Exam 4 Part II Flashcards
The “hypothalamic thermostat”:
The “hypothalamic thermostat”: one of the main areas is the preoptic area, also anterior hypothalamus. Heat-sensitive neurons are found there that respond to direct stimulation and also to signals from warm and cold receptors in the skin.
. Another area associated with heat control is the
. Another area associated with heat control is the posterior hypothalamus.
Heat loss mechanisms (sometimes considered preoptic) prevent a rise in body T
vasodilation in dermis
increased sweating: sym. ACh neurons onto sweat glands (some adrenergic on hands & feet use NE)
Dec. heat production (heat loss mechanisms)
decreased muscle tone – mainly
chemical thermogenesis (caused by Epin. & NE) is reduced.
also: ↓ thyroid hormones
Heat gain mechanisms – heat production and conservation
vasoconstriction in dermis
decreased sweating
increased muscle tone – shivering
chemical thermogenesis by Epin. and NE increased
also: thyroid hormones are secreted when part of the hypothalamus gets colder
___activate the heat gain mechanisms
Pyrogens activate the heat gain mechanisms
At the beginning of a fever a person feels cold (has a chill) because
At the beginning of a fever a person feels cold (has a chill) because the hypothalamic thermostat has been reset upward. Shivering, etc., will raise the body T to the new hypothalamic setting.
Benefits of a fever
Phagocytosis and other WBC activity, e.g., antibody production, are enhanced (some bacteria may be inhibited).
disadvantages of a fever
Higher T makes all chemical reactions faster → metabolic rate is high which increases the body T. Heat stroke can result.
Most CNS synapses are _rather than _, and
Most CNS synapses are chemical rather than electrical, and through chemical synapses, conduction is one-way.
In the synapse, there are ligand-activated (transmitter-activated) channels of mainly 3 types:
mainly Na+, some K+: same channel, excitatory
K+, inhibitory
Cl- and a few anions allowed to pass, inhibitory
Often transmitters do tend to be either excitatory or inhib. but
not always
amine transmitters
NE, Epi., DA (dopamine), 5-HT (serotonin)
Antipsychotics: many block
Antipsychotics: many block DA receptors = reduced dopamine activity.
Antidepressants: many block reuptake of
Antidepressants: many block reuptake of serotonin, some very specifically = SSRIs = specific serotonin reuptake inhibitors. In the short-term at least, SSRIs increase 5-HT in the synaptic cleft.
amino acid anti anxiety drugs
caution: also anti-memory!, also anti-life: 2X risk of death! diazepam = valium), glycine (inhibitory, strychnine blocks: can cause convulsions), glutamate (excitatory, MSG = monosodium glutamate, can be an excitotoxin), aspartate (excitatory)
neuroactive peptides
hypothalamic releasing hormones
- pituitary peptides, e.g., ACTH
- substance P (associated with sensory pathways and pain)
- endorphins = endogenous opiates like enkephalins (block pain) - other peptides, e.g., VIP, insulin, CCK in gut and brain
NO (nitric oxide), CO: both activate
NO (nitric oxide), CO: both activate guanyl cyclase, produce cGMP
Some transmitters can act as __where the main transmitter is something else. They can either enhance or inhibit main transmitter effects.
neuromodulators
An action potential is not generated at the soma (cell body). It is instead generated at
the axon hillock because many times greater number of Na+- channels exist there than at the soma. It is difficult to open the required number of channels at the soma.
Whether there is a net depolarization or hyperpolarization in a postsynaptic cell is a result of the summation of the
the excitatory and inhibitory inputs at synapses.
Spatial summation of postsynaptic potentials at different synapses on a cell
, say, spatial summation of Na+ & Cl-: can’t predict net depolarization or hyperpolarization
Temporal summation of synaptic potentials at the same synapse.
Repetitive firing of the presynaptic cell associated with that synapse causes transmitter to be released repetitively.
Facilitation:
enhancement or reinforcement of nervous activity by arrival of other excitatory impulses. Summation has been going on; it is not yet enough to reach threshold, but brings the membrane closer to it. The synapse is said to be facilitated.
B. Some characteristics of synaptic transmission
fatigue
hypoxia
synaptic delay
fatigue and synaptic transmission
t can occur when an excitatory synapse is fired at a very fast rate. At first a large amount of transmitter is released and then progressively less, and consequently the postsynaptic cell fires progressively less. Synaptic fatigue is considered to be what stops a convulsion. One might consider it protective.
hypoxia and synaptic transmission
“Lack of oxygen supply can cause complete inexcitability.” Some cells may be dormant and not dead, as evidenced by some improvement of people and animals while in a hyperbaric O2 chamber.
synaptic delay
Synaptic delay is the time from the loss of an action potential as it enters the nerve terminal of one neuron to the time an action potential starts in the next neuron. 0.5 msec.
spatial summation
For example, a pin prick stimulates many different fibers simultaneously = spatial summation. A weak stimulus only stimulates a few fibers, a strong one many more. Signal strength can be conveyed by simultaneous activation of a number of fibers.
temporal summation
Signal strength can also be conveyed by increased frequency of nerve impulses in each fiber = temporal summation.
divergence
Divergence: signals can diverge into multiple tracts, going to different places. E.g., the motor cortex sends signals toward the muscles and also to the respiratory center. OPTIONAL It is often necessary for neurons entering a group of neurons or neuronal pool to excite many more neurons leaving the group or pool. This is called divergence.
Divergence can
Divergence can amplify a signal: the input signal spreads to an increasing number of neurons as it goes through successive pools.
Convergence:
Convergence: It can be from a single source (more than 1 cell) or from several different sources onto 1 cell. This can be necessary because neurons normally need more than 1 input to be excited.
Just the right amount of facilitation and inhibition is needed to
Just the right amount of facilitation and inhibition is needed to get a faithful representation transmitted.
The cortex (and other central areas, hypothalamus, amygdala…) can
control the degree of facilitation or inhibition of different sensory pathways by impulses via centrifugal nerve fibers onto sensory pathways. Our private definition of salience!
Lateral inhibition provides contrast in the spatial pattern which is
important in sensory discrimination. Studied in visual discrimination especially. Enhanced visual signals at edges of light & dark
Since every part of the nervous system is connected to every other part, if one part excites another which excites another, it may result in a
a reexcitation, called a reverberating circuit. Reverberating circuits are theorized to operate in some forms of learning and actually do operate in epileptic seizures
Some groups of neurons exert gross inhibitory control over
widespread areas of the brain. Some parts of the basal ganglia are exerting inhibitory influences over the motor control system, for example. The majority of the nervous system is inhibitory.
Mechanoreceptors: -
Mechanoreceptors: - deformation of some kind
skin and deep receptors
Skin tactile senses
free nerve endings – and also pain
Meissner’s corpuscles - touch
Deep tissue
Pacinian corpuscles
Muscle spindle receptors- stretch receptors
Golgi tendon receptors\
Spray-type endings (Ruffini’s endings)
Hearing: hair cells - receptors in cochlea
Equilibrium: vestibular receptors – hair cells
Arterial P: baroreceptors of carotid sinus and aortic arch
Thermoreceptors:
Thermoreceptors: warm and cold
Nociceptors:
Nociceptors: pain: free nerve endings
Electromagnetic receptors:
Electromagnetic receptors: rods and cones. Hyperpolarize to light
Chemoreceptors:
Chemoreceptors: taste, smell, osmolality, blood CO2, H+, O2, blood glucose, amino acids, fatty acids (receptors in hypothalamus)
B. Sensory transduction
B. Sensory transduction = conversion of whatever modality it is to an electrical signal
receptor potentials - graded responses
If the receptor potential is greater than threshold, the more it is above threshold the greater the action potential frequency. Not the greater the action potential amplitude: A.P. amplitudes are all-or-none.
Adaptation of receptors
All sensory receptors adapt. When the stimulus is first applied, there is a response and then with continued application, the receptor ceases to respond or responds less. Some types of receptors are rapidly adapting and some are slowly adapting.
function of rapidly adapting receptors
Such rapidly adapting ones can be used to detect changes in stimulus strength. They are rate receptors, movement receptors or phasic receptors. The Pacinian corpuscle signals every time there is a change. Once there’s no change they cease to respond.
function of slowly adapting receptors
Tonic receptors: They detect stimulus strength e.g., slowly-adapting joint receptors let the brain know of body position. Others are the pain receptors, baroreceptors, chemoreceptors of the carotid and aortic bodies and vestibular receptors. Remember that the carotid and aortic baroreceptors take a few days to adapt.
Touch sensation comes from
Touch sensation comes from stimulating tactile receptors in the skin and immediately below.
Pressure from stimulating
stimulating deeper tissues (there are different specific receptors, but also it’s determined by location. )
Vibration comes from
Vibration comes from rapidly repetitive sensory signals. Some of the same types of receptors that detect touch and pressure are used: the rapidly adapting ones
pacinian corpuscles
superficial and deep
meissner’s corpuscles
superficial
Pacinian corpuscles detect the
Pacinian corpuscles detect the fastest vibrations at 60-500 cps.
Meissner’s corpuscles are
Meissner’s corpuscles are less rapidly adapting and they signal up to 80 Hz = cps= cycles per second.
Dorsal column - lemniscal system:
Fibers go straight up the dorsal columns of the spinal cord and then cross over at the medulla and travel through the medial lemniscus to the thalamus. Not light touch nor feather touch
Anterolateral system (anterior & lateral spinothalamic tracts)
Fibers immediately cross to the opposite side of the spinal cord, travel via the anterior and lateral portions of the spinal cord to go to the brainstem and thalamus.
C. Transmission in the dorsal column-lemniscal system
Transmission is via large, myelinated fibers and is fast with a high degree of spatial orientation of nerve fibers with respect to their origin on the surface of the body. This spatial orientation is preserved all the way up to the somatosensory cortex. “Precise point-to-point somatotopic (topographic) map”
Areas of the body with large representations correspond to the fact that
in these areas of the body, there are the greatest numbers of these specialized receptors.
Two-point discrimination ability is better in areas with
Two-point discrimination ability is better in areas with a lot of receptors, small receptive field size, separate neurons carrying that info away.
If somatosensory area I is removed:
can only localize sensation crudely
unable to judge degrees of pressure against the body, unable to judge wt. of objects
astereognosis - unable to judge shapes or forms of objects
can’t judge textures of materials (requires that one make judgment about what touched
one set of sense receptors immediately before)
All kinds of touch, pressure discriminations are lost.
Somatic association areas are responsible for
Somatic association areas are responsible for interpreting some of the signals to primary cortex. Electrical stimulation can sometimes cause a complex sensation in the body, sometimes even the feeling of an object, like a knife or a ball. When this area is removed, the person can’t recognize complex objects by feeling them.
D. Transmission in the anterolateral syst. (Ant. & lat. spinothalamic
Velocities are only ½ to 1/3 as great as in the dorsal column system and localization is poor.
