The nervous system Flashcards
Sensory
the nervous system senses changes from within the body or from outside of the body and conveys this information to the spinal cord and brain.
Integrating
the sensory information received is analysed, stored and integrated to produce a response.
Body
The integrated response may be to command muscles to move or produce glandular secretions
Nervous tissue is classified as and functions as
Classified as:
neurons (aka nerve cells)
neuroglia (aka glia)
Functions:
neurons conduct nerve impulses,
while neuroglia protect and support neurons
Neuroglia
Special connective tissue of nervous tissue.
Insulate neurons
Nourish neurons
Support neurons
Protect neurons
Neuron’s
Monitor changes in the internal and external environment
Convert stimuli (which are detectable changes in the environment) to nerve impulses
Conduct these impulses to other neurons, muscle cells, or gland cells
Cell body of a neuron
Cell body (Soma or perikaryon): includes nucleus and cytoplasm (containing normal cellular organelles).
Dendrites of a neuron
Highly branched, short, numerous cell processes
Receive stimuli
From other neurons
May be modified as sensory receptors for heat, stretch, pressure
Bring impulses toward the cell body
Axon in a neuron
Single process
Conducts impulses away from cell body to other cells (Nerve, muscle and gland cells).
A single axon may be called a nerve fiber
A single process can be very long i.e. a single axon in the horse may extend over two meters from the spinal cord all the way to the lower leg.
Bundles of axons are called
Tracts in the CNS
Nerves outside the CNS
Axons are covered in myelin called
Cell membrane of glial cells tightly wrapped around axon:
Schwann cells in the PNS
Oligodendrocytes in the CNS
Myelin are
Myelin is an insulator that speeds the conduction of impulses along axons (faster than unmyelinated axons)
Myelin is white
Nervous tissue which contains lots of myelinated axons is called white matter
Nervous tissue which does not contain myelin looks dark, and is called gray matter
This type of nervous tissue often contains the cell bodies.
CNS is
Central nervous system or CNS, is anatomically composed of the brain and the spinal cord.
Both are found in the center (cranial-caudal axis) of the animal’s body.
PNS is
Peripheral nervous system or PNS
Those parts of the nervous system that extend away from the central axis toward the periphery of the body.
Composed of:
Cranial nerves which originate from the brain
Spinal nerves which emerge from the spinal cord.
PNS system is divided into
The peripheral nervous system is subdivided into autonomic and somatic nervous systems based on their function
Motor function
Voluntary control of skeletal muscles, such as conscious movement.
Sensory function
Information that is consciously perceived
Information from the muscles, skin, eye or ear are consciously perceived information about the internal and external environment.
Many fibers are myelinated
Autonomic nervous system: unconscious
Co-ordination of automatic (unconscious) body functions
Sensory and motor to the visceral structures (smooth and cardiac muscle and endocrine glands)
Automatically regulates a wide variety of functions e.g. heart rate and GI motility
Maintains internal balance or homeostasis
Many fibers are non-myelinated
Sodium potassium pump
More sodium leaves than potassium enters
Net negative charge inside cell membrane
Membrane is polarized
Difference in charge across membrane is resting membrane potential
Nerve cells also have specialized sodium and potassium transport proteins
Can be called channels or gates
These are closed at rest
Depolarization of neuron
Upon stimulation, Na+ channels open,
Allow only sodium to enter the cell
Moves down it’s concentration gradient by diffusion
Positive ions neutralize negative charge inside the cell
Called depolarization
Repolarization of neuron
Na+ channel closes
K+ channels open
K+ ions flow out of cell
Outflow of K+ ions restores net negative charge inside of cell
Concentration gradient and electrical charges drive K+
The whole process of charge is rapid and is called an action potential
Return to resting state of neurons
The difference between the repolarized and resting states is that the sodium ions are now on the inside of the membrane and potassium ions are on the outside of the membrane
Active transport by the sodium-potassium pump slowly restores ion concentrations
Refractory period of neurons
During depolarization and early repolarization, the neuron cannot respond to another stimuli until it has re-established the resting membrane potential
Absolute refractory period
Will not respond under any circumstances
Relative refractory period:
During end of repolarization period
Possible to stimulate another depolarization if stimulus is ver large
A nerve impulse
Electric charges cross cell membrane (depolarization), followed by repolarization.
Adjacent area is stimulated because opening of the gates is triggered by the change in voltage, spreading a wave of depolarization i.e. the nerve impulse is conducted along the nerve.
Cannot be conducted backwards because this area is refractory
Threshold stimulus
Depolarization in response to a stimulus is strong enough to depolarize adjacent membrane and generate a nerve impulse
All-or-Nothing principle
If stimulus is too weak to open adjacent channels, the wave will not spread
Saltatory conduction: rapid conduction in myelinated axons
Transmission of the impulse occurs from gap to gap
Speeds the rate of conduction
Called saltatory conduction
Depolarization in myelinated axons can only take place at the gaps called the nodes of ranvier
How neurons communicate
Transmission of impulses from one neuron to the next cannot occur directly because they don’t touch
Synapse is the junction between two neurons or between a neuron and a target cell
The gap is called the synaptic cleft
Many neurons will have more than one synapse, and many will have thousands
Allows complex integration of signals
The presynaptic neuron releases a neurotransmitter
Received by the postsynaptic neuron
Synaptic end bulb is
The synaptic end bulb is the end of the presynaptic axon
Contains many mitochondria
Provide energy
Has neurotransmitters in vesicles
Releases the neurotransmitters when a wave of depolarization reaches the end bulb and opens calcium channels
Neurotransmitters diffuse across the synaptic cleft to bind to receptors specific for that neurotransmitter on the postsynaptic membrane of the other cell
Membrane depolarizes
Types of neurotransmitters and their effect on postsynaptic membrane
Postsynaptic membranes may have receptors for more than one neurotransmitter
There are excitatory and inhibitory types of neurotransmitters (NT)
Excitatory NT will tend to cause depolarization
Inhibitory NT will tend to increase the polarization (hyperpolarize) of the membrane and make it less likely to depolarize/fire (i.e. Will help prevent a signal from being transmitted)
The relative balance of excitatory NT and inhibitory NT determines the overall effect on the postsynaptic membrane – called summation
Acetylcholine
Very common NT in the body
Can be excitatory or inhibitory, depending on where found
Excitatory at somatic neuromuscular junctions
Inhibitory on the heart (slows rate)
Norepinephrine, dopamine, and epinephrine
Group of NT called catecholamines
Norepinephrine – associated with ‘fight or flight’
Epinephrine – also ‘fight or flight’, but can be released from the adrenal medulla as a hormone as well as a NT
Dopamine – found in the brain and is important for autonomic functions, muscular control, and muscular control
Gamma-aminobutyric acid (GABA) and glycine
Inhibitory NT
GABA is in brain
Glycine in spinal cord
Some tranquillizers affect GABA receptors to increase its inhibitory activity in the brain → sedation
Diazepam (valium)
