You will note that the 3 chpts on this test build on the chpt on cells and
on molecules. Review these as well!
NEURAL CONTROL MECHANISMS Chpt 8
The nervous system is divided into two parts:
The central nervous system (CNS) comprises the brain and spinal
cord - integrates incoming information and determines response
The peripheral nervous system (PNS) consists of afferent and efferent
neurons extending from the CNS - routes for input to and output from CNS
Information flow through the NS follows the pattern of the reflex arc
(concept introduced in Ch7)
Structure and Maintenance of Neurons
Neuron - functional unit of the nervous system - the nerve cell
The cell body contains the nucleus, a large variety of organelles
including rough ER, Golgi, neurofilaments and neurotubules
Processes - extended portions of the nerve cell
Dendrites receive information from other neurons;
increase receptive surface area
are single or branched - relatively short
Axon (nerve fiber) transmits outgoing signals to target cells
(other neurons or effector cells)
Axon terminal at distal end contains neurotransmitters which
are released to stimulate the target cell
Axon may be covered with sections of myelin separated
by nodes of Ranvier,
Myelin sheath is extension of membrane of either -
Schwann cell in PNS or oligodendrocyte in CNS
Neurotransmitters are syn on ribosome attached to ER, packaged
into vescicles(by Golgi) and moved 'down' length of axon by
slow axonal transport - axoplasmic flow
fast axonal transport - motor proteins (use ATP) walk vesicles
along microtubules [16in/day]
retrograde transport - return path, also fast;
can be subverted by toxins, viruses
Glial cells - numerous, provide physical support, assist in growth and
development of neurons; regulate the ECF composition and transfer
glucose, ions, etc from capillaries to neurons, blood-brain barrier;
act as immune agents to remove damaged cells or 'invaders'
Functional Classes of Neurons Table 8-1
Neurons are classified in three ways:
a. Afferent neurons transmit information into the CNS from receptors at
their peripheral endings.
b. Efferent neurons transmit information out of the CNS to effector
cells.
c. Interneurons lie entirely within the CNS and form circuits with other
interneurons or connect afferent and efferent neurons.
Information is transmitted between these cells across a synapse
neurotransmitters released by a presynaptic neuron and
combine with receptors on a postsynaptic neuron
Neural Growth and Regeneration *
Neurons develop from neuroepithelial cells of embryonic neural tube,
migrate to their final location, and send out processes to their target cells.
Growth of neuronal processes stimulated by neurotropic growth factors
Cell division to form new neurons is markedly slowed after birth and
practically non-existent by puberty
Repair to neurons is limited:
damaged peripheral neurons may regrow the axon to their target organ
damaged neurons of the CNS do not regenerate or restore significant
function.
How does the NS function?
The Resting Membrane Potential
An electrical disequilibrium exists because of an uneven distribution of
ions across the plasma membrane
Due to the presence of large quantities of proteins with negatively charged
regions in cytoplasm, the interior of a cell is negatively charged
relative to the outside
Also there is a higher conc of K+ inside the cell relative to the exterior
ECF contains lots of Na+ and Cl- ions
This differential is the membrane potential (Vm) see Fig 8-7
can be measured by a voltmeter
Vm expressed as millivolts; avg -70 mV
Membrane potentials are generated mainly by diffusion of ions and are
determined by
(a) the ionic concentration and electrical gradients across
the membrane, and
(b) the membrane's relative permeabilities to different ions
Plasma-membrane Na,K-ATPase pumps maintain intracellular sodium
concentration low and potassium high F 8-13
Other ion channels: Cl- and Ca++
Graded Potentials and Action Potentials - Electrical Signals
Neurons signal information by graded potentials and action potentials (APs).
Excitability - ability to generate and conduct action potential
Graded potentials are local potentials (local current flow = net movement of
positive electrical charge), magnitude can vary; extend over short
distances- within 1 or 2 mm of their site of origin;
Response to opening of chemically-gated ion channels
Often named in relation to function or location Table 8-3.
If GP reaches trigger zone of neuron and depolarizes membrane to threshold
voltage - an AP is generated
Action potential is a rapid change in the membrane potential during which the
membrane rapidly depolarizes and repolarizes. Fig 8-18
APs provide long-distance transmission of information through the NS
APs occur in excitable membranes because these membranes contain
voltage-gated sodium channels which open as the membrane depolarizes;
causes a positive feedback toward the sodium equilibrium (conc)
potential. DEPOLARIZATION
AP ends as the sodium channels close and voltage-gated potassium channels
open, which restores the resting conditions. REPOLARIZATION
**see Fig given in class
Depolarization of excitable membranes triggers APs only when the membrane
potential exceeds the threshold value F 8-20
Regardless of the size of the stimulus, if the membrane reaches threshold,
the APs generated are all the same size.
A membrane is refractory for a brief time even though stimuli that were
previously effective are applied.
APs are propagated without any change in size from one site to another
along a membrane. F 8-21
In myelinated nerve fibers, APs manifest saltatory conduction
Functional Anatomy of Synapses
When the action potential reaches the axon terminal, the depolarization
opens the voltage-gated Ca++ channels; Ca++ enters raising the calcium
concentration within the terminal
Ca++ signals the synaptic vesicles to release, by exocytosis,
neurotransmitter into the synaptic cleft.
A neurotransmitter, which is stored in synaptic vesicles in the
presynaptic axon terminal
The neurotransmitter diffuses across the synaptic cleft and binds
to receptors on the postsynaptic cell; the activated receptors usually
open ion channels.
Activation of the Postsynaptic Cell
Whether a postsynaptic cell fires action potentials depends on the
number of synapses that are active and whether they are excitatory or
inhibitory.
An excitatory synapse brings the membrane of the postsynaptic cell closer
to threshold. F 8-27
At an excitatory synapse, the electrical response in the postsynaptic
cell is called an excitatory postsynaptic potential (EPSP)
An inhibitory synapse hyperpolarizes the postsynaptic cell or
stabilizes it at its resting potential F 8-28
At an inhibitory synapse, response is an inhibitory postsynaptic
potential (IPSP).
Usually at an excitatory synapse, channels in the postsynaptic cell that
are permeable to sodium, potassium, and other small positive ions
are opened;
At inhibitory synapses, channels to chloride and/or potassium
are opened.
The postsynaptic cell's membrane potential is the result of temporal
and spatial summation of the EPSPs and IPSPs at the many active
excitatory and inhibitory synapses on the cell. F 8-30
Synaptic Effectiveness
Synaptic effects are influenced by pre- and postsynaptic events,
drugs, and diseases (Table 8-6).
Electrical Synapses
May occur between cells of approx = size
Requires an area(s) of contact with low electrical resistance
Gap junctions serve as channels to permit rapid flow of ions
Common in embryos; later more restricted in distribution - cardiac
muscle, brain
Electric eel
Neurotransmitters and Neuromodulators Table 8-7.
Both require specific receptors in pm of target cell
In general, neurotransmitters cause EPSPs and IPSPs, and neuromodulators
cause, via second messengers, more complex metabolic effects in the
postsynaptic cell.
The actions of neurotransmitters are usually faster than those of
neuromodulators.
A substance can act as a neurotransmitter at one type of receptor and
as a neuromodulator at another.
Important neural messengers
Acetylcholine (ACh) - in both CNS and PNS
In PNS: Target cell of motor neuron is muscle, neuromuscular
junction localized - EPSPs lead to action potential in
muscle cell
Targets of autonomic motor neurons include glands, heart,
and gut
Removed from receptor by AChE [inhib by nerve gas]
Biogenic amines - family of NTs, similar activity
Serotonin - CNS, regulation of mood, behavior, appetite
Many subtypes of receptors [permit variety
of specialty drugs]
Dopamine - midbrain, motor control [Parkinson's];
mid and forebr, behavior and reward,
sensitive to addictive agents
Norepinephrine - CNS and PNS, behavioral arousal, NT for cardiac
and sm mus.
Amino acids - excitatory activity -memory
inhibitory activity - often involved in motor control
[GABA impt in CNS - deficiency=Huntington's chorea]
Neuropeptides - CNS, Neuromodulators, mechanism complex and poorly
understood
Endogenous opioids - raise pain threshold
Neuropeptide Y - response to stress, circadian rhythms…
TEXT: SecD Summary [edited]
I. Inside the skull and vertebral column, the brain and spinal cord
are enclosed in and protected by the meninges.
Central Nervous System: Spinal Cord
I. The spinal cord is divided into two areas: central gray matter,
which contains nerve cell bodies and dendrites; and white matter,
which surrounds the gray matter and contains myelinated axons organized
into ascending or descending tracts.
II.The axons of the afferent and efferent neurons form the spinal nerves.
Central Nervous System: Brain
I. The brain is divided into six regions: cerebrum, diencephalon, midbrain,
pons, medulla oblongata, and cerebellum. F 8-38,40,41
II.The midbrain, pons, and medulla oblongata form the brainstem, which
contains the reticular formation (medulla - reg. centers for heart
rate, bl. pressure, breathing).
III.The cerebellum plays a role in posture,coordination of movement, and
some kinds of memory.
IV.The cerebrum, made up of right and left cerebral hemispheres, and the
diencephalon together form the forebrain.
The cerebral cortex forms the outer shell of the cerebrum and is
divided into parietal, frontal, occipital, and temporal lobes.
Region links sensory and motor functions (association); interpretaion,
thought, learning;
V. The diencephalon contains the thalamus (relay center) and hypothalamus
(integration center for regulation of internal organs - input to
medulla and sp.cord).
VI.The limbic system is a set of deep forebrain structures associated with
learning and emotions.
Peripheral Nervous System
I. The peripheral nervous system consists of 43 paired nerves¾12 pairs
of cranial nerves and 31 pairs of spinal nerves.
Most nerves contain axons of both afferent and efferent neurons.
II.The efferent division of the peripheral nervous system is divided into
somatic and autonomic parts.
The somatic fibers innervate skeletal-muscle cells and release the
neurotransmitter acetylcholine.
The autonomic nervous system innervates cardiac and smooth muscle,
glands, and gastrointestinal-tract neurons.
Each autonomic pathway consists of a preganglionic neuron with
its cell body in the CNS and a postganglionic neuron with its
cell body in an autonomic ganglion outside the CNS.
a. The autonomic nervous system is divided into sympathetic and
parasympathetic components. The preganglionic neurons in both
sympathetic and parasympathetic divisions release acetylcholine;
the postganglionic parasympathetic neurons release mainly acetylcholine;
and the postganglionic sympathetics release mainly norepinephrine.
b. The adrenal medulla is a hormone-secreting part of the sympathetic
nervous system and secretes mainly epinephrine.
c. Many effector organs innervated by the autonomic
nervous system receive dual innervation.
Pay particular attention to
Tables 1,2,3,7,9,11
Fig 2,3,7,8,11-14,18,20,21,25,27,28,30,31,36,38
We will refer to this portion of Chpt 8 as we continue in the course.
Epsecially Table 8-13. don't memorize it now
Blood Supply, Blood-Brain Barrier Phenomena, and Cerebrospinal Fluid
I. Brain tissue depends on a continuous supply of
glucose and oxygen for metabolism.
II. The brain ventricles and the space within the
meninges are filled with cerebrospinal fluid, which is formed in
the ventricles.
III. The chemical composition of the extracellular
fluid of the CNS is closely regulated by the blood-brain barrier.