Role of neurones
To transmit electrical impulses rapidly around the body to allow the organism to respond to changes in internal and external environment
Parts of a general neurone
Cell body, Dendron, axon,
content and role of cell body
contains the nucleus surrounded by cytoplasm. large amounts of endoplasmic reticulum and mitochondria which are involved in the production of neurotransmitters
What are neurotransmitters?
chemicals used to pass signals from one neurone to the next
Function and structure of dendrons
-short extensions which come from the cell body. divide into smaller branches AKA denrites. -To transmit electrical impulses towards the cell body.
Structure and functions of axons
-Singular elongated nerve fibres. can be v long e.g. those that transmit impulses from the tips of toes and finger to the spinal cord
-cylindrical in shape, consisting of a very narrow region of cytoplasm surrounded by plasma membrane -To transmit electrical impulses away from the cell body
Function of sensory neurones
To transmit impulses from a sensory receptor cell to a relay neurone, motor neurone or the brain.
Has one dendron, which carries the impulse to the cell body, one axon which carries the impulse away from the cell body
Function of relay neurones
To transmit impulses between neurones. e.g. sensory neurones and motor neurones. short axons and dendrons
Function of motor neurones
To transmit impulses from a relay or sensory neurone to an effector, e.g. muscle/gland. one long axon and many short dendrites
Travel of electrical impulses
receptor --> sensory neurone --> relay neurone --> motor neurone --> effector cell
Myelinated neurones
Neurones that have axons covered in myelin sheaths the myelin sheath acts as an insulating layer and allows these myelinated neurones to conduct the electrical impulse at a much faster speed than unmyelinated neurones
What makes the myelin sheath in myelinated neurones?
Schwann cells grow around the axon multiple times, surrounding the axon with layers of membrane
Name for gap between Schwann cells
Node of Ranvier
Why nodes of Ranvier are useful?
Cause signal to jump which allows faster rate of transmission
Why is the rate of transmission slower in non-myelinated neurones?
No nodes of Ranvier so no jumping, continuous transmission is much slower
Types of sensory receptors
Mechano, chemo, thermo, photo
Stimulus mechanoreceptors respond to
Pressure, movment e.g. pacinian corpuscle receptor (detects pressure)
Example of chemoreceptor
Olfactory receptor
Example of thermoreceptor
End bulbs of Krause
Where do you find end bulbs of Krause?
Tongue
Shared features of sensory receptors
Specific to a single type of stimulus, transducers
Role of sensory receptors as transducers
Sensory receptors convert stimulus into a nerve impulse (Generator potential)
Structure of Pacinian Corpuscle
End of neurone surrounded by layers of connective tissue separated by layers of gel, sodium ion channels in membranes, stretch-mediated sodium channels
How Pacinian Corpuscles do transducing
Sodium ion channels too narrow in a normal state, resting potential present, corpuscle changes shape when pressure applied to the corpuscle, membranes stretch, channels widen, sodium ions diffuse in, membrane depolarises, generates generator potential, generator potential creates action potential
What is a resting potential
The potential difference across a neurone's membrane when it isn't transmitting an impulse
When there is a resting potential, where is there a more positive charge?
Outside the membrane
How resting potential develops
3 sodium ions actively transported out of the axon and 2 potassium ions actively transported in by sodium potassium pump, more sodium ions outside the membrane and more potassium ions inside the cytoplasm, sodium ions try to diffuse in and potassium ions try to diffuse out, gated sodium ion channels closed so sodium ions can't diffuse, potassium ions can move freely, more positive ions outside than inside
General value for resting potential
-70mV
Depolarisation
Change in potential difference across a membrane from negative to positive
How generator potential develops
Receptor cells respond to stimuli, gated sodium ion channels open, larger stimuli will open more channels, sodium ions diffuse into the axon, inside of neurone is less negative, change in potential difference across the membrane is a generator potential
How action potential develops
Generator potential reaches threshold
Voltage gated Na+ channels open
Lots of Na+ diffuse into the axon (Positive feedback)
Membrane depolarised, voltage gated Na+ channels close
Voltage gated K+ channels open
K+ diffuse out of membrane and become depolarised
Potential difference overshoots
Membrane becomes hyper polarised
Resting potential restored by sodium potassium pump
Refractory period
Where is there positive feedback in action potentials?
The diffusion of sodium ions into the axon when doing a generator potential will open voltage-gated sodium ion channels so more sodium ions diffuse in
Threshold voltage value
-50mV
Potential difference across membrane when depolarised
+40mV
Name for phase after repolarisation
Refractory period
Role of refractory period
To allow cell to recover, to only allow action potentials to be transmitted in one direction
How an action potential is transmitted down a myelinated neurone
Depolarisation happens at the nodes of Ranvier, sodium ions pass through protein channels at the nodes, localised circuits between nodes, action potential jumps from one node to another
Technical name for transmitting an action potential down a myelinated neurone
Saltatory conduction
Benefits of saltatory conduction
Faster as fewer places where channels have to open, more energy efficient as less repolarisation so less ATP required
All-or-nothing principle
If a stimulus crosses a threshold value, a response will always be triggered. If it doesn't, no action potential will be triggered. Size of action potential not affected by the size of the stimulus
How does size of the stimulus affect action potentials?
Larger stimuli cause more action potentials to be generated in a given time, increasing frequency, increasing degree of response.
Parts of a synapse
Synaptic cleft, presynaptic neurone, postsynaptic neurone, synaptic knob, synaptic vesicles, neurotransmitter receptors
Approximate size of the synaptic cleft
20-30 nm
Organelles the synaptic knob contains
Mitochondria, large amounts of endoplasmic reticulum
Types of neurotransmitter
Excitatory, inhibitory
Excitatory neurotransmitters
Neurotransmitters that result in the depolarisation of the postsynaptic membrane
Inhibitory neurotransmitters
Neurotransmitters that result in the hyperpolarisation of the postsynaptic membrane
Example of excitatory neurotransmitter
Acetylcholine
Example of inhibitory neurotransmitter
GABA
How impulses are transmitted across a synapse
Action potential reaches end of presynaptic neurone, depolarisation causes calcium ion channels to open, calcium ions diffuse to knob, vesicles containing neurotransmitters fuse with membrane, released by exocytosis, diffuse over, bind with receptor on the membrane, sodium ion channels open, sodium ions diffuse into neurone, triggers action potential, propagated along the neurone
Why neurotransmitter must be removed
Prevents response from happening again, neurotransmitter can be recycled
Specifics of the structure of cholinergic synapses
Acetylcholine is the neurotransmitter, hydrolysed by acetylcholinesterase, breaks down to choline and ethanoic acid, reformation requires ATP
Role of synapses
Ensuring impulses are unidirectional, allow impulse from one neurone to be transmitted to a number of neurones, allow an impulse from a number of neurones to feed into one
Summation
When the amount of neurotransmitter builds up to reach the threshold to trigger an action potential
Types of summation
Spatial, temporal
Spatial summation
When a number of presynaptic neurones are connected to one postsynaptic neurone
Temporal summation
When a single presynaptic neurone releases neurotransmitter several times over a short period as a result of several action potentials
Central Nervous System
Brain and spinal cord
Peripheral nervous system
The neurones that connect the CNS to the body,
How is the mammalian nervous system organised functionally?
Somatic and autonomic nervous systems
Somatic nervous system
System under conscious control, carries impulses to the muscles
Autonomic nervous system
System under subconscious control, carries impulses to glands and smooth muscle and cardiac muscle
How is the autonomic nervous system organised?
Sympathetic and parasympathetic nervous systems
Gross structure of the brain
Protected by the skull, surrounded by meninges, five main areas
Main areas of the brain
Cerebrum, cerebellum, medulla oblongata, hypothalamus, pituitary gland
Functions of the cerebrum
To receive sensory information and interpret it with respect to previous experiences, to send impulses along motor neurones to act on the information, used to control both voluntary and involuntary responses
Structure of the cerebrum
Highly convoluted, split into two halves, has discrete areas for different functions, outer layer called cerebral cortex
How is the brain able to judge distance and perspective?
Impulses from right side of the field of vision sent to left hemisphere, impulses from left side sent to right hemisphere, integration gives distance and perspective
Function of the cerebrum
Controls voluntary actions, such as learning, memory, personality and conscious thought.
How does the cerebrum work?
Receives sensory information, interprets it with respect to previous experiences Sends impulses along motor neurones to effectors to produce an appropriate response.
Function of the medulla oblongata
To control reflex activities as part of the autonomic nervous system, such as ventilation and heart rate
Function of the hypothalamus
Main controlling region for the autonomic nervous system, one centre for parasympathetic system, one centre for sympathetic system, controls complex behaviour patterns, monitors composition of blood plasma, produces hormones
Function of the pituitary gland
To control most of the glands in the body
Structure of the pituitary gland
Divided into anterior and posterior pituitary gland
Function of anterior pituitary gland
To produce hormones such as FSH
Function of posterior pituitary gland
To store and release hormones made by the hypothalamus such as ADH
Reflex arc
Receptor detects stimulus, creates action potential for sensory neurone, sensory neurone carries impulse to motor neurone within the spinal cord via a relay neurone, motor neurone carries impulse to effector
What type of reflex is the knee jerk reflex?
Spinal reflex
Spinal reflex
When the neural circuit only goes up to the spinal cord, not the brain
How does the knee jerk reflex work?
Leg tapped just below the patella, stretches the patellar tendon, initiates the reflex arc, extensor muscle on top of the thigh contracts, relay neurone inhibits motor neurone of the flexor muscle so it relaxes, contraction of the extensor muscle causes the leg to kick
What is the knee jerk reflex used for?
Maintaining posture and balance
Examples of reflex actions
Knee jerk, blinking
What type of reflex is the blinking reflex?
Cranial reflex
Optical reflex
Blinking as a reaction to overly bright light
Cranial reflex
A reflex that occurs in the brain
How does the blinking reflex work?
Irritation of the cornea triggers an impulse along the fifth cranial nerve, passes through a relay neurone in the lower brain stem, impulses then sent along branches of the seventh cranial nerve, results in the eyelids closing
Consensual response
Both things respond in the same way to a stimulus
Example of a consensual response
Blinking reflex
How do reflexes increase your chances of survival?
Involuntary responses so the brain can deal with more complex responses, not learnt so provide immediate protection, fast
Stressor
Stimulus that causes the stress response which causes wear and tear on the body's physical or mental resources
Fight or flight response
Full range of coordinated responses of animals to situations of perceived danger
What is the cause of the fight or flight response?
Shift in the balance of stimulation to increase activity of the sympathetic nervous system and a decrease in activity of the parasympathetic nervous system
How is the fight or flight response coordinated?
Hypothalamus activates sympathetic nervous system and the adrenal-cortical system by releasing CRF, sympathetic nervous system activates the adrenal medulla which releases adrenaline and noradrenaline, sympathetic nervous system leads to impulses that activate glands and smooth muscles, anterior pituitary gland releases ACTH which leads to the adrenal cortex which releases hormones
Hormones that are released by the adrenal cortex in the fight-or-flight response
Cortisol, corticosterone
Role of cortisol
To regulate metabolism and blood pressure responses to stress
Role of corticosterone
Regulates immune response and suppresses inflammatory reactions
How does adrenaline use cell signalling?
Binds to its receptor, activates inactive adenyl cyclase to make active adenyl cyclase which is an enzyme, ATP is then converted into cAMP, cAMP is the second messenger
How is the nervous system involved in increasing heart rate?
Centre in the medulla oblongata which increases heart rate sends impulses through the sympathetic nervous system in the accelerator nerve to the SAN
How is the nervous system involved in decreasing heart rate?
Centre in the medulla oblongata which decreases heart rate sends impulses through the parasympathetic nervous system in the vagus nerve to the SAN
Types of receptors involved in changing heart rate
Chemoreceptors, baroreceptors