Nervous Tissue and Neuron Structure Overview

Nervous Tissue Overview

• Nervous tissue is categorized into central and peripheral components
• Central nervous system: brain and spinal cord
• Peripheral nervous system: all other nerve tissue

Neuron Structure

• Neuron structure consists of a cell body (soma), dendrites, and an axon.
• Dendrites receive signals from other neurons.
• The axon transmits signals away from the cell body.
• Axon hillock: where the axon originates
• These communicate between neurons via synapses
• Axon terminals: branches at the end of the axon

Neuron Types

• Sensory neurons: carry sensory information to the central nervous system, generally unipolar.
• Interneurons: connect sensory and motor neurons within the central nervous system, generally multipolar.
• Motor neurons: carry signals from the central nervous system to muscles or glands; generally multipolar.

Neuron Function

• Neurons communicate through nerve impulses (action potentials).
• Nerve impulses travel along the axon to the axon terminals.
• Communication between neurons occurs via chemical or electrical synapses.

Synapses

• Synapses are junctions between neurons that allow communication.
• Chemical synapses use neurotransmitters to transmit signals.
• Electrical synapses use gap junctions for direct transmission.

Myelination

• Myelin sheaths insulate axons, speeding nerve impulse transmission.
• Schwann cells form myelin sheaths in the peripheral nervous system.
• Oligodendrocytes form myelin sheaths in the central nervous system.

Synaptic Transmission

• Action potential reaches the axon terminal of a presynaptic neuron
• Voltage-gated Calcium channels open and calcium enters axon terminal
• Triggers fusion of synaptic vesicles with the plasma membrane and neurotransmitter release into synaptic cleft
• Neurotransmitter diffuses across synaptic cleft and attaches to receptors on post-synaptic cell or muscle cell
• Post-synaptic cell or muscle cell depolarizes which may trigger propagation of an action potential
• Neurotransmitter removal from the synaptic cleft required to terminate the signal.

Action Potentials

• Electrical signals, transmitted along the axon.

• Depolarization: the membrane potential becomes less negative.

• Repolarization: the membrane potential returns to a resting state.

• Voltage-gated channels open and close to regulate the flow of ions.

• Refractory periods: time after an action potential when the neuron cannot fire another action potential.

• Absolute refractory period: no action potential possible.

• Relative refractory period: more difficult to elicit an action potential

Postsynaptic Potentials (graded potentials)

• Excitatory postsynaptic potential (EPSP): brings membrane potential closer to threshold
• Inhibitory postsynaptic potential (IPSP): brings membrane potential further from threshold
• Synaptic potentials integrate to determine whether an action potential occurs.

Temporal and Spatial Summation

• Temporal summation: multiple signals arriving rapidly at a neuron add together
• Spatial summation: signals arriving at different locations on the neuron add together.

Neuromuscular Junction

• The synapse between a motor neuron and a muscle fiber.
• Acetylcholine (ACh) is the neurotransmitter.

Skeletal Muscle Excitation/Contraction Coupling

• The process by which a nerve impulse leads to muscle contraction
• Action potential travels along the axon to stimulate the release of Acetylcholine at the neuromuscular junction.
• Binding of ACh to its receptors opens ion channels which creates an action potential in muscle cell membrane
• Propagation of this action potential along muscle cell membrane
• Leads to release of calcium ions (Ca2+) from sarcoplasmic reticulum
• The calcium ions trigger muscle contraction
• Calcium removal leads to muscle relaxation