Main Division
central nervous system (CNS)
- brain and spinal cord.
- Contains neural tissue, connective tissues, and blood vessels
- The CNS is responsible for higher functions like cognition, decision-making, and controlling voluntary and involuntary actions.
Peripheral Nervous System (PNS)
- All neural tissue outside of CNS
- facilitating communication and control over both voluntary and involuntary functions
Function of CNS
Process and coordinate
- sensory data from inside and outside the body.
- Motor commdans: control activites of peripheral organs (ex skeletal muscle.
- higher functions of the brain, intelligence, memory, learning, emotion
Function of PNS
- Deliver sensory information to the CNS
- Carry motor commands from CNS to PNS
- facilitating communication and control over both voluntary and involuntary functions.
Functional division of PNS
afferent division
- carries sensory information from PNS sensory receptors to CNS
- Special sensory receptors: monitor smell, taste, vision, blanance and hearing.
- Visceral Sensory receptors: Monitor internal organs.
- Somatic sensory receptors: monitor skeletal muscles, joints, and skin surfaces.
Efferent Division
- carries motor commands from CNS to PNS (muscles / glands etc)
Further subdivided into somatic nervous system (SNS) and the autonomic nervous system (ANS)
Subdivision of Efferent/Motor PNS
- Somatic nervous system (SNS) / somatic motor division
- controls voluntary skeletal muscle contractions - Autonomic nervous system (ANS) / visceral motor division
- controls subconscious actions: contraction of smooth muscle, cardiac muscle, and grandular secretion, adipose tissue. Further divided into the sympathetic division (fight of flight), and parasympathetic division (rest and digest)
- Enteric System (ENS): network of neurons located within the gastrointestinal tract that controls digestive functions such as peristalsis, secretion, and blood flow. It operates autonomously but is influenced by both sympathetic and parasympathetic inputs from the autonomic nervous system. Operates semi-independently of the rest of the nervous system (the second brain).
Define parts of nerve
- soma (cell body): Central part of a neuron containing the nucleus, responsible for maintaining cell health and metabolic functions. Everything but axon and dendrites.
- Neuronal Nucleus: Control center of a neuron, housing genetic material and regulating cellular activities like protein synthesis.
- Axon Hillock: Cone-shaped region where action potentials are initiated, connecting the soma to the axon.
- Dendrite: Branched extensions that receive signals from other neurons and transmit them toward the cell body. postsynaptic
- Axon: Long, slender projection that carries electrical impulses away from the cell body toward other cells. presynaptic
- Axolemma: Plasma membrane surrounding the axon, crucial for maintaining the axon’s environment and facilitating signal transmission.
- Axoplasm: This is the cytoplasm within the axon.
- Axon Terminal: Distal end where neurotransmitters are released to communicate with other neurons or target cells.
- Neurilemma: The neurilemma, also known as the Schwann cell sheath or Sheath of Schwann, is the outermost layer of the Schwann cell that surrounds the axons of neurons in the peripheral nervous system. It plays a crucial role in the regeneration of damaged nerves. Only exists in PNS
- Nodes of Ranvier: Gaps in the myelin sheath along an axon that enable rapid conduction of nerve impulses.
- Nissl Bodies: Granular structures within the soma involved in protein synthesis, composed of rough endoplasmic reticulum and ribosomes.
- Neurofilament: Intermediate filaments providing structural support and maintaining cell shape within neurons.
- Telodendria: Fine terminal branches of an axon that form synaptic connections with target cells.
- Axon Terminals: End structures that release neurotransmitters into the synaptic cleft.
- Presynaptic cell: Neuron that releases neurotransmitters into the synapse to transmit a signal.
clusters of ell bodies in CNS are called the nucleus are in gray matter, in the PNS called ganglion
Axons in CNS are called tracts and are in white matters, called nerves in PNS
Describe the organization of a nerve.
Neuron (Individual Cell) → Axon (with myelin sheath) → Endoneurium (surrounds each axon) → Fascicle (bundles of axons) → Perineurium (surrounds each fascicle) → Nerve (bundles of fascicles) → Epineurium (surrounds the entire nerve)
Neuron: the individual nerve cells that carry electrical signals. Each neuron consists of a cell body (soma), dendrites (which receive signals), and an axon (which transmits signals to other neurons or target tissues).
Endoneurium: Delicate layer of areolar CT surrounding each axon, superficial to myelin.
Fascicle: Groups of axons are bundled together. Each fascicle is surrounded by another layer of connective tissue known as the perineurium (dense irregular CT).
Multiple fascicles are bundled together to form a nerve, which is the complete structure that transmits electrical signals throughout the body. The entire nerve is encased in a tough outer layer of connective tissue called the epineurium (dense irregular CT).
Nerve Shapes
- anoxonic neuron: Multiple process, either have no axon or not obvious which is the axon. Rare, found in special sense organs and brain.
- Bipolar Neurons: Contain two processes separated by the cell of the body. One is the axon, one is the dendrite. Rare, only seen in special sense organs
- Unipolar Neurons: Contain a single process with the cell body in the middle. Common in sensory neurons of the PNS.
- Multipolar Neurons: Have more than two processes, a single axon and multiple dendrites. These are the most common.
List Neuroglia, functions, and location
CNS
- Ependymal: Line the central cavity of CNS and resemble simple cuboidal epithelium. They secrete, circulate, and monitor cerebrospinal fluid Serve as stem cells of other glial cells and neurons
- Oligodendrocytes: Myeline CNS axons; provide structural framework.
- Microglia: Make up about 5% of glial cells in CNS. They Monitor the health of neurons and migrate towards injured and dead neurons. They can differentiate into macrophages (other immune components do not have access to the CNS.
- Astrocytes: Maintain the blood-brain barrier; provide structural support; regulate ion nutrient, and dissolved gas concentrations. Absorb and recycle neurotransmitters. form scar tissue after injury.
PNS
- Satellite Cells: Have a similar role as astrocytes in the CNS, surround neuron cell bodies in ganglia where they regulate ion concentration and reabsorbed and recycle neurotransmitters.
- Schwann Cells / Neurolemmacytes: These cells myelinate axons of the PNS. They are also important to the repair of damaged PNS axons.
What is the resting membrane potential?
How is it created?
It is the baseline difference in electrical charge across the cell membrane at rest. For skeletal muscles this difference is normally -85mV and for neurons -70mv.
The sodium-potassium ATPase (Na⁺/K⁺ pump) actively transports three sodium ions (Na⁺) out of the cell for every two potassium ions (K⁺) it pumps into the cell. This activity creates a higher concentration of potassium ions inside the cell and a higher concentration of sodium ions outside the cell.
The cell membrane is more permeable to potassium ions due to the presence of potassium leak channels, which allow K⁺ ions to move out of the cell down their concentration gradient. As K⁺ ions leave the cell, they carry positive charges with them, making the inside of the cell more negative. This movement of K⁺ ions establishes an electrical gradient (negative inside, positive outside) that opposes the further efflux of K⁺.
In most cells, the equilibrium potential for potassium is around -90 mV, which is close to the resting membrane potential. However, because the membrane is also slightly permeable to sodium ions, the actual resting membrane potential is slightly less negative, typically around -70 mV to -90 mV, depending on the cell type
Define Graded Potential and Types of summation
- Local potentials. Cannot spread far from site of stimulation.
- can be inhibitory (IPSP) -> Cl- channel or K+ channel -> hyperpolarization
- or excitatory (EPSP) -> Na+ channel -> depolarization
Summation of PSP (postsynaptic potential) n is the process by which multiple PSPs combine and influence whether or not an action potential is generated. You can have multiple EPSPs, Multiple IPSPs and a combination of EPSPS and IPSPS.
- Temporal summation: the frequency of incoming signals
- Spatial summation the number of presynaptic neurons releasing neurotransmitters simultaneously
Describe Action Potential
- Resting membrane potential: The resting membrane potential is approximately -70 mV, maintained by the sodium-potassium pump, which moves 3 Na⁺ out and 2 K⁺ in, along with K⁺ leak channels that allow K⁺ to exit the cell.
- Threshold is reached: When a graded potential sufficiently depolarizes the membrane (-55mV), voltage-gated sodium channels open, allowing Na⁺ to enter the cell and further depolarize the membrane.
- Depolarization: During depolarization, the membrane potential rapidly rises to about +30 mV as voltage-gated sodium channels fully open, causing a large influx of Na⁺ into the cell.
- Repolarization: Repolarization occurs as the membrane potential returns toward -70 mV, driven by the inactivation of sodium channels (time-dependent inactivation gate) and the opening of voltage-gated potassium channels, which allow K⁺ to exit the cell.
- Hyperpolarization: Hyperpolarization briefly occurs when the membrane potential becomes more negative than the resting potential, typically around -80 mV, due to the continued efflux of K⁺ through voltage-gated potassium channels before they close. K⁺ channels begin to close when the membrane potential returns to resting but it is not an instantaneous process.
- Resting membrane potential is restored: The resting membrane potential is restored as voltage-gated potassium channels close, and the sodium-potassium pump reestablishes the original ion distribution, returning the membrane potential to approximately -70 mV.
compare saltatory and continuous conduction
Where an axon is myelinated, saltatory conduction occurs, where the action potential jumps from node to node. Sodium channels are concentrated at these nodes, allowing the rapid influx of sodium ions, which regenerates the action potential at each node. This process skips over the myelinated internodes, speeding up conduction.
In the absence of myelin, continuous conduction occurs, where the action potential travels along the entire length of the axon membrane. Sodium channels are distributed along the entire length of the axon. The action potential must be regenerated at every point along the axon membrane, leading to a slower transmission of the signal.
Describe: myline, a node, an internode
Myelin is a fatty, insulating sheath that surrounds axons, primarily composed of lipids (about 70-80%) and proteins (about 20-30%). In the CNS, myelin is produced by oligodendrocytes, and in the PNS, it is produced by Schwann cells.
A node, specifically a Node of Ranvier, is a small, unmyelinated gap between adjacent segments of the myelin sheath along an axon. These nodes are crucial for saltatory conduction, where the action potential jumps from one node to the next, increasing the speed of nerve impulse transmission.
An internode is the segment of a myelinated axon between two Nodes of Ranvier. This region is covered by the myelin sheath and helps facilitate the rapid transmission of action potentials along the axon.
What are Cholinergic Synapses
Any synapse that releases ACh. Includes:
- All neuromuscal junctions with skeletal muscle fibers.
- Many synapses in CNS
- All neuron-to-neuron synapses in PNS
- All neuromuscular and neuroglandular junctions of ANS parasympathetic division.
in post synaptic terminal AP triggers opening of Ca++ channels, Ca++ trigger exocytosis of Ach
acetylcholinesterase degrades ACh into acetate and choline which it taking back up by presynaptic neuron
List some important neurotransmitters
- Norepinephrine (NE): Excitatory
- Dopamine: Excitatory and Inhibitory. Controls voluntary movement (basal ganglia) deficiency leads to diseases like parkinsons. Also involved in pleasure / reward.
- Serotonin: Excitatory and Inhibitory. Affects attention and emotional states.
- Gamma aminobutyric acid (GABA): Inhibitory. Not well understood. Reduces anxiety.
Opiods mimic NT such as endorphins and enkephalins
NT can be aa, peptides, gases, prostaglandins
What affects AP conduction velocity
- Axon Diameter: Larger diameter axons have less internal resistance to the flow of ions, allowing action potentials to propagate more quickly. This is because the larger surface area provides more space for ion channels, facilitating faster conduction.
- Degree of Myelination: Velocity is 30 times faster (120 meters /sec) in myelinated axons. Myelin is an insulator and saltatory conduction is more efficient. Large myelinated neurons normally motor-skeletal (120 m/sec). Smaller myelinated go to and from CNS (18 m/sec). Small unmyelinated To and from CNS (1 m/sec)
Defin Refactory period
The refractory period is the time during which a neuron is unable to fire another action potential, or it is significantly more difficult to do so, following an initial action potential. This period ensures that action potentials are unidirectional.
During the absolute refractory period, no new action potential can be initiated, regardless of the strength of the stimulus. This occurs because the voltage-gated sodium channels are inactivated, and they cannot reopen until the membrane potential returns to a level close to the resting potential. Relative refractory period is the period of time in which the AP can be initiated but it takes a stronger than normal stimulus.
What causes a neuron signal to end?
- Diffusion out of the synaptic cleft. Affected by concentration gradient.
- Enzymatic degradation, for example acetylcholinesterase breaks down acetylcholine.
- Reuptake by neurons, receptor-mediated endocytosis.
- Uptake by neuroglia, neurotransmitter transporters.
How does an effector know how strongly and how to respond to a signal
One type of receptor responds to one modality (one receptor responds to temperature, one to pressure, one to pain, etc.). Each nerve only attaches to one category of receptors, so one nerve will only attach to Merkel’s discs or only attaches to Pacinian corpuscles. For example, thermoreceptors in the skin specifically respond to temperature changes. These receptors are linked to nerves that only carry temperature information. When a thermoreceptor detects a change in temperature, it sends a signal along a dedicated pathway, or labeled line to the brain. The brain interprets this signal as temperature because it knows that the specific pathway activated is always associated with temperature sensation
An effector knows how strongly to respond based on the number of neurons firing and the frequency of their action potentials. A higher frequency of action potentials results in a more sustained and forceful response.
Gray Matter Vs White Matter
Gray Matter
CNS: Gray matter in the CNS consists of neuron cell bodies and is found in the cerebral cortex and central regions of the spinal cord; clusters of neuron cell bodies here are called nuclei.
PNS: In the PNS, gray matter consists of neuron cell bodies grouped together in structures known as ganglia.
White Matter
CNS: White matter in the CNS is made up of myelinated axons forming tracts that transmit signals between different brain regions and between the brain and spinal cord.
PNS: White matter in the PNS consists of myelinated axons bundled into nerves that connect the CNS to the rest of the body.
Gray matter is located centrally in the spine, forms an “H”. In the brain gray matter is primarily found in the outer layer, the cerebral cortex.
