WEEK 3 BIOSCIENCE - INTRO TO THE NERVOUS SYSTEM Flashcards
Functions of the nervous system
- sensory function: receptors detect sensory input, which is sent to the control centre
- integrative function: analyses and interprets sensory input, determines an appropriate response, generates motor output that causes the response
- motor function: issues motor function to activate effector
general sensory receptors
- thermoreceptors: detect changes in temperature
- nociceptors: detect painful stimuli
- mechanoreceptors: tactile receptors (touch, pressure & vibration), baroreceptors (changes in blood pressure), proprioceptors (body position)
special sensory receptors
- photoreceptors: detect light
- chemoreceptors: detect chemicals in solution
- mechanoreceptors called hair cells: detect hearing and balance stimuli
motor output
Activates a specific muscle to contract or a gland to secrete to cause a response
CNS
- consists of the brain and spinal cord
- control centre -> performs the function of integration
- controls our emotions, behaviours and personality
- performs intellectual (cognitive) functions
- stores memories
PNS
- consists of sensory receptors and the cranial, spinal and peripheral nerves that link all parts of the body to the CNS
- cranial nerves and their branches primarily innervate structures of the head and neck
- spinal nerves branch to form the peripheral nerves that innervate all parts of the body below the head
PNS sensory division
- afferent
- conveys sensory input from receptors to the CNS
PNS motor division
- efferent
- conveys motor output from the CNS to a muscle or gland
Motor division: somatic nervous system
- conveys “somatic” motor output from the CNS to the body’s skeletal muscles
- somatic motor output controls voluntary skeletal muscle movements and involuntary skeletal muscle movements (somatic reflexes)
Motor division: autonomic nervous system
- conveys “autonomic” motor output from the CNS to the body’s glands, cardiac and smooth muscles
- autonomic motor output controls involuntary activities: heart rate, respiration, blood vessel and pupil diameter, digestion of food, urination and defecation, perspiration and salivation
autonomic NS: sympathetic division
- controls “fight or flight” activities -> activates body functions that support physical activity and inhibits those that don’t
- increases heart rate, respiratory airflow, blood flow to skeletal muscles and sweat gland activity
- dilates pupils
- inhibits digestive functions
- inhibits urination and defecation
autonomic NS: parasympathetic division
- controls “rest and digest” activities -> activates body functions that conserve and restore body energy
- stimulates digestive functions, urination and defecation
- constricts pupils
- decreases heart rate
- decreases respiratory airflow
neuroglia
- support neuron development and function
- six different types of cells which collectively nourish, protect, insulate and structurally support neurons
structural components of a neuron
- dendrites
- cell body
- axon (fiber)
- axon terminals
dendrites
- short processes
- are the central receptive (or input) region of a neuron
- act as sensory receptors to detect stimuli
- receive information from other neurons
- convert the information they receive into a graded potential which conveys the information towards the cell body
cell body
- Contains a nucleus and organelles, e.g., ribosomes, to synthesise chemical neurotransmitters
- Receives information from other neurons & converts this information into a graded potential
- Integrates information (graded potentials) and conveys information towards the initial segment (or first part) of the axon
axon
- A single process that connects to the cell body at the axon hillock
- Is the conducting region of a neuron
- generates & conducts action potentials to convey information from the initial segment to the axon terminals
- Can be covered with a segmented myelin sheath
myelin sheath
- produced by Schwann cells and oligodendrocytes
- increases the speed of signal conduction
- gaps separate segments called nodes of Ranvier (internodes)
- the destruction of myelin (oligodendrocytes) in the CNS à multiple sclerosis
axon terminals
- Form a synapse with another cell i.e. a neuron, muscle or gland
- Are the secretory region of a neuron
- contain synaptic vesicles that store and release
neurotransmitters chemicals that carry the
information from one neuron to another
or to a muscle cell or gland
Neuron cell bodies are organised into…
- nuclei (nucleus) in the CNS
- ganglia (ganglion) in the PNS
Neuron axons are bundled into…
- tracts in the CNS
- nerves in the PNS
classifications of neurons
multipolar
bipolar
unipolar
sensory neurons
- Conduct sensory input from receptors to the CNS
- Unipolar in structure
interneurons
- Conduct information within the CNS
- Multipolar in structure
motor neurons
- Conduct motor output away from the CNS to a muscle or gland
- lower motor neurons conduct somatic motor output
- preganglionic & postganglionic neurons conduct autonomic motor output
- Multipolar in structure
chemically gated channels
- Open in response to a chemical stimulus, e.g. neurotransmitters
- Located along the plasma membrane of the dendrites & cell body
mechanically gated channels
- Open in response to mechanical stimulation e.g. touch, vibration and pressure
- Located along the plasma membrane of
the dendrites
voltage-gated channels
- Open and close in response to voltage changes (i.e. changes in membrane potential)
- Located along the plasma membrane of the axon and axon terminals
depolarization
= membrane potential becomes less negative
- When a stimulus opens Na+ gated channels:
-> influx of Na+ ions into the ICF
-> ICF gains +ve ions cell interior becomes less negative
-> membrane potential becomes less negative e.g. -70 mV to -60 mV
hyperpolarization
= membrane potential becomes more negative
- When a stimulus opens K+ gated channels:
-> efflux of K+ ions out of the ICF
-> ICF loses +ve ions cell interior becomes more negative
-> membrane potential becomes more negative e.g. -70 mV to -80 mV
graded potentials
- Are small changes in the membrane potential (i.e., small depolarisation or hyperpolarisation)
- Originate in the dendrites or cell body of a neuron, when
a stimulus opens chemically-gated or mechanically-gated
channels - Are short-distance signals
- distance travelled is proportional to stimulus strength
- stronger stimulus = bigger change in membrane potential = further signal will travel
for an action potential to occur:
- in response to a stimulus, a graded potential can:
-> travel to the initial segment of an axon
-> depolarise the initial segment to -55 mV = threshold
-> stimulate voltage-gated Na+ channels to open
-> generate an AP
action potentials
- Are long distance signals
- Originate at the initial segment of an axon
- Involve voltage-gated channels
- Are self-propagating
action potential: depolarisation
At threshold (-55 mv):
* voltage-gated Na+ channels open
* Na+ ions enter ICF
* membrane potential becomes LESS negative shifts from -55 mV to +30 mV
action potential: repolarisation
At +30 mV:
* voltage-gated Na+ channels close
* voltage-gated K+ channels open
* K+ ions leave ICF
* membrane potential RETURNS to resting state shifts from +30 mV to -70 mV
action potential: hyperpolarisation
As the membrane potential approaches -70 mV
* voltage-gated K+ channels close slowly
* excess K+ ions leave ICF
* membrane potential becomes MORE negative shifts from -70 mV to -90 mV
continuous conduction
- Occurs in unmyelinated axons
- Action potentials are generated at the voltage-gated channels along the length of the axon
- Conduction occurs at speeds ≤ 2 m/s
saltatory conduction
- Occurs in myelinated axons
- Action potentials are generated at the nodes of Ranvier
- Conduction occurs at speeds >100 m/s
what can impair an action potential?
- Local anesthetics block voltage-gated Na+ channels
-> no action potential
-> no conduction of pain signal to the brain
-> no sensation of pain - Cold and pressure reduced pain sensations by impairing signal conduction
chemical synapse
- A junction that mediates the transfer of information
- At a chemical synapse between two neurons:
-> the neuron sending the information = presynaptic neuron
-> the neuron receiving the information = postsynaptic neuron
-> a synaptic cleft separates presynaptic and postsynaptic membranes
-> signal transmission involves chemical neurotransmitters
information transfer at a synapse
- Action potential arrives at and depolarises axon terminal.
- Voltage-gated Ca2+ channels open.
- Influx of Ca2+ triggers synaptic vesicles to release neurotransmitter into the synaptic cleft.
- Neurotransmitter binds to chemically-gated channels on the postsynaptic neuron (dendrites or cell body).
- Chemically-gated ion channels open Na+ ions enter ICF plasma membrane of postsynaptic neuron depolarises graded potential known as an “excitatory postsynaptic potential” (EPSP) is produced
- EPSP depolarises initial segment of postsynaptic neuron to threshold (-55 mV) action potential generated information successfully transmitted
termination of synaptic transmission
- The neurotransmitter diffuses away from the synaptic cleft
- The neurotransmitter is degraded by enzymes present in the synaptic cleft
- The neurotransmitter re-enters the axon terminal and is destroyed by enzymes or reused. This process is known as reuptake.
