels in MUSCLE FIBERS
voltage-gated Na and K channels are present over entire sarcolemma
depolarization
decrease in membrane potential (becomes less negative than RMP (more positive); decrease in charge separation)
repolarization
returning to RMP
hyperpolarization
increase in membrane potential (becomes more negative than RMP; increase in charge separation)
two types of signals produced by changes in membrane potential
graded potentials and action potentials
graded potentials
short-lived, localized changes in membrane potential; changed cause current flows that decrease in magnitude over distance; "graded" because their magnitude varies directly with stimulus strength
action potentials
signals do not die off over distance; APs are all alike and are independent of stimulus strength; they are the same in neurons and skeletal muscle cells
action potentials don't die off over distance
neurons propagate action potentials over long distances; voltage-gated channels keep action potential alive
action potentials in neurons
nerve impulse
threshold
level of depolarization that must be reached AT THE AXON HILLOCK in order to produce an action potential; all-or-none phenomenon; -55 mV
action potential initiation
initiated at the axon hillock and propagate down the axon;
action potentials occur only in axons
not cell bodies or dendrites; axon is only place where voltage-gated Na and K channels are present
How can CNS determine whether a stimulus is strong or weak?
FREQUENCY of action potentials
strong stimulus
causes action potentials to be generated more often in a given interval
weak stimulus
action potentials generated less often
Refractory periods
absolute and relative refractory period
absolute refractory period
period between voltage-gated Na channels opening and resetting; neuron cannot respond to another stimulus, no matter how strong
relative refractory period
follows absolute refractory period; period when voltage-gated Na channels are closed and voltage-gated K channels are still open (hyperpolarization occurring);
to stimulate relative refractory period again
need stronger than normal stimulus to reopen voltage-gated Na channels (during this period, the threshold for action potential generation is substantially elevated)
conduction velocity
rate of propagation of impulse (action potential)
conduction velocity is influenced by
axon diameter, myelination, temperature and pressure/blood flow, and local anesthetics
axon diameter
larger diameter->less resistance to electrical current (faster propagation)
types of fibers
A fibers, B fibers, and C fibers
A fibers
large, myelinated; fastest conduction; motor neurons to skeletal muscle
B fibers
intermediate size, myelinated; slower conduction; some sensory fibers from skin and visceral organs
C fibers
smallest, unmyelinated; slowest conduction; sensory fibers for pain
unmyelinated
continuous conduction of action potential down the membrane; slower
myelinated
saltatory conduction; better because faster and uses less energy
myelinated=faster
action potentials jump from node to node (myelin does not conduct electricity (insulator)); action potentials only occur at noes of Ranvier
myelinated=uses less energy
easier for Na/K ATPase pump to reestablish RMP over smaller surface of membrane
multiple sclerosis
disease causes demyelination in CNS
temperature and pressure/blood flow
temperature and pressure affect blood flow; cold and/or higher outside pressure->decrease blood flow->reduced oxygen and nutrients to cell->less ATP->slower conduction rate
local anesthetics
block voltage-gated sodium channels (no pain) ->no APs generated
Chemical and Electrical Synapses
sites where impulses are transmitted between cells
electrical synapse (faster)
electrical current (ions) spreads between cells via gap junctions; fast conduction between cells; locations: cardiac muscle, smooth muscle, some areas of brain
chemical synapse (slower)
neurotransmitter released by presynaptic cell binds to receptor on postsynaptic cell
three types of chemical synapse
neuromuscular junction, neuroglandular junction, synapse
neuromuscular junction
neuron to muscle cell
neuroglandular junction
neuron to glandular tissue
synapse
neuron to neuron
presynaptic neuron
conducts impuse toward synapse
postsynaptic neuron
conducts impuse away from synapse
synapse
can occur at dendrites, cell body, or axon hillock of postsynaptic cell; electrical signal (from pre-synaptic neuron) is converted to chemical signal (neurotransmitter release) then converted back to electrical signal (on postynaptic neuron)
stopping neurotransmitter effects
1. enzymatic digestion, 2. reabsorbed by pre-synaptic neuron, 3. diffusion away from synapse
postsynaptic membrane potentials
binding of neurotransmitters to receptors on post-synaptic membranes open ion channels (converts chemical signals to electrical signals); these channels (unlike voltage-gated channels in axon) are insensitive to changes in membrane potential so cannot gen
two types of synapses
excitatory synapse and inhibitory synapse
excitatory synapse
neurotransmitter binding: causes DEPOLARIZATION of postsynaptic membrane; produces local, graded potential called EPSP
inhibitory synapse
neurotransmitter binding: causes HYPERPOLARIZATION of postynaptic membrane; produces local, graded potential called IPSP
EPSP
excitatory postsynaptic potential
EPSP effect on membrane potential
becomes less negative
EPSP membrane potential relative to AP threshold
makes it closer to threshold
EPSP AP generation (at axon hillock)
facilitates it
EPSP mechanism
binding of neurotransmitter opens Na/K channel; allows both Na and K to diffuse across membrane; Na influx > K efflux: net result is more positive on inside= depolarization
IPSP
inhibitory postsynaptic potential
IPSP effect on membrane potential
more negative
IPSP membrane potential relative to AP threshold
makes it further from threshold
IPSP AP generation (at axon hillock)
inhibits it
IPSP mechanism
binding of neurotransmitter opens K+ channel and/or Cl- channel; K efflux and/or Cl- influx: net result is less positive on the inside= hyperpolarization
Integration of postsynaptic events
in neurons, transition between local and graded potentials (EPSPs and IPSPs) and action potentials takes place at the AXON HILLOCK; EPSPs and IPSPs that arrive at the axon hillock are summed, and if they reach the -55 mV threshold, then an AP is produced
two types of summation
temporal summation and spatial summation
temporal (over time) summation
occurs when presynaptic neuron fires rapidly-> increase in neurotransmitter release->more graded potentials produced->graded potentials are added together
spatial (over space) summation
occurs when postsynaptic neuron is stimulated by many axon terminals and/or different neurons at the same time-> more graded potentials produced->graded potentials are added together
no action potential generated
if summed graded responses (temporal and spatial, generated by EPSPs and IPSPs on postsynaptic neuron stimulated by presynaptic neuron) do not reach -55mV threshold at axon hillock
action potential generated
if summed graded responses (temporal and spatial, generated by EPSPs and IPSPs on postsynaptic neuron stimulated by presynaptic neuron) do reach -55mV threshold at axon hillock
1. excitatory > inhibitory, but less than threshold
facilitates only
2. excitatory > inhibitory and reaches threshold
action potential
3. inhibitory > excitatory
no action potential (inhibition only)
Neurotransmitters (NTs)
over 50 different types of NTs have been identified; NTs may be excitatory (generate EPSPs) or inhibitory (generate IPSPs); some neurons release only one type of NT, others release more than one type (different NTs are released at different stimulation fr
NTs chemical classes
Acetylcholine; Biogenic amines: dopamine, norepinephrine, epinephrine, serotonin, histamine; Amino acids: glutamate; Peptides: endorphins; Purines; Dissolved gasses: nitric oxide
serotonin (NT)
mostly inhibitory; located in CNS; plays a role in sleep, appetite, regulation of mood; drugs that block its uptake (ex. Prozac) relieve anxiety and depression
glutamate (NT)
mostly excitatory; located in CNS; plays role in learning and memory
circuits
patterns of synaptic connections among neurons
diverging circuits
when one presynaptic neuron influences many cells; produces a more widespread effect (amplifying effect); ex. single neuron in brain activates many motor neurons in spinal cord which causes thousands of skeletal muscle fibers to contract
converging circuits
when several presynaptic neurons affect 1 postsynaptic neuron; produces a concentrating effect, different stimuli can have same effect; ex. both seeing car approach and hearing brakes squeal cause muscle contraction to move you out of car's path
Central Nervous System (CNS)
includes brain and spinal cord; composed of white and gray matter
nuclei
clusters of cell bodies in the CNS
ganglia
clusters of cell bodies in PNS
tract
bundle of axons in CNS
nerve (nerve fiber)
bundle of axons in PNS
4 regions of the brain
cerebrum/cerebral cortex, cerebellum, diencephalon, brain stem
1. Cerebrum/Cerebral Cortex
conscious mind"; 2 cerebral hemispheres (R and L); 5 lobes: frontal, parietal, occipital, temporal, insula
cortex
outer gray matter (surrounds internal white matter, although there are some islands of gray matter within)
domains
specific areas of cortex for motor and sensory functions
amount of cortex devoted to function
determines the level of control/sensation for body part
contralateral information processing
right side of brain controls left side of body; left side of brain controls right side of body
lateralization or specialization of function
2 hemispheres not equal in function (functional asymmetry)
3 types of functional areas in cerebral cortex
motor areas, sensory areas, and association areas
motor areas
control voluntary muscle movement
1. primary motor cortex
initiation of skeletal muscle contraction
2. premotor cortex
learned motor skills of repetitive nature
3. Broca's area
speech
4. frontal eye field
voluntary eye movements
sensory areas
receive and interpret sensory impulses to provide conscious awareness (sensation)
1. primary somatosensory cortex
receives information from skin and proprioceptors (receptors in skeletal muscle and joints)-> SOMATIC SENSATIONS; provides SPATIAL DISCRIMINATION (ability to determine what body region was stimulated
2. visual cortex
sight
3. auditory cortex
hearing
4. gustatory cortex
taste
5. olfactory cortex
smell
6. visceral sensory cortex
conscious perception of visceral sensations (ex. bladder full)
7. vestibular cortex
balance
Association areas
emotional and intellectual processes (ex. memory, judgement, will, reasoning)
2. Cerebellum
primarily involved in motor activities; COORDINATES MOVEMENTS; processes information from cerebral motor cortex to ensure precise timing and appropriate patterns of skeletal muscle contraction
Cerebellum functions
smooth skeletal muscle movement, provides balance, maintains posture; cerebellar activity occurs SUBCONSCIOUSLY
3. Diencephalon
made up of the Thalamus, Hypothalamus, and Epithalamus
Thalamus
consists of bilateral nuclei (gray matter)
Thalamus functions
relay station: many synapses for information in and out; crude recognition of afferent impulses as pleasant or unpleasant
Hypothalamus
major regulator of homeostasis
Hypothalamus function
control center for the autonomic nervous system (sympathetic and parasympathetic)- controls smooth muscle, gland secretions
Hypothalamus function
emotional and behavior responses (part of limbic system)
Hypothalamus function
body temperature regulation: pre-optic area has heat-losing and heat-promoting centers; initiates cooling (sweating) and heat-generating mechanisms (shivering)
Hypothalamus function
food intake regulation: feeding (hunger) center and satiety center
Hypothalamus function
regulation of H2O balance/thirst: osmoreceptors in hypothalamus are activated when body fluids become too concentrated; (trigger release of antiduretic hormone (ADH) by pituitary gland->causes kidneys to retain water, and stimulated neurons in thirst cent
Hypothalamus function
waking and sleeping patterns: sets timing of sleep cycle in response to daylight-darkness cues
Hypothalamus function
control of endocrine system functioning: controls release of hormones by anterior pituitary gland; produces hormones ADH and oxytocin
Epithalamus
includes pineal gland, which secretes melatonin; along with hypothalamus, helps regulate sleep/wake cycle
melatonin
hormone that induces sleep
4. Brain Stem
made up of 3 parts: the midbrain, pons, and medulla oblongata
Midbrain
includes nuclei and tracts-> connects lower and higher brain centers
substantia nigra
in nuclei, contains neurons that release the neurotransmitter domapine
Parkinson's disease
degeneration of neurons in substantia nigra causes it
nuclei of midbrain
for cranial nerves III-IV
Pons
includes nuclei and tracts-> connects lower and higher brian centers; includes respiratory center that controls DEPTH of breathing (works with medullary respiratory centers)
nuclei of pons
for cranial nerves V-VII
Medulla Oblongata
includes nuclei and tracts->connects brain and spinal cord; part of reticular formation
nuclei of medulla oblongata
for cranial nerves VIII-XII
reticular formation includes (nuclei)
nuclei each control a particular bodily function
cardiac center
regulates heart rate
vasomotor center
regulates blood vessel diameter
respiratory center
regulates RATE of breathing (works with respiratory center in pons)
other centers
control swallowing, vomiting, coughing
functional brain systems
not localized to one region of brain
Limbic system
controls emotional aspects of behavior (emotional brain); involves structures in cerebrum and hypothalamus; connections between limbic system and cerebral cortex explains intimate relationship between thoughts and feelings
Reticular Formation
maintains cerebral cortex in alert, conscious state; acts as a filter for sensory input (repetitive, familiar, weak inputs are filtered out); involves central core of brain stem (medulla oblongata, pons, midbrain);
injury to reticular formation
permanent unconsciousness (irreversible coma); also LSD blocks this formation
Ventricles
4 cavities within brain: 2 lateral, 3rd, and 4th
Ventricles
filled with CSF; continuous with each other and central cavity of spinal cord
apertures
tiny openings that connect ventricles to subarachnoid space
ependymal cells
line ventricles; type of neuroglia that have cilia and tight junctions
choroid plexus
network of capillaries in roofs of the ventricles, just behind ependymal cells
Cerebral Spinal Fluid (CSF)
made by ependymal cells that cover choroid plexus: fluid released by permeable capillaries in choroid plexus passes through ependymal cells, which act as a selective barrier allowing only certain ions to pass
CSF movement
passes from ventricles into central canal of spinal cord and into subarachnoid space (propelled by cilia on ependymal cells)
Functions of CSF
1. protects CNS structures (liquid cushion surrounding brain and spinal cord) 2. maintains optimal chemical environment for neuron function 3. circulation delivers nutrients and removes wastes
cranial meninges
3 connective tissue layers covering the brain
dura mater
periosteal layer- attached to skull; meningeal layer (outermost layer)
arachnoid mater
subarachnoid space=space between arachnoid mater and pia mater; space filled with CSF; carries blood vessels (looks like spider webs)
pia mater
covers brain; continuous with spinal meninges (innermost layer)
blood supply
rich blood supply; high oxygen and glucose requirements (makes ATP using aerobic respiration); no storage capacity for glucose; brief interruption of oxygen- lose consciousness (if > 4 minutes=permanent cell death)
blood brain barrier
protective mechanism to maintain a stable environment for the brain
blood brain barrier is selective
glucose, oxygen, and essential amino acids pass through freely; bloodborne metabolic wastes, proteins, and most drugs are cannot pass through
spinal cord functions
conduct impulses to/from brain; integration of reflexes- direct connections between spinal cord and PNS (without brain involvement)
spinal cord is composed of
grey matter and white matter
grey matter
(inside); nuclei (nerve cell bodies), dendrites, and unmyelinated axons; form horns of spinal cord
white matter
(outside); bundles of myelinated axons; form columns, which include ascending and descending tracts (note: white and grey matter is opposite of brain->white inside, grey outside)
spinal cord is surrounded/protected by
vertebral column; epidural space filled with adipose tissue; spinal meninges; CSF (fills subarachnoid space and central canal)
Spinal roots
formally part of PNS, not spinal cord; includes dorsal and ventral roots
dorsal roots
carry sensory input to spinal cord; cell bodies of sensory neurons in dorsal root ganglia
ventral roots
carry motor output from spinal cord; cell bodies of motor neurons located in grey matter of spinal cord
spinal nerves
fused dorsal and ventral roots; 31 pairs of them that pass between vertebrae (via intervertebral foramen)
ascending tracts
conduct impulses toward higher centers; composed of first-order, second-order, and third-order neurons
1. first-order neurons
transmit impulses from receptors to spinal cord or brain stem; cell bodies in dorsal root ganglion; receptors are cutaneous skin receptors and proprioceptors (receptors in joints, muscles, tendons that detect changes in locomotion, posture, muscle tone)
2. second-order neurons
cell bodies in dorsal horn of spinal cord or in medullary nuclei; transmit impulses to thalamus or cerebellum
3. third-order neurons
cell bodies reside in the thalamus; transmit impulses to somatosensory cortex of cerebrum; no third-order neurons in the cerebellum
main ascending pathways
lemniscal, spinothalamic, spinocerebellar
lemniscal
discriminates touch and conscious proprioception
spinothalamic
discriminates pain and temperature
spinocerebellar
tracts to cerebellum-> contributes to balance and coordination; does not contribute to sensory perception
descending tracts
conduct impulses away from higher centers; composed of upper motor neurons and lower motor neurons
1. upper motor neurons
transmit impulses from motor cortex of cerebrum to lower motor neurons (either directly or via interneuron)
2. lower motor neurons
transmit impulses from spinal cord to effector; cell bodies in ventral horn of spinal cord
Reflex
rapid motor response to a stimulus; unlearned, unpremeditated, involuntary; may be modified by learning and conscious effort
5 parts of a reflex arc
1. receptor, 2. sensory (afferent) neuron 3. integration center (in CNS) 4. motor (efferent) neuron 5. effector
1. receptor
responds to stimulus (changes in external/internal environment); initiates impulses via local electrical changes (graded potentials); graded potential in sensory structure called "receptor potential"; receptor may be part of sensory neuron, or separate st
2. sensory (afferent) neuron
carries impulse from receptor to spinal cord (CNS)
3. integration center (in CNS)
sensory impulse converted to motor impulse: directly or via interneurons; direct=monosynaptic (1 synapse); via interneurons=polysynaptic (2 or more synapses)
4. motor (efferent) neuron
transmits efferent impulse from sensory or interneuron to effector
5. effector
muscle fiber (contraction or elongation); gland (increased or decreased secretion)
somatic reflexes
regulate skeletal muscle contraction/relaxation
autonomic reflexes
autonomic nervous system activates smooth muscle, cardiac muscle, and glands
1. somatic spinal reflexes
(ex. patellar or knee-jerk reflex); muscle stretch results in contraction of that muscle; skeletal via nerves
a. receptor
muscle spindle; composed of intrafusal muscle fibers- can sense stretching; central regions of intrafusal fibers are non-contractile
b. sensory neurons
primary sensory Ia fibers- stimulated by rate and amount of stretching; secondary sensory II fibers- stimulated ONLY by amount of stretching
c. integration center (spinal cord)
synapse of sensory and motor neurons; monosynaptic (no interneuron)
d. motor neurons
alpha motor neurons; stimulate extrafusal muscle fibers to contract
e. effector
extrafusal muscle fiber
agonist
muscle that produces a particular movement
antagonist
muscle that works to reverse that movement
reciprocal inhibition
reflex arc both contracts muscle (agonist) in response to stretching and inhibits contraction of antagonist; sensory neuron synapses with inhibitory interneuron in spinal cord->inhibits motor neuron to antagonist; polysynaptic (involves interneuron)
ipsilaterl
sensory input enters and motor output leaves the spinal cord on the same side (seen in stretch reflex)
contralateral
motor output leaves spinal cord on the opposite side
2. tendon reflex
helps avoid tearing of tendons by avoiding overcontraction
a. receptor
golgi tendon organ (located at junction of muscle and tendon)
tendon reflex causes reciprocal activation
sensory neuron synapses with inhibitory interneuron to inhibit motor neuron to agonist (agonist relaxes) and also with excitatory interneuront o stimulate motor neuron to antagonist (antagonist contracts)
3. withdrawal and crossed-extensor reflexes
withdrawal flexor reflex and crossed-extensor reflex
withdrawal flexor reflex
is automatic withdrawal of threatened body part; receptor: free nerve endings, stimulus: pain or pressure; causes action potential to be generated in sensory neuron (AP propagates down sensory neuron to CNS)
withdrawal flexor reflex has reciprocal inhibition
sensory neuron synapses with excitatory interneuron to stimulate motor neuron to flexor (on ipsilateral side)-> flexor contracts; and synapses with inhibitory interneuron to inhibit motor neuron to extensor (also ipsilateral side)->extensor relaxes
crossed-extensor reflex reciprocal activation
maintains balance; same stimulus (stimulation of same sensory neuron); sensory neuron synapses with inhibitory interneuron to inhibit motor neuron to flexor (on contralateral side)->flexor relaxes; also synapses with excitatory interneuron to stimulate mo
flexor and crossed-extensor reflexes
involve ascending and descending stimulation (via additional interneurons)
intersegmental reflex
additional recruitment of other motor units in acting muscles and/or different muscles; can be overridden by descending signals from the brain (ex. if painful stimulus, brian can prevent withdrawal)
2. somatic cranial reflexes
skeletal via cranial nerves
corneal reflex (blink reflex)
stimulus=touch or foreign body; receptors:=pain receptors in cornea, eyelid, or conjunctiva; sensory neuron=trigeminal nerve (CN V); integration center=brain stem (CNS); motor neuron=facial (CN VII); effector=skeletal muscles of eyelid
3. autonomic reflexes
effectors=cardiac and smooth muscle, glands)
a. pupillary light reflex
pupil constricts in response to light; stimulus=light; receptors=photoreceptors in the eye; sensory neuron=optic nerve (CN II); motor neurons=parasympathetic fibers of ocularmotor nerve (CN III) + ganglionic neuron; effector=smooth muscle in eye (circular
visceral reflex arcs
same components as somatic reflex arcs except visceral reflex arc has 2 motor neurons in its motor component
Autonomic Nervous System (ANS)
motor subdivision of PNS; involuntary; controls internal environment by regulating activity of cardiac and smooth muscle, glands (effectors); divided into parasympathetic and sympathetic
parasympathetic
energy conservation, "resting and digesting
sympathetic
physical or emotional activities, stress, "fight or flight
efferent pathways
in somatic NS, motor neuron cell bodies are in CNS and extend to effector but ANS uses two-neuron chain to effectors (preganglionic and ganglionic neurons)
preganglionic neuron
cell body in CNS; myelinated axons; synapse with ganglionic neuron at ganglion; all preganglionic axons release acetylcholine (ACh)
ganglionic neuron
cell body in autonomic ganglia outside of CNS; axon of ganglionic neuron=postganglionic axon (or fiber); terminates at visceral effectors; unmyelinated axons
parasympathetic postgangionic axons
release ACh
sympathetic postganglionic axons
release NE
dual innervation
ANS has this; most visceral effectors receive nerve fibers for both sympathetic and parasympathetic NS; 2 visceral motor neurons cause opposite effect (counterbalance each other)
neurotransmitters
released by postganglionic axons; can have stimulatory or inhibitory action on effectors
heart rate
in sympathetic it goes up, in parasympathetic it goes down
gut mobility
in sympathetic it goes down, in parasympathetic it goes up
ANS Anatomy
sympathetic and parasympathetic divisions are distinguished by origin sites, lengths of fibers, location of ganglia, and branching and range of effect
1. origin sites
parasympathetic= motor neurons emerge from brain and lower spinal cord (craniosacral); sympathetic= thoracolumbar
2. lengths of fibers
parasympathetic= long preganglionic, short postganglionic axons; sympathetic= short preganglionic, long postganglionic axons
3. location of ganglia
parasympathetic=ganglia in visceral effector organs (terminal ganglia); sympathetic=paravertebral and prevertebral ganglia
paravertebral ganglia
lie alongside spinal cord in sympathetic trunk; innervate organs above the diaphragm
prevertebral ganglia
located anterior to the vertebral column near large abdominal arteries; innervate organs in abdominopelvic cavity
4. branching and range of effect
parasympathetic=localized effect (each preganglionic fiber synapses with few ganglionic neurons; each postganglionic axon innervates a single effector); sympathetic=divergent effect (each preganglionic fiber synapses with many ganglionic neurons; each pos
ganglia
group of cell bodies; where preganglionic axons and ganglionic neurons synapse
visceral sensory neurons
convey information about chemical changes, stretch and irritation to integration centers in the CNS; most are visceral pain afferents; travel along same pathways as somatic pain fibers-> can cause referred pain
referred pain
brain misinterprets visceral pain as coming from more common somatic region source that shares pathway (ex. heart attack may cause sensation of pain in superior thoracic wall and medial part of left arm)
ANS activity
under CNS control even though not voluntary
hypothalamus
top of the ANS control hierarchy
brain stem (especially medulla oblongata)
also provides control
cerebral cortex
may modify working of ANS, but acts at subconscious level through limbic system
hypertension can be caused by
stress, emotion upset->sympathetic fibers constrict blood vessels in abdominal viscera and skin to divert blood to muscles, brain, and heart-> increases blood pressure