Nervous System Overview and Functions

Two control systems coordinate body activities: the nervous system and the endocrine system.
The nervous system rapidly coordinates activities, while the endocrine system manages activities that require longer durations.
Both systems influence target cells via chemical messengers.
The nervous system can control endocrine secretion, as seen in the hypothalamic control over the pituitary gland.

The nervous system has three main functional divisions: the sensory division, the motor division, and the integrating center.
The sensory division receives sensory information from receptors.
The motor division transmits signals from the central nervous system (CNS) to effector organs, including upper and lower motor neurons.
The integrating center, located in the CNS, processes sensory information and generates motor commands. This occurs at three levels: the spinal cord, the lower brain level (brain stem and diencephalon), and the higher brain level (cortex).
The spinal cord controls immediate, automatic, and daily activities.
The lower brain level manages autonomic regulation, unconscious controls of respiration, equilibrium, and posture.
The higher brain level is responsible for sensory functions, motor activities, language, and memory.

A synapse is the junction between two neurons.
Synaptic transmission occurs by the release of chemical messengers called neurotransmitters.
Transmission at the synapse involves the transfer of an impulse (action potential) from one neuron to another.

The synaptic knob contains:
- Clear vesicles containing rapidly acting transmitters, like acetylcholine.
- Mitochondria.

The synaptic cleft is the gap between the presynaptic and postsynaptic membranes.
It is about 30 - 50 nm wide and contains extracellular fluid (ECF) with ions like sodium (Na+) and chloride (Cl-).

The arrival of the action potential at the presynaptic nerve terminal opens voltage-gated calcium (Ca++) channels.
Ca++ enters the synaptic knob, triggering the release of neurotransmitters into the synaptic cleft.
Neurotransmitters bind to receptors on the postsynaptic membrane, causing changes in permeability that lead to:
- Excitatory postsynaptic potential (EPSP) - depolarization
- Inhibitory postsynaptic potential (IPSP) - hyperpolarization
The neurotransmitters are removed from the synaptic cleft by:
- Inactivation by enzymes
- Passive diffusion away from the synapse
- Active reuptake by the axon terminal
- Removal by glial cells

Excitatory Postsynaptic Potential (EPSP): Local depolarization of the postsynaptic membrane.
- Caused by the binding of excitatory neurotransmitters (e.g., acetylcholine) to their receptors.
- Increases excitability of the membrane.
- Occurs due to the opening of ligand-gated sodium (Na+) channels, allowing Na+ entry.
Inhibitory Postsynaptic Potential (IPSP): Local hyperpolarization of the postsynaptic membrane.
- Caused by the binding of inhibitory neurotransmitters (e.g., GABA) to their receptors.
- Decreases excitability of the membrane.
- Occurs due to the opening of ligand-gated chloride (Cl-) and potassium (K+) channels, or the closure of ligand-gated Na+ and Ca++ channels.
Grand Postsynaptic Potential (GPSP): The sum of all EPSPs and IPSPs acting on a postsynaptic neuron.
- If excitatory and inhibitory inputs are equal, GPSP is zero.
- If excitatory is slightly greater than inhibitory, GPSP causes depolarization but doesn't reach firing level.
- If excitatory is much greater than inhibitory, GPSP causes depolarization and reaches firing level.
- If inhibitory is greater than excitatory, GPSP causes hyperpolarization.

Forward Direction: Impulses travel from the presynaptic to the postsynaptic neuron because neurotransmitters are released from the presynaptic neuron.
Synaptic Delay: The time taken for an impulse to cross a synapse, approximately 0.5 milliseconds.
Fatigue: Decrease in the rate of impulse discharge from the postsynaptic neuron after prolonged high-frequency stimulation of the presynaptic neuron.
Synaptic Plasticity: The ability of synapses to change their strength in response to activity.
- Synapses can be strengthened or weakened short-term or long-term.

Changes in Internal Environment:
- pH of blood: Alkalosis (increased pH) increases excitability and synaptic transmission. Acidosis (decreased pH) decreases excitability and synaptic transmission.
- Hypoxia (decreased oxygen supply): Leads to acid accumulation and decreased synaptic transmission.
- Hypoglycemia (low blood sugar): Decreases synaptic transmission because glucose is the energy source for the brain.
- Hormones: Can either facilitate or inhibit synaptic transmission (e.g., thyroid hormones facilitate transmission).
- Water and Electrolytes: Low levels of calcium (Ca++) facilitate synaptic transmission.
Drugs:
- Theophylline and caffeine: Facilitate synaptic transmission.
- Strychnine: Blocks inhibitory chemical transmitters, leaving excitatory transmission unopposed.
- Anesthesia and Hypnotics: Decrease synaptic transmission.