Nervous system Flashcards
How can cells communicate – role of the nervous and endocrine systems
by chemical and electric signals
they integrate and coordinate all body systems
classification of the nervous system (parts and types of neurons)
- Central (CNS); brain and spinal cord – relay neurons/interneurons
- Peripheral (PNS); somatic (voluntary) and autonomic (involuntary) – sensory and motor neurons
nerve – types – largest nerve in the body
a bundle of neurons enclosed in a protective coating
some nerves only contain sensory (optical nerve), some only motor, and some both types of neurons
sciatic nerve
types of neurons and their functions
Sensory – receives information from receptors about internal or external env of the body (eyesight and touch in somatic and internal movements, pH, concentration, temperature in autonomic) and transmits it to the CNS – cell body is positioned in the middle of the cell (on the axon)
Motor – receives information from the CNS and transmits it to the effector (skeletal muscles in somatic and smooth muscles and glands in autonomic NS) – e.g. brain sends an impulse to the adrenaline gland (directly because endocrine system is slow) – cell body is located on the terminal (one side of the neuron)
Interneurons (relay) carry impulses within the CNS from one to another neuron – connecting motor and sensory
Brain function and CNS anatomy (and functions of each structure)
Receives, processes (decision making) and stores information (memory, more synapses = better memory)
1) Cerebrum (big brain) made of:
a) cerebral cortex (surface of the cerebrum made of grey matter (dendrites) that receives and processes the impulse)
b) white matter (inside of cerebrum made of axon terminals)
2) Hind brain made of:
a) cerebellum (small brain) – coordinates and fine tunes contraction of skeletal muscles, regulates balance, timing and coordination of muscles and muscle memory
b) medulla oblongata – autonomic functions like cardiovascular center, pH and temperature control
c) Most of the 12 pairs of cranial nerves are connected to the hind brain
d) Hypothalamus and pituitary gland are a part of the nervous system but have control over the entire endocrine system
3) Spinal cord – inside the vertebral column, closer to the dorsal side (ventral (front) and dorsal (back)) – has white matter on the surface and grey matter on the inside – coordinates unconscious processes, especially reflexes
a) 31 pairs of spinal nerves connected to the spinal cord – sensory nerve roots on the dorsal side and motor nerve roots on the ventral side
What is a stimulus – describe the entire related mechanism
a type of energy from internal or external env that is converted into electrical energy by receptors – nerve impulse conducted by sensory nerves (either to brain by cranial or to spine by spinal nerves) to the relay nerves in CNS where information is processed, and the impulse is then carried by motor neurons to the effectors that carry out the response to the stimulus
The reflex arc – elements (mechanism) of the reflex arc
a rapid, involuntary response to a specific stimulus – the simplest type of coordination by the NS because signals pass through the smallest number of neurons (which speeds up the process) – synapses between the neurons in the arc are either in the spinal cord (pain reflex) or the brain (photo-pupillary)
1. Receptor cells which come in two forms: specialized receptors (stretch, chemoreceptors, photoreceptors in retina) or nerve endings (elongated dendrites) of sensory neurons (touch, heat)
2. Sensory neurons
3. Interneurons – in the grey matter, process signals by combining impulses from multiple inputs and passing impulses to specific other neurons – simple decision-making process because there may be only one interneuron
4. Motor neurons
5. Effector – carry out the response to the stimulus
Example of a reflex arc
the pupillary reflex – protects the retina from overly bright light – bright light detected by photoreceptors (cons and rods) in the retina, signal travels to the optical center in the brain via cranial nerves, signal is sent by motor neurons to the smooth muscles in the iris to contract and thus shrink the pupil
Feedback mechanism – variables and types of receptors
Mechanism that controls internal environment of the body by detecting the change and responding to it by either counteracting the further change in the same direction (negative) or stimulating it (positive)
Variables of internal environment: temp of blood, p(CO2), p(O2), C(glucose) (only in pancreas), blood pressure (baroreceptors), C(blood)
Chemoreceptors, thermal receptors, baroreceptors, stretch receptors, osmoreceptors (C(blood))
Negative feedback mechanism – describe the general process – examples
Nervous system undertakes action to counteract the perceived stimulus.
Receptors detect changes of some variable in the body. This info is relayed to a coordinator that determines the level of response. The coordinator relays a decision to the effector that produces the response. The response will modify the internal env, and new conditions will become new stimuli. The cycle continues until conditions are within acceptable limits.
For example, photo-pupillary reflex, heart rate and ventilation control and peristalsis
Ventilation control
Blood pH is too acidic, chemoreceptors in the vessel walls (aorta, carotid…) sense this, send an impulse via sensory neurons to the coordinator (center in medulla oblongata), impulse is sent back via motor neurons to muscles (diaphragm and intercoastal), ventilation is increased, less CO2 in the blood so blood pH increases to the homeostatic level (7.35-7.45 range)
Heart rate control
Blood pH and pressure (baroreceptors in carotid and aorta) info sent to the cardiovascular center in medulla oblongata => if pH is too low sympathetic nerve stimulates the adrenal gland (effector) to release adrenaline into blood into myocardial cells to increase the heart rate and thus the turnover of blood => if pH is too high parasympathetic nerve stimulates lower heart rate (also called vagabond nerve or nervus vagus)
Peristalsis
Food pushed back by the tongue activates touch receptors in the pharynx, signal passes to the brainstem which stimulates muscle contraction that pushes food into the esophagus. Once there, peristalsis controlled by ENS and is completely involuntary
Peristalsis – longitudinal and circular muscles in the intestine change the diameter and length of the intestine by intermittent contractions and relaxations and thus direct the flow of materials through the intestine – controlled by the enteric nervous system which is the largest and most complex part of PNS (involuntary part peristalsis, striated muscles of tongue and egestion (depending on age)). Anus is a circular muscle – external is striated, internal smooth, rich blood supply needed, when the blood vessels there enlarge called haemorrhoids.
Structures of motor neuron and their functions
1) Dendrites – endings that provide a large SA for connecting with other neurons, they receive the receive nerve impulse and carry it towards cell body
2) Single long axon – carries the nerve impulse away from the cell body and towards the axon terminal
3) Schwann cells – companion cells of neurons
4) Myelin sheath – a thick, insulating lipid layer, formed by Schwann cells wrapping around the axon
5) Nodes of Ranvier – small gaps along the axon (Myelin sheath not continuous)
Why are nodes of Ranvier important? Disease connected with it?
They enable saltatory conduction that is propagation of the nerve impulse along a non-myelinated axon – when the impulse “jumps” from node to node the conduction is faster than through the axon – the larger the axon diameter the smaller the resistance
Multiple sclerosis – myelination, no nodes of Ranvier slow nervous system
Membrane potential
Voltage, unequal distribution of charges between inside and outside of a neuron – more positive outside (greater concentration of cations (Na+) and inside there are some anions (negatively charged proteins and HCO3-) along with a smaller contraction of cations (K+))
In a resting cell (neuron)…
Ions diffuse down their C gradients through leaking channels (voltage-gated) causing the electrochemical gradient to dissipate. – the resting potential (-75 mV) is restored by Na/K pump (pumps 3 Na+ out and 2 K+ in) – membrane is said to be polarized
In an activated cell (neuron)…
1) Activated when a stimulus is applied: Voltage-gated channels open when a high charge is applied (it changes their tertiary and quaternary conformation)
2) Na+ ions diffuse inside, membrane is depolarized as resting potential goes to action potential (+35 mV)
3) When +35 mV is reached, Na+ channels close and K+ channels open
4) K+ ions flow out of the neuron and the membrane is repolarized
5) The channels stay open for a little longer than necessary so more K+ flows out, making the final potential below the resting (refractory period at -80 mV) – tis hyperpolarization is inevitable and it ensures unit directional propagation of the impulse (no backflow)
6) Refractory period is a “rest period” for ion channels before they open again, and it lasts around 2 ms
7) Na+/K+ pump is used to reestablish the needed distribution of ions
Oscilloscope traces – phases of impulse transmission
resting potential – (depolarization) –> action potential – (repolarization) –> refractory period
What are local currents?
The diffusion of Na+ down C gradient outside and inside the neuron – charge increases in the previously resting area, their voltage-gated channels open and Na+ diffuses inside – thus propagation of the nerve impulse in one direction – prevents backflow
Nerve impulse definition – what enables it?
A change of the voltage (action potential) that travels along the axon
Possible due to fact that the membranes of nerve and muscle cells are electrically excitable (membrane potential can be changed) – a stimulus changes the permeability of the membrane to ions
What happens if a stimulus is too weak?
Filed initiation happens because only some channels will open, and the change of charge will be insufficient to reach the threshold potential (-50 mV) and create an action potential (stimulus not perceived)
Synapse
Neurotransmitters
A narrow and fluid filled junction between two neurons or a neuron and an effector (muscle or gland)
Proteins that are used to propagate the nerve impulse from one neuron to another by crossing the synapse (exocytosis from presynaptic neuron)
Propagation of the nerve impulse – describe the mechanism
1) Ca2+ leaking channels on the axon terminal opened by a nerve impulse
2) Ca2+ ions diffuse into the neuron
3) Increased concentration of calcium ions triggers the exocytosis of neurotransmitters by pushing the vesicles that contain them (called synaptic vesicles) towards the membrane
4) Vesicles fuse with the membrane and release NT into the synapse which diffuse towards the postsynaptic neuron
5) On the postsynaptic neuron, there are Na+ channels that are also binding sites for specific NT called neuroreceptors
6) Binding of NT opens the channel, Na+ ions diffuse in and the nerve impulse is created in the other neuron
7) NT later digested by an enzyme in the synaptic cleft and the breakdown products are absorbed by the pre-synaptic neuron and used to re-synthesize more neurotransmitters – this prevents overstimulation of the neuron and constant nerve impulse creation
Two types of transmitters – describe their role and give examples
Each neuron produces a single type of neurotransmitter
Excitatory transmitters – stimulate nerve impulse propagation (e.g. acetylcholine, glutamate) – bind with Na channels – acetylcholine broken down by acetylcholinesterase
Inhibitory transmitters – inhibit transmission by causing hyperpolarization of the neuron (e.g. GABA) – bind to Cl- channels on the postsynaptic neuron, open them and cause Cl- ion diffusion inside – makes the membrane hyperpolarized and the action potential cannot be reached because the charge is pushed further away from the threshold potential value (hyperpolarization at -80 mV, TP at -55 mV)
Exogenous chemicals – what are they, examples and what is their effect?
chemicals originating from outside of the body that affect the synaptic transmission – pesticides and drugs (intake through skin barrier, inhalation, digestion, injection…) – some excite and some inhibit the nerve impulse
1. Neonicotinoid pesticide – synthetic derivative of nicotine, binds to acetylcholine receptors on the postsynaptic neuron, blocks receptors (cannot be digested by acetylcholinesterase), overstimulates the receiving neuron (constantly transmitting NI), leads paralysis and eventual death (insect)
2. Cocaine – joins with dopamine reuptake channels on the presynaptic neuron, blocks them (dopamine constantly in the junction and binds to postsynaptic neuron’s receptors), overexcited, NI constantly transmitted (feeling of euphoria)
Perception of pain mechanism
stimuli like chemical substances, heat and puncturing identified by nerve endings and NI sent to sensory areas of the cortex – from there signal carried to the prefrontal cortex where pain is recognized, brain evaluates the situation in order to send an impulse to the effectors to reduce the exposure to the pain stimulus
What is summation?
the multiple releases of either only excitatory or both excitatory and inhibitory neurotransmitters that results in the creation of an action potential (needs to be more excitatory than inhibitory) – either multiple times by a single cell in a short time period or by multiple cells simultaneously
Consciousness
the state of complex awareness and an emergent property (result of interactions between many individual elements of a system/brain nerves) – reduced in sleep (partial consciousness)
