Nervous System: CNS, PNS Functions

Nervous System Functions

• Sensory input is the detection of changes inside and outside the body by millions of receptors.
• Integration refers to the processing and interpretation of sensory input, leading to a decision for response.
• Motor output involves effector organs like muscles and glands causing a response.

Central Nervous System (CNS)

• The CNS is the nervous system's integrating and control center.
• The CNS consists of the brain and spinal cord.
• The brain has approximately 85 billion neurons, while the spinal cord contains about 100 million neurons.

Peripheral Nervous System (PNS)

• The PNS is the portion of the nervous system outside the CNS.
• The PNS includes nerves and ganglia, which are groups of neuron cell bodies extending from the brain and spinal cord.
• The PNS acts as communication lines connecting the CNS to different body parts.
• The PNS has two subdivisions: sensory (afferent) and motor (efferent).
• The sensory division transmits impulses to the CNS from sensory receptors.
• The motor division transmits signals from the CNS to effector organs, such as muscles and glands, resulting in a motor response.
• The motor division has two parts: somatic and autonomic.
• The somatic nervous system sends impulses from the brain to skeletal muscles for voluntary action.
• The autonomic nervous system (ANS) regulates smooth muscle, cardiac muscle, and glands for involuntary action.
• The ANS consists of parasympathetic and sympathetic divisions.

Cells of the Nervous System

• Nervous tissue is made of cells with two main roles, neurons and supporting cells.
• Neurons are nerve cells, they are excitable, responding to stimuli by changing their membrane potential, and they transmit electrical signals.
• Supporting cells, also called neuroglia or glial cells, are more numerous than neurons, and provide homeostatic support, protection, and defense to the nervous tissue.

Neuroglia of the CNS

• Astrocytes support and guide neuron movement and regulate synapse formation. They control the chemical environment and mop up K+ and released neurotransmitter.
• Microglial cells are small cells monitoring neuron health, that transform into a type of macrophage to eat unwanted material through phagocytosis.
• Ependymal cells line the central cavities of the brain and spinal cord, forming a permeable barrier between neuronal tissue and cerebrospinal fluid, and produce cerebrospinal fluid.
• Oligodendrocytes wrap around nerve fibers and produce an insulating covering called a myelin sheath, which helps propagate impulses.

Neuroglia of the Peripheral Nervous System

• Satellite cells are similar in function to astrocytes in the CNS, but they regulate neurons in the PNS.
• Schwann cells surround all nerve fibers in the PNS and form myelin sheaths, similar to oligodendrocytes, and help with the regeneration of damaged peripheral nerve fibers.

Neurons

• Neurons are large nerve cells that conduct impulses around the body.
• Neurons have high longevity, lasting for an entire lifetime.
• Neurons, once active as communication links, cannot divide and be replaced.
• Neurons have a very high metabolic rate, requiring a continuous supply of oxygen and glucose.

Neuron Structure

• Neurons contain a cell body with a nucleus and abundant mitochondria.
• Neurites, or processes, extend from the cell body in two types.
• Dendrites are multiple short, branching extensions that receive signals from other neurons.
• Axons act as the conducting region of a neuron and each neuron has only one, but can be very long, over 1 meter.

Neuron Classes

• Sensory neurons receive sensory input.
• Interneurons connect sensory neurons to motor neurons, they communicate forming complex networks.
• Motor neurons control voluntary and involuntary functions of muscles and glands.

Nerve Insulation

• Schwann cells (in the PNS) and oligodendrocytes (in the CNS) wrap around nerve fibers, forming a myelin sheath.
• This myelin sheath protects nerves and increases the transmission speed of nerve impulses.
• The myelin sheaths have regular gaps so axon side-branches (collaterals) can contact other nerve fibers.

Nerves

• Neurons and nerves are not the same thing, a nerve is a collection of neurons.
• Neurons are arranged within the nerve in bundles called fascicles, separated by connective tissue.
• Individual neurons are surrounded by endoneurium.
• Groups of neurons (fascicles) are separated by perineurium.
• The whole nerve is coated in connective tissue called epineurium.

Action Potentials

• Neurons have a resting membrane potential where the inside of the nerve cell is more negative than outside.
• Neurons can rapidly change their membrane potential as the inside of the cell becomes less negative, called depolarisation.
• Ion channels in the plasma membrane selectively allow movement of certain ions into and out of the cell based on stimuli.
• Action potentials (APs) are brief long-distance signals within a neuron.
• There is a reversal in membrane potential of around 100 mV, from -70 mV to +30 mV, followed by repolarisation and a period of hyperpolarisation.
• APs are self-propagating and move away from the initial stimuli.
• After depolarisation, each axon segment repolarises, restoring its resting membrane potential.
• Chemically gated ion channels open in response to neurotransmitter binding to specific receptors.
• Voltage gated ion channels open in response to an electrical impulse like an action potential.

Synapses

• Synapses transmit signals between neurons.
• A neuron conducting an impulse towards the synapse is the presynaptic neuron.
• A neuron that transmits the signal away from a synapse is a postsynaptic neuron.
• Chemical synapses are the most abundant type of synapse.
• They release neurotransmitter following arrival of an AP to the axon termini.
• Depolarisation causes vesicles filled with neurotransmitter to fuse with the membrane and release into the synaptic cleft.
• Voltage gated calcium channels open and allow calcium into the axon termini, causing depolarisation.
• Neurotransmitters diffuse across the synaptic cleft and bind to receptors on the postsynaptic neuron.