Nervous system
the master control and communication system of the body
(afferent) monitoring stimuli occurring inside and outside of the body
Sensory output
Integration
interpretation of sensory input by the central nervous system (CNS)
Motor output
(efferent) response to stimuli by activating effector organs
Nervous is anatomically organized into 2 parts
central nervous system
peripheral nervous system
Central nervous system parts. _____ and _____ center
-consists of the brain and the spinal cord
-integration and command center
Peripheral nervous system (PNS)
-consists of paired spinal and cranial nerves
-carries messages to and from the central nervous system
The two principal cell types of the nervous system are
-neurons
-supporting cells (glial cells=neuroglia)
Neurons are _____ cells that transmit _______ (___ of cells in CNS are neurons)
excitable cells that transmit electrical signals
(10% of cells in CNS are neurons)
Supporting cells (glial cells=neuroglia)
cells that surround and wrap neurons
-provide a supportive scaffolding for neurons
-segregate and insulate neurons
-guide young neurons to the proper connections during development
-promote growth during development and maintains neuronal health after dev
Astrocytes
most abundant, versatile, and highly branched glial cells
Astrocytes cling to .... and their ... and cover...
neurons
synaptic endings
capillaries
Functionally, astrocytes (4)
1. Support and brace neurons
2. Anchor neurons in their nutrient supplies (via blood vessels)
3. Guide migration of young neurons
4. Control the chemical environment
Microglia
small, ovoid cells with spiny processes
-transform into phagocytes that monitor the health of neurons and "gobble up" debris and infectious microorganisms
Oligodendrocytes
branched cells that wrap and insulate the CNS nerve fibers from other neurons
Schwann cells
cells that wrap around fibers of the PNS neurons
serve to insulate and increase conduction velocity of action potentials down nerve fibers
Nerve cells (neurons)
basic structural and functional units of the nervous system
Nerve cells are composed of a ..., one ..., and many ...
cell body (soma)
axon
dendrites
neurons are amitotic meaning that
they must be long-lived (last throughout a lifetime)
Neurons have a high... so they require a constant source of ______ and _______
metabolic rate
oxygen
glucose
Soma
processing center and biosynthetic center of neuron
Dendrites
receive information from other neurons
Axon
output information to effectors via action potentials
Axon (transmits output signal)
slender processes of uniform diameter arising from the axon hillock
Axons are also called
nerve fibers
There is usually ... unbranched axon per ...
one
neuron
Rare axon branching, if present are called
axon collaterals
Axon terminal
branched end of an axon that forms a synapse with an effector
Axons generate and transmit
action potentials
Axons secrete ...from the axonal terminals
neurotransmitters
Movement of substances inside the axon occurs in two directions
anterograde
retrograde
Anterograde
toward the axon terminal
Retrograde
away from the axonal terminal
Functional classification of neurons
1. Sensory (afferent)
2. Motor (efferent)
3. Interneurons (association neurons)
Sensory (afferent) neurons
transmit impulses toward the CNS
Motor (efferent) neurons
carry impulses away from the CNS
Interneurons (association neurons)
shuttle signals through the CNS PATHWAYS
Structural classification of neurons
multipolar
bipolar
unipolar
Multipolar
three or more processes (99% +) i.e. multiple separate dendrites
Bipolar
two processes (axon and dendrite), mostly found in the olfactory epithelium and retina
Unipolar
single, short process (humans do not have true unipolar neurons, they have pseudo-unipolar neurons and are exclusively sensory neurons)
The brain is protected by .... (3)
bone (cranium)
meninges
cerebrospinal fluid (CSF)
The spinal cord is protected by .... (3)
bone (vertebral column)
meninges
CSF
Harmful substances in the blood are prevented from reaching the brain by the ...
blood-brain barrier
Meninges
three connective tissue membranes lie external to the CNS
3 membranes of the meninges
dura mater
arachnoid mater
pia mater
Functions of the meninges
cover and protect CNS
protect blood vessels and enclose venous sinuses
form partitions within skull
Dura mater
leathery, strong structure composed of fibrous connective tissue
Arachnoid mater
It is separated from the dura mater by the subdural space
Beneath the arachnoid is a wide subarachnoid space filled with CSF and large blood vessels
Arachnoid villi protrude... and permit...
superiorly
CSF to be absorbed into venous blood
Pia mater
deep meninx composed of delicate connective tissue that is tightly attached to the brain/spinal cord
Cerebrospinal fluid (CSF)
watery solution similar in composition to blood plasma
CSF contains less... and slightly ... ion concentrations than ...
less protein
different ion concentrations
plasma
CSF forms a .... that gives buoyancy to the brain (effectively reducing its weight by ...%)
liquid cushion
96%
The liquid cushion prevents the brain from
crushing under its own weight
protects from blows and other trauma
CSF ...the brain and carries ... signals throughout the CNS
nourishes the brain
chemical throughout the CNS
Choroid plexus
clusters of capillaries that form tissue fluid filters, which hang from the roof of each brain ventricle
Ependymal cells
surround the capillaries and make and secrete CSF
help cleanse CSF by removing wastes
~150 mL of CSF is viable, but ~... mL/day is actually made
500
Brain ventricles
hollow interconnected spaces within the brain that are filled with CSF
Blood-brain barrier is
-protective mechanism that helps maintain a stable environment for the brain
Blood-brain barrier is a neurovascular unit made up of
-continuous endothelium of capillary endothelial cells
-pericytes
-astrocytes
Blood-brain barrier is a selective barrier that allows
nutrients to pass freely through
The blood-brain barrier is INEFFECTIVE against substances that can diffuse through ... (____)
the plasma membrane
hydrophobic
Blood-brain barrier is absent in some areas (...), allowing these areas to monitor the ....of the blood
vomiting center
hypothalamus
chemical composition
Stress makes the blood-brain barrier more ..., meaning that it ...
permeable
increases the ability of chemicals to pass through
Adequate blood flow to the brain/spinal cord is ...at all times
absolutely required
CNS is ~5% of body mass, but receives ... of the cardiac output
15-20%
Reduction/stoppage of blood flow to the brain for a few seconds causes ..., permanent .. or even ...
unconsciousness
permanent brain damage
death within a few minutes
Hypoxia
Blood is supplied to the brain via the ... and ...
internal carotid
vertebral arteries
Vertebral arteries interconnect via the ...and ...
Circle of Willis
distributes blood to the entire brain through several cerebral arteries
Action potentials (nerve impulses) are
-electrical impulses originating at the soma and carried along the length of axons
-always the same regardless of stimulus
-the primary functional activity of the nervous system
Voltage (V)
measure of potential energy generated by separated charge
Potential difference
voltage measured between two points
Current (I)
the flow of electrical charge between two points
Resistance (R)
hindrance to charge flow
Insulator
substance with high electrical resistance
Conductor
substance with low electrical resistance
Physiological electrical current reflects the flow of ... rather than electrons
ions
There is a potential on either side of membranes when
-the concentration of ions is different across the membrane
-the membrane provides a resistance (barrier) to ion flow
Plasma membrane ion channels provide a defined ... for ions to cross the plasma membrane
path
Types of plasma membrane ion channels
-passive (leak) channels
-chemically-gated channels
-voltage-gated channels
-mechanically-gated channels
Passive (leak) channels are
always open
Chemically-gated channels
open with binding of a specific neurotransmitter
Voltage-gated ion channels
open and close in response to membrane potential
Mechanically-gated channels
open and close in response to physical deformation of the channel or membrane
Electrochemical gradient
the electrical and chemical gradients taken together
Ions flow with their ... when they move from an area of high concentration to an area of low concentration
chemical gradient
Ions flow with their ... when they move toward an area of opposite charge
electrical gradient
When gated channels are open... (4)
-specific ions more quickly across the membrane
-movement is in the direction of their electrochemical gradients
-an electrical current is created
-voltage changes across the membrane
Resting membrane potential (Vr)
the potential difference (~ -70 mV) across the membrane of a resting neuron
Resting membrane potential is generated by different concentrations of
Na+
K+
Cl-
protein anions (A-)
Ionic differences are the consequence of
-differential permeability of the plasma membrane to Na+ (low) vs. K+ (high)
-operation of the Na+/K+ ATPase pump
The plasma membrane is more permeable to ... meaning more ...
K+, leaks out
Plasma membrane is less permeable to... meaning ...
Na+
very little leaks in
Na+/K+ ATPase pumps ____ Na+ out per ___ K+ in
3
2
Na+/K+ ATPase pumps work ... and uses a lot of ...
continuously
ATP
Na+/K+ ATPase pumps are a primary reason neurons require a constant supply of ...
O2 and glucose
Most cytoplasmic anions (phosphate, nucleotides, organic acids, proteins) cannot
leak out of the membrane due to size or charge
Depolarization
the inside of the membrane becomes less negative
Repolarization
the membrane returns to its resting membrane potential
Hyperpolarization
the inside of the membrane becomes more negative than the resting potential
Graded potentials
-short-lived, local changes in membrane potential
-decrease in intensity with distance (decremental in nature)
-their magnitude varies directly with the strength of the stimulus
-sufficiently strong graded potentials can initiate action potentials
-this i
Action potentials total amplitude is _____ mV
a fast reversal of membrane potential with a total amplitude of ~100 mV
Action potentials do not decrease in ...over ...
strength
distance
Action potentials are the principal means of ... and ... communication to effectors
interneural communication
neural communication to effectors
An action potential in the axon of a neuron is also called a
nerve impulse
All-or-none phenomenon
action potentials either happen completely, or not at all
Absolute refractory period
Time from the opening of the Na+ activation gates through a return to their closed state
The absolute refractory period (prevents, ensures, enforces)
-prevents the neuron from generating an action potential
-ensures that each action potential is separate
-enforces one-way transmission of nerve impulses (anteriograde)
Relative refractory period
the interval following the absolute refractory period when
-VG Na+ channels are closed
-VG K+ channels are in the process of closing
-repolarization is occuring
In the relative refractory period, the threshold level is elevated, so...
only strong stimuli will cause another action potential
Most neuronal axons are
myelinated
Myelination
whitish, fatty (20% protein/80% lipid), segmented sheath of Schwann cells (in PNS) surrounding the axons
Myelination functions to
protect the axon
electrically insulate fibers from one another
increase the speed of nerve impulse transmission
In the CNS, Myelin sheaths are formed by ... instead of Schwann cells, and each ... myelinates axons of .... neurons
oligodendrocytes
multiple
Conduction speed of nerve impulse depends upon (2)
-diameter of axon (larger=faster)
-myelination state (myelinated=faster)--makes biggest difference
Fast signal examples efferent and afferent
motor neuron signaling to skeletal muscle, sensory afferent signaling to CNS
Slow signals
neuron signaling to visceral organs (stomach, intestines, etc.)
Saltatory conduction
Current passes through a myelinated axon only at the Nodes of Ranvier
Nodes of Ranvier
gaps between the ends of the Schwann cells
VG Na+ in saltatory conduction are concentrated at
the nodes of ranvier
Action potentials are triggered only at ... and .... from one node to the next in saltatory conduction
nodes
jump
Saltatory conduction is ... than conduction along unmyelinated axons
much faster
Synapse
a junction that mediates information transfer from one neuron to either
-another neuron
-an effector cell
Presynaptic neuron
conducts impulses toward the synapse
Postsynaptic neuron
transmits impulses away from the synapse
Chemical synapses
specialized for the release and reception of neurotransmitters
Chemical synapses are composed of 2 parts
-axonal terminal of presynaptic neuron
-receptor region on the dendrite of postsynaptic neuron
Axonal terminal located in
presynaptic knob or terminal buttons
Receptor region located in
postsynaptic membrane
Synaptic cleft
fluid-filled space separating the presynaptic and postsynaptic neurons
Synaptic cleft prevents nerve impulses from ...
directly passing from one neuron to the next
Transmission across the synaptic cleft is... and ensures...
a chemical event
ensure unidirectional communication between neurons
Mechanism of chemical communication
i.e. what channel is opened at the presynaptic neuron terminal button
1. nerve impulses (action potentials) reach the axonal terminal of the presynaptic neuron and open voltage-gated Ca2+ channels
2. Neurotransmitter is released from vesicles into the synaptic cleft via exocytosis
3.Neurotransmitter diffuses across the syna
Termination of neurotransmitter signaling
neurotransmitter bound to a postsynaptic neuron produces a continuous postsynaptic effect and must be removed from its receptor in order to stop the signal
Removal of neurotransmitters occurs when they (3)
1. Degraded by enzymes
2. Reabsorbed by astrocytes or the presynaptic terminals
3. Diffuse out of the synaptic cleft
Temporal summation
2 excitatory stimuli close in time cause EPSPs (excitatory post synaptic potential) that add together
Spatial summation
2 simultaneous stimuli AT DIFFERENT LOCATIONS cause EPSPs that add together
Neurotransmitters
chemicals used for neuronal communication with the body and the brain
Neurotransmitters can be classified
chemically or functionally
Chemical classification of neurotransmitters (5 main classes)
-acetylcholine
-biogenic amines--serotonin
-amino acids--glutamate
-peptides--endorphins
-novel messengers (ATP, dissolved gases, NO, CO)
Acetylcholine
First neurotransmitter identified
Released at the neuromuscular junction
degraded by the enzyme acetylcholinesterase (AChE)
Acetylcholine is released by
-all neurons that stimulate skeletal muscle
-some neurons in the autonomic nervous system
Biogenic amines include
CATECHOLAMINES:
dopamine, norepinephrine (NE), and epinephrine
Indolamines:
serotonin and histamine
Biogenic amines play roles in
emotional behaviors, biological clock, autonomic nervous system
Amino acid neurotransmitters
-GABA
-Glutamate
Found only in CNS
Can be excitatory or inhibitory
Excitatory amino acid neurotransmitters
generate EPSPs (hypopolarization)
Inhibitory amino acid neurotransmitters
generate IPSPs (hyperpolarization)
Peptide neurotransmitters include
Substance P--mediator of pian signals
Beta endorphin, dynorphin, enkephalins
Peptide neurotransmitters act as
natural opiates, reducing our perception of pain
Functional classification of neurotransmitters
Excitatory or inhibitory
Excitatory neurotransmitters cause
depolarizations
ex. glutamate
Inhibitory neurotransmitters cause
hyperpolarizations
ex. GABA, Glycine
Some neurotransmitters have both excitatory and inhibitory effects, determined by the... of the postsynaptic neuron
receptor subtype
Example of an excitatory and inhibitory neurotransmitter
acetylcholine
excitatory skeletal muscle
inhibitory cardiac muscle
Mechanisms of action of neurotransmitter receptors
direct or indirect
Direct mechanism of action of a neurotransmitter receptor
neurotransmitters that open ion channels
-promote rapid responses
-ACh and amino acids
Indirect mechanism of action of a neurotransmitter receptor
neurotransmitters that act through a second messengers
-slower but more long-lasting effects
-Biogenic amines, peptides, and dissolved gases
Neuronal pools (circuits)
functional groups of neurons that
-integrate incoming information
-forward the processed information to its appropriate destination
-varied neuron number
-diverse patterns of information flow
Two patterns of neuronal processing
1. Serial processing
2. Parallel processing
Serial processing
-input travels along one pathway to a specific destination
-works in an all-or-none manner
-Example: Spinal reflexes
Parallel processing
-input travels along several pathways
-pathways are integrated in different CNS systems
-one stimulus promotes numerous responses
-Example: a smell may remind one of the odor and associated experiences
CNS is composed of
brain and spinal cord
Cerebrum has ridges (...) and shallow grooves (...)
gyri
sulci
Cerebrum deep grooves
fissures
Two hemispheres separated by the
longitudinal fissure
3 basic regions of cerebrum
-outer cortex (grey matter)
-inner white matter
-basal nuclei
Deep sulci divide the hemispheres into 5 lobes
frontal
parietal
temporal
occipital
insula
Central sulcus separates
the frontal and parietal lobes
Parieto-occipital sulcus separates the
parietal and occipital lobes
Lateral sulcus
separates the parietal and temporal lobes
Cortex
superficial gray matter--accounts for 40% of the mass of the brain
Cortex largely consists of neuron
soma
Cortex enables (5)
sensation
communication
memory
understanding
voluntary movements
Each hemisphere acts .... (controls the opposite side of the body)
contralaterally
Hemispheres are not equal in
function
No functional area acts alone, conscious behavior involves the
entire cortex
Three cortical functional areas of cerebrum are
-motor areas
-sensory areas
-association areas
Motor areas
control voluntary movement
Sensory areas
conscious awareness of sensation
Association areas
integrate information
Frontal lobe
voluntary motor functions and areas for planning, mood, smell
Parietal lobe
sensory reception and integration of sensory information
Occipital lobe
visual reception and processing
Temporal lobe
hearing, smell, learning, memory, emotional behaviors
Motor control areas
primary motor cortex
Primary motor cortex
located in precentral gyrus
allows conscious control of precise, skilled, voluntary movements
Motor homunculus
caricature of relative amounts of cortical tissue devoted to each motor function
Sensory Areas: Primary Somatosensory Cortex
located in postcentral gyrus
receives information from skin and skeletal muscles
exhibits spatial discrimination
Sensory areas within the cerebrum
1. Primary somatosensory cortex
2. Somatosensory association cortex
3. Visual and auditory areas
4. Olfactory, gustatory, and vestibular cortices
5. Somatosensory homuncuclus
Somatosensory homunculus
Caricature of relative amounts of cortical tissue devoted to each sensory function
Lateralization of cortical function
each hemisphere has abilities not shared with its partner
Cerebral dominance
designates the hemisphere dominant for language
Left hemisphere
Primarily associated with language, math, and logic
Right hemisphere
primarily associated with visual-spatial skills, emotions, and artistic skills
White matter of the cerebrum
consists of deep myelinated fibers and their tracts
White matter responsible for
communication between the cerebral cortex and lower CNS centers, and between areas of the cerebral cortex
Connections between hemispheres are made by 3 types of fibers
-commissural fibers
-association fibers
-projection fibers
Commissural fibers
connect corresponding gray areas of the two hemispheres
Association fibers
connect different parts of the same hemisphere
Projection fibers
enter the hemisphere from lower brain or spinal cord centers
Basal nuclei (ganglia)
masses of gray matter found deep within the cortical white matter
The corpus striatum is composed of 3 parts
1. caudate nucleus
2. lentiform nucleus
3. Fibers of internal capsule
Lentiform nucleus composed of
putamen and globus pallidus
Fibers of internal capsule running between and through
caudate and lentiform nuclei
Diencephalon
central core of the forebrain
Diencephalon consists of 4 paired structures
thalamus
subthalamus
hypothalamus
epithalamus
Diencephalon encloses the
third ventricle
Thalamus
afferent impulses from all senses converge and synapse in the thalamus
Impulses of similar function in the thalamus are
sorted out, edited, and relayed as a group
All inputs ascending to the cerebral cortex pass through the ____
thalamus
Thalamus plays a key role in
mediating sensation
motor activities
cortical arousal
learning
memory
Hypothalamus
located below the thalamus, it caps the brainstem and forms the inferolateral walls of the third ventricle
HOMEOSTASIS
Infundibulum
stalk of the hypothalamus
connects to the pituitary gland
Hypothalamus regulates
blood pressure
rate and force of heartbeat
digestive tract motility
visceral activities
Hypothalamus involved with perception of
pleasure
fear
rage
Hypothalamus controls mechanisms needed to maintain normal body...
temperature (thermoregulation)
Hunger and satiety regulated by
hypothalamus
Regulation of sleep and sleep cycle by
hypothalamus
Hypothalamus functions as an endocrine gland
-releases hormones that control secretion of hormones by the anterior pituitary
-the supraoptic and paraventricular nuclei produce ADH and oxytocin
Brain stem consists of 3 regions
midbrain
pons
medulla oblongata
Brain stem similar in structure to spinal cord but contains
embedded nuclei
Brain stem controls .... necessary for survival
automatic (autonomic) behaviors
Midbrain location
between diencephalon and pons
Midbrain structures include
Cerebral peduncles
cerebral aqueduct
VARIOUS NUCLEI
Midbrain functions
coordination of head/eye movement
startle reflexes
integration with basal nuclei
relay stations for descending motor pathway
Pons
bulging brainstem region between the midbrain and the medulla oblongata
Functions processed through the pons
sleep
hearing
balance
taste
facial expression
sensation
respiration
swallowing
bladder control
posture
Medulla oblongata has pyramids which are
two longitudinal ridges formed by corticopinal tracts
Decussation of the pyramids in the medulla oblongata
crossover points of the corticospinal tracts
Ascending sensory tract nuclei in medulla oblongata
somatic sensory information is relayed to somatosensory cortex
Visceral motor nuclei in the medulla oblongata
-cardiovascular center
-respiratory centers
-specialized reflexes (vomiting, sneezing, hiccuping, coughing)
Cerebellum provides precise ... of skeletal muscle contraction
timing and appropriate patterns
Cerebellar activity occurs
subconsciously
Cerebellum has 2 bilaterally symmetrical hemispheres connected medially by the
vermis
Each hemisphere has 3 lobes in the cerebellum
anterior
posterior
flocculonodular
Arbor vitae in the cerebellum
distinctive treelike pattern of the cerebellar white matter
Superior, middle, and inferior peduncles in the cerebellum
connect the cerebellum to the midbrain, pons, and medulla respectively
Function/integration with cerebral cortex (cerebellum)
-cerebellum receives impulses of the intent to initiate voluntary muscle contraction
-proprioceptors and visual signals "inform" the cerebellum of the body's condition
-Cerebellar cortex calculates the best way to perform a movement
-A "blueprint" of coor
Functional brain systems (2 types)
Networks of neurons working together and spanning wide areas of the brain
-limbic system
-reticular formation
Limbic system
deep structures located in the cerebral hemispheres and diencephalon
Limbic system parts
amygdala
cingulate gyrus
hippocampus
Amygdala
anger, danger, and fear responses
Cingulate gyrus
role is in expressing emotions via gestures, and resolves mental conflict
Hippocampus
important in memory
Reticular formation
composed of 3 broad columns of gray matter forming the core of the brain stem
Wide ranging axonal connections
Reticular activating system component
filters sensory input
keeps cortex alert and active
regulates sleep/consciousness
Normal brain function involves
continuous electrical activity
An Electroencephalogram (EEG)
records electrical activity (brain waves)
Brain waves
patterns of neuronal electrical activity
Alpha brain waves
regular, rhythmic, low-amplitude, slow, synchronous waves indicating an "idling" brain
Beta brain waves
rhythmic, more irregular waves occurring during the awake and mentally alert state
Theta brain waves
more irregular than alpha waves, common in children but abnormal in adults (pathological)
Delta brain waves
high-amplitude waves seen in deep sleep and when reticular activating system is damped
2 types of sleep
NREM
REM
NREM
Non-rapid eye movement
REM
rapid eye movement
One passes through ____ stages of NREM during the first 30-45 minutes of sleep
4
REM sleep deprivation causes
moodiness/depression
Memory is the
storage and retrieval of information
2 stages of memory
Short term
long term
Short term memory
lasts a few minutes and is very limated
Long term memory
has practically limitless capacity
Factors that effect transfer of memory from STM to LTM include
-emotional state
-rehearsal
-association
-automatic memory
Traumatic brain injuries (TBI)
due to concussive events as a result of the brain hitting the inside of the cranium
Blows to the head can also result in ruptured blood vessels in the brain or meninges, causing
increased intracranial pressure
Cerebrovascular accident (CVA, stroke)
caused when blood circulation to a portion of the brain is blocked and brain tissue dies
Transient ischemic attacks (TIAs)
temporary episodes of reversible cerebral ischemia
--spasm in smooth muscle of artery
Alzheimers disease
a progressive degenerative disease of the brain that results in dementia
Parkinsons disease
degeneration of the dopamine-releasing neurons of the substantia nigra (basal nuclei)
Huntingtons disease
a fatal hereditary disorder caused by a mutation of the protein huntingtin that leads to degeneration of the basal nuclei
Spinal cord (what marks each end)
CNS tissue is enclosed within vertebral column from the foramen magnum to L1
-provides 2 way communication
Spinal cord protected by
bone, meninges, CSF
Epidural space
space between the vertebrae and the dural sheath (dura mater) filled with fat and a network of veins
Conus medullaris
terminal portion of the spinal cord
Cauda equina
collection of nerve roots at the inferior end of the vertebral canal
Spinal nerves
31 pairs attach to the cord by paired roots
named by the vertebral column region from which they originate
Cervical and lumbar enlargements
sites where nerves serving the upper and lower limbs emerge
Organization of gray matter
Dorsal half
Ventral half
Roots fuse laterally
4 zones
Dorsal half
sensory roots and ganglia
Ventral half
motor roots
Dorsal and ventral roots fuse laterally to form
spinal nerves
Ascending (input) pathways
-The axons of FIRST-ORDER SENSORY NEURONS branch diffusely as they enter the spinal cord and medulla
-some take part in spinal cord reflexes
-Others synapse with SECOND-ORDER NEURONS in the spinal cord or medullary nuclei and continue to ascend to the tha
Descending spinal cord tracts are
output tracts, delivering efferent impulses from the brain to the spinal cord
Descending spinal cord tracts: motor pathways involve two neurons...
upper
lower
Upper neurons
soma located in the brain, axon extends down the spinal cord
Lower neurons
soma located in the spinal cord, axon extends out of the spinal cord in the spinal nerves
Paralysis
loss of motor function
Flaccid paralysis
severe damage to the ventral root or anterior horn cells
-lower motor neurons are damaged and impulses do not reach muscles
-no voluntary or involuntary control of muscles
Spastic paralysis
only upper motor neurons of the primary motor cortex are damaged
-spinal neurons remain intact and muscles are stimulated irregularly
-no voluntary control of muscles
Transection
cross sectioning of the spinal cord at any level results in total motor and sensory loss in regions inferior to the cut
Paraplegia
Transection between T1 and L1
Quadriplegia
transection in the cervical region
Amyotrophic lateral sclerosis (Lou Gehrig's disease)
neuromuscular condition involving destruction of ventral horn motor neurons and fibers of the pyramidal tract
-death will occur