Nervous System Function

• Nerves refers to bundles of neurones

Nervous Systems

• Central nervous system includes the brain and spinal cord
• Peripheral nervous system includes the somatic, enteric, and autonomic systems
• Enteric controls the digestive system
• The autonomic nervous system consists of sympathetic and parasympathetic branches

Bioelectricity

• Membrane potential arises from ion gradients
• Some cells change membrane potential in response to stimuli; these are excitable cells, such as neurones
• Nerve impulses are changes in membrane potential that travel down nerves

Ion Gradients

• Cell membranes are highly permeable to ions which, allows the formation of ion gradients
• Membrane potential allows electrical signaling and excitability Resting membrane potential is -70 mV
• Membrane potential underlies neurotransmission, muscle contraction, secretion, and immune responses

Transport Across Cell Membranes

• Passive diffusion moves materials across cell membranes
• Active transport moves materials across cell membranes
• Facilitated diffusion moves materials across cell membranes

Ion Gradients

• Membrane potential occurs due to unequal ion distribution, which is important for speedy transmission
• The Nernst Equation describes membrane potential at equilibrium
• Potential of 1 ion: Sum every ion to get total Em
• Accounts for ion permeability

Neurones

• Neurones are highly specialised cells
• Neurones transmit info as electrical signals, in the form of nerve impulses or action potentials (APs)
• APs only travel one way, from dendrites to axons

Action Potential

• Electrical impulses are formed by ions moving into a neurone
• A signal received at dendrites results in dendritic depolarisation
• This depolarisation opens voltage-gated sodium channels (Nav)
• Signals can be received by ligand gated ion channels, or metabotropic channels

Action Potential Details

• Stimulus is from electrical, mechanical or chemical sources
• Stimulus must be strong enough to reach threshold potential
• Enough depolarisation is needed to open the first Nav channel

Myelination

• Myelination in CNS cells is from oligodendrocytes
• Myelination in peripheral NS cells is from Schwann cells
• Axons in most vertebrae are myelinated with myelin
• Only Nodes of Ranvier are exposed
• Action potential 'jumps' from one node to the next
• Results in faster neurotransmission, and is more energy efficient
• Saltatory conduction refers to the events when AP jumps from node to node

Repolarisation

• Not discussed

Summary

• Membrane potential is key to all cells; it’s the sum of all ion gradients
• Excitable cells depolarise in response to stimuli
• Neurones, skeletal muscle, and hormone-secreting cells are excitable
• Neurones transmit electrical signals
• Synapses are points of communication between neurones
• Converts the electrical signals to chemical signals, which causes neurotransmitter release

Synapses

• Synapse where one neurone meets another
• Electrical AP triggers release of neurotransmitter
• Neurotransmitters bind to receptors on postsynaptic neurone, triggering depolarisation

Neurotransmitter

• Neurotransmitter used for neurological information from one cell to another
• A chemical is released from the nerve cell that transmits an impulse from a nerve cell to another, muscle, organ, or other tissue

Types of Neurotransmitter

• Excitatory transmitters include glutamate, monoamines, and acetylcholine
• Inhibitory neurotransmitters include GABA in the brain, and glycine in the spine
• Endorphins inhibit pain sensations
• Neuromodulators include neuropeptides which function as neurotransmitters and hormones
• Also, endocannabinoids which activate the cannabinoid system

Neurotransmitter Receptors

• Ionotropic receptors bind glutamate, GABA, and glycine and uses ion channels
• Faster reaction
• Metabotropic receptors bind monoamines and histamine, are G-protein coupled, and produce more diverse effects
• Some neurotransmitters, such as glutamate and GABA, have both types of receptors

Synaptic Transmission

• Multiple synapses onto same dendrite
• Can be excitatory, inhibitory
• Whether a neurone fires depending on sum of all synaptic inputs

Nervous Tissue Cells

• Unipolar neurones: sensory, sensory receptor, synapse that releases neurotransmitter
• Bipolar neurones: 1 neurone - neurone
• Multipolar neurones: lots from inputs
• Glial cells occupy 50-80% of the brain

Glial Cells

• Glial cells outnumber neurones
• They provide supporting roles, are vital for neuronal health and the neuronal immune system
• Glial cells also conduct CSF production

Astrocyte

• Astrocytes outnumber neurones and support neurones
• Astrocytes are component of the BBB, and aid in communication
• They also deal with NT waste and formation of scar tissue in the brain in the event of an injury.

Tripartite Synapse

• Astrocytes form many of these synapses and maintains the synapse's integrity

Oligodendrocyte

• Aids in myelination of CNS neurones which is attacked by immune system in MS

Microglia

• Brain-specific immune system, which is a type of macrophage
• Can play potential role in many diseases like MS and Alzheimer's
• MS leads to destruction of the myelin, and Alzheimer's leads to the production of pro-inflammatory cytokines
• Neuroinflammation is also largely mediated by communication between microglia and astrocytes
• Plays vital role in synapses and the brain

Neuromuscular Junctions

• A specialised synapse at either skeletal or smooth muscle
• Presynaptic neurone makes synapses with the muscle cell

Skeletal Muscle NMJ

• Acetylcholine neurotransmitter
• Signal ends by acetylcholinesterase
• Ionotropic receptor in NMJ will activate nicotinic acetylcholine receptor

Summary

• Synaptic structures enable neurotransmitter release and detection
• Neurotransmitters can be excitatory or inhibitory
• Sum of all synaptic inputs determines firing of neurone
• Most of the brain is glial cells
• Glial cells play supporting roles to neurones
• NMJ is a specialised synapse
• Skeletal muscle synapse is ionotropic, smooth muscle synapse is metabotropic

The Somatic NS

• Part of peripheral NS
• Controls movement of skeletal muscles

Ascending and Descending Tracts

• Ascending tracts relay information from the spinal cord to the sensory cortex
• Descending tracts relay information from the motor cortex to the spinal cord

Sensory Neurones

• Senses touch, scent, pain, etc
• Relays info to spinal cord and brain
• Enters spine at the dorsal horn, via dorsal root
• Unipolar neurones - cell body is at the dorsal root ganglion
• Myelinated

Motor Neurones

• Relay nerve impulses from the spine to trigger contraction of skeletal muscle
• Exits the spine via ventral root
• Alpha motor neurone, Multipolar, and Myelinated

NMJ

• Acetycholine binds to and activates Nicotinic acetylcholine receptor
• Ionotropic

Drugs and Toxins Targeting NMJs

• Botulinum toxin inhibits acetylcholine secretion
• Flaccid paralysis (weakness or paralysis in the proximal limb muscles)
• Can lead to paralysis of diaphragm or being unable to breathe
• Organophosphates inhibit acetycholinesterase
• Can lead to muscle spasms

Myasthenia Gravis

• Autoimmune disease
• autoantibodies to acetylcholine receptors (85%), or due to muscle receptor tyrosine kinase (15%)
• Double vision and ptosis are distinguishing features

Treatment

• Main symptomatic treatment is cholinesterase inhibitors
• Increase acetylcholine levels in synaptic cleft
• Increases activation of remaining AcH receptors that aren't damaged
• Similar in action to organophosphates

Spinal Control of Movement - Reflexes

• Voluntary movements initiated in brain
• Many aspects of movement are controlled at spine
• Stretch reflex from muscle spindles (knee jerk), inhibitory feedback from golgi tendon organs, withdraw reflex, Reciprocal inhibition of extensor and flexor muscles
• These movements do not require brain participation

The Five Components of Reflex Arc

• Not discussed

Muscle Spindle Stretch Reflexes

• Muscle spindles are specialised muscle fibres surrounded by a capsule inside skeletal muscle
• Proprioceptors sense muscle length and activate sensory neurones (la axons)

Stretch Reflex

• Increase muscle tension to counter stretching, such as bearing body weight

Golgi Tendon Organs + Tension

• The organs sense tension
• Proprioceptors between muscle and tendon sense tension and activate sensory neurones
• Contracts the muscle against resistance

Autogenic Inhibition

• type Ib axons - synapse onto inhibitory spinal interneurones
• These axes inhibit motor-neurones via (GABA/ glycine), which reduces muscle contraction
• Regulates muscle tension within a normal range
• Important for fine motor skills

Other Proprioceptors

• Many are found in connective tissues of joints
• Especially in joint capsules
• Provide info of positioning within the joint
• Knowing where limbs are when eyes are closed

Inhibitory Interneurones

• Vital for stretch reflex, stops opposing reflexes
• Reciprocal inhibition: have to inhibit tricep to contract the bicep

Knee Jerk Reflex

• Combo of muscle spindle stretch receptor and reciprocal inhibition

Excitatory Interneurones

• Withdrawal reflex - Ipsilateral: stimulus and effector on same side of the body

Crossed Extensor Reflex

• Maintains balance during withdraw reflex
• Painful stimulus on one foot causes withdraw of the foot, and simultaneous contraction of the opposite leg muscles
• Contralateral: stimulus and effector on different sides of body

Brain Control of Movement

• Complex brain network integrates sensory input and coordinates motor output

Basal Ganglia

• Centers for controlling and initiating voluntary movement

Cerebellum

• Coordinates complex series of movement
• Inhibited by ethanol intoxication Majority of neurones are small granule cells
• Ccerebellar granule cells = neurones in the rest of CNS
• Highly complex info processing centre

Summary

• SNS coordinates and controls skeletal muscle movement
• Muscle control can be coordinated by spinal cord
• Sensory (afferent) neurones and Motor (efferent) neurones are involved in Reflexes (Me)

Spinal Control of Muscles

• spinal control of muscles can be overridden by areas of the brain
• Dedicated brain areas control voluntary movements through cooperation and collating sensory information from Somatosensory cortex, Motor cortex, Basal ganglia.
• The motor cortex is the main contributor

Explore the roles of inhibitory spinal interneurons, reciprocal inhibition, and the crossed extensor reflex. Understand the basal ganglia's role in motor control and the complexity of cerebellar granule cells. Learn about action potentials, myelin sheaths, and neurotransmitters.