OIA1004 NERVOUS SYSTEM

Nervous System Functions

Overview: The nervous system is responsible for sensing changes in the environment, processing that information, and initiating appropriate responses. It operates through three primary functions: sensory, integrative, and motor, facilitated by specialized neurons.

Sensory Function:

Senses changes in internal and external environments via sensory receptors.

Involves sensory (afferent) neurons to transmit sensory information to the central nervous system (CNS).

Integrative Function:

Analyzes sensory information, stores relevant data, and makes decisions regarding behaviors.

Carried out by association or interneurons within the CNS.

Motor Function:

Responds to stimuli by initiating muscle actions or glandular secretions.

Utilizes motor (efferent) neurons to convey commands from the CNS to effectors (muscles and glands).

Signal Transmission

Overview: Signal transmission refers to the process by which neurons communicate with each other or with effector cells at synapses. This involves electrical and chemical signals that lead to postsynaptic potentials, ultimately resulting in action potential generation.

Synapses:

Regions where communication occurs between two neurons or a neuron and an effector cell.

Communication is typically unidirectional (from neuron to target cell).

Two main types: Electrical synapses and Chemical synapses.

Electrical Synapses:

Direct connections between neurons via gap junctions.

Allow for rapid signal transmission.

Chemical Synapses:

Involve neurotransmitter release from presynaptic neurons.

Steps of transmission:

Nerve impulse arrives at synaptic end bulb.

Voltage-gated Ca²⁺ channels open.

Neurotransmitters are released into the synaptic cleft.

Neurotransmitters bind to postsynaptic receptors.

Ion channels open in the postsynaptic membrane.

Postsynaptic potential occurs (either depolarization - EPSP or hyperpolarization - IPSP).

Action potential may be generated if threshold is reached.

Postsynaptic Potentials:

EPSP (Excitatory Postsynaptic Potential): Depolarization leading to increased likelihood of action potential.

IPSP (Inhibitory Postsynaptic Potential): Hyperpolarization leading to decreased likelihood of action potential.

Summation:

Temporal Summation:

Occurs when stimuli happen at the same location on the postsynaptic membrane but at different times.

Can lead to cumulative effects on postsynaptic potential.

Spatial Summation:

Occurs when stimuli happen at different locations on the postsynaptic membrane simultaneously.

Can also lead to cumulative effects on postsynaptic potential.

Nervous System Organization

Overview: The nervous system is a complex network that coordinates body functions and responses. It is divided into the Central Nervous System (CNS) and Peripheral Nervous System (PNS), each with distinct roles in processing sensory information and controlling motor functions.

Central Nervous System (CNS):

Comprises the brain and spinal cord.

Responsible for processing and integrating sensory information.

Coordinates voluntary and involuntary actions through neural pathways.

Peripheral Nervous System (PNS):

Consists of all nervous tissue outside the CNS.

Divided into:

Somatic Nervous System (SNS):

Voluntary control over skeletal muscles.

Includes sensory neurons from cutaneous and special receptors to the CNS.

Autonomic Nervous System (ANS):

Involuntary control over smooth muscle, cardiac muscle, and glands.

Further divided into sympathetic and parasympathetic divisions.

Enteric Nervous System (ENS):

Controls gastrointestinal tract functions independently of the CNS and ANS.

Involves involuntary sensory and motor neurons.

Sensory and Motor Pathways:

Sensory receptors detect stimuli and send signals to the CNS.

Motor neurons transmit commands from the CNS to effectors (muscles and glands).

Action Potentials

Overview: Action potentials are rapid, transient changes in the membrane potential of neurons that allow for the transmission of electrical signals. They follow an all-or-none principle and involve a sequence of depolarization and repolarization phases, crucial for neuronal communication.

Resting Membrane Potential:

Maintained at approximately -70 mV when the neuron is not firing.

Only leakage channels are open; voltage-gated Na+ and K+ channels are closed.

Graded Potentials:

Localized changes in membrane potential that can vary in amplitude.

Can be hyperpolarizing (making the inside more negative) or depolarizing (making the inside less negative).

If graded potentials reach threshold (-55 mV), they trigger an action potential.

Depolarization:

Initiated when the membrane potential reaches the threshold.

Voltage-gated Na+ channels open, allowing Na+ ions to flow into the neuron.

This influx causes a rapid increase in positive charge inside the cell, leading to depolarization.

Repolarization:

Occurs after the peak of the action potential.

Voltage-gated K+ channels open, allowing K+ ions to exit the neuron.

This outflow restores the negative membrane potential, returning it towards resting state.

Propagation:

The action potential travels along the axon without losing strength.

Each segment of the axon undergoes depolarization and repolarization sequentially, propagating the signal down the length of the neuron.